Mars is the next-most distant planet from the Sun after Earth, and the fourth planet in the solar system as a whole. It is also the outermost of the small, rocky ("terrestrial") planets; beyond Mars lies the diffuse Asteroid Belt, followed by the four gas giants (Jupiter, Saturn, Uranus and Neptune). The planet is best-known for its distinctly rust-red colour. Due to its proximity to Earth and suspicions that its surface once resembled Earth's (and therefore could have harboured primitive life), Mars is currently the most important target for interplanetary space probes.
- Position in the Solar System -
Mars's orbit is about 50 million miles from that of Earth, placing it 143 million miles away from the Sun. The planet is only about half the size of Earth, and completes one orbit around the Sun in about 687 Earth-days, or a little less than two Earth-years. However, in a unique case in the solar system, the Martian day is actually almost identical to an Earth-day, lasting about 24 hours and 40 minutes. All other planets in the solar system have days lasting either much longer or much shorter than those of Mars and Earth.
Two moons circle Mars, making it the only inner rocky planet (apart from Earth itself) to possess moons, and the only one to have multiple moons. However, neither of the Martian moons are as impressive as our own. Neither Phobos nor Deimos, as they are known, are large enough to have coalesced into a spherical shape. Phobos, the largest of the two, has a radius of just 7 miles, and orbits only a few thousand miles from Mars's surface. It is gradually being pulled closer and closer, and is expected to be broken apart by Mars's gravity within the next 100 million years.
- Surface and Composition of Mars -
Mars is about half the size of Earth, and has about one-sixth the mass (and therefore a much weaker gravity). Its rocky red surface has fascinated generations of amateur astronomers. The surface is pockmarked by features greater than their counterparts on Earth. For instance, the largest volcano on Mars, Olympus Mons, is 27 kilometres high - several times the height of Mount Everest. The largest canyon, the Valles Marineris, is nearly ten times the length and over three times the depth of the Grand Canyon.
Unlike Earth, Mars has no magnetic field - and unlike Earth or Venus, it also has a very weak atmosphere consisting almost entirely of carbon dioxide, with smaller amounts of nitrogen, argon, and oxygen. The Martian magnetic field was lost several billion years ago, and the thin atmosphere of the present is a product of billions of years of interactions with the solar wind, stripping away layer after layer from the upper atmosphere.
Although Mars is barren and hostile today, the evidence indicates that in the distant past, perhaps when its atmosphere was richer, circumstances were quite different. Water still exists at both poles, in ice form, and the evidence suggests that liquid water once flowed across the planet. It is this historic and current presence of water which has fuelled much of the current interest in the history of Mars, since it now appears to be the most likely place in the solar system where life apart from that on Earth might have evolved - if, of course, such life ever did.
- Exploration of Mars -
Mars is both the easiest planet to reach with a space probe launched from Earth, as well as one of the most interesting objects of study in the inner solar system. (Admittedly, Venus might turn out to be equally interesting were more attention directed to its study.) A large number of European, Soviet, and American space probes have been sent to the planet since the 1960s, although most have failed. It is also generally expected that the first interplanetary manned spacecraft would be a Mars mission, although no such mission has yet been announced by any space agency.
The Soviets have had poor luck exploring Mars, losing the majority of their probes. In contrast, NASA has been much luckier, the embarrassing loss of the Mars Climate Orbiter due to a metric-imperial conversion error notwithstanding. Mariner 4 flew past Mars in 1964, Mariner 9 became the first to enter its orbit in 1971, and then the Viking 1 and Viking 2 probes carried out lengthy landing missions during the mid-1970s. NASA's Spirit and Opportunity rovers, which landed in 2004, are both still operational on the surface six years later, though one is stuck and has recently been written off as permanently lost. The much larger Curiosity rover, formerly the Mars Science Laboratory, is currently scheduled to launch in 2011, followed by a Russian mission to Phobos shortly thereafter and then a European rover to Mars again in 2018.