Released April 1, 2012: This observation shows a portion of the
wall (light-toned material) and floor of a trough in the Acheron
Fossae region of Mars. Many dark and light-toned slope streaks
are visible on the wall of the trough surrounded by dunes.
Slope streak formation is among the few known processes
currently active on Mars. While the mechanism of formation
and triggering is debated, they are most commonly believed to
form by downslope movement of extremely dry sand or very
fine-grained dust in an almost fluidlike manner (analogous
to a terrestrial snow avalanche) exposing darker underlying
material. Some of the slope streaks show evidence that
downslope movement is being diverted around obstacles, such
as large boulders, and a few appear to originate at boulders or
clumps of rocky material. These slope streaks, as well as others
on the planet, do not have deposits of displaced material at
their downslope ends. The darkest slope streaks are youngest
and can be seen to cross cut and lie on top of the older and
lighter-toned streaks. The lighter-toned streaks are believed
to be dark streaks that are lightening with time as new dust is
deposited on their surface.
Released Sept. 7, 2011: Mars has extremely large temperature changes from winter to summer compared to the Earth. It gets cold enough to
freeze carbon dioxide out of the atmosphere during the winter, but this ice is unstable when the warmer summer arrives and forces it to sublimate
(transform directly back into a gas) away. Near the South pole though, it stays cold enough for some of this seasonal ice to stick around all year and
even accumulate from year to year. This slab of ice is a few meters (about 10 feet) thick and is penetrated by the flat-floored pits shown here. The
quasi-circular pits in the center of the scene are about 60 meters (200 feet) across. The distinct color of the pit walls may be due to dust mixed into the
ice. For most of the year these walls are covered with bright frost, but they defrost and show their true colors at the end of the summer.
Released Sept. 30, 2015:
This cliff is the site of
the most frequent frost
avalanches seen by HiRISE.
In this area in northern
spring, frost avalanches
are common and HiRISE
monitors this cliff, or
scarp, to learn more about
the timing and frequency
of the avalanches, and
their relationship to the
evolution of frost on the
flat ground above and
below the scarp. The small
white cloud in front of
the brick red cliff is likely
carbon dioxide frost dislodged from the layers above, caught in the act of cascading down the cliff. It is larger than it looks, more than 20 meters
across, and it probably kicked up clouds of dust when it hit the ground. The avalanches tend to take place when the North Polar region is warming,
suggesting that the avalanches may be triggered by thermal expansion.
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Released Feb. 22, 2012: These barchan (crescent-shaped) sand dunes are found within the North Polar erg of Mars. This type of dune provides a great
record of the wind environment when they formed and moved: barchan dunes’ horns point downwind. Although the question of present-day sand motion
is still open, it appears possible that these dunes are active (when not covered in frost) as their crestlines are very sharp and their slipfaces (the inner
curved region between the horns/downwind surface) appear very smooth and steep. In this image, taken during the northern spring season, the dunes
and ground are still covered in seasonal frost. The speckled appearance is due to the warming of the area -- as the carbon dioxide frost and ice on the
dunes warm, small areas warm and sublimate (turn from solid to gas) faster, creating small jets that expose/deposit dark sand and dust onto the surface.
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