Mars

General characteristics

• Diameter = 6787 km (.53 of Earth's diameter)
  Its size is intermediate between the Earth and the Moon

• Density = 3.94 g/cm3 (rocks at the surface have density of about 3 g/cm3)
  This suggests that it is probably differentiated with a small iron core.

• Magnetic field is negligible.

• Range of elevations is much greater than than either Venus or Earth.
  Highest peak is about 30 km high.

• Surface temperature
  • Maximum = 295 K or 22 C or 72 F
  • Minimum = 190 K or -83 C or -117 F

• Day length = 24 hours and 40 minutes (24.45 hours)

• Year is 686.98 days (about 2 Earth years).

• Has seasons; axial tilt is approx. 25 degrees (compared to 23.5 for Earth)
  Seasons are characterized by changes in brightness and color of surface markings.
  Polar ice caps grow and shrinking with the seasons.
  There is a permanent polar cap of water ice and a seasonal cap consisting of a thin layer of dry ice (carbon dioxide ice)

  Winds re-distribute dust seasonally. Active sand dunes are present.
  Dust storms are seen as yellow clouds.
  Streaks on the surface show wind directions.

  Dark areas on the surface are coarse, rocky material (no dust)
  Areas covered with dust are bright.

• Mars is named for the "God of War"

• Has 2 moons
  • Phobos (fear) -larger (about 21 km)
  • Deimos (dread) - smaller (about 13 km)

  Both are irregular, not spherical.
  May be captured asteroids
  Heavily cratered

  Phobos has an 8-hour orbit and crosses the sky 2 - 3 times per day
  Tidal effect is slowing Phobos; may crash into Mars in 100 million years

  Like our moon, Deimos is slowly moving away.
  Moons have a low density (2 g/cm3), suggesting light elements or volatiles such as water.
  May be similar to carbonaceous chondrites.

Atmosphere

Composition

• 95.3% Carbon dioxide (CO2)
• 2.7% Nitrogen (N2)
• 1.6% Argon (Ar), an inert gas
• Traces of (0.23%) :
  • Oxygen (O2)
  • Carbon monoxide (CO)
  • Water (H2O)

The upper atmosphere lacks ozone and other UV absorbers

Chemical composition of the atmosphere is similar to that of Venus, but the atmospheric pressure is much less.

Atmospheric pressure

Becasue of the thin atmosphere, the greenhouse effect causes the temperature to be increased by only about 3 degrees C.

No liquid water exists because the atmospheric pressure is too low.

Circulation

• Very cold poles.
  In winter, CO2 freezes out of the atmosphere.
  This CO2 frost causes the atmospheric pressure to decrease
  Creates winds from pole to pole as one freezes and the other thaws.
  Ice goes directly to gas through sublimation.

• Large temperature difference between day and night
  Atmosphere is so thin that it can't store heat
  Daily temperature change of 60 degrees K
  Rapid night cooling
  Quick day heating
  Strong winds blow across the terminator (line separating light from dark) from the cold side to the warm side.

Spacecraft Observations

1. July 1965 Mariner 4 flyby saw craters

2. Flybys also by Mariner 6, 7, and 9

3. 7 USSR missions; orbiters and landers

4. In 1971, Mariner 9 and 2 USSR missions.
   All arrived during a dust storm
   As the dust settled, Mariner 9 saw:
   • 4 very large volcanoes
     Olympus Mons is the largest volcano in the solar system

   • Large canyon - Valles Marinaris
     4000 km long
     cliffs 7 km high

     The Grand Canyon in Arizona is only 1/10 as long and 1/3 as deep

   • Dry dendritic channels interpreted as riverbeds

5. In 1976 - surface pictures returned from Viking 1 and 2

   Experiments to look for life in the Martian soil
   No definite evidence for or against.
   No organic compounds were detected.

Surface Features

Southern Hemisphere - heavily cratered like lunar highlands (old; about 4 billion years)

Cause of dichotomy unknown - relict scar of an ancient impact?

Boundary between the two is a cliff 1-2 km high (Northern hemi. is 3 - 5 km lower than southern)

N. Hemi. has craters (2 - 3 times the number on lunar maria) and craterless plains.

Tharsis bulge (or rise or uplift) on N-plains:
5000-6000 km across; 10 mi high
Center of volcanism and extensive fracture system

Elysium rise or uplift also in N
1500 km diameter, 4 km high
0.5 billion years older than Tharsis

Several large basins in cratered highlands:

1. Hellas
2. Argyre

Craters have lobes of ejecta
Craters over 5 km diameter have fluidized ejecta indicative of water or water-ice in the crust
May be permafrost to 1 km deepth, and liquid water below.

Two types of craters:

1. Large (over 20 km), eroded, flat floored
   erosion? volcanics? ejecta?

2. Small and fresh with internal details such as terraces

Mars is closer to the Asteroid Belt than the moon and may have had more impacts

No evidence of plate tectonics

A few "Mars-quakes" detected by Viking seismographs suggesting a very thick lithosphere;
Core and Mantle may be solid

Volcanism

Volcano is located above a stoaionery mantle plume or hot spot.
No plate tectonics, allowing lava flows to concentrate in one area over a long period of time
Lower gravity allows surface to support a larger volcano than Earth can
Also, smaller planet has cooled, and thicker crust can support larger volcano without subsidence

Probably active overlast 2 - 3 billion years; probably dormant now.

Water

Many canyons are on E slope of Tharsis uplift

Channels show evidence of fluid flow.
Start abruptly without tributaries, suggesting that the surface collapsed as the fluid flowed away.
Islands and deep scours sugges a catastrophic flood.

Channels found throughout heavily cratered terrain.

Atmospheric pressure was problably higher in the past; so low now that water evaporates away.

Loss of volcanism and outgassing reduced atmospheric pressure

This page created on February 25, 1996
by Pamela J. W. Gore
DeKalb College, Clarkston, GA
pgore@gpc.edu