Deciphering weather and global climate on another planet is not easy. As anyone knows from watching local news meteorologists, it is not easy right here on Earth. Mars, the Red Planet, is a special partner in the solar system, being a terrestrial world that is relatively close by and characterized by a desert-like climate, sandy dunes, wind storms, polar ice caps, and other features that make it seem similar to various locales on Earth.

But in reality, Mars is terrifically different than our world. With an equatorial diameter of 6,792 kilometers, the Red Planet is slightly more than half Earth's size, and it contains only a tenth of Earth's mass. The martian orbit carries it around the Sun once every 687 days, making its year equivalent to 1.9 Earth years.

Like Earth, Mars is differentiated – that is, it has a dense metallic core overlain by a rocky mantle. The planet's familiar orange color comes from copious amounts of iron oxide, like rust, richly coating the planet's rocks. Mars’ two moons, Phobos and Deimos, were discovered by American astronomer Asaph Hall (1829–1907) at the US Naval Observatory in 1877; they are probably captured asteroids, and measure a mere 27 by 22 by 18 kilometers (Phobos) and 15 by 12 by 10 kilometers (Deimos). Alternatively, some planetary scientists believe they may be pieces of Mars from large impacts.

Over the years, spacecraft missions have added enormously to our understanding of Mars. Missions got off to a rocky start, however. Following a series of unsuccessful missions by the Soviet Union and the failed US probe Mariner 3, the first flyby of Mars took place when the Mariner 4 spacecraft flew past the Red Planet in 1965. Mariner 9 produced a flood of martian imaging in 1971–1972. Substantial progress commenced with the US Viking 1 and Viking 2 landers, which touched down on Mars in 1976 and conducted various experiments. More significantly than the landers, however, were the Viking orbiters, which conducted vast amounts of science after the landers proved somewhat disappointing.

Twenty years later, in 1997, the Americans landed the Mars Pathfinder spacecraft in Chryse Planitia, a smooth circular plain. The craft consisted of the Carl Sagan Memorial Station, named after the then recently deceased astronomer, and a small rover called Sojourner.