Thanks
to a flotilla of Mars orbiters,
there's been a steady flow of information streaming in from that puzzling
world. Scientists are piecing together a far more coherent view of "real time"
versus "geological time" in dealing with the whole of Mars today.
One
of the more perplexing finds on Mars are features that look like the product of
groundwater seeping to the surface. These gullies could be formed by flowing
water--perhaps fed by a groundwater supply. Deposits of soil and rocks
transported by these flows have been found too.
More
importantly, gully features appear to be young. So young, in fact, they might
be forming today. Clearly, gullies may well be areas of astrobiologial
interest--a
niche for life that could be present and accounted for on Mars.
It
was back in June 2000 when Mars gullies became big
news--new landforms that had never been seen before as revealed in images
taken from the Mars Global Surveyor. Spacecraft have now been sending back
views of the planet and its ever-changing face over the course of the past five
Martian years.
Mars
gullies remain controversial as to how they are created. But now gully watching
scientists are nearing a watershed moment in unraveling the story behind the
formations.
Mars, by gully
Gullies
are high on a "change detection" target list for NASA's newly positioned Mars
Reconnaissance Orbiter (MRO).
Onboard that Mars-circling craft is the ultra-powerful High Resolution Imaging
Science Experiment (HiRISE)
camera.
"HiRISE
can do an excellent job of change detection due to the high resolution" and
other attributes of the imaging system, said Alfred McEwen, Director of the Planetary
Image Research Lab at the University of Arizona in Tucson, Arizona. He is MRO's
HiRISE principal investigator.
"We
are planning a major effort for change detection over many terrains on Mars,"
McEwen told SPACE.com, such as polar layered
deposits, dunes
and gullies.
MRO's
point-and-shoot skills also enable re-imaging of terrains at the exact same
season of different Mars years to match illumination angles.
"Viewing
angles will differ, but we plan to acquire stereo images and produce digital
elevation models of any site that shows evidence for change ... so we can
correct for any viewing effects and make precise measurements of changes,"
McEwen noted.
"We
will of course see changes in color and albedo [variations in the amount of
sunlight reflected by the martian surface] but interpretation of such surficial
changes can be controversial," McEwen said. "So our hope is to detect and
measure actual changes in the topography."
Therefore,
MRO can constrain the current rates of change of features, McEwen added, like
the age of younger features such as gullies--even if the spacecraft and Mars
investigators doesn't actually spot new gully erosion.
Research focus
The
true nature of the Mars gullies remains a work in progress, said Linda Martel,
a geologist and an educational outreach staffer at the Hawaii Institute of
Geophysics and Planetology at the University of Hawaii.
"Most
researchers are still favoring two models of formation for gullies on Martian
slopes--erosion caused by groundwater discharge or by the melting of
near-surface ice or snow," Martel advised.
Mapping
the locations and orientations of gullies continues, with Mars scientists
looking at temperature and pressure conditions in the subsurface that allow
liquid water to exist where gully recesses are found. Also, they are evaluating
atmospheric conditions and exposure of the surface to martian sunshine.
"They
are coming up with very convincing reasons why either model makes sense,"
Martel said. "But when researchers say that gullies are recent they aren't
talking about today, right now."
For
one, Martel said, it's impossible to carve the larger--about 4 miles (7
kilometers) long--gullies by flowing pure water across the surface under today's
temperature and pressure conditions on Mars. An on-going research focus, she
said, is appreciating what the gullies say about changes in the stability of
water or changes in atmospheric pressure, temperature, humidity or exposure to
sunshine on Mars.
Dry landslides
Evidence
that water is carving out martian gullies doesn't, well, hold water in some
circles. The Moon has gullies that look strikingly familiar to the ravines on
Mars--and water certainly didn't form gullies on that bone-dry world.
Mars
gullies are made up of a deep channel with a collapsed region at its upper
end--an "alcove"--and at the other end an area of accumulated debris--an "apron"
that appears to have been transported down the slope.
Gwendolyn
Bart, a University of Arizona lunar and
planetary scientist, has made a comparison of small lunar landslides and martian
gullies. Some lunar landslides have an alcove-channel-apron
morphology--structure and form of a feature--similar to that of the martian
gullies
"Because
the Moon is devoid of geologically active water, we know the lunar features
formed by dry landslides," Bart observed. "As a result, it is impossible to
rule out dry landslides as the formation mechanism for martian gullies, based
solely on the alcove-channel-apron morphology."
Bart
said that more high resolution lunar data--like that expected from NASA's super-snooping
Lunar Reconnaissance Orbiter to be launched in 2008--will give researchers the
ability to identify more lunar features, and to see whether the Moon reproduces
the great variety of gullies spotted on Mars.
Protected snowpacks
"It
is my opinion that there easily could be active melting or release of water
from sub-surface
aquifers," said Philip Christensen at Arizona State University (ASU) in Tempe, a leading Mars
researcher. He is Principal Investigator for the 2001 Mars
Odyssey's Thermal Emission Imaging System (THEMIS) instrument.
Christensen
said that he believes some of the Mars gullies emanate from dust-covered
surface snowpacks. Small, local changes in the conditions of these snowpacks--such
as wind or landslides removing the dust cover--could allow these snows to begin
to melt and release water today, he advised.
"The
main argument against a lot of current activity is that these snowpacks likely
formed during the previous climate cycles and are 50,000 to 300,000 years old,"
Christensen told SPACE.com. "If they had remained active all that time
they would be completely gone by now."
However,
Christensen added, if the snowpacks get covered by a protective layer of dust,
then the snow can be stable for a long time. Remove the dust and the melting is
reactivated, then gullies could be eroding in some places on Mars today, he
said.
"Alternatively,
if the water is coming from sub-surface aquifers or ground ice ... local
heating or other local changes could also release water and these gullies could
also be active," Christensen suggested.
Key questions
In
his view, Christensen said that all this gully-speak leads to some key
questions regarding the red planet.
For
one, what is the nature of near-surface snow and water in the mid-latitude
regions?
"These
regions are extremely interesting," Christensen remarked, "because they're cold
enough for snow and ice to accumulate during climate cycle--but warm enough for
melting and release of liquid water to occur at other times ... perhaps current
times."
The
equatorial regions appear very dry today, Christensen noted, and the poles are
very cold throughout the year.
"The
gullies indicate that there is snow or water near the surface in the
mid-latitudes," Christensen pointed out. "This is where I think a lot of the
action and excitement on Mars is occurring today."
Off limits, no wheel zones
So
why not send a robot to get up-close and personal with a Mars gully?
NASA's
Mars Science Laboratory (MSL),
now being built for a 2009 sendoff to the red planet, won't be up to sterility
snuff to explore a gully site due to planetary protection rules.
Gullies
for MSL are off limits, no wheel zones. You don't want to infect possible martian
microbes or water sources with hitchhiking bacteria brought from Earth.
"Gullies
appear to be the best
chance we will have--or perhaps could have--for seeking present-day life on
the martian surface," suggested John Rummel, Senior Scientist for Astrobiology
in the Science Mission Directorate at NASA Headquarters in Washington, D.C.
For
astrobiologists, Rummel continued, there is no more compelling target than
gullies for exploration on Mars. But there's need for the right rover that's
outfitted with the right instruments before we go, he said.
Delving
into Mars gullies, Rummel said, can be extended from orbit around the planet--or
even from platforms flying through the martian atmosphere, like robotic
airplanes or balloons.
"But
the most important phase of the exploration of the gullies can only be done by
visiting them and making measurements on site," Rummel said. "To do this
requires an agile, sterile rover and finely tuned analytical instruments ...
but with those tools, we have perhaps the best chance this century to discover
whether or not Mars is alive."
This article is part of SPACE.com's weekly Mystery Monday
series.
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