Gusev & Meridiani
Gusev Crater
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Gusev crater is a complex
impact feature in the general Elysium area, located at 14.64°S 175.36°E. Running into it is
Ma’adim Vallis, a channel running 900 km, part of a vast fluvial system
converging towards the Elysium Basin. Where Ma’adim Vallis enters Gusev there are structures generally consistent with fluvial deltas, such as is found at the mouths of major rivers on Earth. It is hard to escape the fact that water once flowed here. |
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But when? How? And for
how long? By using a combination of visible image mapping, thermal image mapping, altimeter data, thermal inertia data, and crater densities, it is possible to make some educated guesses as to the history of the region. Unfortunately, the picture isn’t exactly crystal-clear yet. |
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Cabrol & Grin suggested
the following scenario for a combination of fluvial (river) and lacrustine
(lake) deposition:
Ma’adim Vallis may have
originally carried water sporadically throughout a period of time stretching
billions of years.
At some point in the past,
Gusev was formed, and was filled by groundwater from the nearby Ma’adim region.
At this point it was a lake, but may have spent most of this time covered with
a sheet of ice.
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A pair of 30 km impact
craters were formed on the SE rim of Gusev. These filled with water from
Ma’adim Vallis and eventually opened up into Gusev, allowing Ma’adim Valis to
empty directly into Gusev. During this time, most of the delta formations and
related deposits were made. The increased ability of Ma’adim Vallis to
channel all of its flow into one region may have abetted the deeper
channeling seen in the middle of the valley. |
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At some point the NW wall
of Gusev gave way and water could drain from the crater.
This process began around
the Middle Noachian period, was most intensive throughout the Hesperian, and
continued through much of the Amazonian.
Milam and collaborators
were not convinced that this model explains everything, and suggest that
Aeolian (wind-produced) processes, and ash from Appolinaris Patera (a volcanic
feature less than 300 km away) may have also contributed to what we currently
see in and around Gusev. Only
after the arrival of the Mars Exploration Rovers in 2004 could these issues be
addressed with “ground truth” information.
Nevertheless, the evidence
suggests that if you want to look for a place where liquid water was present
for a long stretch of time (long enough for life to have gained a foothold),
Gusev would be a prime target.
For that reason, it was
chosen as one of the landing sites for the Mars Exploration Rovers (MERS):
Spirit.
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Recent thermal models of
the Gusev floor by Helbert & Benkhoff suggest that ice might be present
there a mere 1.5 m from the surface! Unfortunately, the MERs
have no way to reach this. But the recent MGS images of gully formation may support this idea. |
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Meridiani Planum
Hematite in northern Sinus Meridiani
One of the important finds
of the Mars Global Surveyor’s Thermal Emission Spectrometer (TES) was the
detection of hematite
in the Sinus Meridiani region of Mars. See http://www.psrd.hawaii.edu/Mar03/Meridiani.html
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The IR spectrum of Meridiani Planum. Compared to that of crystalline
hematite
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Hematite usually forms in
the presence of water. It can also form in volcanic regions, but no volcanoes
are found in this region. So this was a telltale signature of a region that
probably had liquid water for an extensive period of time. However, the
“watery” scenario of production was bee disputed by some planetary
scientists. The only way to settle the issue is to land at the site and take
a closer look. It is for this reason
that the Mars Exploration Rover Opportunity was sent to Meridiani Planum. |
To glimpse the lines of
thought investigator were using to justify sending one of the MERs to
Meridiani, take a look at this presentation by H.E. Newsom, et al. which I came
across in 2003, before the MERS arrived on Mars:
http://www.physics.uc.edu/~sitko/AdvancedAstro/22-Gusev-Meridiani/MER_4th_newsom.pdf