SETI bioastro: Could Mars Have Ever Supported Life?

From: Larry Klaes (lklaes@bbn.com)
Date: Tue May 02 2000 - 14:18:02 PDT


S P A C E V I E W S

                             Issue 2000.18
                              2000 May 1
                  http://www.spaceviews.com/2000/0501/

 *** Articles ***

                  Could Mars Have Ever Supported Life?
                            by Bruce Moomaw

[Editor's Note: This is the latest in a series of articles by writer
Bruce Moomaw on last month's Astrobiology Science Conference. Two
previous articles, on the prospects of life on Europa and the future
of Mars exploration, were published in our April 17 issue. Additional
articles about Mars and extrasolar planets will be published in a
future issue.]

        One of the major topics of interest at the first Astrobiology
Science Conference, held last month at NASA's Ames Research Center in
California, was the prospects for life -- past or present -- on Mars.
The conference presentations made it clear that scientists are still
far from knowing whether or not life once existed or exists today on
Mars, and have not yet reached consensus on controversial topics such
as possible evidence of life in Martian meteorite ALH84001. One of
the key topics of discussion at the conference was whether Mars was
even hospitable enough for the formation of life at any time in its
history.

Keeping Mars Warm

        There is general agreement that Mars, during its early days,
was far more hospitable for the appearance of microbial life than it
is now -- but was it hospitable enough? We know with near-certainty
(from the erosion rates of ancient Martian terrain) that, until about
3.5 billion years ago, Mars had a dense carbon dioxide atmosphere,
maybe several times denser than Earth's atmosphere is today, and that
it then lost all of it over a period of less than a billion years.
But there is still a furious debate over whether Mars, during that
period, ever had a surface temperature above freezing, so that large
amounts of liquid water could exist on its surface.

        For example, Mars' branching "valley networks" that look like
dried-up riverbeds have some structural peculiarities which suggest
that they were carved instead by much slower trickles of ground water
a short distance below the surface, which carved out tunnels whose
roofs finally caved in, a process called "groundwater sapping". In
addition, the much bigger "catastrophic outflow" floods that carved
vast channels in Mars' surface were caused by eruptions of underground
water from underneath Mars' thick permafrost layer (up to 2 billion
years ago) that were huge but very brief, lasting only a few days
before freezing and later sublimating into vapor in Mars' new near-
vacuum atmosphere. In both cases, the underground water could have
been geothermally heated.

        Dense carbon dioxide (CO2) produces a strong greenhouse
effect, but during those early days of the Solar System, the Sun was
about one-quarter less bright than it is now. Calculations indicated
that Mars' early air -- no matter how thick it was -- could not
possibly have warmed its surface above about -43 degrees Celsius (-45
degrees Fahrenheit), because beyond a certain point a thicker
atmosphere would just have scattered and reflected more sunlight back
into space, both from the gaseous CO2 itself and from the thicker
layer of frozen "dry ice" clouds that would form in it.

        Calculations by atmospheric scientist James Kasting had
indicated that a layer of frozen CO2 clouds in Mars' upper atmosphere
would significantly cool the planet. The clouds would reflect most of
the sunlight that hit them back into space, but are relatively
transparent to infrared radiation and so would not trap the heat
radiation that Mars' surface gave off when it was warmed by the
remaining sunlight. But in 1997, Francois Forget and Raymond
Pierrehumbert carried out new studies indicating that dry ice clouds
are far better at reflecting infrared radiation than had been thought.
They concluded that the thick dry-ice cloud layer that would form in a
Martian atmosphere twice as thick as Earth's might actually warm the
planet's surface to a toasty 27 deg C (80 deg F)!
  
        Kasting took them seriously, but decided to do a still more
thorough study of CO2 clouds, and reported it at the Conference. His
new conclusions fall in the middle: whether CO2 clouds have a net
warming or cooling effect seems to depend both on how thick they are
and what altitude they form at. If they have a high "optical depth"
(visible-light opacity) or are at a low altitude, they can still cool
a planet. His overall conclusion is that we simply do not know nearly
enough yet about the formation processes of clouds -- or even have
accurate enough 3-D computerized climate models -- to know whether
early Mars could have had a surface temperature above freezing.
Kasting did add that even a small trace of methane could have warmed
Mars above freezing -- although such methane would itself have to be
produced by Martian bacteria. Monika Kress added in a poster that
methane might instead have been supplied by comet impacts on early
Mars.

Other Ways to Support Life

        Even if the surface of early Mars was mostly frozen, though,
there is no question that it had far more volcanic activity than it
does now, simply because the planet was still cooling down from its
creation -- and the resultant hot springs could have provided a fairly
large supply of near-surface liquid water that might still have
allowed the evolution of life. There is also strong evidence that
some of Mars' giant craters -- and the canyons in its huge Valles
Marineris -- were filled early in its history by lakes that could have
been warmed by subsurface heat and preserved under a surface layer of
ice for hundreds of millions of years after the rest of the planet's
surface had turned hostile.

        A new idea that turned up in a whole series of conference
posters is that volcanoes might not be the only Martian areas
geothermally warmed enough for liquid water to exist: big meteor
craters might have been warm enough after their formation to maintain
near-surface springs, or even lakes, for tens of thousands of years.
Jeff Plescia suggested the Hellas Basin -- Mars' most gigantic impact
crater, in the southern hemisphere -- as the biggest such region; it
is flanked by ancient volcanoes and valley networks suggesting just
how warm it made its surroundings. C.S. Cockell pointed out that
ancient impact craters on Earth have made the local igneous rocks
porous enough to serve as a habitat for bacteria. Teresa Segura
suggested that a single Mars impact that gouged a 50-km crater could
raise the entire planet above freezing for decades.

        Hellas is interesting for another reason as well. Robert
Haberle pointed out in his talk that the common belief that Mars' air
pressure is too low for liquid water to exist anywhere on its surface
today is not quite true -- there are still a few lowlands where the
air pressure is just high enough for it to exist briefly. He
identified seven such regions where the temperature is high enough,
for at least a few weeks each year, for liquid water to exist, but the
air pressure is still so low that it evaporates after only a few
minutes, which means that it would have to be replenished from some
groundwater source. Five of these lowlands are in the equatorial
regions and have long since dried out, but two others -- the huge
Hellas and Elysium craters -- are in the far south and may still have
a substantial amount of ground ice mixed into their soil (originally
poured into them by runoff from those valley networks created by the
heat of the giant impacts that formed the basins). If their soil is
actually moist, even for a few weeks a year, it might allow live
microbes to thaw out and carry out their reproductive cycle before
refreezing. Addition evidence suggests that at least a few big
Martian craters, including Hellas, were still filled by lakes of
liquid water (probably covered by a layer of ice) as recently as a few
hundred million years ago.

The Mystery of the Carbonates

        There is another problem, though: one of the biggest
geological puzzles on Mars today. Although some of ancient Mars'
original dense atmosphere -- given the planet's weak gravity -- may
have been blasted into space by the huge asteroidal impacts of the
"heavy bombardment" period, the feeling is that most of it disappeared
because it reacted with early Mars' surface or near-surface liquid
water and the surface rocks to form carbonate minerals. On Earth,
crustal tectonics (or "continental drift") recycles these minerals by
dragging them back down into Earth's hot interior every few hundred
million years, so that they are broken down and the CO2 released again
through our volcanoes -- but Mars is too small to have crustal
tectonics, and so most of its early CO2 would have vanished forever,
reducing the air pressure and lowering greenhouse warming until water
could virtually always exist on Mars' surface only as ice or vapor.
And indeed most of the 15 Mars meteorites identified on Earth contain
significant amounts of carbonates (in which the possible microfossil
evidence in the famous ALH84001 rock is found).

        But planetologists have been going nuts trying to confirm any
carbonate deposits on Mars' surface today. Earth-based telescopic
searches have been ambiguous -- and the thermal IR spectrometer
("TES") on Mars Global Surveyor, which was expected to find such
deposits, has so far not detected a confirmable trace. Where are the
carbonates? If they don't exist in large amounts, does this mean that
Mars never had much liquid water anywhere near its surface after all
-- or could there be some other explanation? Was there some surface
process that breaks surface carbonates back down -- but allows them to
build up in near-surface deposits formed by subsurface liquid water?

        The question led to one of the more interesting debates from
the conference. Allen Treiman thinks the explanation is that the
surface carbonate minerals are indeed there, but that the TES is
simply much less sensitive to them than had been thought because their
surfaces are rough. Disagreeing with Treiman was Steven Ruff, who
said in his own talk that the TES has proven that the famous "White
Rock" -- a 12 by 15-km (7.4 by 9.3 mi.) patch of light-colored
material on the floor of a Martian crater, which has been thought by
many to be a deposit of carbonates or other water-deposited salts --
is nothing of the kind. Ruff claimed that the TES' infrared spectra
of the Rock show conclusively that it is basically the same regular
Martian soil as the dark-colored dunes that surround it (although it
has caked into a lighter-colored and firmer form). He added that it's
actually no lighter in color than many other regular areas of Martian
soil and looks light only by comparison with those surrounding dunes;
and that the spectrometer's temperature measurements show that it is
regular soil rather than harder rocky material.

        Treiman responded that any individual spectrum by TES has been
shown by his tests to be hopelessly insensitive to carbonates. Ruff
replied that his conclusions are based on combining a whole series of
TES spectra, thus increasing their sensitivity. Treiman responded
that since the spacecraft is sailing over the surface of Mars, such a
series of spectra never really cover the same spot -- but Ruff replied
to that by saying that the TES took enough spectra of the White Rock
that one can accurately say by combining them that the Rock contains
no carbonates. This intellectual ping-pong match ended
inconclusively. However, astrogeologist Jack Farmer later said that
his own tests lead him to a centrist position: he agrees with Treiman
that the TES is disastrously insensitive to carbonate deposits because
they're usually small and are thus washed out of its blurry wide-angle
vision, but he thinks (unlike Treiman) that the sharper-resolution
"THEMIS" multispectral IR camera on the upcoming 2001 Mars Surveyor
Orbiter will definitely be able to detect them. Apparently we'll have
to wait and see.

========
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