December 18, 2017 – A paper published today in Nature Geoscience by an international research team recommends an unconventional strategy to look for the possibility of life on Mars.
While sedimentary rocks are commonly targeted by those searching for evidence of life on Mars, the article suggests that examining Martian hydrothermal and subsurface environments may be a far more effective approach.
“Much of what we’ve learned about life over the course of our planet’s past has stemmed from studying the sedimentary rock record,” says Sarah Stewart Johnson, an assistant professor of planetary science in Georgetown's biology department and the Science, Technology, and International Affairs program, and co-author of the research. “That’s largely because Earth’s biomass has been dominated by photosynthetic life. Evidence suggests that Mars, however, may have had a very different history.”
The earliest forms of life on Earth survived on chemical energy.
Comparing Planetary Crusts
“It wasn’t until partway through Earth’s history that life figured out how to harness energy from the sun, and by the time photosynthesis evolved on Earth, Mars was a hyperarid, frozen desert, with a surface bombarded by high-energy solar and cosmic radiation,” she says.
In the paper, which Johnson co-authored with lead author Joseph Michalski at the University of Hong Kong, and colleagues from Princeton University, the University of Colorado, Brown University and NASA, the researchers compare aspects of the planetary crusts and key environmental events on the two planets.
They also identify sites where hydrothermal and exhumed subsurface deposits appear to be located on Mars, and pinpoint different lines of evidence that could indicate that life once inhabited these environments.
‘Groundswell of Attention’
“Looking for life in non-sedimentary rocks isn’t as easy,” says Johnson, whose work focuses on detecting biosignatures in planetary environments. “We have fewer analogies to draw from, but these ideas are beginning to attract a groundswell of attention in the Mars science community.”
NASA recently funded a high-profile workshop on rock-hosted biosignatures at Caltech, and research increasingly is being conducted on how life survives in the deep, dark biosphere on Earth.
The discovery of new hydrothermal deposits on Mars that may once have lined the floor of a large, long ago sea was announced earlier this year.
Understanding Life’s Origins
The paper underscores how targeting primitive, non-photosynthetic life dovetails with discovering evidence of prebiotic chemistry.
Earth’s early history has vanished, erased by erosion and plate tectonics, whereas Mars retains its ancient crust.
“A search strategy based on hydrothermal and subsurface environments thus not only maximizes the chance of finding primitive life but also finding clues to the origin of life itself,” Johnson says. “The best lens for understanding the beginning of life on our own planet, as it turns out, may be Mars.”