Does anything call Mars home? JPL/NASA

If life still exists on the Red Planet, it must be very rare – or so an unexploited energy source in the atmosphere suggests.

The Martian atmosphere is unusually rich in carbon monoxide, which many microbes here on Earth can convert to carbon dioxide to yield energy for growth.

“It’s a free lunch, just sitting in the atmosphere, that microbes could be eating,” says Steven Sholes, an astrobiologist at the University of Washington. The persistence of that leftover lunch suggests that Martian life must be nonexistent, or at least very rare.


To determine just how rare, Sholes gathered estimates of how quickly solar radiation generates carbon monoxide in the Martian atmosphere, and how fast it diffuses down to the planet’s surface and into subsurface rocks, where any Martian life would be sheltering from deadly radiation. Then he used these estimates to calculate the maximum subsurface microbial biomass that could be consuming carbon monoxide, yet still leave the observed amount of leftovers.

Martian microbes?

By this calculation, Mars could harbour no more than one billionth of Earth’s biomass, or less than one microbial cell per cubic centimetre of soil, he told the Astrobiology Science Conference in Mesa, Arizona, last week.

His analysis doesn’t prove the existence of life on Mars today one way or the other, only that if it is there it is rare. The calculation also only accounts for microbes that metabolise carbon monoxide, he notes. Other microbes, using other metabolic pathways, could be present – though it would be surprising if they didn’t adapt to make use of the free lunch, he says.

Even the high end of Sholes’s estimate suggests that spacecraft looking for extant life on Mars face a daunting task. The least productive environments on Earth contain 100 times more cells per cubic centimetre as the calculated Martian maximum, says Tori Hoehler, an astrobiologist at NASA Ames Research Center in California.

“It’s not easy to find those organisms,” says Hoehler, so finding even sparser life on Mars will be even tougher.