Astronomy

Mercury was named by the Romans after the messenger of the gods because it seemed to move more quickly than any other planet. Mercury is the innermost planet in our solar system and is the second smallest one. Pluto is the smallest. Both Saturn and Jupiter have moons that are larger than Mercury, such as Titan and Ganymede. Jupiter’s moons Io, Europa, and Callisto are very close in size to Mercury.

Mercury resembles our moon with lunar-like terrain but differs with respect to its density. Mercury has a density of 5.43 gm/cm³ which is similar to the density of the Earth. This density indicates that its core has an iron composition like the Earth. The core probably takes up about 70% to 80% of the planet’s radius with the outer region largely composed of silicate rocks.

Mercury has almost no atmosphere. The atmosphere on Earth helps keep a uniform temperature from day to night. On Mercury, due to its closeness to the Sun, the temperature rises to over 400° C during the day. At night, because of the lack of atmosphere to help retain heat, the temperature drops to -180° C.

Mercury Statistics

Characteristic	Measurement
Mass (kg)	3.303e+23
Mass (Earth = 1)	5.5271e-02
Equatorial radius (km)	2,439.7
Equatorial radius (Earth = 1)	3.8252e-01
Mean density (gm/cm^3)	5.42
Mean distance from Sun (km)	57,910,000
Mean distance from the Sun (Earth = 1)	0.3871
Rotational period (days)	58.6462
Orbital period (days)	87.969
Mean orbital velocity (km/sec)	47.88
Orbital eccentricity	0.2056
Tilt of axis	0.00°
Orbital inclination	7.004°
Equatorial surface gravity (m/sec^2)	2.78
Escape velocity (km/sec)	4.25
Visual geometric albedo	0.10
Magnitude (Vo)	-1.9
Mean surface temperature	179°C
Maximum Surface Temperature	427°C
Minimum Surface Temperature	-173°C

Â	Percent
Helium	42%
Sodium	42%
Oxygen	15%
Other	1 %

Views of Mercury

Mercury

This photomosaic of Mercury was constructed from photos taken by Mariner 10 six hours after the spacecraft flew past the planet on March 29, 1974. The north pole is at the top and the equator extends from left to right about two-thirds down from the top. A large circular basin, about 1,300 kilometers in diameter, is emerging from the day-night terminator at left center. Bright rayed craters are prominent in this view of Mercury. One such ray seems to join in both east-west and north-south directions.

Mercury

This two-image mosaic of Mercury was constructed from photos taken by Mariner 10 a few hours before the spacecraft’s closest and first encounter with the planet on March 29, 1974.

Hills of Mercury

Weird terrain” best describes this hilly, lineated region of Mercury. This area is at the antipodal point from the large Caloris basin. The shock wave produced by the Caloris impact was reflected and focused to this antipodal point, thus jumbling the crust and breaking it into a series of complex blocks. The area covered is about 100 kilometers on a side.

Southwest Mercury

This image is a portion of the southwest quadrant of Mercury taken March 29, 1974, by the Mariner 10 spacecraft. The picture was taken four hours before the time of closest approach when Mariner was 198,000 kilometers from the planet. The largest craters seen in this picture are about 100 kilometers in diameter.

Caloris Basin

This mosaic shows the Caloris Basin (located half-way in shadow on the morning terminator). Caloris is Latin for heat and the basin is named this because it is near the subsolar point (the point closest to the sun) when Mercury is at aphelion. Caloris basin is 1,300 kilometers in diameter and is the largest know structure on Mercury. It was formed from an impact of a projectile with asteroid dimensions. The interior floor of the basin contains smooth plains but is highly ridged and fractured. North is towards the top of this image.

Caloris Basin Floor

This image is a high resolution view of the Caloris Basin shown in the previous image. It shows ridges and fractures that increase in size towards the center of the basin (upper left).

Bright Rayed Craters

This image shows two prominent craters (upper right) with bright halos on Mercury. The craters are about 40 kilometers in diameter. The halos and rays cover other features on the surface indicating that they are some of the youngest on Mercury.

Double Ring Basin

This image shows a double-ring basin which is 200 kilometers in diameter. The floor contains smooth plains material. The inner ring basin is at a lower elevation than the outer ring.

Large Faults on Mercury

This Mariner 10 image shows Santa Maria Rupes, the sinuous dark feature running through the crater at the center of this image. Many such features were discovered in the Mariner images of Mercury and are interpreted to be enormous thrust faults where part of the mercurian crust was pushed slightly over an adjacent part by compressional forces. The abundance and length of the thrust faults indicate that the radius of Mercury decreased by 1-2 kilometers after the solidification and impact cratering of the surface. This volume change probably was due to the cooling of the planet, following the formation of a metallic core three-fourths the size of the planet. North is towards the top and is 200 kilometers across.

Antoniadi Ridge

This is an image of a 450 kilometer ridge called Antoniadi. It travels along the right edge of the image, and transects a large 80 kilometer crater about half way in between. It crosses smooth plains to the north and intercrater plains to the south [Strom et al., 1975].

Discovery Quadrangle

Mosaic of the Discovery quadrangle of Mercury.