As long as humans have explored the planets, from the time of crude telescopes through the age of sophisticated robotic spacecraft, Mars has stirred great scientific and human interest. As early as the mid-1600's, the astronomers Huygens and Cassini were able to note the planet's major markings and determine its rotation period. Based on observations of Mars between 1777-83, William Herschel suggested that Mars has a thin atmosphere, polar ice caps, and seasons similar to Earth's. In 1877, Schiaparelli made detailed maps of Mars' surface and popularized the notion that long canali, or channels, cover the surface. But it was nineteenth-century astronomer Percival Lowell's description of an Earth-like Mars, criss-crossed with long canals and covered with seasonal vegetation, that drew an intense public interest in Mars (this drawing comes from his book Mars). Lowell's books describing civilizations on Mars were among the most popular scientific writings at the turn of the century, yet many scientists of his day felt that he crossed the line between science and science fiction.

Ideas of civilizations and Earth-like life were dashed when the first spacecraft to reach Mars revealed a dry, cold, moon-like landscape. Mariner 4 flew by the planet in 1965 and returned a few images of the southern cratered highlands. Scientists noted how the spacecraft's radio signal diminished as Mars passed between it and the Earth, calculating that Mars' atmospheric pressure was 150 times less than Earth's. Mariners 6 and 7 in 1969 returned many images that showed more varied landscapes. An infrared radiometer found that the temperature of the south polar cap was 150 K (-193 F), definitively showing that it was composed of solid carbon dioxide, not water ice. In 1971-72, Mariner 9 conducted the first global mapping of Mars' surface and atmosphere from orbit. A vast number of features and phenomena were seen for the first time, revolutionizing the current view of Mars. Meanwhile, the Soviet Union launched an ambitious series of probes to Mars with some novel results, although all unfortunately prematurely failed.

The landmark spacecraft missions to Mars were the pair of Viking Orbiters and Landers which arrived in 1976. The orbiters mapped the entire surface at all seasons, measured the amount and distribution of atmospheric water vapor through time, determined the thermal properties of all surface materials including ices, and monitored dust storms. Their discoveries included the fact that, unlike the south, the north polar cap was composed of water ice. The primary goal of the landers was to conduct biology experiments designed to detect signs of past or present life. No unambiguous signs of biological activity were found. The landers also returned imagery and meteorology from the surface for the first time.

While these spacecraft did not find aqueducts, civilizations, or even bacteria, they did find signs of a past era that must have been quite different from the present. The older highlands, which cover roughly the southern hemisphere of Mars, are dissected by networks of valleys that, although now dry, were probably formed by flowing water. In other locations there is evidence for massive flooding, larger than any floods on Earth, that carved wide paths through the landscape. The valley networks and the flood channels flow into the northern plains of Mars, yet there is no sign of the vast volumes of water that they once carried. Has this water been frozen into an ocean beneath northern plains, locked in the polar ice caps, or lost to space? Why was liquid water once so abundant but now is completely absent? The Mars Polar Lander mission continues the quest to understand Mars by addressing these and other questions left unanswered by previous explorations.