Modern
Mars has an arid climate combined with an atmosphere to thin to
support liquid water and a surface temperature seldom warm enough to
allow ice to melt. However much of the surface of Mars shows evidence
of a wetter climate in the past, with river systems, lakes and even
shallow seas being identified by planetary scientists. This has led
to the theory that during the Noachian Period, more than 3.7
billion years ago, Mars had a warmer, wetter climate with extensive
precipitation (rainfall) and water dominated environments covering
much of the surface of the planet. However some of the ancient
terrains predicted by this model as having been likely to have had
very wet climates during the Noachian are devoid of any signs of
water. This has led to the development of an alternative climate
model for ancient Mars, the Icy Highlands model, in which the climate
of ancient Mars was still predominantly cold and dry, but with
extensive glaciation forming in highland regions, particularly around
the equator, and the water-created landforms of Mars being created by
rare episodes of warming which melted these ice caps leading to
catastrophic but short-lived floods.
In a
paper published in the journal Geology on 23 August 2016, Joel Davies
of the Department of Earth Sciences at University College London, Matt Balme of the Department of Physical Sciences at the Open University,
Peter Grindrod of the Department of Earth and Planetary Sciences at
Birkbeck College, University of London, Rebecca Williams of the Planetary Science Institute and Sanjeev Gupta of the Department of Earth Sciences and Engineering at Imperial College London describe evidence for a
network of fluvial (river) channels covering much of the Arabia
Terra, the largest of the 'dry' Noachian Terrains of Mars, and
therefore one of the biggest obstacles to the Warm an Wet model of
the ancient Martian climate.
These
channels are detected in images produced by the Context Camera on the
Mars Reconnaissance Orbiter, and take the form of sinuous ridges and
branching systems rising up to 60 m above the surrounding terrain and
flowing with and around topological features in a way that would be
predicted for fluvial channels. Ridges are not an obvious sign of
fluvial activity, but can be formed if river channels develop a more
robust structure that the surrounding terrain, for example a stony
riverbed or even river-bottom carbonate deposit in an area otherwise
covered by loess (dusty, easily wind-blown soil), which can lead to
the riverbed remaining as a raised, or 'inverted', channel after the
surrounding terrain has been eroded away. Such inverted riverbeds are
not purely theoretical they are familiar from desert regions on
Earth, and are therefore to be expected on Mars if that planet once
had a wet climate but has subsequently aridified.
Map
showing distribution of inverted channels (black lines) and valley
networks (white lines) in Arabia Terra, Mars, study area. Many of the
inverted channels (e.g., southwest region of study area) are not
associated with valley networks. Background image is Mars OrbitalLaser Altimeter gridded topographic map. Davies et al.
(2016).
These
channels are not evenly distributed across the Arabia Terra, but
rather are concentrated in the southwest, with the northwest and east
of the terrain largely devoid of such structures. This is consistent
with large-scale water-flows from the north to the south, draining
into the Meridiani Planum to the south and southwest of the Arabia
Terra, though a number of streams appear to drain into other
features, particularly craters that may once have held ancient lakes.
Mars
Reconnaissance Orbiter Context Camera mosaics of (A) anabranching and
sinuous inverted channel system (Aram Dorsum; candidate ExoMars
landing site) in southwest Arabia
Terra; and (B) branching and sinuous inverted channel system in
southwest Arabia Terra, terminating in terraced, sub-circular feature
consistent with inverted paleolake deposit. Both inverted channels
are unconformably overlain by regional etched units. Davies et al.
(2016).
Based
upon the relationships of the channels to other geographic features,
Davies et al. conclude that
the channels were probably active in the Middle-to-Late Noarchian.
The presence of the channels on the Arabia Terra, one of the largest
areas on Mars on which fluvial structures had not previously been
identified, lends considerable support to the warm and wet hypothesis
of early Martian climate, a model which predicts a wet climate in the
Arabia Terra region, and which was stretched by the absence of such
evidence.
See also...
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Hydrated silicate minerals in the Mariner Valley, Mars.
Hydrated minerals (minerals
containing water) are considered to be evidence of the former presence
of
liquid water on Mars. They have been observed at a...
Interpreting landslide deposits in the Mariner Valley, Mars Landslides on Mars typically have much greater runout distances than
those on Earth, due to the planets lower gravity and thinner atmosphere. This
can...
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