As NASA's Perseverance rover begins its search for ancient life on the surface
of Mars, a new study suggests that the Martian subsurface might be a good
place to look for possible present-day life on the Red Planet.
The study, published in the journal Astrobiology, looked at the chemical
composition of Martian meteorites -- rocks blasted off of the surface of
Mars that eventually landed on Earth. The analysis determined that those
rocks, if in consistent contact with water, would produce the chemical
energy needed to support microbial communities similar to those that survive
in the unlit depths of the Earth. Because these meteorites may be
representative of vast swaths of the Martian crust, the findings suggest
that much of the Mars subsurface could be habitable.
"The big implication here for subsurface exploration science is that
wherever you have groundwater on Mars, there's a good chance that you have
enough chemical energy to support subsurface microbial life," said Jesse
Tarnas, a postdoctoral researcher at NASA's Jet Propulsion Laboratory who
led the study while completing his Ph.D. at Brown University. "We don't know
whether life ever got started beneath the surface of Mars, but if it did, we
think there would be ample energy there to sustain it right up to today."
In recent decades, scientists have discovered that Earth's depths are home
to a vast biome that exists largely separated from the world above. Lacking
sunlight, these creatures survive using the byproducts of chemical reactions
produced when rocks come into contact with water.
One of those reactions is radiolysis, which occurs when radioactive elements
within rocks react with water trapped in pore and fracture space. The
reaction breaks water molecules into their constituent elements, hydrogen
and oxygen. The liberated hydrogen is dissolved in the remaining
groundwater, while minerals like pyrite (fool's gold) soak up free oxygen to
form sulfate minerals. Microbes can ingest the dissolved hydrogen as fuel
and use the oxygen preserved in the sulfates to "burn" that fuel.
In places like Canada's Kidd Creek Mine, these "sulfate-reducing" microbes
have been found living more than a mile underground, in water that hasn't
seen the light of day in more than a billion years. Tarnas has been working
with a team co-led by Brown University professor Jack Mustard and Professor
Barbara Sherwood Lollar of the University of Toronto to better understand
these underground systems, with an eye toward looking for similar habitats
on Mars and elsewhere in the solar system. The project, called Earth 4-D:
Subsurface Science and Exploration, is supported by the Canadian Institute
for Advances Research.
For this new study, the researchers wanted to see if the ingredients for
radiolysis-driven habitats could exist on Mars. They drew on data from
NASA's Curiosity rover and other orbiting spacecraft, as well as
compositional data from a suite of Martian meteorites, which are
representative of different parts of the planet's crust.
The researchers were looking for the ingredients for radiolysis: radioactive
elements like thorium, uranium and potassium; sulfide minerals that could be
converted to sulfate; and rock units with adequate pore space to trap water.
The study found that in several different types of Martian meteorites, all
the ingredients are present in adequate abundances to support Earth-like
habitats. This was particularly true for regolith breccias -- meteorites
sourced from crustal rocks more than 3.6 billion years old -- which were
found to have the highest potential for life support. Unlike Earth, Mars
lacks a plate tectonics system that constantly recycle crustal rocks. So
these ancient terrains remain largely undisturbed.
The researchers say the findings help make the case for an exploration
program that looks for signs of present-day life in the Martian subsurface.
Prior research has found evidence of an active groundwater system on Mars in
the past, the researchers say, and there's reason to believe that
groundwater exists today. One recent study, for example, raised the
possibility of an underground lake lurking under the planet's southern ice
cap. This new research suggests that wherever there's groundwater, there's
energy for life.
Tarnas and Mustard say that while there are certainly technical challenges
involved in subsurface exploration, they aren't as insurmountable as people
may think. A drilling operation wouldn't require "a Texas-sized oil rig,"
Mustard said, and recent advances in small drill probes could soon put the
Martian depths within reach.
"The subsurface is one of the frontiers in Mars exploration," Mustard said.
"We've investigated the atmosphere, mapped the surface with different
wavelengths of light and landed on the surface in half-a-dozen places, and
that work continues to tell us so much about the planet's past. But if we
want to think about the possibility of present-day life, the subsurface is
absolutely going to be where the action is."
Reference:
J.D. Tarnas, J.F. Mustard, B. Sherwood Lollar, V. Stamenković, K.M. Cannon,
J.-P. Lorand, T.C. Onstott, J.R. Michalski, O. Warr, A.M. Palumbo, A.-C.
Plesa. Earth-like Habitable Environments in the Subsurface of Mars.
Astrobiology, 2021; DOI:
10.1089/ast.2020.2386
Tags:
Space & Astrophysics