Mars & Venus
Mars
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.
Venus
Venus is the second planet from the Sun and the sixth largest in the Solar System. It is the brightest object in the sky except for the Sun and the Moon. Like Mercury, it was popularly thought to be two separate bodies: ancient astronomers called the object that appeared in the morning Phosphorus, and the object that appeared in the evening Hesperus. Later, they realized these objects were the same planet. They named Venus in honor of the Roman goddess of love and beauty.
Astronomers refer to Venus as Earth's sister planet. Both are similar in size, mass, density and volume. Both formed about the same time and condensed out of the same nebula. However, during the last few years scientists have found that the kinship ends here. Venus is very different from the Earth. It has no oceans and is surrounded by a heavy atmosphere composed mainly of carbon dioxide with virtually no water vapor. Its clouds are composed of sulfuric acid droplets. At the surface, the atmospheric pressure is 92 times that of the Earth's at sea-level.
The first spacecraft to visit Venus was Mariner 2 in 1962. It was subsequently visited by many others (more than 20 in all so far), including Pioneer Venus and the Soviet Venera 7 the first spacecraft to land on another planet, and Venera 9 which returned the first photographs of the surface. Most recently, the orbiting US spacecraft Magellan produced detailed maps of Venus' surface using radar.