Hungry on Mars

Mars has never had enough geochemical energy to allow numerous or complicated organisms to evolve during the last 4 million years, scientists report in today's Journal of Geophysical Research. The finding suggests that even if simple life does exist on Mars, it will be difficult to find.

Life requires liquid water and the building blocks of organic molecules--elements such as carbon, hydrogen, oxygen, and sulfur. Billions of years ago on Earth, these elements are thought to have assembled into primitive cells that relied on chemical energy to survive; only millions of years later did the complicated machinery of photosynthesis evolve. Mars once had water and the necessary elements, but planetary scientist Bruce Jakosky of the University of Colorado, Boulder, and microbiologist Everett Shock of Washington University in St. Louis suspected that the available energy might have curtailed the development of life on the red planet.

To test the theory, "we determined how much energy might be given off by interactions between water and rock," says Jakosky. This energy is released, for example, when basaltic rock weathers to clay, releasing hydrogen ions that primitive life-forms could use as chemical energy. "If microbes can tap into those reactions, then they can run their metabolism or build structures," says Jakosky. By compiling geologic maps and running computer models, the researchers estimated the abundance of these energy sources.

The result was "a surprisingly low number," says Jakosky. The researchers estimated that any Martian organisms that might have existed could have produced only 5 grams of biomass per square centimeter every billion years--4 million times less than the amount produced by Earth's organisms over the same period. This suggests that Mars never had more than five microorganisms per square centimeter, says Jakosky. The total energy was "large enough to imagine that life could have evolved, but also small enough that you don't expect complicated organisms to have evolved," he says.

"No one had really considered [energy limitations] until now," says Michael Meyer, an astrobiologist at NASA Headquarters in Washington, D.C. While the finding explains why there aren't any signs of life-forms teeming on Mars, he says, it doesn't rule out the possibility of limited enclaves. If life has existed on Mars, Meyer says, finding traces of it will require targeting specific sites, such as fossil hot springs.