Credit: J. Bell (Cornell U.), M. Wolff (SSI) et al., STScI, NASA

MARS

My ochred cliffs and rusted sands stand regal and serene,
But oh, my wan and wasted world, I miss your blue and green.
— "Lament for a Red Planet," Jonathan Eberhart

Vital Stats:

Orbital radius: 1.5 Earth's
Diameter: 0.5 Earth's
Mass: 0.1 Earth's
Gravity: 0.38 g
Year: 2.0 Earth's (225 days)
Day: 1.0 Earth's (24.6 hours)

Points of Interest:

Mars has ice caps, like Earth. In winter, carbon dioxide snows out on top of the water-ice, forming a layer of dry ice on caps.
Mars has some spectacular geography. Its biggest volcano is the size of New Mexico; its caldera could hold Hawaii.
Mars's volcanos are strongly concentrated in northern hemisphere; its moon-like craters are concentrated in the south.
The whole northern hemisphere of Mars is smoother and a kilometer or more lower than the southern. Perhaps due to a very large, very long-ago impact.
Mars's biggest canyon, Valles Marineris, runs a fifth of the way around the planet and is as long as Europe.
Mars had a lot of water in its ancient past. It also has dry river beds, which may be more recent. This, of course, bears on the question of whether Mars has or had life.
Plumes of methane gas have been detected on Mars. Produced by life in the soil?

Mars on Earth:

Mars is the only planet with a clear color – red. It looks rusty, and in fact it is. Therefore Mars has been associated with blood and iron and war, and given to warrior gods like Nergal, Ares, Mars, and Tyr. The Romans identified their Mars with the Norse Tyr, so Mars's day is Tuesday, Tyr's Day, French Mardi, Spanish Martes, Italian Martedi. The alchemical metal for Mars is of course iron.

Johannes Kepler figured out the laws of planetary motion by using Mars as his test case – a difficult job he called his "war with Mars." Kepler's laws in turn guided Newton in his discovery of the law of gravity as an inverse-square force, the first of the forces that populate physical theory.

Mars is THE alien planet par excellence. The first alien invasion story, Wells's The War of the Worlds, was launched from Mars, and many another invasion from there has been launched since. If someone wants to talk about how things would appear to an objective or naïve observer, one with no human preconceptions, they'll often talk about a hypothetical Martian, or Martian scientist, or Martian anthropologist. Sometimes they speak of a generic "alien," but usually it's a Martian.

Mars has seasonal color changes. That, plus some streakiness in images from early telescopes, plus wishful thinking, led us to hope for canals on Mars, created by real Martians, irrigating their deserts. Alas, various probes, like the Mariners, the Viking landers, Spirit, and Opportunity, have found no Martians. Unless they are bacteria, or deep underground. Hope springs eternal.

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