Sky and Telescope - July 2018 - 15
NASA's MAVEN mission has conﬁrmed how our planetary
neighbor lost its protective gas envelope.
M A R S TE R R A IN : ESA / D LR / FU B E R LIN , CC BY- SA 3 .0 IGO; ILLUSTR ATIO N S: N ASA GS FC
ention anything about the potential for life on Mars,
and you'll get people's attention. The conversation
often centers on water. Life as we know it requires
liquid water, and the evidence that Mars has had liquid water
in various forms throughout its history means that it could
have supported life.
But Mars is a frozen, desert planet today, with a thin
atmosphere and temperatures typically well below the melting point of water ice. So why do we think Mars had abundant liquid water in the past? And what caused the climate to
change to one unfriendly to life?
The evidence that Mars once had liquid water came initially from orbiter images. Thanks to them, we've known for
decades that Mars has systems of valleys that look like they
formed via water runoff. We've seen what appear to be deposits of sediments carried by water as it ﬂowed into the enclosed
basins of impact craters. We've also observed that the oldest
surfaces look worn down, and that few impact craters smaller
than about 10-15 km wide mar the landscape, which combined tell us that signiﬁcant erosion has occurred - with
erosion by water runoff being the favored explanation.
Starting with the Mars Pathﬁnder landing in 1997, we also
began observing small-scale surface features that suggested
water once ﬂowed. We've identiﬁed sediment deposits that
bear all the hallmarks of having been laid down in liquid
water. And we've seen small, round deposits formed by the
buildup of waterborne material, called concretions, and even
speciﬁc minerals that require liquid water in order to form.
By estimating dates for these various features, planetary
scientists have put together a rough timeline for water on
Mars. It reveals that the strongest signs of liquid water tend
to be very old. In fact, all of the geological and geochemical evidence points toward Mars having had a climate that
allowed liquid water to be widespread on the surface up until
about 3.5 to 3.7 billion years ago. Conditions then changed
rapidly (geologically speaking), and whatever allowed water
to be present disappeared over a period of only a few hundred
million years, leaving behind a colder and drier planet.
There is some evidence for water in later epochs - large
ﬂood channels that appear to have been formed by the
catastrophic release of water from the subsurface, very recent
(within the last couple million years) gullies that may involve
water - but not as an abundant, stable liquid. These later features would all involve subsurface sources and do not require
a different climate than what we see today.
What would the climate on early Mars have looked like?
We expect that the average temperature likely was near 0°C
or higher in order to support extensive liquid water. This
compares to today's average temperature of around −60°C.
The simplest explanation for the higher earlier temperatures
is the presence of an abundant greenhouse gas that would
trap the heat from the Sun and raise the global temperature.
Carbon dioxide is the best greenhouse gas candidate, and it's
the primary component of Mars's current, thin atmosphere.
Scientists have not worked out the details of this ancient
warming - whether CO2 alone could raise the temperatures
or whether another greenhouse gas such as hydrogen or
methane would have been required - but most expect that a
thicker atmosphere early in Mars's history will turn out to be
the right explanation.
This is the picture we had prior to the Mars Atmosphere
and Volatile Evolution (MAVEN) mission, which launched in
2013: that there must have once been a thicker atmosphere
and that CO2 played a role (S&T: Sept. 2014, p. 20). How
did that atmosphere disappear? That is the mystery we sent
MAVEN to solve.
My fellow planetary scientists and I had already spent
many years thinking about where the CO2 could have gone.
Observations had revealed CO2-derived minerals (such as
p POOR MARS Top: The Sun strips Mars of its atmosphere, both via its
ever-present wind of charged particles (streaks) and occasional violent
eruptions (brownish band). Above: Close-up, with atmospheric escape.
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