Gullies on Mars
Martian gullies are small, incised networks of narrow channels and their associated downslope sediment deposits, found on the planet of Mars.
[6] A recent study examined 54,040 CTX images that covered 85% of the Martian surface found 4861 separate gullied landforms (e.g., individual craters, mounds, valleys, etc.
Because the gullies are so young, this would suggest that liquid water has been present on Mars in its very recent geological past, with consequences for the potential habitability of the modern surface.
On July 10, 2014, NASA reported that gullies on the surface of Mars were mostly formed by the seasonal freezing of carbon dioxide (CO2), and not by that of liquid water as considered earlier.
Various measurements and calculations show that liquid water could exist in aquifers at the usual depths where gullies begin.
[13] One variation of this model is that rising hot magma could have melted ice in the ground and caused water to flow in aquifers.
[22][23][24] This ice-rich mantle, a few yards thick, smooths the land, but in places it has a bumpy texture, resembling the surface of a basketball.
The water comes back to ground at lower latitudes as deposits of frost or snow mixed generously with dust.
It is in the right latitude range, but its altitude is so high that there is not enough pressure to keep ice from sublimating (going directly from a solid to a gas); hence it does not have gullies.
[5] In general, it is now estimated that during periods of high obliquity, the ice caps will melt causing higher temperature, pressure, and moisture.
Direct evidence for these snowpacks was recently discovered for the first time, showing that this mantle is indeed composed of <~1% dust and ice [35] Changes observed within gullies over multiple Mars Years shows that dusty ice being exposed today is disappearing, and potentially melting to form channels within the mantle, and the rock underneath.
[35] The third theory is that climate changes may be enough to allow ice deposited from atmospheric vapor into the ground to melt and thus form the gullies.
During a warmer climate, the first few meters of ground could thaw and produce a "debris flow" similar to those on the dry and cold Greenland east coast.
[36] Since the gullies occur on steep slopes only a small decrease of the shear strength of the soil particles is needed to begin the flow.
[50] However, some gullies in the past may have been aided by weather changes that involved larger amounts of water, perhaps from melted snow.
Recent studies describe using the High Resolution Imaging Science Experiment (HiRISE) camera on MRO to examine gullies at 356 sites, starting in 2006.
Before-and-after images demonstrated the timing of this activity coincided with seasonal carbon dioxide frost and temperatures that would not have allowed for liquid water.
[56][60] Simulations described in a 2015 conference, show that high pressure CO2 gas trapping in the subsurface can cause debris flows.
[62] This research was described in a later article entitled, "Formation of gullies on Mars by debris flows triggered by CO2 sublimation.
When the higher intensity sunlight of spring begins, light penetrates the translucent dry ice layer, consequently warming the ground.
The dirt particles mix with the pressurized gas and act as a fluid that can flow down the slope and carve gullies.
Dry ice, or solid carbon dioxide, accumulates in the cold winter and then when it is starting to warm up, changes appear in gullies.
A team of researchers examined changes in a sand dune gully in Matara Crater (49.5°S; 34.9°E - Noachis quadrangle) for 5 years.
Although there is considerable evidence for CO2 frost transporting loose materials, it seems unlikely that sublimating CO2 gas can erode and weather rocks to form gullies.
Using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter researchers studied over 100 Martian gully sites and found no evidence for specific minerals being more likely to be associated with gullies, or with the formation of hydrated minerals that would have been made by recent liquid water.
[70][71] However, as described above, the amounts of liquid water thought to be generated in near-freezing conditions from melting snowpacks are unlikely to cause chemical weathering in the first place.
In the spring at certain point, the ground will be warm enough and the air pressure high enough for liquid water to form at certain times of the day.
Models support the idea that pressure/temperature changes during high obliquity times are enough to allow liquid water to be stable in places where gullies are common.
Indeed, a large team of researchers published a paper in Science that showed that water would have existed where the gullies formed when the tilt of Mars went to 35 degrees.
In the winter dry ice accumulates on the dunes and then in the spring dark spots appear and dark-toned streaks grow downhill.
