An Introduction to Solar System Astronomy
Prof. Scott Gaudi
Mars is the fourth planet from the Sun
- About half the size of Earth
- Two Moons: Phobos & Deimos
- Cratered Highlands & low-lying Plains
- Extinct Volcanoes
- Deep canyons & flow channels
- Polar caps of CO2 and H2O ice
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
- Size: 14 x 10 km
- Orbit radius: 9380 km; Period: 7.66 hours
- Size: 8 x 6 km
- Orbit radius: 23,460 km; Period: 30.3 hours
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.
- Mariner 4: 1965 July 14
- Mariners 6 & 7 (1969)
- Mariner 9 (US 1971)
- Viking Orbiters (US 1976)
- Mars Global Surveyor (US 1998-present)
- Mars Climate Orbiter (US 1998-1999): burned up on orbit insertion.
- Mars Odyssey Orbiter (US 2001-present)
- Mars Express (Europe 2003-present)
- Mars Reconnaissance Orbiter (US 2006-present)
- Mars 2 & 3 (USSR 1971), one crashed, the other failed
20 seconds after landing.
- Viking 1 & 2 (US 1976)
- Mars Pathfinder & Sojourner rover (1997)
- Mars Polar Lander (US 1999): crashed on landing
- Mars Exploration Rovers (US 2004): Spirit & Opportunity.
- Beagle 2 Lander (UK 2004): failed on landing in Dec 2003
- Phoenix Lander (NASA 2007): landed in north polar region, found water ice.
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
- 95% CO2
- 2.7% N2
- 1.6% Argon
- Traces of H2O (0.03%)
Thin, dry atmosphere:
- Pressure is only 0.007 that of Earth
- Like being at an altitude of 30 km on Earth.
- Drops as much as 25% from summer to winter due to
condensation of CO2 at the poles.
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
Evolution of Mars' Atmosphere
Might have been warm enough for liquid water during the first Gyr:
- CO2 got locked into carbonaceous rocks? This
is a controversial idea.
As Mars cooled:
- H2O froze out (most may already have been
frozen into saturated rocks)
- Much of the remaining CO2 and N2
escaped the low gravity of Mars
Resulted in a thin, cold, dry, CO2 and N2
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.
- Daytime: 244 K (-20º F) max
- Nighttime: 187 K (-123º F) minimum
There is, however, enough of an atmosphere to sustain moderately
strong Surface Winds:
- 17 km/h (11 mph), up to 30 km/h (31 mph)
- Seasonal high winds can raise dust storms
- Some storms have covered the entire planet
High thin clouds, morning fog & frost are seen on Mars. The sky
appears reddish because of suspended dust particles from the dust
The Surface of Mars
Old, heavily cratered surface dominating the southern
hemisphere. The Martian highlands are like the Moon's but
with important differences:
- Craters show signs of erosion
- Crossed by deep valley networks
- Smooth areas between craters due to volcanism
Low-lying plains dominate the northern hemisphere
- Lightly cratered and so younger
- Tharsis plain may be only 500 Myr old.
- North & South poles are capped by deposits of CO2
and H2O ices mixed with dust
- Poles grow and shrink with the seasons
Tharsis region of Mars is a volcanic plateau ~4000 km across
& 10km high.
- Largest volcano in the Solar System: 24 km high, &
600 km across at the base.
- Stationary hotspot - get one big shield volcano instead
of a chain of smaller volcanoes.
- Last erupted ~300 Myr ago judging from the number of
craters on its sides.
Canyons & Channels
- Vast canyon (rift valley) 4000 km long, 2-7 km deep, up
to 600 km wide.
- Formed by faulting (crust pulling apart), not water
- Deep channels & flood plains
- Signs of sudden flows of liquid water (these are not
steady-state flows like rivers on Earth).
Water on Mars
Results from the Mars orbital surveys:
- Relatively recent rapid floods leaving behind gullies like
seen on Earth, could be water or something else (unclear).
- Layered terrains laid down early in Mars' history (early
means ~3.5 Gyr ago), that resemble similar
layered sedimentary terrains on Earth (e.g., the Grand Canyon).
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.
- Find layered sedimentary rocks with flow patterning
- Salt deposits laid down by evaporating water
- Hydrated minerals like Hematite
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,
Is it possible that there is life now, but buried deep underground?
- No evidence of present life from the Viking Lander experiments.
- Strong evidence of standing water in the past, but no
liquid water on the surface now.
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
- Is there liquid water buried deep below the surface where we haven't
- Such life would be shielded from the Sun's UV deep underground as
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)
- Launched Nov 1996, arrived Sept. 1997.
- Orbiter designed to make a high-resolution imaging survey, as
well as to study the atmosphere and interior of Mars.
- Laser altimeter has provided detailed 3D viws of Mars.
- Primary mission ended in 2001, an extended mission phase
lasted until November 2006 when contact was lost with the spacecraft.
2001 Mars Odyssey Orbiter (2001-2007)
- Launched April 2001, arrived in orbit October 2001
- Carries a gamma-ray spectrometer, infrared thermal imager, and a
radiation environment experiment.
- High spatial & spectral resolution mapping of Mars surface
mineralogy and morphology.
- Measure near-surface radiation environment
- Global map of the surface elemental composition
- Also acts as a communications relay for other Mars missions.
Mars Express (2003-present)
- ESA & Italian Mission, Launched June 2003, arrived in orbit
- Polar Orbit, designed to study the atmosphere, structure, and
geology of Mars.
- Found evidence for subsurface water ice, and detected abundant water
ice at the Mars south pole
- It also deployed the Beagle 2 lander which failed after landing
(no signals were heard after re-entry).
Mars Exploration Rovers (2004-present)
- Two large advanced robotic rovers, named Spirit and
Opportunity were launched in May 2003, and landed safely
in January 2004. As of this writing (Nov 2006), they are still
going strong, much longer than expected.
- Mobile field-geology systems which can rove around and study
selected rocks and soil features.
- Spirit landed in Gusev Crater, and Opportunity landed
in a small crater in the Meridiani Planum, on the opposite side
of the planet. Both sites are near the Martian equator.
- A major achievement of these rovers has been to find direct evidence
of past liquid water action (e.g., sedimentary rock
layers, hydrated minerals, etc.)
Mars Reconnaissance Orbiter (2006-present)
For more details on these and future missions, see the NASA's Mars Exploration Program web
page at JPL.
- Launched in 2005, orbit insertion in March 2006, aerobraking maneuvers
completed in August 2006.
- High- and Low-resolution cameras (HiRISE, CTX, MARCI)
to image terrain in unprecedented detail and monitor weather
- Spectral mapper to do remote minerology
- Climate radiometer (MCS) to profile the atmosphere measuring
temperature, pressure, dust and water-vapor content
- Shallow ground-penetrating radar (SHARAD) to probe for ice beneath
more than 1meter below the Mars surface.
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