Overview, Appearance

• Name and appearance: Aris (god of war) and Mars, from the color; Brighter than many stars, reddish; From Earth one can sometimes see white (icy) regions near the poles with a telescope.
• Motion: 1.52 AU from the Sun on an eccentric, almost 2-year orbit; rotates once every 24.6 hours, with a tilted axis (it has seasons!).
• Size and Mass: Radius about half that of Earth [3400 km]; [Mass 11% of Earth's; density 3.9 times that of water, the surface gravity 0.38 g].

Atmosphere and Weather

• Composition: Less than 1% of Earth's pressure, made of 95% CO₂ [we can see this from Earth]; Little water, no rain; Dusty and reddish.
• History: The atmosphere was probably thicker in the past; Was it blown away in an impact? Gradually lost to space because of a weak gravity? The interior is not active enough to keep replenishing it with volcano eruptions.
• Effect: The atmosphere is too thin to protect, to cause greenhouse effect or affect the temperature significantly, but thick enough to erode features.
• Weather: Winds, dust storms and cyclones; Mostly colder than Earth, with daily and seasonal changes [from 150 K (–220°F) to 295 K (70°F)], probably warmer in the past.

Magnetic field: Almost none, so there is no protection from the solar wind, which is also continuously blowing away the atmosphere. (But parts of the crust are highly magnetized, so Mars does have weak auroras!)

 Exploration

• [Early missions: In 1965, after other attempts failed, Mariner 4 reached Mars, took close-up pictures, and ended any hopes of finding canals, cities, Martians. 1965-1971, US Mariner orbiters; 1989, Soviet Phobos 2 orbiter (Phobos 1 was lost); 1971, Soviet Mars 3 lander; 1976, US Viking landers ($3 billion, 8 years to develop!).]
• Recent missions: 1997, Mars Pathfinder [$225 million, 4 years to develop]; Climate Orbiter, lost 09.99; Polar Lander, lost 12.99; the Japanese Nozomi abandoned in 2003 due to technical problems; NASA's Mars Global Surveyor was in orbit from 1999 to 2006.
• Orbiting missions: NASA's Mars Odyssey has been operational since 2001, ESA's Mars Express since late 2003 (Beagle 2 lander lost during the descent), and NASA's Mars Reconnaissance Orbiter since early 2006; Why every 2 years?
• Landing missions: The rovers Spirit and Opportunity have been on the surface since Jan 2004, the Phoenix Mars Lander since 2008.
• Plans: More missions are in the planning stages, including the Mars Science Laboratory rover for 2009, and later sample returns to Earth; Issues to consider are the budget, possible planet contamination; Human missions are now being considered.

Surface

• Color: It really is reddish (or "butterscotch"!), from rusting iron; Covered with rocks and sand.
• Northern hemisphere: Younger (3 Gyr) volcanic plains; Southern hemisphere: Older (4 Gyr), heavily cratered highlands, higher and thicker; Polar ice caps; Why the difference? A collision with a very large (500 miles or so) asteroid is the currently favored possibility.
• Features: Tharsis bulge (4000 km), with 3 prominent volcanos; Mariner valley stretch fracture, longer than the US; Craters can be used to estimate the age of the surface; Their shape indicates permafrost underneath.
• Volcanos: There are many, including the largest in the solar system, Olympus Mons [more than 15 mi high, 340 mi wide!] How can they be taller that on Earth? Are some of them active?

  Water?

• In the past: It definitely existed, from the runoff channels (highlands) and outflow channels and islands (equatorial) we see on the surface, and from analysis of soil by Opportunity and Spirit rovers; Whether water was present for long or brief periods, and whether there were oceans, is unclear.
• Now: It may exist only in a frozen form, at the poles (both polar caps contain large amounts of water ice, in addition to carbion dioxide ice), and in permafrost under the surface, but there is a huge quantity of it; Recent gullies may indicate that water flowed recently, but they may have a different explanation.

Interior

• Situation: Very little known, no seismic data, some from Global Surveyor, probably inactive.
• Geological activity: None now? Possibly plate tectonics in the past, and active volcanoes (not clear how recently the last eruptions occurred).
• Core: Not so hot, from current absence of geological activity; Long thought to be either not molten or not metallic, from absence of magnetic field, but years of spacecraft tracking data indicate molten iron in the core.

  Moons

• How many? Two, 28 km Phobos ("fear") and 16 km Deimos ("panic"), the sons of Ares (Mars) and Aphrodite (Venus); [Discovered only in 1877; But Jonathan Swift and Kepler had guessed that Mars had two moons!].
• Origin: Thought to be captured asteroid(s), but the details are not clear [they may be fragments of an old, larger moon]; Made of lighter material than Mars, which probably includes ice.
• Properties: Small, dark, irregularly shaped (Why?), and cratered, on very low orbits (Phobos is closer than synchronous).

Life on Mars?

• 1800's: Schiaparelli ("canali", 1877), Lowell ("canals"), and others.
• 1976: The Viking lander analyzed samples and found no evidence, but the experiments are still being debated.
• 1996: Rock from Mars, meteorite ALH84001 (left Mars 16 Myr ago, arrived on Earth 13 kyr ago, found in Antarctica in 1984), showed chemical evidence like carbonate globules, and tubular fossil-like forms.
• Present: 1996 evidence disputed (Earth contamination? Carbonates have also been seen in nebulae around stars); The basic idea remains as a possibility, but studies by the rovers so far are inconclusive, and currently few people believe there is life; New studies focus on where and when water existed, and minerals present.