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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 35:

Key Ideas:

Mars is the fourth planet from the Sun



Water on Mars

Mars at a Glance

Orbital Data
Semi-Major Axis: a = 1.52 AU
Orbital Period: P = 1.88 years
Eccentricity: e = 0.0934
Inclination: i = 1.85°

Planetary Data
Radius: R = 3393 km (0.532 REarth)
Mass: M = 0.107 MEarth
Rotation Period: 24h 37m
Axis Tilt: 25° (get seasons like Earth)
Two Moons: Phobos & Deimos

The Moons of Mars

Mars has 2 tiny, irregularly shaped moons

Phobos ("fear")

Deimos ("panic")

The appearance and composition is like outer-belt asteroids. Are they captured asteroids? There are compelling arguments on both sides. Given our general lack of data, no clear resolution is currently in sight.

Mars Exploration




The US is currently undertaking a regular, intensive program of robotic Mars exploration both orbiters and lander/rover spacecraft. For more details on these and future missions, see the NASA's Mars Exploration Program web page at JPL.

The Atmosphere of Mars


Thin, dry atmosphere:

The composition of Mars' atmosphere is a lot like that of Venus, except that Venus' atmosphere is about 10,000 times heavier. These conditions make water unstable on the Martian surface, although there could be substantial amounts of water frozen below the surface (permafrost).

Evolution of Mars' Atmosphere

Might have been warm enough for liquid water during the first Gyr:

As Mars cooled:

Resulted in a thin, cold, dry, CO2 and N2 atmosphere.

Weather on Mars

Mars experiences extreme Day/Night temperatures The reason is that the thin atmosphere on Mars does not retain much heat (unlike the heavier atmospheres of Earth and Venus), so radiative cooling on the night side is very efficient.

There is, however, enough of an atmosphere to sustain moderately strong Surface Winds:

High thin clouds, morning fog & frost are seen on Mars. The sky appears reddish because of suspended dust particles from the dust storms.

The Surface of Mars

Cratered Highlands:

Old, heavily cratered surface dominating the southern hemisphere. The Martian highlands are like the Moon's but with important differences:


Low-lying plains dominate the northern hemisphere

Polar Caps:

Mars Volcanoes

Tharsis region of Mars is a volcanic plateau ~4000 km across & 10km high.

Olympus Mons:

Canyons & Channels

Valles Marineris:

Flow Channels:

Water on Mars

Results from the Mars orbital surveys:

Mars Exploration Rover results:

Evidence is of geologic features that formed in the presence of standing water sometime in Mars' past. This is the strongest direct evidence to date of liquid water on Mars.

Life on Mars?

If there was liquid water in Mars' past, does this also mean that there is the possibility of life, most likely microbiological life, as well? Is it possible that there is life now, but buried deep underground? We simply do not have answers to these questions, but the strong geologic evidence of processes that occured in the presence of standing liquid water in the past are a strong motivation to take these questions seriously.

In many ways, this is becoming one of the primary motivations for increasing robotic exploration of Mars. Finding evidence of past life would be a tremendous discovery. This is a story which has to play out for some time to come before we can do more than speculate.

Supplement: Current Mars Exploration

Mars Global Surveyor (1996-2006)

2001 Mars Odyssey Orbiter (2001-2007)

Mars Express (2003-present)

Mars Exploration Rovers (2004-present)

Mars Reconnaissance Orbiter (2006-present)

For more details on these and future missions, see the NASA's Mars Exploration Program web page at JPL.
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