Sky & Telescope - January 2020 - 39

q UNDERGROUND WATER This map from the ExoMars Trace Gas Orbiter shows hydrogen's distribution (bluer colors mean more hydrogen) in the
uppermost meter of Mars's surface. Hydrogen might indicate water, water absorbed into the surface, or minerals formed in water.

Water signature in equatorial regions may signify shallow permafrost, hydrated
minerals, or the former locations of the planet's poles in ancient times.
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Earth soon after, and by the time it became widespread enough
to dramatically boost our atmosphere's oxygen content, Mars's
surface had been a frozen and hyperarid desert bombarded by
high-energy radiation for more than a billion years.
For this reason, some researchers think that life on Mars
might have arisen underground instead of migrating there,
avoiding the surface altogether. In their view, searching for
evidence of life on the Martian surface is a biased approach
fueled by what we see on Earth. Here, photosynthesizing surface life is extremely abundant, favored as it is by a protective
atmosphere and magnetic ﬁeld, a moderate climate, and easy
access to water. One way or the other, scientists increasingly
think that Mars research needs to shift its focus from the
surface to the underground. Even if life got a foothold above
ground, the harsh surface conditions might have wiped out
any organic remains or other revealing signs, thwarting any
life-searching missions based on surface features.
"Anything related to life, extinct or extant, leads us to
the subsurface," says Vlada Stamenkovic´ (NASA Jet Propulsion Laboratory), an ardent advocate for subsurface Martian
exploration. "It's clear that if we really want to understand
if there ever was or is life on Mars, then we really have to dig
into the subsurface and explore what's beneath."

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Looking for Subsurface Life
Finding out if there is or has ever been underground life on
Mars is not a straightforward proposition. First of all, we don't
really know the physical properties of what lies below the surface. Things like temperature and water availability remain big
unknowns. We only know there is a frozen layer close to the
surface and a core that is probably still warm, making it likely
that a temperate zone exists somewhere in between.
"These are reasonable conjectures, but we won't really
know until we study Martian geophysics," says Ricardo Amils
(Center for Astrobiology, Spain), who has worked extensively
in characterizing southern Spain's Rio Tinto region, considered
one of the best Martian analogs we have on Earth. Still, he's
conﬁdent scientists need to look beneath the surface to fully
assess the Red Planet's habitability. "If there is life on Mars it
has to be in the subsurface, there is no doubt about that."
That's why NASA's Insight lander is an important ﬁrst
step towards the exploration of the Martian underground.
The probe landed on Mars on November 26, 2018, carrying a seismometer, a thermal probe, and radio antennas,
which together will reveal key aspects of Martian geophysics,
such as the size and physical properties of the planet's core,
mantle, and crust, as well as details about its inner heat ﬂow.
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Sky & Telescope - January 2020

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