If Mars has Life, its Total Mass is 10,000 Times Smaller than Earth's

NewScientist | Apr. 12, 2019 | By: Michael Marshall

NASA's Curiosity Rover is Drilling into the Martian Surface - there might not be much life down there. Image: NASA/JPL-Caltech/MSSS

We still don’t have hard evidence for life on Mars, but a new study suggests that if it does exist there cannot be much of it.

The absolute maximum seems to be 200 million tonnes of living matter, according to a team led by Steven Sholes at the University of Washington in Seattle – the equivalent, they say, of about a million blue whales. That sounds like a lot but is no more than 1/10,000th the amount of living matter on Earth – far less, even considering that Earth’s surface area is about 3.5 times larger than Mars’s.

As researchers continue to explore Mars they might ultimately conclude the planet has no life at all. But many hold out hope that Martian microbes exist somewhere on, or more likely below, the Martian surface. If Sholes and his colleagues are correct, it might be very thin on the ground.

The key piece of evidence is that Mars’s air contains two rich sources of chemical energy: carbon monoxide and hydrogen. The fact these abundant food sources are not being consumed suggests either that there is no life on Mars, or that it is scarce.

The carbon monoxide and hydrogen may be “anti-biosignatures” that tell us Martian life is either absent or extremely limited, says Sholes. Astrobiologists have often focused on identifying biosignatures that reveal the presence of life, but have paid less attention to signatures of its absence.

A study published in 2000 highlighted the energy available from carbon monoxide and hydrogen, which are produced by the action of sunlight. The study suggested that liquid water might instead be the limiting factor on Martian life.

To find out how much life could be there, Sholes and his colleagues built a model of the atmosphere’s chemistry. They simulated how much carbon monoxide and hydrogen should be forming in the air and how much could be drawn out of the air by microorganisms, while keeping the simulated levels in the air in line with observations.

To estimate the maximum amount of life these chemical flows could support, the team assumed that each cell is “just barely staying around” says Sholes, by using the minimum possible amount of energy. “They’re not increasing in size, not reproducing, not moving, not wasting any energy, being 100 per cent efficient, going very slow, just enough to keep their DNA or RNA intact,” says Sholes. Even allowing for this, Mars probably supports a very small biomass.

It is possible that Martian microbes are living off other chemicals, as some organisms do on Earth. But Sholes says there is no sign of them doing so. For example, some Earthly microbes turn carbon dioxide into methane – but there is only limited evidence of methane on Mars.

Any microbes must be a few kilometres below the surface, the team says. That’s because the only liquid water on Mars is underground where it is warmer. Sholes says life could only survive within about 10 kilometres of the Martian surface, where it is both deep enough for liquid water and shallow enough for carbon monoxide and hydrogen to diffuse downwards in large quantities. Finding these microscopic Martians would likely require drilling down into Mars’s crust.

Sholes previously described a preliminary version of the study at an astrobiology conference in 2017.