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Martian spiders
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Small Troughs Growing on Mars May Become 'Spiders'

Erosion-carved troughs that grow and branch during multiple Martian years may be infant versions of larger features known as Martian "spiders," which are radially patterned channels found only in the south polar region of Mars.
Researchers using NASA's Mars Reconnaissance Orbiter (MRO) report the first detection of cumulative growth, from one Martian spring to another, of channels resulting from the same thawing-carbon-dioxide process believed to form the spider-like features.

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RE: Mars' South Pole
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Citizen Scientists Seek South Pole 'Spiders' on Mars

Ten thousand volunteers viewing images of Martian south polar regions have helped identify targets for closer inspection, yielding new insights about seasonal slabs of frozen carbon dioxide and erosional features known as "spiders."
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Christiaan Huygens discovered the Martian south polar cap on the 13th August, 1642.



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Springtime at Mars' south pole

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Springtime at Mars' south pole

ESA's Mars Express celebrates eight years in space with a new view of ice in the southern polar region of Mars. The poles are closely linked to the planet's climate and constantly change with the seasons. Their study is an important scientific objective of the mission.

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Martian South Pole
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Title: Sublimation of the Martian CO2 Seasonal South Polar Cap
Authors: Frederic Schmidt, Bernard Schmitt, Sylvain Doute, Francois Forget, Jeng-Jong Jian, Patrick Martin, Yves Langevin, Jean-Pierre Bibring, the OMEGA Team
(Version v2)

The polar condensation/sublimation of CO2, that involve about one fourth of the atmosphere mass, is the major Martian climatic cycle. Early observations in visible and thermal infrared have shown that the sublimation of the Seasonal South Polar Cap (SSPC) is not symmetric around the geographic South Pole. Here we use observations by OMEGA/Mars Express in the near-infrared to detect unambiguously the presence of CO2 at the surface, and to estimate albedo. Second, we estimate the sublimation of CO2 released in the atmosphere and show that there is a two-step process. From Ls=180° to 220°, the sublimation is nearly symmetric with a slight advantage for the cryptic region. After Ls=220° the anti-cryptic region sublimation is stronger. Those two phases are not balanced such that there is 22% ±9 more mass the anti-cryptic region, arguing for more snow precipitation. We compare those results with the MOLA height measurements. Finally we discuss implications for the Martian atmosphere about general circulation and gas tracers, e.g. Ar.

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Circular Feature at South Pole
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This 4 kilometre diameter feature near the edge of the south polar residual cap was recognised in Mariner 9 and Viking Orbiter images taken in the 1970s, but its origin could not be inferred. It was therefore targeted for HiRISE stereo imaging.

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Credit: NASA/JPL/University of Arizona

The bright areas in this image are covered by carbon dioxide frost, and the "swiss cheese" terrain typical of the south polar residual cap covers much of the imaged area. The dark walls of the circular depression do not have as much frost on them, and are fractured in a polygonal pattern. Apparently the surface of the walls has been extensively modified by thermal expansion and contraction of water ice.
It also appears that the "swiss cheese" terrain of the residual cap has buried the floor of the circular depression, as well as the terrain surrounding the feature, making it difficult to infer the origin of this depression. Its circular symmetry is consistent with an impact origin, but there is no evidence of a crater rim or ejecta (perhaps because they have been buried). The depression may have formed by collapse, but there is little evidence of extensional fractures that would be expected around a collapse pit. Analysis of HiRISE stereo data may help the interpretation of this feature.


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RE: Mars' South Pole
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An MIT-led team of planetary scientists has found that the southern pole of Mars contains the largest deposit of frozen water in the inner solar system, outside of Earth.
The new results show that water, not carbon dioxide, is the predominant frozen liquid found in the southern polar region of Mars, said Maria Zuber, MIT professor of geophysics.

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Ice on Mars
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Wobbles on Mars cause ice ages that are much more dramatic than those on Earth, says astronomer Norbert Schörghofer of the University of Hawaii.
Thanks to our large, stabilizing Moon, Earth's rotation axis is always tilted by about 23 degrees. The tilt of Mars, however, can wobble by as much as10 degrees from its current 25 degrees. Wobbles cause big changes in the amount of sunlight reaching different parts of Mars, so vast amounts of ice shift between the poles and the rest of the planet every 120,000 years.
Surface temperature and atmospheric humidity changed because of varying sunlight. When the climate was dry, the ice receded to a greater depth or disappeared entirely except at the highest latitudes. Dust contained in retreating ice eventually covered the ice, making it no longer visible on the surface.
timeline
The history of subsurface ice layers on Mars over the last few million years. The tilt of the rotation axis changes with time, and the planet periodically experiences dry and humid climates.

timeline
The history of subsurface ice layers on Mars over the last few million years. The tilt of the rotation axis changes with time, and the planet periodically experiences dry and humid climates.

So much of this subsurface ice has been detected that its only plausible origin was thought to be massive snowfall. However, Schörghofer's theory suggests that a lot of that snowfall ice has since been lost to the atmosphere. It has been replaced by a new layer of ice, formed not from snowfall, since the climate had meanwhile turned less humid, but by diffusion of water vapour into the soil. Atmospheric vapor can freeze inside the soil and form "pore-ice," which is mainly soil with some ice in pore spaces.

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-- Edited by Blobrana at 00:16, 2007-09-16

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RE: Mars' South Pole
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Thanks to data from ESA's Mars Express mission, combined with models of the Martian climate, scientists can now suggest how the orbit of Mars around the Sun affects the deposition of water ice at the Martian South Pole.
 Early during the mission, the OMEGA instrument (Visible and Infrared Mineralogical Mapping Spectrometer) on board Mars Express had already found previously undetected perennial deposits of water-ice. They are sitting on top of million-year old layered terrains and provide strong evidence for a recent glacial activity.
However, only now a realistic explanation for the age of the deposits and the mechanism of their formation could finally be suggested. This was achieved thanks to the OMEGA mapping and characterisation of these ice deposits, combined with the computer-generated Martian Global Climate Models (GCMs).
Franck Montmessin, from the Service d'Aéronomie du CNRS/IPSL (France) and lead author of the findings, explains how the deposits of water ice at the Martian's poles 'behave'.

"We believe that the deposits of water-ice are juggled between Mars North and South Poles over a cycle that spans 51 000 years, corresponding to the time span in which the planet's precession is inverted."

Precession is the phenomenon by which the rotation axis of a planet wobbles.
Montmessin and colleagues came to the conclusion by turning back time in their Mars climate computer model. This was done by changing the precession together with other orbital information.
The scientists set the clock 21 000 years back, when the closest vicinity of the planet to the Sun corresponded to the northern summer a situation opposite to that of today.
The model has shown that water at the North Pole was in an unstable condition and was easily transported to the South Pole in the form of water vapour, to then re-condense and freeze on the surface. Up to 1 millimetre of water ice was deposited at the South Pole every year. After Mars has spent more than 10 000 years in that climatic configuration, this accumulation led to a layer up to 6-metre thick.
About 10 000 years ago the precession cycle was inverted, and started to return to its current configuration. Water-ice at the South Pole became unstable, and was forced to progressively return back to the North.
About 1000 years ago, by a not-yet-well explained trigger mechanism, the erosion of the water-ice deposits at the South pole was blocked as soon as layers of CO2 ice were deposited on the water-ice and trapped it, as OMEGA has observed them.
 
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Carbon Dioxide Jets
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Peculiar spots, fan-like markings, and spider-shaped features on Mars' southern ice cap are seasonal formations, researchers announced today.
The shapes are formed by thin layers of dark dusty material that are sprayed by roaring jets of carbon dioxide that erupt through the ice cap.

marscrystal
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This dusty material may also be the reason that the southern ice cap doesn't reflect much light.
The mystery markings, generally 50 to 150 feet wide, appear every southern spring as the Sun rises over the red planet's ice cap. They last about three to four months.

"Originally, scientists thought the spots were patches of warm, bare ground exposed as the ice disappeared" - Philip Christensen, study co-author, Arizona State University.

But observations made with the Thermal Emission Imaging System (THEMIS), a multi-wavelength camera on NASA's Mars Odyssey orbiter, revealed that the spots were nearly as cold as the carbon dioxide ice, which along with water ice, make up the ice caps.
With more than 200 visible and infrared images from THEMIS, Christensen and his team studied an area of the southern ice cap from the end of winter in that region through the middle of summer.
They watched how the spots appeared and developed into fan shaped markings and "spider's" grooves eroded into the surface under ice. Their finding is detailed in the August 17 issue of the journal Nature.

The whole process begins during Mars' frigid Antarctic winter, when temperatures drop to minus 200 degrees Fahrenheit. It's so cold that the Martian air - 95 percent carbon dioxide - freezes directly onto the surface of the permanent polar cap, which is made of water ice covered with layers of dust and sand, Christensen explained.
This layer of dusty carbon dioxide frost re-crystallises, becomes denser throughout winter and starts to slowly sink into the frost.
By spring, the frost layer becomes a slab of semi-transparent ice of about 3 feet thick, sitting on a layer of dark sand and dust. Sunlight passing through the slab reaches the dark material and warms it enough that the ice touching the ground turns directly to gas without going through the liquid phase. This process is called sublimation.

The warmed material produces a reservoir of pressurized gas under the slab, lifting it off the ground. The weak spots in the slab break through and high-pressure gas shoot up at speeds of almost 100 mph carrying loose sand and particles into the Martian air, the researchers propose.
The large particles, too heavy to go far, land around the vents to make the spots, while the lighter sand grains blow downwind, creating the fans that scientists see.
The lightest pieces drift away to form a thin layer of dust.

"It's like separating wheat and chaff. The finest-grained materials are carried off by the wind, while coarser grains are sifted again and again, year after year" - Philip Christensen.

The jets continue to erupt until the entire ice slab sublimates and vanishes. This entire process will repeat again at the end of the following winter.
In a related study reported in this week's journal Nature, the low reflectivity of the southern ice cap is also attributed to the dust in the region. Scientists initially thought this might be because the region is ice-free.
But the mystery deepened when temperature measurements obtained the Thermal Emission Spectrometer (TES) on board the Mars Global Surveyor spacecraft revealed that this dark region was nearly as cold as the bright regions surrounding it, said lead study author Yves Langevin from Institut d'Astrophysique Spatiale, Orsay, France.
The next thought was that the ice is so see through that the ground can be seen and thus there is reduced reflectivity.
What we observed is that light rays don't penetrate deeply into carbon dioxide ice, hence the "clear slab" idea must be revised, Langevin explained. What is needed is a layer of carbon dioxide (CO2) ice with heavy surface contamination by dust.

"This dust can be either carried into the region from a large basin called "Hellas" by winds (then later carried away) or brought to the surface by CO2 gas bubbles building up below the ice layer, then blowing up" - Yves Langevin.

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