For Life, Size Matters: Mars Might Be Too Small To Be Habitable

Artist's rendition of Mars with Earth-like surface water. Image Credit: NASA Earth Observatory/Joshua Stevens; NOAA National Environmental Satellite, Data, and Information Service; NASA/JPL-Caltech/USGS; Graphic design by Sean Garcia/Washington University

New research suggests that the cut-off for the habitability of rocky worlds could have something to do with their size. It seems that some planets and moons might not be big enough to hold on to molecules that might be crucial for the formation of life — and Mars is among them.

Reporting in the Proceedings of the National Academy of Sciences, researchers looked at the presence of different types (isotopes) of potassium present in Martian meteorites. Potassium is a moderate "volatile", the group of elements and compounds that can be lost by planetary bodies. Water is another, slightly more volatile, example. 

We know Mars once had water. The Martian landscape is marked by ancient lakes and river valleys, though where it all went is less clear. In this study, potassium is used as a tracer for water. The team found that Mars has lost a lot more potassium than Earth (bigger and wetter than Mars), but not as much as the Moon or asteroid Vesta (much smaller and drier). So the size of the world might be key to its ability to hold on to those molecules, like water, that could make life germinate.

“Mars’ fate was decided from the beginning,” senior author Professor Kun Wang from Washington University said in a statement. “There is likely a threshold on the size requirements of rocky planets to retain enough water to enable habitability and plate tectonics, with mass exceeding that of Mars.”

This relationship between a body’s gravity and the amount of volatiles have implication for how wet Mars might have been in the past.

“It’s indisputable that there used to be liquid water on the surface of Mars, but how much water in total Mars once had is hard to quantify through remote sensing and rover studies alone,” Wang added. “There are many models out there for the bulk water content of Mars. In some of them, early Mars was even wetter than the Earth. We don’t believe that was the case.”

Volatiles are also lost due to how much light a planetary body gets, among other factors. Scientists have to take this and more into account when considering if Mars could (or has ever) support life and what the chances are that exoplanets – worlds beyond the Solar System – might be habitable.

"This study emphasizes that there is a very limited size range for planets to have just enough but not too much water to develop a habitable surface environment," said co-author Klaus Mezger of the Center for Space and Habitability at the University of Bern, Switzerland. "These results will guide astronomers in their search for habitable exoplanets in other solar systems."


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