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OCR for page 8
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he first high-resolution martian
images acquired by the Mariner 4
~ spacecraft in 1965 shattered pop-
ular notions of Mars. Far from being
an oasis, the surface of Mars appeared
instead to be as battered and barren as
the Moon. With its thin atmosphere
and bitterly cold temperatures, Mars
seemed more parched than the driest
places on Earth. The prospect that life
could have evolved there seemed dim.
Each subsequent mission to Mars
has changed that impression in sur-
prising ways. Mariner 9 revealed
~..........................................
UNIX ,,,,,,
.............. -.
Of,
~~-.-.-.-.-.-.-.-...........
an..
ancient towering volcanoes, extensive
polar caps, and immense canyons
apparently cut by water. Systematic
observations of the surface and atmos-
phere by Viking led to a dramatic
increase in our knowledge of the
breadth of martian geological history
and the dynamics of the current cli-
mate. The recent Mars Global
Surveyor (MGS) mission has again rev-
olutionized our understanding of mar-
tian evolution. Far from having no
magnetic field as previously believed,
MGS discovered that large portions of
the surface of Mars were strongly mag-
netized early in martian history.
Moreover, MGS has seen indications of
recent or ongoing climate change, and
has found small gullies with character-
istics suggesting that they were recent-
ly carved by fluid flow. Additionally,
fundamental information has been
derived from the study of martian
meteorites. Detailed analysis of these
samples has invigorated the debate
over whether life ever existed on Mars.
Despite studies to date, we still do
not know fully where water exists on
Mars today. There are direct observa-
tions of four exposed martian water
reservoirs, which include water vapor
in the atmosphere, water ice in the
atmosphere, seasonal water ice
deposits at the surface, and perma-
nent water ice deposits at the north
and south poles. Of the four reser-
voirs, the martian polar caps are by
far the most massive. Recent MGS
data sets indicate that the mass of
water ice contained within the mar-
tian north and south polar caps is
equivalent to a global ocean some 22
to 33 meters deep. Recent observa-
tions from the Mars Odyssey space-
craft also suggest a patchy reservoir of
water ice beneath the martian surface.
At increasing depth, where the rock is
warmer, liquid water may be present
in pore spaces.
5.....................................................
; ............................................
$~x-:-:-:::::::::::::
The Mars Pathfinder landing site (above), like much if not all of the martian surface, is drier than Earth's driest desert. But this
may not always have been the case. Gullies on canyon and crater walls, such as these (top) in Sirenum Terra seen by Mars
Global Surveyor, are possible evidence that water has flowed on or near to the martian surface in the geologically recent past.
OCR for page 9
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_
On Earth, life is found wherever
there is liquid water. On Mars,
although the peak daytime surface
temperature near the equator can rise
above the freezing point of water, the
average surface temperature is about
-55°C. The surface of Mars today is
cold, dry, oxidized, and exposed to an
intense amount of solar ultraviolet
radiation. These factors are likely to
limit or even to prohibit life at or near
the surface of the martian soil.
The surface environment of Mars,
however, may not always have been as
hostile to life as it is today. The geo-
logical evidence, especially in the val-
ley networks, indicates that the mar-
tian climate could have been apprecia-
bly more hospitable to life about 3 bil-
lion years ago the atmosphere
appears to have been warmer and
more dense, and liquid water existed
on the surface. In such a climate, life
could have developed, possibly leav-
ing behind fossil evidence in mineral
deposits created by surface water.
To date, a single set of robotic stud-
ies has searched directly for existing life
on Mars: the Viking life-detection
experiments, which were designed to
test for organisms using carbon dioxide
or organic molecules in a manner anal-
ogous with terrestrial organisms. The
results of the Viking experiments very
strongly suggest that the materials test-
ed were devoid of organic compounds
or other signs of life, but this conclu-
sion has been debated. The lack of
unanimity in the scientific community
highlights the difficulties inherent in
the detection of microorganisms by
robotic means. Indeed, even if it were
generally acknowledged that the Viking
experiments did not show the presence
of life, the experiments could still be
criticized as being overly geocentric in
showing only a lack of evidence for
lifeforms on or near the surface of Mars
that were similar to life on Earth.
The pace of Mars exploration is
currently breathtaking! NASA current-
ly has two spacecraft Mars Global
Surveyor and Mars Odyssey operat-
ing in orbit about the Red Planet, and
two more the twin Mars Exploration
Rovers, Spirit and Opportunity are
en route. The NASA missions are
At 3000 km long and up to 8 km deep, Mars's Valles Marineris canyon system
dwarfs any such feature on Earth. Mosaic constructed from Viking 1 orbiter images.
scheduled to be joined by a Japanese
orbiter, Nozomi, and Europe's Mars
Express orbiter and Beagle 2 lander.
Moreover, NASA has well-defined
plans to launch additional Mars mis-
sions at each launch opportunity for
the remainder of this decade (see the
diagram on page 10~.
Since Mars exploration activities
for the rest of this decade are well in
hand, the SSE Survey concentrated on
identifying gaps in the existing pro-
gram and laying the groundwork for
activities in the decade beyond 2013.
The SSE Survey concluded that, with
our present state of knowledge and
technological expertise, it is unlikely
that robotic techniques will be able to
conclusively prove whether there is or
has been life on Mars. Results
obtained from life-detection experi-
ments carried out by robotic means
on the martian surface can be
challenged as ambiguous for the
following reasons:
· Results that show an absence of life
may not be accepted because the
experiments yielding them were too
geocentric or otherwise limited;
· Results consistent with, but not
definitive of, the existence of life (e.g.,
the detection of organic compounds
of unknown, either biological or non-
biological, origin) may be regarded as
incapable of providing a clear-cut
answer; and
· Results interpreted as showing the
existence of life will be regarded as
necessarily suspect, since they might
reflect the presence of terrestrial con-
taminants instead of true martian life.
Definitive answers about the exis-
tence of martian life will require labo-
ratory analysis of Mars samples
returned to Earth. Samples provide
the ultimate ground truth for the
wealth of data returned from tele-
scopes, orbiting sensors, and in situ
missions thoughout the solar system.
g
OCR for page 10
NASA, together with other national
and international space agencies, has
detailed plans for missions (above) to
follow on from the current Mars Global
Surveyor and Mars Odyssey. These
missions include the Mars Exploration
Rovers (right), Mars Express, and
Nozomi currently en route to Mars.
Missions beyond 2009 are being
planned.
The SSE Survey recommends that
NASA should spend the next 10 years
preparing for a Mars Sample Return
campaign near 2015. Sample return
should be conducted to obtain rocks
from a variety of geological settings.
Moreover, to best assess Mars's
potential for life, robotic techniques
should be devised to collect samples
from beneath the martian surface
where conditions are more hos-
pitable for living organisms. (For
more discussion of sample-return
missions, see the Technology
Development section on page 25.)
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OCR for page 11
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Although technically challenging, collection and return of martian samples to Earth for intensive study in terrestrial laboratories |
is a key scientific priority for the decade beginning in 2013. Advanced rovers may be used to collect samples. I
Guiding Themes Addressed Important Planetary Science Questions Addressed |
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Representative terms from entire chapter:
martian surface
