PAST AND FUTURE STUDIES OF MARS Mars played an important role in resolving the orbital motions of the planets, because of
its puzzling looping motion in the heavens as viewed from Earth. When Johannes Kepler determined in 1609 that the orbital path of Mars is elliptical, he abolished the older Ptolemaic theories based on circular orbits of the planets. The very next year Galileo made the first telescopic
observations of the planet and recorded its phases. In the late 1600s other
scientists made closer observations of Mars, determining that its rotational period is close to Earth's, that it has polar caps that show seasonal change, and that dark areas could be distinguished. These were thought to be seas. In the late 1700s, William Herschel noted bright, changing patches that he thought were clouds, suggesting that the planet had an atmosphere. He and later astronomers therefore shared the view that life thrives on Mars. In the late 1800s astronomers also began to observe linear markingsÑtermed "canali" by Giovanni Schiaparelli, though he simply meant channels and not "canals"Ñthat connected broad, dark areas. These observations increased speculation about intelligent life on Mars, culminating in the popularization of this idea by Percival Lowell. (The "canali" are now known not to be real features but probably to result from a visual illusion in which dark areas appear connected by lines.) The Martian satellites were discovered by Asaph Hall in 1877. Knowledge of the planet has greatly increased through spacecraft missions, beginning with Mariner 4 in 1965 and the Mariner 6 and 7 flybys in 1969. The highly successful Mariner 9 mission in 1971Ð72 and the Viking orbiters launched in 1976 together imaged the entire Martian surface and revealed the geologic diversity of the planet. The Viking landers made long-term observations of the surface and atmosphere and performed experiments to determine the composition of surface materials. Their biological experiments found no evidence of organic material. Its apparent absence, together with the planet's thin, dry atmosphere, has led most researchers to believe that no near-surface life exists on Mars. Hospitable conditions for life may be present, however, in warm, wet subterranean environments, if such exist. Other scientists think that life may have existed early in Mars's history, when climate conditions were probably more favorable. In fact, study of the rare SNC (shergottite-nakhlite-chassignite) meteorites collected on Earth, which are thought to have been ejected from Mars by impacts, provided great excitement in this respect in 1996. NASA and other scientists studying the meteorites claimed that they yielded strong evidence of past life on Mars, in the form of tubelike structures resembling microbial fossils. This startling claim has yet to be widely endorsed by other scientists, but it added increased interest to the robotic mission of the Mars Pathfinder, one part of NASA's Discovery Program. Launched toward Mars on Dec. 2, 1996, the craft parachuted safely down to the planet's surface on July 4, 1997, bouncing to a stop by means of air bags that then deflated. It then released a small, six-wheeled rover called Sojourner to conduct chemical and geological tests on Mars rocks and soil. Pathfinder also recorded wind data that scientists on Earth later converted into soundÑthe first time such sounds from another world have been heard. Although NASA lost contact with the lander in October 1997, it had by that time completed its original planned schedule and returned many remarkable photographs and important data. (The latter include the finding of a higher percentage of silica in the rocks than expected, suggesting that perhaps plate tectonics did take place on Mars after all.) Launched on Nov. 6, 1996, Global Surveyor, the other part of the Discovery program, entered Martian orbit on Sept. 11, 1997. It began slow maneuvers to enter a final, closer orbit and begin its two-yeaping mission in 1999. On Dec. 11, 1998, NASA launched the Mars Climate Orbiter to study the planet from an orbit 422 km (262 mi) above the surface. It was followed on Jan. 3, 1999, by the Mars Polar Lander; targeted at the planet's south pole, it will dig for signs of water and transmit sounds as well as pictures. The lander also carries the Deep Space 2 probes, which are intended to penetrate up to 90 cm (3 ft) into the surface.