Photo: NASA

The mystery of whether there is or has been life on Mars may be clarified or even resolved soon, with four new spacecraft—two from the European Space Agency (ESA) and two from NASA—arriving at the planet. They join two NASA probes already in orbit, though one of them—Odyssey—had a radiation sensor knocked out by the unusually intense solar storms in October. A fifth spacecraft, Japan's Nozomi, suffered problems that were aggravated by the solar activity and it will probably miss Mars altogether.

Such mishaps are a reminder of the "Mars curse" that has seemed to dog so many attempts to reach the red planet. No doubt the curse was on the minds of European flight controllers as Beagle 2 approached the planet in hopes of making a safe landing on 25 December.

The search for life on Mars got a big boost last November when NASA and its Jet Propulsion Laboratory in Pasadena, Calif., together announced that they had smoking-gun evidence of liquid water's having played a major role in shaping meandering channels on Mars. From that perpective, the most intriguing of the fleet of spacecraft heading toward Mars is the Beagle 2 Lander mission, carried onboard ESA's Mars Express Orbiter. Beagle 2 will look directly for evidence of life, while the orbiter will survey the planet for underground water, minerals, and weather, relaying signals to Earth from the Beagle.

If the ESA and NASA missions meet with success, they will bring to conclusion one of humankind's great quests. Not surprisingly, the venture has had its share of extraordinary personalities and curious anecdotes. The last dedicated life sciences landers sent to Mars were NASA's twin Viking spacecraft in 1976. The scientist who designed and built one of the three biology experiments, the controversial Gilbert Levin, began his career as a specialist in sterilizing municipal water supplies. In 1997, Levin attracted attention and derision with a claim that Viking had discovered microbial life after all. If that claim is now borne out, it will be in no small part because of a drilling device on Beagle 2 that was invented by a Hong Kong dentist [see "Mars: Dead or Alive?," IEEE Spectrum, May 2003, pp. 36-41].

The failure of Viking's mass spectrometer to find any traces of organic molecules when its data were first analyzed convinced many of the scientists working on the project that the planet was lifeless. Beagle 2 will reexamine the soil and rocks of Mars with an instrument called GAP (Gas Analysis Package), which is 100 times more sensitive to organic molecules than Viking's spectrometer. Beagle 2 also carries the first oxidant sensor. Its purpose is to detect the presence of strong chemicals in the Martian atmosphere that oxidize and destroy organic molecules, one of the primary theories put forth to explain the Viking results. Based on what the oxidant sensor and organic analysis instruments show, the Beagle 2 science team could announce the discovery of life on Mars, or at least reopen the discussion.

Meanwhile, the first of NASA's Mars Exploration Rovers, Spirit, was due to land on 4 January in Gusev Crater, a formation thought to have once been a liquid-water lake. Several weeks after that, a duplicate rover named Opportunity will land on the opposite side of the planet, on a flat plain known as Meridiani Planum.

The rovers are designed to travel about 40 to 100 meters each Martian day. Describing NASA's experimental strategy, James Garvin, the lead scientist for NASA's Mars exploration program, says it is "to explore the rocks in two localities where the preponderance of evidence from orbital reconnaissance tells us that liquid water was once there, for an unknown length of time."

"Persistent warm and wet environments leave their signature in the rocks, even after billions of years," he explained to Spectrum.

The scientific instruments aboard Spirit and Opportunity will try to determine the history of water and climate on Mars and to resolve whether or not it was ever conducive to life. But will the Beagle 2 Lander have already settled the question of life on Mars by the time the NASA rovers arrive?

Says Garvin: "NASA would welcome any Beagle 2 GAP observations that supported the existence of organics in a locality such as Isidis Planitia," the site of a presumed ancient ocean basin on Mars. But he cautioned that reproducibility and measurement precision, as well as preflight controls and calibrations, must be evaluated before measurements of organic molecules will be widely accepted.

"This in no way reflects any doubts about the quality of the GAP experiment, but rather the realities of the scientific method," he says.