Missions to Mars
Of all the wanderers in the night sky, the planet Mars has haunted humanity the most, for centuries stimulating conjecture about extraterrestrial civilizations. Even many scientists speculated that Mars might harbor at least primitive life. But early fly-bys of Mars by NASA's Mariner probes in the 1960s showed a cratered landscape little different from the Earth's moon, and most scientists gave up hope that Mars was anything but a dead world. Those spacecraft, however, photographed only a small portion of the Martian surface. When Mariner 9 reached Mars in 1971 and began to send back photographs of the entire surface, opinions about the planet changed quickly. Evident in the new images were valleys and channels that appeared to have been carved by flowing water, as well as giant canyons and huge volcanoes. These discoveries showed that Mars had once been surging with great geologic upheavals and running water. But was there life on the red planet?
An interplanetary armada of two Viking orbiters and two landers from the United States arrived at Mars in 1976. The landers were equipped with chemical laboratories to detect signs of metabolic (living) processes in the soil, and to search for organic (carbon-containing) molecules that might be the product of life. No organic compounds were found, but the soil seemed unusually reactive in a way that was initially interpreted as a possible sign of life. Further study of the results, however, showed that the chemistry of the soil itself, which is rich in certain iron oxides, had caused the reactions—the landers had not detected life.
The Viking orbiters, meanwhile, studied Mars from high above. Their images of the Martian surface showed even more dramatically the valley networks, channels, volcanoes, and canyons seen earlier by Mariner 9. The much finer detail of the Viking images enabled scientists to suggest that some geological features on Mars were the remnants of glacial erosion and to identify sediments that may have been left behind by ancient lakes that had long since dried up.
Given the abundant evidence for liquid water on Mars in the past, scientists began to develop theories to explain why Mars today is a cold, dry planet. Because Mars is considerably farther from the sun than the Earth is, its early wet climate must have been sustained by a much thicker atmosphere. That atmosphere may have been dominated by carbon dioxide, which is still the main component of the now-thin envelope of gases surrounding Mars. Some of the early atmosphere was probably lost as the result of large impacts by asteroids. A larger portion of it may be locked in the crust as carbonates that formed when the planet had abundant water. Like Venus, Mars shows no sign of active plate tectonics, which on Earth returns carbon dioxide to the atmosphere by melting and squeezing carbonates in the crust. On Mars, carbon dioxide, once locked in carbonates, could not be recycled in this way.
To test this theory, NASA in 1992 launched a probe called Mars Observer check for the presence of carbonate minerals on the planet. But the spacecraft was lost when its propulsion system failed in August 1993, just days before reaching Martian orbit, and the search for carbonates on Mars was put on hold. Instead, a series of Mars orbiters, beginning with the one launched in 1996, will conduct the search.