The Odds for Life on Mars

Science fiction has long portrayed Mars as home to intelligent life, and, as we saw in Chapter 13, "So Close and Yet So Far: The Inner Planets," Percival Lowell, early in the twentieth century, created a great stir with his theory of Martian canals. Actor-director-playwright Orson Welles triggered a nationwide panic with his 1938 radio dramatization of H. G. Wells's War of the Worlds, about a Martian invasion of Earth. People were ready to believe.

Given our fascination with the red planet and its proximity, it is no wonder that Mars has been the target of a number of unmanned probes (see Chapter 9, "Space Race: From Sputnik to the International Space Station"). In the mid- to late 1970s, the Viking probes performed robotic experiments on the Martian surface, including tests designed to detect the presence of simple life forms such as microbes. These tests yielded positive results, which, however, were subsequently reinterpreted as false positives resulting from chemical reactions with the Martian soil. However, on August 7, 1996, scientists announced that, based on its chemical composition, a meteorite recovered in Antarctica had originated on Mars and possibly contained fossilized traces of molecules that, on Earth, can be produced only by bacteria.

Most recently, the Mars Pathfinder (1997) and Mars Global Surveyor (1998) missions have added detailed panoramic views from the Martian surface and high-resolution satellite images of the surface. Both of these missions suggest the presence at some time in the past of liquid water on the Martian surface. Barry E. Digregorio, Gilbert V. Levin, and Patricia Ann Straat, authors of Mars: The Living Planet (North Atlantic Books, 1997), present evidence of possible subsurface water, though most evidence suggests that the Martian surface has been dry for a very long time.

All of these results are intriguing, but, so far, none of the Martian probes has found clear evidence of life (past or present) on Mars. In the past, the Martian atmosphere, now very thin, was thicker and, as a consequence, the planet's surface warmer and wetter. There is a possibility that microbial life once existed on Mars. Under the planet's current cold, dry, and generally harsh conditions; however, the presence of life is highly unlikely.

The Mars Orbiter Camera continues to transmit high-resolution images of the Martian surface that suggest at least the periodic presence of surface water.

(Image from NASA)

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