Mars is the fourth planet from the Sun and the second smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature. Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Mars trojan.
Water – a key necessity for life as we know it – still flows sometimes on the surface of Mars, a new study suggests. Strong evidence for seasonal flows of liquid salty water have been detected by NASA's Mars Reconnaissance Orbiter, report scientists in a paper published Monday in the journal Nature Geoscience. "It suggests that it would be possible for life to be on Mars today," said John Grunsfeld, NASA's associate administration for science, at a news conference detailing the discovery.Previously, scientists have found ample evidence that Mars was once much warmer and wetter billions of years ago than it is now, and could have supported life then. It hasn't been clear whether the colder, drier Red Planet of today could still support microbial life. Many scientists think life could still survive deep underground, but not necessarily on the surface where it could be more easily detected.
Until the first successful Mars flyby in 1965 by Mariner 4, many speculated about the presence of liquid water on the planet's surface. This was based on observed periodic variations in light and dark patches, particularly in the polar latitudes, which appeared to be seas and continents; long, dark striations were interpreted by some as irrigation channels for liquid water. These straight line features were later explained as optical illusions, though geological evidence gathered by unmanned missions suggests that Mars once had large-scale water coverage on its surface at some earlier stage of its life.[21] In 2005, radar data revealed the presence of large quantities of water ice at the poles[22] and at mid-latitudes.[23][24] The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008
