that will gain us access to biosignatures in a better
state of chemical preservation,” protected from the
cosmic radiation that, due to Mars’ weak magnetic
field and thin atmosphere, bathes the planet’s surface,
Vago says. Over long periods of time, this radia-
tion degrades organic matter, further complicating
attempts to discern whether material might have
had a biological origin.
Another major factor in the search for biosigna-
tures is where the rovers end up landing. The NASA
2020 rover team is evaluating three remaining candi-
date locations — two in the northern hemisphere, as
well as one in the southern hemisphere, the Colum-
bia Hills, where the Spirit rover previously explored
— with a final decision expected in summer or fall
2018, Farley says. The ExoMars team, meanwhile,is still considering two northern hemisphere sites
apart from NASA’s options.
Finding a location where the topography,
altitude and climate are amenable to successful
spacecraft landings is vital. But to increase the odds
of finding potential life, the goal is to select an
area where sediments were deposited in a watery
environment roughly 3.5 billion to 4 billion years
ago, when Mars is thought to have had a thicker
atmosphere and a more temperate, Earth-like cli-
mate. Ideally, after deposition, these sediments
would have been covered under layers — hundreds
to thousands of meters thick — of cap rock or soil,
called overburden, before then being mostly or
completely unburied by impact- or wind-driven
erosion relatively recently in Mars’ history. Each
of the NASA and ExoMars candidate sites feature
unique characteristics and challenges, although all
meet these fundamental criteria.
The idea is that the overburden “would have
afforded an added level of protection,” Vago says,
so that any long-buried organic material would
only have been exposed to cosmic radiation for a
few hundred million years, say, instead of billions
of years. “That’s the strategy to try to get the best
possible molecular biosignatures,” he says, perhaps
including relatively hardy molecules like lipids, the
main component in cell membranes.
Whichever sites are chosen for the rovers, sci-
entists will “wind up looking at a rock record that
does not exist on Earth,” Farley says, which is one
of the most appealing aspects of the missions. On
Earth, very little rock older than 3.5 billion years
is preserved because most of it has been recycled
through erosion and plate tectonics. But “a large
fraction of those rocks are preserved on the surface
of Mars,” he says. “And so by going to Mars, we can
actually learn about what a terrestrial planet looked
like in its earliest history,” and also perhaps learn
about the sorts of environments where life might
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