Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 9:30

Lecture 35:

The Deserts of Mars

Key Ideas:

Mars is the fourth planet from the Sun

• About half the size of Earth
• Two Moons: Phobos & Deimos

Atmosphere:

• Thin, dry CO₂ atmosphere

Surface:

• 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 R_Earth)

Mass: M = 0.107 M_Earth

Rotation Period: 24^h 37^m

Axis Tilt: 25° (get seasons like Earth)

Two Moons: Phobos & Deimos

The Moons of Mars

Phobos ("fear")

• Size: 14 x 10 km
• Orbit radius: 9380 km; Period: 7.66 hours

Deimos ("panic")

• 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.

Mars Exploration

• Mariner 4: 1965 July 14
• Mariners 6 & 7 (1969)

Orbiters:

• 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)

Landers:

• 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, still going strong...
• Beagle 2 Lander (UK 2004): failed on landing in Dec 2003

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 (permafrost).

Evolution of Mars' Atmosphere

• 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 atmosphere.

Weather on Mars

• 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 storms.

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

Plains:

Low-lying plains dominate the northern hemisphere

• Lightly cratered and so younger
• Tharsis plain may be only 500 Myr old.

Polar Caps:

• North & South poles are capped by deposits of CO₂ and H₂O ices mixed with dust
• Poles grow and shrink with the seasons

Mars Volcanoes

Olympus Mons:

• 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 erosion.

Flow Channels:

• Deep channels & flood plains
• Signs of sudden flows of liquid water (these are not steady-state flows like rivers on Earth).

Water on Mars

• 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:

• Find layered sedimentary rocks with flow patterning
• Salt deposits laid down by evaporating water
• Hydrated minerals like Hematite

Life on Mars?

• 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.

• Is there liquid water buried deep below the surface where we haven't yet probed?
• Such life would be shielded from the Sun's UV deep underground as well.

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 December 2003.
• 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)

• 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.