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 surface of Mars. Recent missions have shown that the soil includes necessary chemicals for life, like sodium and potassium.
Even more interesting, however, has been the search for water. Substantial amounts of water ice have been found in the polar regions in the last several years. Still, the evidence currently suggests that the Martian surface has not been home to large quantities of liquid water for most of the planet's long history.
Nor, it follows, could it have been home to any form of advanced life during that same time period. The best-case scenario for life on Mars today would be some sort of bacteria-sized organisms specially adapted to the harsh cold and heat of the Martian year, similar to the so-called extremophiles which have been found in the harshest environments on Earth, like deep-sea vents and volcanically active pools. The lack of even microorganism-sized life on Mars has come as something of a disappointment: since the discovery of "canal"-like structures on the surface centuries ago, Mars had always been seen as the most likely candidate for evolving life apart from Earth.
- Moons -
Mars has two moons, Deimos and Phobos. Neither is of particular astronomical interest: indeed, both are probably captured asteroids, too small even to have forced themselves into spherical shapes through gravity. New analyses of Phobos have shown that, however it was captured by Mars, it has ended up in much too low an orbit. Every time Phobos makes its 8-hour orbit of Mars, it is pulled in a little closer. In several million years, it will probably be wrenched apart by Martian gravity, after which the fragments will rain down on the Martian surface, causing a fresh new set of sizeable craters.
The Russians have tentatively scheduled a probe to visit Phobos in 2017, which would be the first spacecraft to visit a Martian moon.
- Exploration -
Mars has been the major subject of American interplanetary probe missions since the arrival of the first spacecraft, Mariner 4, during the mid-1960s. (In contrast, the Soviet program focused more on Venus.) Some of NASA's landmark missions to Mars include the Viking landers, which operated for several years on the surface taking pictures and conducting basic geology and chemistry experiments; Pathfinder-Sojourner, which successfully landed the first mobile probe on the surface of Mars; and, most recently, the Spirit and Opportunity probes, much larger versions of Sojourner.
Today, there are several operational American spacecraft either on or orbiting Mars, including the Odyssey, Express, and Reconnaissance Orbiter spacecraft in orbit, and the long-lived Spirit and Opportunity rovers on the surface. The two rovers have been carrying out their record-setting mission since touching down in January 2004, although Spirit recently became stuck and will likely never move again.
If humans one day travel to another planet, their first target will almost certainly be Mars. However, so far no space agency has been willing to undertake the considerable risk and exorbitant expense of a manned mission to the red planet. The next mission to Mars will be another unmanned probe: Curiosity (formerly the Mars Science Laboratory), an even larger, faster, and more powerful version of the successful mobile probe design. Curiosity will be twice the size of its predecessors, roughly as large as a small car.