On July 20, 1976, a 572-kilogram spacecraft, supported on three sturdy legs, touched down on the orange sands of Mars in the western part of Chryse Planitia. Six weeks later, its sister craft made landfall about 200 kilometers west of the crater Mie in Utopia Planitia. The twin Viking landers were the first purpose-built attempts to hunt for life on another world.
Each carried a miniature laboratory containing experiments designed to show if Martian microbes were alive in the soil of the Red Planet. Initially, there was great excitement because a couple of the experiments gave signs that soil samples delivered to the onboard lab by the Viking robotic arm were feeding on the nutrients offered to them. But as time went on, the results became more puzzling—something was definitely active in the soil, but was it chemical or biological? Opinion shifted emphatically the chemical way, when an instrument called the gas chromatograph–mass spectrometer (GC-MS) failed to detect any trace of organic (carbon-bearing) compounds in the soil samples.
A few scientists, notably Gil Levin, the principal investigator of one of the Viking experiments, continued to argue that Viking really did find life. Serious doubts were cast on the sensitivity of the GC-MS to low concentrations of organics, and it remains a fact that not all Viking’s data can be squared with a simple chemical interpretation.
Mars isn’t a friendly place today for terrestrial life. Its thin atmosphere—which exposes the surface to harsh ultraviolet rays from the Sun—its generally low temperatures, and its lack of water make it hostile to any but the most hardy bacteria. But in the remote past, the fourth planet was warmer and wetter, and had a much denser atmosphere. In some ways, given that life started on Earth so quickly, it would be surprising if, under similar conditions on Mars, it didn’t also develop there. If it did, the question is whether it could have survived to the present, having gradually adapted to a more and more challenging environment. One possibility is that it may have retreated underground, to places shielded from solar radiation and where supplies of liquid water may still be found.
The latest spacecraft to arrive on Mars, the Curiosity rover, isn’t intended to look for life. However, what it can do is detect any organic compounds there might be in the top few inches of soil. Its first serious attempt to do this, using a sample of windblown sand and dust from a site called Rocknest, drew a blank. Likewise, the rover has failed to sniff any of the methane in the atmosphere that earlier measurements by orbiting spacecraft and ground-based telescopes seemed to have picked up. One of the possible explanations for this short-lived atmospheric component—if it exists—is methane-producing microbes. But for now, any clues to past or present Martian life remain elusive.
As for the future, only the European Space Agency’s ExoMars multi-vehicle mission, which may also involve the Russians, has a strong focus on searching for bio-signatures. NASA’s withdrawal from this mission has left it financially insecure. But, assuming it still goes ahead, it is scheduled to involve two launches—that of an orbiter and stationary lander in 2016, and of a rover in 2018. Meanwhile, astrobiologists have widened their net in the search for alien life to include several other worlds in the solar system.