Astronomy

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. Illumination is from the left at an incidence angle of 38 degrees.

Dark Splotches on Lava Flows

The splotch at far right contains a crater while the others (at center and far left) do not. The diffuse boundaries of the splotches and the wind streaks from the splotch at the left indicate that the splotches are composed of fine debris. Radar illumination is from the left at an incidence angle of 30 degrees.

Somerville Crater

29.95Ý Latitude, 282.90Ý Longitude; 37 kilometers in diameter; central peak crater.

Somerville crater is split in half by a rift valley. A north-south profile through the crater is visible in the rift. The eastern half of the crater is visible on the opposite side of the rift. Illumination is from the left at an incidence angle of 42 degrees.

Alcott Crater

-59.50Ý Latitude, 354.55Ý Longitude; 62.7 kilometers in diameter; structureless floor crater.

Alcott crater was extensively flooded by lava. A remnant of radar-bright ejecta is preserved outside the crater’s southeast rim. Illumination is from the left at an incidence angle of 27 degrees.

Addams Crater

-56.10Ý Latitude, 98.90Ý Longitude; 90 kilometers in diameter; double-ring crater.

Addams crater is remarkable for the extensive outflow that extends 600 kilometers from the crater rim. Because of the high temperature and pressure on the Venusian surface, impacts produce more melt than on other planets. Outflow deposits are very thin. Their direction is controlled by the local topography

Venus, the planet the jewel of the sky, was once know by ancient astronomers as the morning star and evening star. Early astronomers once thought Venus to be two separate bodies. Venus, which is named after the Roman goddess of love and beauty, is veiled by thick swirling cloud cover.

Astronomers refer to Venus as Earth’s sister planet. Both are similar in size, mass, density and volume. Both formed about the same time and condensed out of the same nebula. However, during the last few years scientists have found that the kinship ends here. Venus is very different from the Earth. It has no oceans and is surrounded by a heavy atmosphere composed mainly of carbon dioxide with virtually no water vapor. Its clouds are composed of sulfuric acid droplets. At the surface, the atmospheric pressure is 92 times that of the Earth’s at sea-level.

Venus is scorched with a surface temperature of about 482° C. This high temperature is primarily due to a runaway greenhouse effect caused by the heavy atmosphere of carbon dioxide. Sunlight passes through the atmosphere to heat the surface of the planet. Heat is radiated out, but is trapped by the dense atmosphere and not allowed to escape into space. This makes Venus hotter than Mercury.

A Venusian day is 243 Earth days and is longer than its year of 225 days. Oddly, Venus rotates from east to west. To an observer on Venus, the Sun would rise in the west and set in the east.

Until just recently, Venus’ dense cloud cover has prevented scientists from uncovering the geological nature of the surface. Developments in radar telescopes and radar imaging systems orbiting the planet have made it possible to see through the cloud deck to the surface below. Four of the most successful missions in revealing the Venusian surface are NASA’s Pioneer Venus mission (1978), the Soviet Union’s Venera 15 and 16 missions (1983-1984), and NASA’s Magellan radar mapping mission (1990-1994). As these spacecraft began mapping the planet a new picture of Venus emerged.

Venus’ surface is relatively young geologically speaking. It appears to have been completely resurfaced 300 to 500 million years ago. Scientists debate how and why this occurred. The Venusian topography consists of vast plains covered by lava flows and mountain or highland regions deformed by geological activity. Maxwell Montes in Ishtar Terra is the highest peak on Venus. The Aphrodite Terra highlands extend almost half way around the equator. Magellan images of highland regions above 2.5 kilometers are unusually bright, characteristic of moist soil. However, liquid water does not exist on the surface and cannot account for the bright highlands. One theory suggests that the bright material might be composed of metallic compounds. Studies have shown the material might be iron pyrite (also know as “fools gold”). It is unstable on the plains but would be stable in the highlands. The material could also be some type of exotic material which would give the same results but at lower concentrations.

Venus is scarred by numerous impact craters distributed randomly over its surface. Small craters less that 2 kilometers are almost non-existent due to the heavy Venusian atmosphere. The exception occurs when large meteorites shatter just before impact, creating crater clusters. Volcanoes and volcanic features are even more numerous. At least 85% of the Venusian surface is covered with volcanic rock. Hugh lava flows, extending for hundreds of kilometers, have flooded the lowlands creating vast plains. More than 100,000 small shield volcanoes dot the surface along with hundreds of large volcanoes. Flows from volcanoes have produced long sinuous channels extending for hundreds of kilometers, with one extending nearly 7,000 kilometers.

Giant calderas more than 100 kilometers in diameter are found on Venus. Terrestrial calderas are usually only several kilometers in diameter. Several features unique to Venus include coronae and arachnoids. Coronae are large circular to oval features, encircled with cliffs and are hundreds of kilometers across. They are thought to be the surface expression of mantle upwelling. Archnoids are circular to elongated features similar to coronae. They may have been caused by molten rock seeping into surface fractures and producing systems of radiating dikes and fractures.

Venus Statistics

Characteristic	Measurement
Mass (kg)	4.869e+24
Mass (Earth = 1)	0.81476
Equatorial radius (km)	6,051.8
Equatorial radius (Earth = 1)	0.9488
Mean density (gm/cm^3)	5.25
Mean distance from Sun (km)	108,200,000
Mean distance from the Sun (Earth = 1)	0.7233
Rotational period (days)	-243.0187
Orbital period (days)	224.701
Mean orbital velocity (km/sec)	35.02
Orbital eccentricity	0.0068
Tilt of axis	177.36°
Orbital inclination	3.394°
Equatorial surface gravity (m/sec^2)	8.87
Escape velocity (km/sec)	10.36
Visual geometric albedo	0.65
Magnitude (Vo)	-4.4
Mean surface temperature	482°C
Atmospheric pressure (bars)	92

Atmospheric Composition	Percent
Carbon Dioxide	96%
Nitrogen	3+%
Other	Less than 1%*

*Includes trace amounts of sulfur dioxide, water vapor, carbon monoxide, argon, helium, neon, hydrogen chloride, and hydrogen fluoride.

Views of Venus

Mariner 10 Image of Venus

This beautiful image of Venus is a mosaic of three images acquired by the Mariner 10 spacecraft on February 5, 1974. It shows the thick cloud coverage that prevents optical observation of the surface of Venus. Only through radar mapping is the surface revealed.

Galileo Image of Venus

On February 10, 1990 the Galileo spacecraft acquired this image of Venus. Only thick cloud cover can be seen.

Hubble Image of Venus

This is a Hubble Space Telescope ultraviolet-light image of the planet Venus, taken on January 24, 1995, when Venus was at a distance of 113.6 million kilometers from Earth. At ultraviolet wavelengths cloud patterns become distinctive. In particular, a horizontal “Y” shaped cloud feature is visible near the equator. The polar regions are bright, possibly showing a haze of small particles overlying the main clouds. The dark regions show the location of enhanced sulfur dioxide near the cloud tops. From previous missions, astronomers know that such features travel east to west along with the Venus’ prevailing winds, to make a complete circuit around the planet in four days.

Venus

This is a global view of the surface of Venus centered at 180 degrees East longitude. Simulated color is used to enhance small-scale structure.

Five Global Views of Venus

The surface of Venus is displayed in these five global views. The center image (A) is centered at Venus’ north pole. The other four images are centered around the equator of Venus at (B) 0 degrees longitude, (C) 90 degrees East longitude, (D) 180 degrees and (E) 270 degrees East longitude. The bright region near the center in the polar view is Maxwell Montes, the highest mountain range on Venus. Ovda Regio is centered in the (C) 90 degrees East longitude view. Atla Regio is seen prominently in the (D) 180 East longitude view.

Hemispheric View of Venus

This hemispheric view of Venus, as revealed by more than a decade of radar investigations culminating in the 1990-1994 Magellan mission, is centered at 0 degrees East longitude. The effective resolution of this image is about 3 kilometers. It was processed to improve contrast and to emphasize small features, and was color-coded to represent elevation.

Additional Hemispheric Views of Venus

• View centered at 90 degrees East longitude.
• View centered at 180 degrees East longitude.
• View centered at 90 degrees West longitude.
• View centered at the north pole.
• View centered at the south pole.

Venusian Map

This image is a Mercator projection of Venusian topography. Many of the different regions have been labeled. The map extends from -66.5 to 66.5 degrees in latitude and starts at 240 degrees longitude.

Venusian Topography

This image is a Mercator projection of Venusian topography The highland regions such as Ishtar Terra, Aphrodite Terra, Alpha Region and Beta Regio are shown in yellow and orange. The low-lying regions are shown in blue.

Cylindrical Map of Venus

Venus is displayed in this simple cylindrical map of the planet’s surface. The right and left edges of the image are at 240 degrees East longitude. The top and bottom of the image are at 90 degrees North latitude and 90 degrees South latitude, respectively. The bright region at the top, left of center, is Maxwell Montes, the highest mountain range on Venus. Aphrodite Terra, a large highland region, extends along the equator to the right of center. The scattered dark patches in this image are halos surrounding some of the younger impact craters. This global data set reveals a number of craters consistent with an average Venus surface age of 300 million to 500 million years.

Gula Mons and Crater Cunitz

A portion of Western Eistla Regio is displayed in this three dimensional perspective view of the surface of Venus. The viewpoint is located 1,310 kilometers southwest of Gula Mons at an elevation of 0.78 kilometers. The view is to the northeast with Gula Mons appearing on the horizon. Gula Mons, a 3 kilometer high volcano, is located at approximately 22 degrees North latitude, 359 degrees East longitude. The impact crater Cunitz, named for the astronomer and mathematician Maria Cunitz, is visible in the center of the image. The crater is 48.5 kilometers in diameter and is 215 kilometers from the viewer’s position.

Eistla Regio - Rift Valley

A portion of Western Eistla Regio is displayed in this three dimensional perspective view of the surface of Venus. The viewpoint is located 725 kilometers southeast of Gula Mons. A rift valley, shown in the foreground, extends to the base of Gula Mons, a 3 kilometer high volcano. This view is facing the northwest with Gula Mons appearing at the right on the horizon. Sif Mons, a volcano with a diameter of 300 kilometers and a height of 2 kilometers, appears to the left of Gula Mons in the background.

Eistla Regio

A portion of Western Eistla Regio is displayed in this three dimensional perspective view of the surface of Venus. The viewpoint is located 1,100 kilometers northeast of Gula Mons at an elevation of 7.5 kilometers. Lava flows extend for hundreds of kilometers across the fractured plains shown in the foreground, to the base of Gula Mons. This view faces the southwest with Gula Mons appearing at the left just below the horizon. Sif Mons appears to the right of Gula Mons. The distance between Sif Mons and Gula Mons is approximately 730 kilometers.

Lakshmi Planum

The southern scarp and basin province of western Ishtar Terra are portrayed in this three dimensional perspective view. Western Ishtar Terra is about the size of Australia and is a major focus of Magellan investigations. The highland terrain is centered on a 2.5 to 4 kilometers high plateau called Lakshmi Planum which can be seen in the distance at the right. Here the surface of the plateau drops precipitously into the bounding lowlands, with steep slopes that exceed 5% over 50 kilometers.

Alpha Regio

These images show the Alpha Regio. The bright lineated terrain is a series of troughs, ridges, and faults that are oriented in many directions. The lengths of these features generally range from 10 to 50 kilometers. The topographic elevation within Alpha Regio varies over a range of 4 kilometers. Local topographic lows, whose outlines are generally controlled by structures within the central region, are relatively radar-dark and filled with volcanic lavas. Source vents for this volcanism appear as bright spots within the smooth plains units.

Arachnoids

Arachnoids are one of the more remarkable features found on Venus. They are seen on radar-dark plains in these Magellan image mosaics of the Fortuna region. As the name suggests, arachnoids are circular to ovoid features with concentric rings and a complex network of fractures extending outward. The arachnoids range in size from approximately 50 to 230 kilometers in diameter. Arachnoids are similar in form but generally smaller than coronae (circular volcanic structures surrounded by a set of ridges and grooves as well as radial lines). One theory concerning their origin is that they are a precursor to coronae formation. The radar-bright lines extending for many kilometers might have resulted from an upwelling of magma from the interior of the planet which pushed up the surface to form “cracks.” Radar-bright lava flows are present in the 1st and 3rd image, also indicative of volcanic activity in this area. Some of the fractures cut across these flows, indicating that the flows occurred before the fractures appeared. Such relations between different structures provide good evidence for relative age dating of events. At present, arachnoids are found only on Venus and can now be more closely studied with the high-resolution (120 meter) radar imagery from Magellan.

Parallel Lines

Two groups of parallel features that intersect almost at right angles are visible. The regularity of this terrain caused scientists to nickname it graph paper terrain. The fainter lineations are spaced at intervals of about 1 kilometer and extend beyond the boundaries of the image. The brighter, more dominant lineations are less regular and often appear to begin and end where they intersect the fainter lineations. It is not yet clear whether the two sets of lineations represent faults or fractures, but in areas outside the image, the bright lineations are associated with pit craters and other volcanic features.

Surface Photographs from Venera 9 and 10

The Soviet Venera 9 and 10 spacecraft were launched on 8 and 14 June 1975, respectively, to do the unprecedented: place landers on the surface of Venus and return images. The two spacecraft successfully landed descent crafts on 16 and 23 October 1975. These images were obtained on 22 and 25 October 1975. Venera 9 landed on a slope inclined by about 30 degrees to the horizontal whereas Venera 10 was inclined about 8 degrees. The two spacecraft were separated by about 2,100 kilometers. Most of the rocks in the images are between about 0.3 and 1 meter.

Color Surface Photographs from Venera 13

On March 3, 1982, the Venera 13 lander touched down on the Venusian surface. It was the first Venera mission to include a color TV camera. This image is the left half of the Venera 13 photo.