Mars is the fourth planet from the Sun and the seventh largest in the
Solar System. Mars has been known since prehistoric times. It has been extensively studied with ground-based observatories. But even very large telescopes find Mars a difficult target, because it is too small.
Mars has the largest canyon in the solar system. It would reach from Los Angeles to Chicago if it was on Earth!
Mars has a very thin atmosphere, mostly carbon dioxide, but dust storms can cover the whole planet for months at a time. About every two years the Earth and Mars come close together. The planet has two moons, Diemos and Phobos.
The first spacecraft to visit Mars was Mariner 4 in 1965. Several others followed including Mars 2, the first spacecraft to land on Mars and the two Viking landers in 1976. Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on 1997 July 4. In 2004 the Mars Expedition Rovers "Spirit" and "Opportunity" landed on Mars sending back geologic data and many pictures.
Scientists have found strong evidence that water once flowed on the surface of Mars. The evidence includes channels, valleys, and gullies on the planet's surface. If this interpretation of the evidence is correct, water may still lie in cracks and pores in subsurface rock. A space probe has also discovered vast amounts of ice beneath the surface, most of it near the south pole.