Mars

Mars is the fourth planet from the Sun, and has been known since ancient times for its reddish color. Mars is named after the Roman god of war, and takes that name from its color. Perhaps because its bloody hue, its close proximity to Earth, and its seasonally-changing surface features, Mars has played a larger role in human culture and mythology than any other planet.

Orbit and Observation

Mars orbits the Sun at about 1.5 times the average distance of the Earth, with a an orbital period of 687 days. Mars's orbit eccentricity (0.0934) is about six times greater than the Earth's, so its distance from Earth varies widely - from 35 million miles (59 million km) at a "favorable" opposition near the orbit's perihelion, to 248 million miles (399 million km) at superior conjunction near its aphelion. Because of this, Mars varies greatly in its apparent size, from 3.5 to 25 arc seconds, and in brightness from magnitude -2.9 to +1.7.

In a small telescope, Mars shows many of the surface features that sparked the imagination of science fiction writers. Prominent white polar caps are visible, as are odd dusky markings on its surface. These markings show that Mars rotates once every 24 hours and 37 minutes - so its day is almost the same length as Earth's. Mars also has an axial tilt very similar to Earth's, and has seasons like the Earth. The polar caps shrink and expand during the Martian summer and winter, and the dark patterns on its surface also display seasonal changes. Mars has an atmosphere with sparse clouds, and exhibits occasional dust storms which sometimes grow to cover the entire planet's surface for a few weeks.

Exploration and the Possibility of Life

Once we began to study Mars with telescopes, that planet's intriguing similarities with Earth became apparent. It was the American astronomer Percival Lowell in the early 1900s who first proposed that Mars was the home of a dying civilization, which built canals to carry water across its rusty deserts, from the polar ice caps to cities along the equator. That vision was popularized in the novels of H. G. Wells and Edgar Rice Burroughs, and captured the popular imagination, but it ultimately proved to be false.

The first spacecraft mission to fly by Mars (Mariner 4, in 1965) revealed a vast, barren wasteland of craters. Mars's atmosphere was only 1% as dense as Earth's at the surface, and composed of 95% carbon dioxide (with small percentages of nitrogen and argon). There was no protective ozone layer, and no magnetic field to shield the surface from deadly solar radiation. Its surface was cold - the average temperature is -81° F (-63° C), with a minimum of -200° F (-140° C), and a maximum of 68° F (20° C) on the warmest days at the equator. These conditions made the surface of Mars completely inhospitable to life as we know it. The large number of craters seemed to indicate that Mars was a dead world, geologically speaking, as well.

However, subsequent spacecraft exploration showed that Mars was perhaps not quite such a dead place after all. Mariner 9, which entered orbit around the planet in 1971 and mapped its surface in high detail for the first time, revealed the presence of huge volcanos and vast canyon systems. The tallest of the volcanoes, Olympus Mons ("Mount Olympus"), is about 17 miles (27 km) high - about three times the elevation of Mount Everest! - and is 340 miles (550 km) across at its base. Olympus Mons is both the largest volcano and the tallest mountain in the solar system, and is aptly named for the home of the gods.

The deepest and longest canyon system on Mars is Valles Marineris (the "Mariner Valley"), and is up to 4 miles (7 km) deep, 120 miles (200 km) wide, and 3000 miles (5000 km) long. It is the largest known crevice in the solar system, and if placed on Earth, it would span the entire United States. Mars was clearly home to some significant geological activity in its past.

More importantly, spacecraft images showed surface features that seemed to indicate the presence of water: channels, dry riverbeds, and flood plains. Features such as these look strikingly similar to features on Earth which have been created by liquid water. But on Mars, unlike Earth, liquid water is all but nonexistent.

Today most of Mars's water is buried beneath the surface or frozen as ice in the polar caps. Mars's permanent polar caps are made of water ice; the seasonal expansion and contraction of the polar caps is actually due to the presence of carbon dioxide ice freezing out of the atmosphere - Mars's polar regions never become warm enough for water to melt.

The first spacecraft to successfully land on Mars (Viking 1, in 1976) detected minute trace amounts of water vapor in the atmosphere, but failed to find any conclusive evidence of life. Later missions, particularly NASA's Spirit and Opportunity rovers which landed on Mars in 2004, confirmed that the Martian surface was once covered by abundant amounts of liquid water. The Opportunity rover photographed mineral formations, dubbed "blueberries", which could have only formed in the presence of liquid water.

The Mars Phoenix mission, which landed near Mars's north polar region in 2008, may have photographed deposits of water ice directly under the lander itself. These ice deposits were revealed when the lander's rockets blasted away the overlying topsoil. Later, photographs showed what appeared to be droplets of liquid water condensing briefly on the lander's legs, before evaporating into the thin, dry atmosphere.

Meteorites from Mars have landed on Earth, blasted from the Red Planet's surface by enormous asteroid impacts. Preserved in Antarctic ice, these meteorites are known to have originated on Mars, because the composition of gases trapped inside their porous interiors exactly matches that of the Martian atmosphere.

One particular Martian meteorite discovered in Antarctica in 1984, called ALH84001, became the subject of great controversy in 1996. That year, NASA scientists announced that amino acids and other organic compounds had been discovered inside ALH84001. They also showed images of microscopic structures inside the meteorite, resembling (but much smaller than) fossilized bacteria on Earth. These discoveries were first announced as solid evidence that life had actually arisen on Mars. But this conclusion was immediately disputed by other scientists, who argued that the evidence could also be explained by non-biological processes. The situation is unresolved, and in late 2009 some scientists reasserted that Martian meteorites still provide strong evidence of life on ancient Mars.

The evidence is clear that Mars once had a much warmer, wetter past. If so, there is a possibility that life once existed, or still exists, on Mars. Because of that possibility, Mars remains a primary target of our space exploration.

Moons of Mars

Mars has two tiny moons, Phobos and Deimos. Both moons were discovered by Asaph Hall in 1877. Both are tidally locked with Mars, always showing the same face to the planet; and both orbit Mars very close to the plane of its equator. Phobos and Deimos are both small rocky bodies, resembling asteroids. This has fueled speculation that they actually are captured asteroids.