Geoscience Reference
In-Depth Information
valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Large
areas in the southern hemisphere contained branched valley networks, suggesting
that rain once fell. Many craters look as if the impactor fell into mud. When they
were formed, ice in the soil may have melted, turned the ground into mud, and
then the mud flowed across the surface. Regions, called “chaotic terrain,” seemed
to have quickly lost great volumes of water which caused large channels to form
downstream. Estimates for some channel flows run to 10,000 times the flow of the
Mississippi River. Underground volcanism may have melted frozen ice; the water
then flowed away and the ground collapsed to leave chaotic terrain. Also, a general
chemical analysis by the two Viking landers suggested that the surface has been
either exposed to or submerged in water in the past.
8.3.9.5
Mars Global Surveyor
The Mars Global Surveyor's thermal emission spectrometer (TES) is an instrument
able to determine the mineral composition on the surface of Mars. Mineral
composition gives information on the presence or absence of water in ancient times.
TES identified a large (30,000 km
2
) area in the Nili Fossae formation that contains
the mineral olivine. It is thought that the ancient asteroid impact that created the
Isidis basin resulted in faults that exposed the olivine. The discovery of olivine is a
strong evidence that parts of Mars have been extremely dry for a long time. Olivine
was also discovered in many other small outcrops within 60
ı
north and south of the
equator. The probe has imaged several channels that suggest past sustained liquid
flows, two of them are found in Nanedi Vallis and in Nirgal Vallis.
8.3.9.6
Mars Pathfinder
The Pathfinder lander recorded the variation of diurnal temperature cycle. It was
coldest just before sunrise, about
78
ı
C, and warmest just after Mars noon, about
8
ı
C. These extremes occurred near the ground which both warmed up and cooled
down fast. At this location, the highest temperature never reached the freezing point
of water (0
ı
C), too cold for pure liquid water to exist on the surface.
Surface pressures varied diurnally over a 0.2 millibar range but showed two
daily minima and two daily maxima. The average daily pressure decreased from
about 6.75 millibars to a low of just under 6.7 millibars, corresponding to when
the maximum amount of carbon dioxide had condensed on the South Pole. The
atmospheric pressure measured by the Pathfinder on Mars is very low - about 0.6 %
of Earth's - and it would not permit liquid water to exist on the surface.
Other observations were consistent with water being present in the past. Some
of the rocks at the Mars Pathfinder site leaned against each other in a manner
geologists term imbricated. It is suspected that strong flood waters in the past pushed
the rocks around until they faced away from the flow. Some pebbles were rounded,
perhaps from being tumbled in a stream. Parts of the ground are crusty, maybe due to
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