Astronomy

(left) The Lunar Phases (from new, waxing crescent, first quarter, waxing gibbous, full waning gibbous, last quarter, waning crescent and back to new)

Earth’s Moon

The Moon has fascinated mankind throughout the ages. By simply viewing with the naked eye, one can discern two major types of terrain: relatively bright highlands and darker plains. By the middle of the 17th century, Galileo and other early astronomers made telescopic observations, noting an almost endless overlapping of craters. It has also been known for more than a century that the Moon is less dense than the Earth. Although acertain amount of information was ascertained about the Moon before the space age, this new era has revealed many secrets barely imaginable before that time. Current knowledge of the Moon is greater than for any other solar system object except Earth. This lends to a greater understanding of geologic processes and further appreciation of the complexity of terrestrial planets.

On July 20, 1969, Neil Armstrong became the first man to step onto the surface of the Moon. He was followed by Edwin Aldrin, both of the Apollo 11 mission. They and other moon walkers experienced the effects of no atmosphere. Radio communications were used because sound waves can only be heard by traveling through the medium of air. The lunar sky is always black because diffraction of light requires an atmosphere. The astronauts also experienced gravitational differences. The moon’s gravity is one-sixth that of the Earth’s; a man who weighs 82 kilograms on Earth weighs only 14 kilograms on the Moon.

The Moon is 384,403 kilometers distant from the Earth. Its diameter is 3,476 kilometers. Both the rotation of the Moon and its revolution around Earth takes 27 days, 7 hours, and 43 minutes. This synchronous rotation is caused by an unsymmetrical distribution of mass in the Moon, which has allowed Earth’s gravity to keep one lunar hemisphere permanently turned toward Earth. Optical liberations have been observed telescopically since the mid-17th century. Very small but real liberations (maximum about O°.O4) are caused by the effect of the Sun’s gravity and the eccentricity of Earth’s orbit, perturbing the Moon’s orbit and allowing cyclical preponderances of torque in both east-west and north-south directions.

Four nuclear powered seismic stations were installed during the Apollo project to collect seismic data about the interior of the Moon. There is only residual tectonic activity due to cooling and tidal forcing, but other moonquakes have been caused by meteor impacts and artificial means, such as deliberately crashing the Lunar Module into the moon. The results have shown the Moon to have a crust 60 kilometers (37 miles) thick at the center of the near side. If this crust is uniform over the Moon, it would constitute about 10% of the Moon’s volume as compared to the less than 1% on Earth. The seismic determinations of a crust and mantle on the Moon indicate a layered planet with differentiation by igneous processes. There is no evidence for an iron-rich core unless it were a small one. Seismic information has influenced theories about the formation and evolution of the Moon.

The Moon was heavily bombarded early in its history, which caused many of the original rocks of the ancient crust to be thoroughly mixed, melted, buried, or obliterated. Meteoritic impacts brought a variety of “exotic” rocks to the Moon so that samples obtained from only 9 locations produced many different rock types for study. The impacts also exposed Moon rocks of great depth and distributed their fragments laterally away from their places of origin, making them more accessible. The underlying crust was also thinned and cracked, allowing molten basalt from the interior to reach the surface. Because the Moon has neither an atmosphere nor any water, the components in the soils do not weather chemically as they would on Earth. Rocks more than 4 billion years old still exist there, yielding information about the early history of the solar system that is unavailable on Earth. Geological activity on the Moon consists of occasional large impacts and the continued formation of the regolith. It is thus considered geologically dead. With such an active early history of bombardment and a relatively abrupt end of heavy impact activity, the Moon is considered fossilized in time.

The Apollo and Luna missions returned 382 kilograms of rock and soil from which three major surface materials have been studied: the regolith, the maria, and the terrae. Micrometeorite bombardment has thoroughly pulverized the surface rocks into a fine-grained debris called the regolith. The regolith, or lunar soil, is unconsolidated mineral grains, rock fragments, and combinations of these which have been welded by impact-generated glass. It is found over the entire Moon, with the exception of steep crater and valley walls. It is 2 to 8 meters thick on the maria and may exceed 15 meters on the terrae, depending on how long the bedrock underneath it has been exposed to meteoritic bombardment.

The dark, relatively lightly cratered maria cover about 16 percent of the lunar surface and is concentrated on the nearside of the Moon, mostly within impact basins. This concentration may be explained by the fact that the Moon’s center of mass is offset from its geometric center by about 2 kilometers in the direction of Earth, probably because the crust is thicker on the farside. It is possible, therefore, that basalt magmas rising from the interior reached the surface easily on the nearside, but encountered difficulty on the farside. Mare rocks are basalt and most date from 3.8 to 3.1 billion years. Some fragments in highland breccias date to 4.3 billion years and high resolution photographs suggest some mare flows actually embay young craters and may thus be as young as 1 billion years. The maria average only a few hundred meters in thickness but are so massive they frequently deformed the crust underneath them which created fault-like depressions and raised ridges.

The relatively bright, heavily cratered highlands are called terrae. The craters and basins in the highlands are formed by meteorite impact and are thus older than the maria, having accumulated more craters. The dominant rock type in this region contain high contents of plagioclase feldspar (a mineral rich in calcium and aluminum) and are a mixture of crustal fragments brecciated by meteorite impacts. Most terrae breccias are composed of still older breccia fragments. Other terrae samples are fine-grained crystalline rocks formed by shock melting due to the high pressures of an impact event. Nearly all of the highland breccias and impact melts formed about 4.0 to 3.8 billion years ago. The intense bombardment began 4.6 billion years ago, which is the estimated time of the Moon’s origin.

Moon Statistics

Characteristic	Measurement
Mass (kg)	7.349e+22
Mass (Earth = 1)	1.2298e-02
Equatorial radius (km)	1,737.4
Equatorial radius (Earth = 1)	2.7241e-01
Mean density (gm/cm^3)	3.34
Mean distance from Earth (km)	384,400
Rotational period (days)	27.32166
Orbital period (days)	27.32166
Mean orbital velocity (km/sec)	1.03
Orbital eccentricity	0.05
Tilt of axis	6.68°
Orbital inclination	18.3-28.6°
Equatorial surface gravity (m/sec^2)	1.62
Equatorial escape velocity (km/sec)	2.38
Visual geometric albedo	0.12
Magnitude (Vo)	-12.74
Mean surface temperature (day)	107°C
Mean surface temperature (night)	-153°C
Maximum surface temperature	123°C
Minimum surface temperature	-233°C

Views of the Moon

The following is a collection of images showing the moon.

Apollo 17 - Whole Moon View

This full disc of the Moon was photographed by the Apollo 17 crew during their trans-Earth coast homeward following a successful lunar landing mission in December 1972. Mare seen on this photo include Serentatis, Tranquillitatis, Nectaris, Foecunditatis and Crisium.

Far Side of the Moon

This image was taken by Apollo 11 astronauts in 1969. It shows a portion of the Moon’s heavily cratered far side. The large crater is approximately 80 kilometers in diameter. The rugged terrain seen here is typical of the farside of the Moon.

Lunar South Pole

This mosaic is composed of 1,500 Clementine images of the south polar region of the Moon. The top half of the mosaic faces Earth. Clementine has revealed what appears to be a major depression near the lunar south pole (center), evident from the presence of extensive shadows around the pole. This depression probably is an ancient basin formed by the impact of an asteroid or comet. A significant portion of the dark area near the pole may be in permanent shadow, and sufficiently cold to trap water of cometary origin in the form of ice. The impact basin Schrodinger (near the 4 o’clock position) is a two-ring basin, about 320 kilometers in diameter which is recognized to be the second youngest impact basin on the Moon. The center of Schrodinger is flooded by lavas. A volcanic vent seen in the floor of Schrodinger is one of the largest single explosive volcanoes on the Moon. The Apollo 11 Lunar Module (LM) ascent stage, with Astronauts Neil A. Armstrong and Edwin E. Aldrin Jr. aboard, is photographed from the Command and Service Module (CSM) during rendezvous in lunar orbit. The LM was making its docking approach to the CSM. Astronaut Michael Collins remained with the CSM in lunar orbit while the other two crewmen explored the lunar surface. The large, dark-colored area in the background is Smyth’s Sea, centered at 85 degrees east longitude and 2 degrees south latitude on the lunar surface (nearside). This view looks west. The Earth rises above the lunar horizon.

Apollo 11 - Flag

Astronaut Edwin E. Aldrin Jr., lunar module pilot, poses for a photograph beside the deployed United States flag during Apollo 11 extravehicular activity on the lunar surface. The Lunar Module Eagle is on the left. The footprints of the astronauts are clearly visible in the soil of the Moon. This picture was taken by Astronaut Neil A. Armstrong, commander, with a 70mm lunar surface camera.

Apollo 11 - Earth from the Moon

This view of the Earth rising over the Moon’s horizon was taken from the Apollo 11 spacecraft. The lunar terrain pictured is in the area of Smyth’s Sea on the nearside.

Apollo 11 - Footprint on the Moon

A close-up view of an astronaut’s footprint in the lunar soil, photographed with a 70mm lunar surface camera during the Apollo 11 extravehicular activity (EVA) on the Moon.

Apollo 15 - Lunar Roving Vehicle

This is a view of the Lunar Roving Vehicle photographed alone against the desolate lunar background during an Apollo 15 lunar surface extravehicular activity (EVA) at the Hadley-Apennine landing site. This view is looking north. The west edge of Mount Hadley is at the upper right edge of the picture. Mount Hadley rises approximately 4,500 meters above the plain. The most distant lunar feature visible is approximately 25 kilometers away.

Apollo 17 - Taurus-Littrow Landing Site

This is the landing site of the last Apollo mission (Apollo 17). It was in the valley among the Taurus-Littrow hills on the southeastern rim of Mare Serenitatis. Astronauts Eugene Cernan and Harrison H. Schmitt explored the valley with the aid of an electrically powered car. This image shows Schmitt inspecting a huge boulder that has rolled down the side of an adjacent hill.

Apollo 17 - Large Lunar Boulder

Earth in the far distant background is seen above a large lunar boulder on the Moon. This photo was taken with a handheld Hasselblad camera by the last two Moon walkers in the Apollo Program.

Apollo 17 - Lunar Scape

This image is an excellent view of the desolate lunar space at Station 4 showing scientist-astronaut Harrison H. Schmitt, lunar module pilot, working at the Lunar Roving Vehicle during the second Apollo 17 extravehicular activity at the Taurus-Littrow landing site. This is the area where Schmitt first spotted the orange soil which is visible on either side of the Lunar Roving Vehicle in this picture. Shorty Crater is to the right, and the peak in the center background is Family Mountain. A portion of South Massif is on the horizon at the left edge.

Apollo 17 - Orange Soil

These orange glass spheres and fragments are the finest particles ever brought back from the Moon. The particles range in size from 20 to 45 microns. The orange soil was brought back from the Taurus-Littrow landing site by the Apollo 17 crewmen. Scientist-Astronaut Harrison J. Schmitt discovered the orange soil at Shorty Crater. The orange particles, which are intermixed with black and black-speckled grains, are about the same size as the particles that compose silt on Earth. Chemical analysis of the orange soil material has show the sample to be similar to some of the samples brought back from the Apollo 11 (Sea of Tranquility) site
several hundred miles to the southwest. Like those samples, it is rich in titanium (8%) and iron oxide (22%). But unlike the Apollo 11 samples, the orange soil is unexplainably rich in zinc. The orange soil is probably of volcanic origin and not the product of meteorite impact.

Apollo 17 - Oblique view of Copernicus

This is an oblique view of the large crater Copernicus on the lunar nearside, as photographed from the Apollo 17 spacecraft in lunar orbit.

Impact cratering is a process found everywhere in the solar system except on the giant gaseous planets. Earth has been heavily impacted but erosion has removed most of the craters.

Perhaps the finest surviving impact crater on Earth is the Barringer Meteor Crater near Winslow, Arizona. It is 1.2 kilometers across and 200 meters deep. It was formed about 49,000 years ago when a 50 meter nickel/iron meteorite struck the desert at a speed of 11 kilometers per second. Native Americans living in the region observed the impact and felt the tremendous shock wave that must have moved through the atmosphere.

An examination of actual craters, almost any image of the Moon will do, will prepare the students for this activity. Just about all craters have deep central depressions, raised rims, and a blanket of ejected material surrounding them.

You and your students can observe the Moon directly during daylight. Check your newspaper for the phases of the Moon and observe it in the afternoon during “first quarter” and in the morning during “third quarter.” The Moon will be separated from the Sun by 90 degrees to the east (left) at first quarter and 90 degrees to the west (right) during third quarter. The large dark regions are the remains of very great impacts and many retain their circular boundaries. Binoculars on a tripod provide a spectacular view.

You can create craters in the classroom with a box, lined with a trash bag, with sides at least 10 centimeters high (the lid to photocopier paper boxes is perfect); flour (8 to 10 centimeters deep with at least 2.5 centimeters of clearance to the box rim), some dry (powdered) tempra paint (red or blue), and some marbles.

Place the flour in the box and smooth and firmly pack it (experiment with different degrees of firmness). Place a dusting of the paint powder over the flour (colored water in a spray bottle works, but not as well). Use the marbles to bombard the surface (one at a time). Look for classical cratering features: basin, raised rim, ejecta blanket (material excavated from the crater and dumped around it, visible as white flour on the colored powder), and rays (material shot out at high velocity forming lines pointing directly away from the impact site).

Students should keep careful records and can do top and profile drawings of the craters and compare craters formed by different size projectiles, different velocities, and different angles of impact. Different size projectiles can be dropped from measured heights so that they will have common velocities. They should also remember that the quality of their tests is more important than quantity.

After several craters, the flour and tempra can be mixed and re-smoothed without changing the white of the flour too much. Then a new layer of tempra can be applied and additional experiments conducted. In real impacts the impacting object is destroyed or broken up into small chunks. Of course the marble will not do this and will remain whole in the crater.

Vocabulary

Central Peak

A mountain found in the center of large craters. It is formed by a “rebound” of the rock at the impact site (the marble will be sitting there in this activity).

Crater

A (usually) circular depression in a surface caused by an impact.

Ejecta

Material tossed out of the crater.

Ejecta Blanket

Ejecta tossed out at low speed. The material lies like a blanket around the crater.

Floor

The interior of the crater. It is flat in large craters (the marble will be there in this activity).

Rays

Ejecta tossed out of the crater at high speed. The material forms long lines pointing directly away from the crater.

Rim

The raised edge of the crater. It is formed by the outwards and upwards compression of the crater walls, not ejecta.

Clouds from Space. Shuttle astronauts are clearly fascinated by the topside view of Earth’s atmospheric patterns that space flight provides, since every space shuttle crew takes a significant number of photographs of clouds. In the past two years, interest in clouds has increased considerably as scientists attempt to understand global warming and the greenhouse effect. Efforts to predict climatic changes associated with global warming have focused new attention on the warming and cooling properties of clouds. The picture is a complex one, involving competing feedback mechanisms, and is not fully understood at this time. All clouds block some fraction of the incoming solar radiation, and absorb some fraction of the heat radiated back from the Earth’s surface, and the balance between these two processes is hard to quantify. However, contemporary thinking suggests that the lower altitude cumulus clouds (such as pictures Thunderstorms, Brazil and Cumulus Cloud Tops) have a net cooling effect on Earth’s surface, reflecting heat back to space. Conversely, the higher, thin cirrus clouds (such as pictures Jet Stream Cirrus and Jet Stream Cirrus, Saudi Arabia) trap heat, reflecting it back to the surface of Earth.

Current data suggest that the cooling effects of great masses of cumulus storm clouds over the ocean at mid latitudes outweigh the heating effects of the upper-level cirrus clouds when considered on a global scale. Nevertheless, there is cause for concern because many models of global warming predict a decline in heavy mid-latitude cumulus storm clouds in the future. The amount of high-level cirrus cloud is predicted to rise as the cumulus decreases. If environmental and climatic changes result in altered weather and atmospheric patterns that adhere to these models, such changes will in turn induce accelerated global warming

Images of Clouds From Space

Jet Stream Cirrus

This photograph taken from about 320 kilometers above the Earth shows a band of cirrus clouds produced by a westerly jet stream that stretches across the Red Sea from Sudan to Saudi Arabia. The contained uniformity of the cloud formation reflects the narrow track of the jet stream moving from left to right across the frame. The shuttle photo shows that the cloud band comprises a series of distinct and precisely spaced roll clouds. These are created by a rolling motion in the upper level air current.

Florida Squall Line

The shuttle crew approached this storm system from its southern margin in the Gulf of Mexico. The margins are clearly defined. The clouds in the storm system rise to about 16,500 meters. April squall lines,/b> of this type are often associated with tornado development across the southeastern states.

Thunderstorms, Brazil

These cumulus thunderheads near São Paulo, Brazil, where photographed from almost directly overhead by the STS 41-B crew. This perspective conveys something of the energy that drives these cloud columns to punch up into the atmosphere,/b>. The foreshortening resulting from the near-vertical viewing angle disguises the fact that the cloudheads so prominently in view are but the tops of massive thunderhead storm clouds that can tower up to 18,000 meters in the tropics.

Cumulus Cloud Tops

The STS 41-B crew shot this oblique photograph just moments after the previous picture was taken. Some more fully developed thunderheads can be seen in the same Brazilian storm. When the rising cumulus columns meet the tropopause, or base of the stratosphere, at about 15,000 kilometers, they reach a ceiling and can no longer rise buoyantly by convection. The stable temperature of the stratosphere suppresses further adiabatic ascent of moisture that has been driven through the troposphere by the 5-6.8 degree/kilometer lapse rate. Instead, ice clouds spread horizontally into the extended cirrus heads seen in this photograph, forming the “anvil heads” that we identify from the ground. The finer, feathery development around the edges of some of the thunderheads is glaciation - water vapor in the cloud is turning to ice at high altitude.

Cloud Margin, Bering Sea

All that can be seen in this photograph is cloud stretching several hundred kilometers to the limb of the Earth, yet it tells us a great deal about the water in the Bering Sea below. The line or cloud margin running diagonally across the frame with dense, thick cloud to the right and lighter, more broken cloud to the left reflects an ocean current margin. A difference in water temperature on either side of the margin is reflected in the cloud forms condensing above. This striking cloud boundary stretches for 800-960 kilometers in this photograph.

Coastal Current, Namibia

Condensing moisture from ocean currents in some parts of the world creates clouds that stay uniformly in position above that current for months at a time. This example shows clouds hanging above the cold Benguela current, which travels northward along the Atlantic coast of southwestern Africa. It is interesting that while the ocean is densely cloud-covered and the clouds lap at the coast, they never cross the coastline. The pinkish-colored Namib Desert is one of the driest places on Earth, confirming that the cloud associated with the ocean current does not stray off its prescribed track. Indeed the Namib Desert is home to unique inhabitants - insects with leg hairs especially adapted to collect moisture from morning dew - a strange irony of life on Earth where moisture-laden clouds hang so closeby.

Unique Cloud Lanes, Oman

These wispy rows of cloud or “cloud lanes” are recognized as a “landmark” by successive shuttle crews. This unique cloud formation off Oman is virtually constant at certain times of year. The clouds are created by a small vortex in the low level wind current. There is little difference between the ocean and atmosphere temperatures here, but the air current may have been subjected to heating from the Somali Current.

Jet stream Cirrus, Saudi Arabia

This series of cirrus clouds is know as “roll clouds” because they are sculpted into tight rolls by air currents from the jet stream over Saudi Arabia and the Red Sea. The crest-to-crest spacing of the cloud bands can be used to calculate the velocity of the jet stream.

Jet Stream Convergence

This photograph taken over Namibia reveals another effect of jet streams. Here two streams converge; cloud has formed in the corridor between the two streams. Turbulence along the margins of the jet stream may explain the sharp boundary. The point of convergence of the two air streams is precisely located by this photograph. Shadows mark the cloud edges against a sunlit Namibian backdrop.

Cloud Streets, Tiladumati Atoll, Maldive Islands

Small cumulus clouds frequently form in parallel rows or “cloud streets” in stable air conditions. These cloud streets over the reefs of the Maldive Islands in the Indian Ocean denote the prevailing wind direction, the cloud streets lying parallel with the wind. Turbulent air lifted by the windward portions of the islands promotes cloud formation downwind.

Island Wake, Hawaii

The combination of warm water temperature and hundreds of square kilometers of ocean, uninterrupted by land masses, results in a regular cumulus and stratocumulus cloud formation. In the Pacific Ocean the Trade Winds propel the clouds from east to west across the ocean. When the air current is intercepted by a sufficiently high land mass, such as the Big Island of Hawaii, the stable cloud pattern is interrupted and the clouds divide to bypass the island in a wide arc forming an “island wake”. In addition to illustrating how gracefully the clouds circumnavigate Hawaii’s volcanic peaks, the photograph shows how the prevailing wind direction dictates that the north and northeast of the island are wetter than the western side of the island and frequently under cloud. The clouds deposit rain on the low ground before dividing and spinning out to sea when they meet the Kohala Mountains and Mauna Kea with its summit at 4,205 meters.

Cloud Tail, Lake Tana

Islands or high land, elevated above the surroundings and interrupting the air stream, can produce “tails” as well as “wakes.” Shuttle astronauts have frequently observed Dek Island in Lake Tana in Ethiopia, the source of the Blue Nile, with a well-developed cloud tail. This occurs when the land mass disrupts the air flow, creating downwind turbulence that promotes condensation. The lake stands at 1,800 meters above sea level.

Open Cells over Ocean

Open cell formations like this are frequently found over ocean. The cells are denser to the left of the frame than to the right, suggesting a gradual warming in water temperature. By looking at this photograph and studying the water color and cloud density, an expert could tell you which ocean you are looking at, the time of year, and the temperature of the water below. This picture was taken in the Indian Ocean, north of Australia.

Anticyclonic Clouds

This pinwheel of anticyclonic clouds was photographed by the STS 41-B crew over the southern hemisphere of the Pacific Ocean. The ground winds at the center of this cyclonic system reach 80 kilometers per second. Circular storms in the northern hemisphere produce spiraling clouds with a clockwise pattern, while southern latitude storms have a counterclockwise cloud motion.

Eye of Hurricane Kamysi

During the Solar Maximum Satellite Repair Mission, astronauts had an excellent opportunity to look down the eye of Hurricane Kamysi over the Indian Ocean. Clear blue water can be seen through the hurricane’s eye, and the crew reported that they could see the ocean wave below. Unfortunately, the camera film could not pick them out.

Typhoon Odessa

Odessa is one of the strongest circular storm patterns seen by shuttle crews to date and has a superb tightly formed eye. The tighter the eye in a circular storm, the stronger the winds underneath. Mission STS 51-1 came to be known as the mission of all the hurricanes, tracking no less than four circular storms around the globe. Live pictures from Discovery of Hurricane Elena in the Gulf of Mexico were transmitted directly from Mission Control in Houston to the National Hurricane Center in Florida for correlation with conventional weather satellite and high level aircraft data.

Sunrise

Space shuttle crews see a sunrise or sunset every 45 minutes as they circle the Earth at 27,300 kilometers per hour, crossing the surface at 6.4 kilometers per second. From their unique perspective they see clearly defined bands of color through the atmosphere as the sun rises. High-peaking cumulus clouds, topping out in anvil-head cirrus can be seen as black shadows against the sunlit horizon. The brightness of the colors in the atmosphere in this photograph taken over the South China Sea is due to concentrations of dust in the atmosphere. Greater concentrations of dust are found in equatorial regions. There are various sources for such upper level dust. Many dust storms in Africa, intensified by several years of drought, have been responsible for putting large amounts of dust into the atmosphere in recent times. Ash clouds from major volcanic eruptions can have a similar effect. Recent discussion of the climatic and environmental effects of a “nuclear winter” centering on upper atmosphere pollution has drawn from the atmospheric effects of catastrophic volcanic eruptions.

Weather System Margin

The Discovery crew photographed this very distinct stripe running through the clouds for several hundred kilometers. Two weather systems are sliding past each other like crustal plates on the Earth’s surface. The one at the top of the photographM (geographical north) is moving up and curing away slightly to the north, while the system at the bottom of the frame is moving westward and curving gently to the south in conjunction with a cyclone located several hundred kilometers away. The miniature cold-water gyres on the fringes of the two weather systems indicate that a channel of colder water runs under the break in the clouds and is reflected above where colder air runs between the two cloud masses.

Like Sigmund Jähn, those who have gone into space have come back with a changed perspective and reverence for the planet Earth. Gone are the political boundaries. Gone are the boundaries between nations. We are all one people and each is responsible for maintaining Earth’s delicate and fragile balance. We are her stewards and must take care of her for future generations.

Our perspective on Earth can be very narrow. We may not see the effects of one tree that is cut down. Only by expanding our perspective can we see entire rain forests that have been devastated. Humans can destroy in a matter of days that which nature took thousands of years to create. We might ask what harm can one factory do to the environment by not meeting proper pollution controls. The effect from space is obvious. Pictures taken by Gemini astronauts almost 30 years ago are much clearer than those taken by space shuttle astronauts today.

The following quotations are taken from astronauts who have gone into space and the effect that it had upon them:

Quotes From Astronauts

My first view - a panorama of brilliant deep blue ocean, shot with shades of green and gray and white - was of atolls and clouds. Close to the window I could see that this Pacific scene in motion was rimmed by the great curved limb of the Earth. It had a thin halo of blue held close, and beyond, black space. I held my breath, but something was missing - I felt strangely unfulfilled. Here was a tremendous visual spectacle, but viewed in silence. There was no grand musical accompaniment; no triumphant, inspired sonata or symphony. Each one of us must write the music of this sphere for ourselves. — Charles Walker, USA

My view of our planet was a glimpse of divinity. — Edgar Mitchell, USA

For the first time in my life I saw the horizon as a curved line. It was accentuated by a thin seam of dark blue light - our atmosphere. Obviously this was not the ocean of air I had been told it was so many times in my life. I was terrified by its fragile appearance. — Ulf Merbold, Federal Republic of Germany

A Chinese tale tells of some men sent to harm a young girl who, upon seeing her beauty, become her protectors rather than her violators. That’s how I felt seeing the Earth for the first time. “I could not help but love and cherish her. — Taylor Wang, China/USA

Images From Space

Glaciers, lakes and fault zone, Tibet

The Tibet plateau is the largest and highest elevated region in the world. The plateau is 1,200 kilometers from east to west and 900 kilometers north to south, with a mean elevation of more than 400 meters. Because the plateau rises above so much of the atmosphere, photographs are typically brilliantly crisp and clear. A plethora of geological features are visible in any frame. This picture shows the northwest corner of the plateau near the point where the ground falls away to the Tarim Basin. The impressive snow-capped mountain at top right with well-developed valley glaciers is Muztag Ulu, which has an elevation of 7,282 meters. The plateau was elevated as a consequence of the collision between India and Asia, which resulted in extensive shortening by overthrusting and folding.

A second important consequence of the collision was major strike slip faulting, facilitating the tectonic “escape” of China like a squeezed melon seed. The linear valley with two lakes may be the site of a strike slip fault. At the lower right corner, two light-toned outcrops are also apparently displaced some 300 kilometers by the left lateral fault.The blue lake at center shows extensive terraces around its northern shores. During glacial times, lake levels on the plateau stood as much as 300 meters higher than at the present day. Since the end of the Ice Age, the climate has become increasingly arid and lakes have shrunk. The bounding Himalayan and Kun Lun mountain ranges act as effective barriers to moisture-laden winds.

Radar Image of Mount Everest

This is a radar image of Mount Everest and its surroundings, along the border of Nepal and Tibet. The peak of Mount Everest, the highest elevation on Earth at 8,848 meters, can be seen near the center of each image. It shows an area approximately 70 by 38 kilometers that is centered at 28.0 degrees north latitude and 86.9 degrees east longitude. North is toward the upper left. Many features of the Himalayan terrain are visible in the image. Snow covered areas appear bright blue in the image which was taken in early spring and shows deep snow cover. The curving and branching features seen are glaciers. Radar is sensitive to characteristics of the glacier surfaces that are not detected by conventional photography, such as the ice roughness, water content and stratification. For this reason, the glaciers show a variety of colors (blue, purple, red, yellow, white) but only appear as gray or white in an optical photograph.

Lost City of Ubar, Southern Oman, Arabian Peninsula

This is a radar image of the region around the site of the lost city of Ubar in southern Oman, on the Arabian Peninsula. The ancient city was discovered in 1992 with the aid of remote sensing data. Archeologists believe Ubar existed from about 2800 B.C. to about 300 A.D. and was a remote desert outpost where caravans were assembled for the transport of frankincense across the desert. The prominent, magenta colored area is a region of large sand dunes. The prominent green areas are rough limestone rocks, which form a rocky desert floor. A major wadi, or dry stream bed, runs across the middle of the image and is shown largely in white due to strong radar scattering.

Dune fields, Namibian Coastal Desert

The arid coastal plain that forms the Namib Desert extends the entire length of the Atlantic coast of South West Africa, a total of more than 800 kilometers. Its width varies between 40 and 140 kilometers. The intricate pattern of large sand dunes is caused mainly by dry westerly winds cooled by the offshore Benguela current. Some of the dunes are extremely large, exceeding 300 meters. Running diagonally downward from the upper right corner is a dune-free tongue of alluvial gravel known as the Sossusvlei. This is formed by occasional flash floods draining from the barren, rocky hills on the right of the picture.

Oblique View, Galapagos Islands, Pacific Ocean

The Galapagos archipelago lies 1,000 kilometers west of Ecuador and 1,500 kilometers southwest of the Panama Canal. Geologically the islands sit on the Galapagos rift, an offshoot of the East Pacific Rise. The chain of young volcanic islands - 13 large islands and many smaller ones - straddles the equator, stretches between 1° north and 1°3′ south, and lies between 89 and 92° west longitude. With the exception of Isabella, the largest island, the islands are roughly circular in shape with high volcanic craters at the island centers, that rise to 1,520 meters. Numerous eruptions have taken place on the islands within historic times. However, the detailed geology of the islands is only now coming under investigation, since most are extremely inaccessible. A major eruption on Fernandina Island in 1974 went unnoticed on the ground until it was observed by astronauts aboard the Skylab 4 spacecraft. The islands are largely desolate lava piles with little vegetation along the coastlines. However,
the high volcanic mountains generate rains that have mantled the summits with dense jungle. In this photograph, clouds can be seen forming over the high points of individual islands. The islands are famous not only for their volcanic associations but also for the peculiar flora and fauna that result from isolation from any continental mainland.

Canton Atoll, Phoenix Islands, Pacific Ocean

Canton Atoll is a good example of a long-lived coral atoll. Like Tupai, it probably originated as a fringing reef developed around a volcanic island that has long since disappeared. Unlike Tupai, however, it is far distant from any above-surface volcanic structure. Its parent volcano long ago subsided deep beneath the sea. The atoll lies only 2.5° from the equator and is subjected to long periods of drought. Although it is the largest island in the Phoenix group, only 9 square kilometers rise above sea level. The island was discovered in the early 19th century and was named after an American whaling ship wrecked there in 1854. For several decades, American companies extracted the valuable guano. However, in the 20th century, Canton’s attraction was as a refueling stop for aircraft on long-haul flights across the Pacific. Hence, the island has a long runway on the north shore and the designation, on maps, of the lagoon as a seaplane anchorage.However, advances in aircraft design eliminated the island’s role as a refueling stop. With no economic role and insufficient soil to support crops, the island does not support permanent habitation. Patterns of coral heads growing within the shallow water of the lagoon are clearly visible as a thin white network.

Tahaa, Raiatea, Bora Bora and Tupai Atolls, Pacific Ocean

This chain of coral-fringed islands forms the Leeward Island chain within the French Society Islands. At bottom right are the islands of Tahaa and Raiatea. They are old, eroded volcanoes, fringed by a coral reef. Northward along the chain, the original central volcanoes are older and more heavily eroded. On Bora Bora (center), the reef is prominently developed and the island significantly eroded. The northernmost island, Tupai, is merely an atoll, having lost any relic of the volcano around which the reef originally grew, except for the shallow floor of the lagoon, showing up in turquoise. This sequence provides an excellent illustration of the hypothesis first propounded by Charles Darwin to explain the origin of coral reefs in deep oceans. Reef-building corals can only live in shallow waters of 20 meters, in temperatures over 21° centigrade. Initially, corals formed fringing reefs around volcanic islands. Old volcanoes are very rapidly eroded in tropical climates until they reach sea level. Below sea level, the rate of erosion is much slower, and atolls such as Tupai might exist for long periods. If for geological reasons the original volcano subsides below sea level at a slow enough rate, corals will continue to build, thus preserving an atoll at the surface long after the original volcanic edifice has been deeply submerged.

Great Barrier Reef, East Coast of Australia

The Great Barrier Reef is the largest structure ever built by living organisms. At least 350 different species of coral are found in the reef, which is 2,000 kilometers long and forms a natural breakwater for the east coast of Australia. Underlying sediments, twice as old as the reef itself, indicate that the region was once above sea level. Geological evidence shows that the reef began growing more than 25 million years ago. As the image shows, the “reef” is in fact composed of many individual detached reefs, separated by deep water channels. The calcareous remains of tiny creatures called coral polyps and hydrocorals provide the basic building material for the reefs while the remains of coraline algae and organisms called polyzoas provide the cement that holds the structure together. When fossilized, such reefs and the debris eroded from them form thick limestone units. The Great Barrier Reef is the largest reef on Earth at the present day. The reasons for its size and longevity are the very stablegeological setting of the Australian platform, and the favorable oceanic circulation. Coral cannot exist at temperatures below 21° centigrade. The warmth of the waters of the Australian continental shelf varies little with depth because of the stirring action of the southeast trade winds. These winds pound the outer edge of the reef for nine months of the year, and this also keeps the reef supplied with seawater rich in the organic material needed by the growing coral.

Brandberg intrusion, Namibia

The Brandberg is an isolated massif reaching 2,606 meters, and rises much higher than any other feature for hundreds of kilometers around. It is composed of a single mass of granite that rose through the Earth’s crust some 120 million years ago. Slightly south and to the west of the Brandberg is the much-eroded Messum Intrusion. Both of these intrusions reflect a period of extraordinarily widespread geological unrest in the Earth’s history, which preceded the opening of the Atlantic Ocean and the effusion of vast volumes of basaltic lavas of the Karoo formation that form the Drakensberg plateau. Karoo lavas are exposed immediately to the west of the intrusion. Rocks forced aside by the upward movement of the intrusion are visible encircling the margin of the Brandberg, tilted sharply upward. Ancient gneisses, distinguished by their lineated texture, are conspicuous along the dry river valley in the center of the frame. The existence of a set of lavas in South America of the same age and type as those of the Karoo was used for many years by some geologists as strong evidence that Africa and South America had once been united. However, their arguments were not widely accepted until geophysical data demonstrated the reality of plate tectonics.

Anticlines and salt domes, Gulf coast, Iran

One of the most spectacular examples of anticlinal fold structures lie on the north shore of the Strait of Hormuz in the Persian Gulf. Located near the important city of Bandar Abbas, these folds form the foothills of the Zagros Mountains, which run north-northwesterly through Iran. The folds were formed when the Arabian shield collided with the western Asian continental mass about 4 to 10 million years ago. Subduction still continues slightly further east, beneath Baluchistan, but is inactive in the Gulf itself. Although not obvious in the photograph, the shortening expressed by the folds is accompanied by extensive thrusting on the easterly dipping planes. All the deformation is geologically young; the folded sediments are Paleogene and Neogene. Simple anticlinal structures are well know as classic traps for hydrocarbons, and some producing wells are located in the area. The other features that are prominent in this photograph are the dark circular patches. These represent the surface expression of salt domes that have risen diapirically from the Cambrian Hormuz salt horizon through the younger sediments to reach the surface. Only in a hot arid environment such as that of the Gulf can the soluble salt escape rapid erosion. Salt domes also are frequently favorable sites for trapping hydrocarbons.

Dendritic drainage pattern, Yemen

The Republic of South Yemen lies on the edge of one of the world’s great sand seas, the Rubh-al-Khali, but even this dry desert region bears the unmistakable imprint of flowing streams and rivers. The branching pattern in the photograph could only have been produced by running water, draining off the surrounding land. These filigree patterns are termed “dendritic drainages” because of their similarity to the way in which trees branch out into progressively finer twigs. The term comes from the Greek dendrites, meaning tree-like. The dry gullies or waids appear to pose something of a paradox in an area that is apparently exceptionally arid desert, with no vestige of plant life. Freak rainstorms and flash flooding might deepen and extend the gullies, but they are far too infrequent at the present day to have produced the pattern seen here. The drainage pattern is clearly a fossil. When the Earth emerged from the last Ice Age, the Sahara and the Rubh-al-Khali were savanna grasslands with a more temperate climate and much higher rainfall than they experience today. Runoff from the coastal mountains carved the dendritic drainage pattern, which was then “fossilized” when the climate became more arid.

Sediment laden drainage, Betsiboka River, Madagascar

The Betsiboka is Madagascar’s main river, flowing for a total of 525 kilometers from north of Tananarive. The river is navigable for at least 130 kilometers inland and the lower reaches pictured here are noted for their extensive rice fields. While the red sediment being transported provides an attractive and informative example of a river estuary, it is a symptom of an ecological disaster for Madagascar. Humans have felled and cleared the island’s natural cover of tropical forest so extensively that soil erosion has been vastly accelerated. Much of the sediment visible in the river represents an irreplaceable natural asset. Brick-red lateritic soils, the result of tropical weathering, are responsible for the strong color of the sediments. Most of the deforestation in Madagascar has taken place over the last 20 years, the same period during which observations from space have been conducted. Recent observations show that very little of the original forest remains.

Uplifed Basalt Plateau Somalia Coast

Prior to the opening of the Red Sea and the separation of Arabia and Africa, the site of the future ocean was marked by regional doming, rifting, and effusion of basaltic lavas. A thick pile of dissected basalt is visible in this photograph of the north coast of Somalia, which originally joined the south coast of Arabia. The lavas from a conspicuous, dark sequence with four or five topographic steps and their upper surface exhibits a prominent paleo-drainage pattern. An unconformity separates the basalts from the underlying Precambrian basement gneisses. The photograph also reveals the hot climate and harsh desert terrain of the Somali Republic. Nothing grows on the coastal strip where rain rarely falls. The land rises in steps to a highland plateau. At an elevation of 1,500 meters the climate is more pleasant than on the coast but, at a latitude only 10° from the equator, the sun is blistering and only scrub can survive.

Folds in metasediments, Belcher Islands, Canada

What at first glance may look like swirls of paint on a blue canvas are in act the Belcher Islands in Hudson’s Bay. These unusual low-lying islands extend over about 13,000 square kilometers but have a land area of only about 2,800 square kilometers. Their ribbon-like appearance is the result of the submergence of an eroded sequence of thinly bedded, folded metasedimentary rocks, of which the harder, more resistant emerge above sea level. The rocks are of Aphebian age, 1.64 to 2.34 billion years old. The weight of the great continental ice sheets lying on northern Canada was sufficient to push the existing land below sea level by perhaps as much as 1,000 meters around Hudson’s Bay. Now that the ice has gone, the land is recovering isostatically, so the highest of the areas below sea level a few thousand years ago are now just emerging. The rate of uplift immediately after the Ice Age was about 12 centimeters per year; it has now slowed to 1 centimeter per year and this will continue for some time into the future. The rate of uplift may be slow enough that erosion is able to maintain the islands’ topography at a steady level.

Coast and Andes Mountains, Chile

The Andes mountains form one of the longest continuous mountain ranges on Earth, extending from the shores of the Caribbean as far south as the Magellan Straits. Perhaps the most surprising aspect of this range is how narrow it is over much of its length - the high part of the range is typically less than 150 kilometers broad. Illustrated is the section of the Andes near Coquimbo, Chile, where the highest peaks are 6,300 meters. Low lighting and the oblique perspective emphasize the narrowness of the range, which forms a formidable natural obstacle, and explains how the improbably long and thin country of Chile acquired its identity. In this part of the range, active volcanism is absent. The Benioff zone in this region has a very shallow dip (10°). To both north and south, the Benioff zone dips more steeply (30°) and volcanism is well developed. Clouds illuminated by the low sun hang over the Argentine Pampas beyond the Andes and illustrate the marked climatic differences between different sides of the Andes. In the south, the Chilean side of the Andes tends to be well watered and fertile, while the pampas are in rain shadow and tend to be very dry. Further north, the Chilean coast is exceptionally dry (and forms the Atacama desert) while the eastern slopes are much wetter.

Benguela Current, Plankton Bloom

Plankton find a rich feeding ground in the cold waters lying off the Namibian Desert coast. They have found a narrow corridor of cold, nutrient-rich water in the Benguela Current along the coast. Just a few kilometers out to sea, the warmer waters of the Atlantic do no support the plankton. The band of clouds across the top right of the frame has been created by the interaction of the colder waters of the current and the atmosphere, so the boundary between the cold coastal waters of the Benguela Current is clearly evident to the space observer. It is one of the subtle wonders of the fragile earthly environment that plankton, and the fish that feed on them, should find such attractive feeding grounds sandwiched between the Namibian desert, one of the driest places on earth, and the warm, nutrient-poor waters of the central Atlantic.

The Magellan spacecraft was the first planetary explorer to be launched by a space shuttle when it was carried aloft by the shuttle Atlantis from Kennedy Space Center in Florida on May 4, 1989. Atlantis took Magellan into low Earth orbit, where it was released from the shuttle’s cargo bay and fired by a solid-fuel motor called the Inertial Upper Stage (IUS) on its way to Venus. Magellan looped around the Sun one-and-a-half times before arriving at Venus on August 10, 1990. A solid-fuel motor on the spacecraft then fired, placing Magellan into a near-polar elliptical orbit around Venus.

The spacecraft carried a sophisticated imaging radar, which was used to make the most highly detailed map of Venus ever captured during its four years in orbit around Venus from 1990 to 1994. After concluding its radar mapping, Magellan also made global maps of Venus’s gravity field. Flight controllers then tested a new maneuvering technique called aerobraking, which uses a planet’s atmosphere to slow or steer a spacecraft. The spacecraft made a dramatic plunge into the thick, hot Venusian atmosphere on October 12, 1994, and was crushed by the pressure of Venus’s atmosphere. Magellan’s signal was lost at 10:02 Universal Time (3:02 a.m. Pacific Daylight Time) that day.

The Magellan mission was divided up into “cycles” with each cycle lasting 243 days (the time necessary for Venus to rotate once under the Magellan orbit). The mission proceeded as follows:

Date Mission Event

• 04 May 1989 - Launch
• 10 Aug. 1990 - Venus orbit insertion and spacecraft checkout
• 15 Sep. 1990 - Cycle 1: Radar mapping (left-looking)
• 15 May 1991 - Cycle 2: Radar mapping (right-looking)
• 15 Jan. 1992 - Cycle 3: Radar mapping (left-looking)
• 14 Sep. 1992 - Cycle 4: Gravity data acquisition
• 24 May 1993 - Aerobraking to circular orbit
• 03 Aug. 1993 - Cycle 5: Gravity data acquisition
• 30 Aug. 1994 - Windmill experiment
• 12 Oct. 1994 - Loss of radio signal
• 13 Oct. 1994 - Loss of spacecraft

In all, the highly successful imaging radar mapped more than 98 percent of the planet’s surface and collected high-resolution gravity data of Venus. The lessons learned from Magellan’s aerodynamic dive into the Venusian atmosphere will be applied to future planetary missions.

Views of Magellan

Magellan Assembly

On May 4, 1989, the Magellan spacecraft was deployed from the shuttle. The spacecraft is topped by a 3.7-meter diameter dish-shaped antenna that was a spare part left over from the Voyager program. The long, white, horn-shaped antenna, attached just to the left of the dish antenna, is the altimeter antenna that gathers data concerning the surface height of features on Venus. Most of the spacecraft is wrapped in reflective white thermal blankets that protect its sensitive instruments from solar radiation.

Deployment

The Magellan spacecraft’s deployment from the shuttle Atlantis’ cargo bay was captured by an astronaut with a hand-held camera pointed through the shuttle’s aft flight deck windows. Deployment occurred in the early evening of May 4, 1989, after Atlantis had carried Magellan and its Inertial Upper Stage (IUS) booster rocket, into low Earth orbit. Once the shuttle was safely away from the spacecraft, the IUS ignited and placed Magellan on course for its 15-month journey to Venus.

Magellan Orbiting Venus

On August 10, 1990, Magellan entered into orbit about Venus, as depicted in this artist’s view. During its 243-day primary mission, referred to as Cycle 1, the spacecraft mapped well over 80 percent of the planet with its high-resolution Synthetic Aperture Radar (SAR). The spacecraft returned more digital data in the first cycle than all previous U.S. planetary missions combined.

Magellan Mapping Venus

The sequence of events that comprise a Magellan mapping orbit are shown in this artist’s conception. For the first 37.2 minutes of each orbit, the Synthetic Aperture Radar measures and records a 20-kilometer wide swath of the planet’s surface. When Magellan reaches the high point of its orbit, the spacecraft turns its antenna toward Earth and transmits the data. After 113.8 minutes of transmitting, the antenna is repositioned for another orbit about Venus.

Volcanic features are numerous and widely scattered on Venus. These features include widespread lava plains, extensive flows, lava channels, small shields, cones, domes, intermediate to large shields, and caldera-like structures not associated with shield volcanoes. Scientists suspect that some volcanoes are still active; however, clouds from volcanic eruptions are invisible to radar. In order to detect volcanic activity, scientists must compare images taken at different times and look for changes on the planet’s surface.

The majority of volcanic materials on Venus are thought to be basaltic in composition, partly because of their appearance. This view is supported to some extent by data returned from the Soviet Vega and Venera space probes [Barsukov et al., 1982 and 1986].

Volcanic Plains

Volcanic plains typically cover large lowland areas separated by mountains or ridge belts; they extend over 85% of the planetary surface. The plains range in elevation from about 1.5 kilometers below to 2 kilometers above the mean planetary radius of 6,051 kilometers [Guest et al., 1992]. They consist of extensive sheets of flood lavas hundreds of kilometers in width and mostly 100 to 700 kilometers in length. Some plain regions are bright in radar images, but most are dark, indicating a smooth surface.

Lava Flows

Lava flows may extend from a few to hundreds of kilometers in length. They can originate from volcanoes, cracks in the crust and depressions in the surface. Often, the sources of the flood lavas are unseen.

Lava flows can be characterized by smooth surfaces and the presence of channels, levees, pressure ridges, flow margins, and flow directions. These characteristics provide important information about lava flow formation and the underlying topography. Many flows contain small volcanic shields and cones.

Lava Flow Field South of Ozza Mons

A flow field south of Ozza Mons in Atla Regio consists of numerous adjacent and overlapping flows with varying degrees of brightness. Brightness in radar images is related to several factors such as surface roughness, emissivity and the dielectic constant of the material. Emissivity is a measure of how well an object approximates a perfect blackbody radiator. It is controlled primarily by the dielectric constant. Material of high dielectric constant is generally a good reflector.

Flow fields of Mylitta Fluctus in Lavinia Planitia

Mylitta Fluctus is a complex of six flow fields in the southern hemisphere of Venus [Roberts et al., 1992]. The area is similar in size to the Columbia River flood basalt province on Earth. Each flow field is composed of numerous individual flows. Many of the flows are hundreds of kilometers long, comparatively narrow (tens of kilometers), Many scientists believe the flow fields are generally basaltic in composition [Campbell and Campbell, 1992].

Festoon Flows

A volcano on the plains between Artemis Chasma and Imdr Regio displays a sheet of thick radar-bright flows and broad flow lobes. This type of flow has been name “festoon” and only three have been found on Venus [Head et al., 1992]. The lobes and flows show prominent transverse ridges that have an average spacing of about 750 meters. The flow features are associated with a complex domical structure about 100 kilometers across and 1 kilometer in relief. They are surrounded at a lower elevation by plains surfaces that expose radar-bright volcanic deposits [Moore et al., 1992]. These materials extend some 360 to 400 kilometers from the volcano. They appear to overlie the radar-dark, lowland plains that dominate this region of the surface.

Festoon Flow Diagram

This diagram shows the volcano broken down into six types of regions. Measurements indicate that in the ridged mesas the scarps have relief up to 205 meters; in the lobate mesas, the relief is from 133 to 723 meters.

Lava Channels

Lava channels extending from hundreds to thousands of kilometers in length are conspicuous on the Venusian plains. Simple channels typically show little or no branching. They include long sinuous forms, termed “canali”, and sinuous rilles. Canali are best preserved in regions of subdued relief. They have a high width-to-depth ratio and maintain a remarkably constant width over very long distances. Images reveal the presence of meanders, point bars, cut banks, and abandoned channel segments.

Both the source and the distal ends of many canali are buried or extensively subdued by lava flows younger than those that formed the channels. Measurements have shown considerable relief in longitudinal channel profiles, implying significant tectonic deformation of the plains since the channels formed [Parker et al., 1992]. Wrinkle ridges and ridge belts commonly transect canali. Vertical displacements of hundreds of meters over horizontal distances of a few kilometers are common at ridge crossings.

Sinuous Channel

A Sinuous segment of a simple radar-dark channel about 200 kilometers long and 2 kilometers wide is shown in this image. Channel outlines at both ends are indistinct, probably because of infilling by younger lavas. Thin bright returns from channel walls denote steep slopes. A transecting relict channel of approximately similar width is denoted by parallel bright margins (levees) that cross the lava plains in a northwest direction on each side of the radar-dark channel.

Sinuous Rilles

Sinuous rilles emanate from depressions and enlarged fractures south of Ovda Regio. They become progressively narrower and more shallow in the downstream direction. They are typically 1 to 2 kilometers wide and tens to hundreds of kilometers in length. Channel walls form a distinct boundary between the channel floor and the surrounding terrain. Channel material is similar to that of the surrounding terrain. An impact crater about 12 kilometers in diameter has disrupted the eastern channel at center right.

Ammavaru Lava Flows & Topography

This is a mosaic of the Ammavaru volcanic complex and associated outflow channel in the Lada Terra region. The channel displays a broad U-shaped outline across the image that extends from a collapse source on the southwest flank of Ammavaru (upper left), through reaches that are anastomosing (lower left center) and distributary (lower center), to terminal flow deposits east of a breach in the north-trending ridge (upper right). The channel is more than 1,200 kilometers long. The southern, topographically lower end of the trough was flooded with lavas that appear radar-dark in the image. At 51.5 degrees South, 25.5 degrees East, the flood spreads into a broad anastomosing reach. East of the highlands at this locality the channel branches into a distributary reach for about 130 kilometers. Three radar-dark distributaries change to radar-bright with dark margins about midway along this reach. Bright flow deposits with lobate morphology are extensively distributed here. The deposits are ponded on the west side of a north-trending ridge belt for over 300 kilometers. The main distributary channel extends through these deposits and terminates eastward at an extensive radar-bright plain east of a breach in the ridge belt. The radar-bright deposits from the outflow channel cover an area of about 100,000 km2; they show broad lobate margins typical of lava flows.

Ammavaru Outflow Channel

This image shows the collapse source and the upper reach of outflow channel on southwest flank of the volcano Ammavaru. The main channel, about 5.5 kilometers wide, is contained within a linear trough that extends south-southeast for about 300 kilometers. A subsidiary channel about 1 kilometer wide divides at the south limit of a linear scarp and reunites farther downstream.

Ammavaru - Anastomosing Reach of Outflow Channel

Anastomosing reach of outflow channel shows streamlined islands that point eastward in the flow direction of the lava deposits. Radar-dark embayments in highland areas denote lava ponding and flooding that occurred prior to eastward channel cutting and the formation of the distributary reach east of the highlands (right center).

Small Volcanoes

Volcanic constructs and edifices on Venus have been classified and subdivided on the basis of their size and morphology [Slyuta and Kreslavsky, 1990; Head et al., 1992]. Centers with a diameter less than 20 kilometers are considered small. They occur typically on the plains, but are also found on the flanks of large volcanoes and in association with coronas and arachnoids. They consist of small shields, cones, and some domes.

Small shields have circular to elongated outlines. In general, they have very shallow slopes and are not associated with flow deposits. Many small shields have been identified by their smooth circular outlines and image tones that are darker than the surrounding plains. The outlines may also be diffuse and may be distinguished by the presence of a centrally located, circular summit pit about 1 kilometer or less in diameter. Clusters of small shields approximately 10 kilometers in diameter are widely distributed, often in association with linear fracture belts on the plains.

Cones are circular features with steep slopes and a centrally located summit pit. They range in height from 200 to 1,700 meters with slopes from 12 to 23 degrees. Individual flows are not usually visible. Cones tend to occur in clusters on the plains. A temporal relation between cones or shields in a cluster and fractures on the plains is evident in cases where some of the cones or shields are cut by fractures and therefore are older. Other cones or shields in the cluster are superposed on the fractures and thus are younger.

Cluster of Cone Volcanoes

The cone volcanoes in this cluster are about 2 kilometers in diameter, 200 meters high, with 12-degree steep slopes overlying a fracture network in Niobe Planitia. Some cones are cut by younger, more widely spaced, north-striking fractures with curvilinear outlines.

Intermediate Volcanoes

Intermediate volcanoes are defined as centers between 20 to 100 kilometers in diameter. Typically they consist of relatively symmetrical shields characterized by radial lava flows and fracture patterns. Domes are prominent features in this size class.

Anemone Type Volcano

Scientists have named this type of volcano “anemone” because of its petallike lava flows and radiating radar-bright patterns. They normally occur in association with fissure type eruptions. This volcano is 40 by 60 kilometers in size and has a dark central edifice with bright central flows. It has elongated summit pits and an arcuate graben along the southern summit [ Head et al., 1992].

Tick

Scientists nicknamed this type of volcano a tick. About 65.6 kilometers across at the base, this volcano has a flat, concave summit 34.8 kilometers in diameter. The sides of the volcano are characterized by radiating ridges and valleys. The rim of the volcano to the west appears to have been breached by dark lava flows that emanated from a shallow summit pit 5.4 kilometers in diameter and traveled west along a channel. A series of coalescing, collapsed pits 2 to 10 kilometers in diameter is 10 kilometers west of the summit rim. The black square represents missing data.

Domes

The majority of Venusian domes range in diameter from less than 10 kilometers to about 100 kilometers with a mean of about 24 kilometers. Their height range from 70 to 2000 meters with a mean of about 700 meters above the surrounding terrain. They are usually surrounded by a steep perimeter and have a relatively flat top. Images reveal that these features are remarkably circular in outline. The surfaces are slightly rough and have a slightly lower reflectivity and correspondingly higher emissivity than the surrounding terrain. They may have formed from viscous lava that erupted uniformly from a central vent [Pavri et al., 1992].

Small craters are a common feature of the surfaces of all domes; they may or may not be central. Breakouts occur on the flanks of some domes and radial fractures extend down the slopes into the surrounding plains. Many domes show evidence of gravitational collapse, slumping, tectonism, impact, and lava flooding.

Domes occur singly, in pairs, groups, or overlapping clusters. Many are associated with coronas, but the eruptive mechanism is not clearly understood. The domes are concentrated at elevations near or just below the mean planetary radius of 6052 kilometers.

Pancake Volcanoes

This cluster of four overlapping domes is located on the eastern edge of Alpha Regio. The domes average about 25 kilometers in diameter with maximum heights of 750 meters. These features can be interpreted as viscous or thick eruptions of lava coming from a vent on the relatively level ground allowing the lava to flow in an even lateral pattern.

Collapse Features

Some volcanic domes have steep scalloped margins. The outline of the scallops and the presence of debris aprons in places around the margins suggests that the scallops were formed by slope failure. In addition, scalloped-margin domes are often surrounded by concentric fractures.

Small Dome in Navka Planitia

This 17.4 kilometers dome in Navka Planitia shows collapsed margins and landslide deposits in both the northwest and the northeast quadrants. The landslide deposits show hummocky surfaces that extend up to 10 kilometers out on the plains. The dome is about 1.86 kilometers high and has a slope of about 23 degrees. In general, the scale of lava domes and collapse features on Venus is orders of magnitude larger than that on Earth.

Large Volcanoes

Large volcanoes have diameters mostly in the 100 to 600 kilometer range. Such edifices are characterized by a dominance of radial lava flows in association with positive topography. They occur mostly at higher elevations in broad rises and at tectonic junctions.

Sapas Mons

Sapas Mons is a large volcano approximately 400 kilometers in diameter and 1.5 kilometers high located on a topographic rise in Atla Regio. The summit consists of two mesas with flat to slightly convex tops and smooth surfaces that appear radar-dark in the image. The sides of the volcano show numerous bright overlapping flows that provide the edifice with a roughly radial outline. Many of the flows appear to be flank eruptions. Radial fractures clearly transect the flows to the east and south. Darker flows in the southeast quadrant are probably smoother than the bright flows closer to the eruptive center. An impact crater with a diameter of 20 kilometers located in the northeast quadrant is partially buried by lava flows. A medium-to-light gray flow appears to be ponded to the west by the crater. This flow has been diverted south and east where it has buried a portion of the hummocky ejecta on the southeast side of the crater.

Calderas

Calderas on Venus have been defined as circular to elongate depressions not associated with well-defined edifices. Characteristically they show the concentric patterns of surrounding fractures [Head et al., 1992]. They may lie in a broad region of elevated topography. They are distinct from impact craters in lacking a hummocky raised rim and an associated ejecta pattern.

Sacajawea Patera

Sacajawea Patera is an elliptical caldera measuring 260 by 175 kilometers that forms a depression about 2 kilometers deep. The depression is enclosed by a zone of concentric troughs that show radar-bright outlines extending from 60 to 130 kilometers outward from the caldera floor. The floor is covered with smooth mottled plains. The brightest deposits occur around the periphery and near the center of the caldera floor where there is a ponded leveed flow. Linear to sinuous scarps show bright outlines that extend southeast from the eastern margin of the caldera. A small shield measuring 12 kilometers in diameter is transected by one of these features.

Impact craters are a common feature on most planetary bodies because projectiles (such as meteoroids, asteroids, and comets) have collided with planetary surfaces for billions of years. Projectiles able to penetrate a planet’s atmosphere impact the surface at a velocity of tens of kilometers per second with enough energy to generate shock waves in the crustal rocks. These shock waves propagate to produce craters by the ejection of vapors, melted rocks, hot particles and fragments, sheared and fractured rocks, and large blocks [Melosh, 1989].

During this process, the deepest target material is exposed closest to the crater rim and the most shallow material is deposited farthest from the rim. Generally, impact craters have a circular outline, a raised rim, and a depth that is shallow relative to the diameter. The crater is surrounded by ejecta deposits that decrease in thickness outward from the crater rim. Because of the dense Venusian atmosphere, some aspects of crater formation and morphology on Venus are different from thse on other bodies.

There is a general progression in morphology between large, intermediate, and small craters: large craters might have several rings and smooth floors; intermediate craters tend to have a central peak and smooth floors; small craters have a simple bowl floor that is rough. Impact craters exhibit a wide range of degradation on different planets, so they are useful indicators of resurfacing and modification of surfaces. On Earth, craters are rapidly degraded and destroyed by surficial weathering processes. In contrast, Venusian craters remain pristine because they are young, and there is very little weathering that affects them.

General Characteristics

Impact craters are found to be distributed randomly but uniformly over the surface of Venus. Cratering of the terrestrial planets shows a record of two distinct periods, one from the late period of heavy bombardment and the other from a bombardment of asteroids and comets which occurred more recently. Venus shows no record of the heavy bombardment period, indicating that it was resurface about 300 to 500 million years ago. The majority of Venusian craters appears pristine because they were formed after Venus was resurfaced; there has been very little geologic activity and weathering since then to degrade and destroy the craters [Schaber et al., 1992]. Venus has a fewer number of small craters than any other planet. Small projectiles vaporize or break up in the Venusian atmosphere before they reach the surface. Many craters display radar-bright or -dark halos, and a number of craters have extended deposits that are parabolically shaped and open to the west [Campbell et al., 1992]. A feature unique to Venusian craters is radar-bright outflow deposits that extend over great distances, following the local topography [Asimow and Wood, 1992].

Impact origin of craters is determined from a group of criteria. The features that best determine an impact origin for a crater are: (1) a circular rim crest outline; (2) flanks that gently rise above the surrounding terrain; (3) floors with elevations lower than those of the surrounding terrain; (4) an ejecta blanket surrounding the crater; and (5) an inner basin that might be present in very large craters. The Venusian impact crater Danilova is 48 kilometers in diameter. It has a central peak, a crater wall, a crater floor, an ejecta blanket, and crater outflow deposits. (See also the geological sketch map for the crater.)

The ejecta blanket and the circular rim of a crater are very bright in radar images because both are rough, with many facets oriented perpendicular to the radar illumination. Typically, the crater wall that slopes toward the radar appears compressed while the wall that slopes away from the radar appears expanded. Walls parallel to the illumination have intermediate brightness and widths.

The symmetry of a crater depends on the angle of impact of the projectile that formed it. Craters produced by an impact that is normal to the surface tend to be radially symmetric: rim crests have roughly equal elevations everywhere and are concentric with the crater floor outline, while flanks appear the same in all radial directions. However, most impact craters are produced by projectiles with trajectories that are oblique to the surface. When the angle is very oblique, the crater has a bilateral symmetry about the plane of the trajectory, with rim crests highest on the down-trajectory side and lowest on the up-trajectory side, while crater flanks extend to distances greater on the down-trajectory sides than on the up-trajectory sides.

Venusian impact craters have either a radar-bright or -dark crater floor, or both. The brightness of the crater floor in Magellan images appears to depend on the incidence angle of the radar, the size of the crater, the terrain on which the crater formed, and the amount of infilling by lava or impact melt.

Impact Crater Classification

Schaber et al. [1992] have classified the Venusian impact craters seen in Magellan images into six morphologic types.

• Multi-ringed craters. These are similar to the larger multiringed basins on the Moon, Mercury, and Mars. The type includes all craters larger than 100 kilometers in diameter.
• Double-ring craters. These have an outer rim and an inner ring. Most craters with diameters larger than 40 kilometers are in this classification.
• Central peak craters. These craters account for approximately 37% of the craters on Venus; they have central mounds or a radar-bright jagged central peak.
• Craters with structureless floors. They generally have terraced walls and flat floors.
• Irregular craters. This type represents the smallest craters of less than 16 kilometers in diameter. Their floors are usually radar bright because they are rough and complex.
• Multiple craters. They are characterized by two or more craters produced by projectiles that impacted very close to each other; in some cases, the crater rims may overlap.

Small, simple bowl-shaped craters, which are quite common on the Moon and Mars, are scarce on Venus. Instead, small Venusian craters form tight clusters and they overlap.

Distinguishing Impact Craters from Volcanic Craters

The Venera 15/16 and Arecibo images of Venus reveal several circular features that resemble both impact craters and volcanic features. Cleopatra, 105 kilometers in diameter and lying on Maxwell Montes, is one of these controversial features. Cleopatra was originally interpreted as a caldera on top of a giant volcanic construct [Masursky et al., 1980]. From the Venera 15/16 and Arecibo images, Basilevsky and Ivanov [1990] interpreted Cleopatra to be a peak-ring structure, which supported an impact origin for the crater. However, the lack of evidence for crater-rim deposits, the large depth of Cleopatra, and extensive plains deposits to the east that apparently emanated from the crater led Schaber et al. [1986] to suggest a possible volcanic origin.

Scientists used Magellan’s high-resolution images to solve the mystery of Cleopatra crater. These images show an inner basin, an outer basin, and rough ejecta deposits. Although the crater rim resembles that of a volcanic caldera, the ejecta and the inner basin provide compelling evidence that the structure is an impact crater. The ejecta deposits surrounding Cleopatra appear to be incomplete and do not extend as far as they should for a crater of this size, possibly because the crater was produced on the highlands rather than the plains. The ejecta can be identified as such because they are rougher and have more large-scale slopes than those of the surrounding terrain.

Flows of impact melt or lavas of impact-triggered volcanism breached the crater rim and filled the troughs in the upper-right corner of the image. These flows and the floor of the crater are radar-dark because they are smoother than the surrounding terrain.

Large Crater (Meade) Properties

Mead crater, with a diameter of 280 kilometers, is the largest impact crater on Venus. The inner ring is thought to represent the original rim of the crater cavity, while the outer scarp is thought to be the expression of a ring fault that has downdropped the flank terrace [Schaber et al., 1992]. The surrounding plain is covered by fine debris that decreases the return to the radar, and appears darker on the image. The floor of the crater has several large cracks that show as bright lines. Ejecta from the crater that appear as diffuse patches surrounding the crater rim are brighter than the surrounding plain because they are rougher and have more slopes facing the radar. The drop in elevation from the crater rim to the center of the crater is approximately 1 kilometer. This is quite shallow for a crater the size of Mead; it may be that Mead has experienced relaxation of its floor, or a large amount of material has flooded the crater floor.

Halos, Outflow Deposits, and Splotches

Yablochkina crater exhibits several interesting features: a radar-dark halo and radar-bright outflow deposits. The crater and its ejecta are surrounded by a dark halo. Approximately half of the impact craters on Venus are partially or wholly surrounded by halos [Phillips et al., 1991]. These halos possibly represent smooth areas with little surface roughness. Atmospheric shock waves produced as the meteoroid passed through the thick atmosphere might have removed wavelength-size structures from the existing terrain and pulverized the surface materials to produce these dark margins. Alternatively, fine debris produced by the destruction of the target material or the meteoroid as it passed through the atmosphere and exploded at the surface might have been deposited before the crater formed. In addition to dark halos, many Venusian craters have bright halos, also thought to have formed from atmospheric shock waves.

Also surrounding the Yablochkina crater in many locations, but particularly to the northeast, are deposits or flows that are often brighter than the crater ejecta. These flows originate predominantly downrange from the point of the impact. The great distances that these deposits travel and the fact that they follow the topography suggest that they consist of low viscosity material [Schaber et al, 1992].

Smaller impactors might be broken up as they enter the Venusian atmosphere [Basilevsky et al., 1987]. Except for the smallest members of some crater clusters, no craters smaller than 3 kilometers in diameter have been observed [Phillips et al., 1991]. The image Crater Cluster shows an irregular crater of approximately 14 kilometers mean diameter. The crater is actually a cluster of four separate craters in rim contact. The noncircular rims and multiple, hummocky floors are probably the result of the breakup and dispersion of a meteoroid during its passage through the dense Venusian atmosphere; subsequently, the meteoroid fragments impacted simultaneously to create the cluster. Meteoroids that would form craters smaller than the observed cutoff diameter of 3 kilometers either are not able to penetrate the atmospheric column or they decelerate to velocities insufficient to form impact craters [Phillips et al., 1991]. However, the shock or pressure wave created as such a meteoroid travels through the atmosphere may still have energy sufficient to deform the surface.

The image “Dark Splotches on Lava Flows” shows three dark splotches on the plains of Venus. The impact crater in the splotch at the right indicates that the meteoroid was not completely destroyed and reached the surface to produce a crater. Much of the meteoroid, however, was destroyed and its remnants and/or shock wave produced the large dark margin that surrounds the crater. The other two splotches, at near center and the extreme left, have no associated impact crater, indicating that only a shock wave disturbed the surface. Evidence that these splotches represent a deposit of material is the change in brightness of the underlying lava flows from the center of the splotches outward. The dark margin to the left has associated wind streaks, suggesting that the splotch is composed of material fine enough to be moved by the wind.

Crater Modification

The majority of impact craters on Venus (62%) are pristine and unmodified [Schaber et al., 1992]. This indicates that impact craters have not been significantly altered by surficial processes. In only a few cases have craters been modified by lava flows or tectonism. One example is crater Somerville, a 37-kilometer diameter crater in Beta Regio, which has been cut by many fractures and faults. The crater was split in half during the formation of a rift that is up to 20 kilometers wide and apparently quite deep. A north-south profile through the center of this crater is visible as a result of the downdropping. Most of the central peak is visible as a bright spot in the middle of the crater. A radar-bright ejecta blanket is also visible through the fractures. A small portion of the eastern half of the crater can be seen on the far side of the rift. While the majority of large impact craters on Venus have floors that are radar-dark (probably due to flooding of the crater floor by lavas from below after the crater was produced), craters that have been modified by volcanism not associated with the impact process are rare on Venus.

One such rarity is Alcott, a 63 kilometer diameter crater extensively flooded by lava. A remnant of rough, radar-bright radial ejecta is preserved outside the crater’s southeast rim. The large, trough-like depression to the southwest is a rille or channel through which lava once flowed; the radar-bright eastern side and radar-dark western side indicate that this feature is a trough with steep slopes. The presence of partly lava-flooded craters like this one is important in understanding the rate of resurfacing on Venus by volcanism. The lack of flooded craters suggests that the surface might have been covered by lava flows about 500 million years ago, burying all existing craters [Schaber et al., 1992]. According to this model, the craters now visible in the Magellan images represent impacts that occurred after this resurfacing event, and there has been very little volcanic and other resurfacing since that time.

Views of Venusian Craters

Danilova Crater

-26.4Ý Latitude, 337.2Ý Longitude; 49 kilometers diameter; central peak crater.

The first image shows the Venusian impact crater Danilova as seen by the Magellan spacecraft. The crater has a central peak, a crater wall, a crater floor, an ejecta blanket, and crater outflow deposits. The second image is a geologic sketch map of the crater.

Golubkina Crater

60.30Ý Latitude, 286.55Ý Longitude; 30.1 kilometers in diameter; central peak crater

This is a Magellan image of Crater Golubkina. The 30.1-kilometer diameter crater is characterized by terraced inner walls and a central peak, typical of large impact craters on the Earth, the Moon and Mars. The terraced inner walls take shape late in the formation of an impact crater, due to the collapse of the initial cavity created by the meteorite impact. The central peak forms due to the rebound of the inner crater floor. This crater is named after the Russian sculptor Anna Golubkina.

Golubkina in 3D

This is a computer generated, 3D perspective view of the Golubkina crater.

Cleopatra Crater

- 65.90Ý Latitude, 7.00Ý Longitude; 105 kilometers in diameter; double ring crater.

Once believed to be a volcanic caldera, Cleopatra was shown by Magellan data to have an impact origin. A small ejecta blanket is visible surrounding the crater rim and an inner, radar-dark basin can be seen on the crater floor. Illumination is from the left at an incidence angle of 25 degrees.

Mead Crater

12.50Ý Latitude, 57.20Ý Longitude; 280 kilometers in diameter; multi-ring crater.

Mead crater is the largest impact crater on Venus, with a diameter of 280 kilometers. The crater has an inner and an outer ring and a small ejecta blanket surrounding the outer ring. The crater floor looks very similar in morphology to the surrounding plain. The dark vertical bands running through the image are artifacts associated with processing the synthetic aperture radar (SAR) data. Illumination is from the left at an incidence angle of 45 degrees.

Yablochkina Crater

48.27Ý Latitude, 195.15Ý Longitude; 63 kilometers in diameter; double-ring crater.

Yablochkina crater exhibits two interesting features: a radar-dark halo and radar-bright outflow deposits. The crater and its ejecta are surrounded by a dark halo which possibly represents smooth areas with little surface roughness.

Crater Cluster

- 25.6Ý Latitude, 336.0Ý Longitude; 1.5 kilometers diameter; irregular crater.

A small projectile broke up in the atmosphere to form four smaller impactors that struck nearly simultaneously to form this crater cluster. Illumi