by D. Vogt

Created on: March 05, 2010

Mars is the fourth planet in the solar system, located in an orbit between Earth and Jupiter (this also makes it the last of the rocky planets; all more distant planets are gas giants). Viewed from Earth, Mars has a perpetually orange or reddish tinge, resulting from its iron-rich rocky surface.

Like the Earth, Mars once had water and still has a nearly 24-hour day; unlike the Earth, however, its atmosphere is rich in carbon dioxide and is perilously thin. Notable features include the probable presence of liquid water (in some areas of the planet), and Olympus Mons, the largest mountain on any known planet. To date, Mars is the most heavily explored planet, although thus far it has been approached only by unmanned probes.

- Roughly Earth-Like -

Although Venus is closest to Earth in terms of size, in most other respects Mars is our closest relative in the solar system. Despite being roughly half the Earth's size, it has a day lasting 24.5 hours (slightly longer than Earth's), and a year equivalent to slightly less than two Earth-years. This longer year results from its orbit: Mars is 50 million miles away from us, and roughly half again as far from the Sun as Earth. Mars also has seasons much like Earth's, although because it orbits farther from the Sun and has a more strikingly elliptical (oval-shaped) orbit, its winters are colder and its summers are warmer than any here on Earth.

One of the most significant differences between Earth and Mars, however, is the Martian atmosphere. Mars lost its magnetic field billions of years ago, presumably through planetary cooling; after this, the outer reaches of the atmosphere were permanently exposed to solar radiation, and have been gradually stripped away.

This leaves an atmosphere which is both lower in pressure and, more importantly, much thinner, reaching just a few short miles above the surface of the planet. Spacecraft have confirmed that this thin atmosphere is made up almost entirely of carbon dioxide, with some nitrogen, argon, and oxygen, and trace amounts of chemicals like methane. On Earth, the atmosphere is quite different, dominated by nitrogen and oxygen. Earth's rich plant life continually maintain a high ratio of oxygen to carbon dioxide; on Mars, there is no such process to produce an oxygen-rich atmosphere.

- Life on Mars? -

To date, the most interesting and high-priority experiments by NASA and the European Space Agency have involved the search for water and building blocks of organic life on the

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