Martian gullies

Martian gullies are narrow channels and their associated downslope sediment deposits, they are found on steep slopes on Mars. They were first discovered on Mars Global Surveyor images especially on the walls of craters. Usually, each gully has a ‘‘alcove’’ at its head, a fan-shaped apron at its base, and a single thread of incised channel connecting the two. The whole gully resembles an hourglass.

They are estimated to be relatively young because they have few, if any craters. 

Most gullies appear 30 degrees poleward in each hemisphere, with greater numbers in the southern hemisphere. Some studies have found that gullies occur on slopes that face all directions.

Although thousands of gullies have been found, they appear to be restricted to only certain areas of the planet. In the northern hemisphere, they have been found in Arcadia Planitia, Tempe Terra, Acidalia Planitia, and Utopia Planitia.

<refname="2007Icar..188..324H">last1=Heldmann|first1=J|last2=Carlsson|first2=E|last3=Johansson|first3=H|last4=Mellon|first4=M|last5=Toon|first5=O|title=Observations of martian gullies and constraints on potential formation mechanisms. The northern hemisphere|journal=Icarus|volume=188|pages=324–344|date=2007|doi=10.1016/j.icarus.2006.</ref>

In the south, high concentrations are found on the northern edge of Argyre basin, in northern Noachis Terra, and along the walls of the Hellas outflow channels.

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.), which totaled tens of thousands of individual gullies. This number may represent a fairly accurate census of gullies since it is estimated that CTX can resolve 95% of gullies. This article reviews some significant history of the discovery Martian gullies. On the basis of their form, aspects, positions, and location amongst and apparent interaction with features thought to be rich in water ice, many researchers think (especially when they were first discovered) that the processes carving the gullies involve liquid water. Because the gullies are so young, this would suggest that liquid water has been present on Mars in its very recent geological past, consequently added to the possibility of living forms on the present surface.

 After being discovered, many hypotheses were put forward to explain the gullies.

However, as in the usual progression of science, some ideas came to be more plausible than others when more observations were made, when other instruments were used, and when statistical analysis was employed. Even though some gullies resembled debris flows on Earth, it was found that many gullies were on slopes that were not steep enough for typical debris flows. Although it was suggested that liquid carbon dioxide could cause gullies, calculations showed that the pressure and temperatures were not suitable for liquid carbon dioxide.

Moreover, the winding shape of the gullies suggested that the flows were slower than what would be produced in debris flows or eruptions of liquid carbon dioxide. Liquid carbon dioxide would explode out of the ground in the thin Martian atmosphere. Because the liquid carbon dioxide would throw material over 100 meters, the channels should be discontinuous, but they are not.

Eventually, most hypotheses focused on liquid water coming from an aquifer, from melting at the base of old glaciers (or snowpacks), or from the melting of ice in the ground when the climate was warmer.

Close-up images with HiRISE showed details that support the idea that a fluid was involved. Images show that channels formed at various times--smaller channels were found in larger valleys, suggesting that after a valley formed another formed at a later time. Many cases showed channels took different paths at different times. Streamlined forms like teardrop-shaped islands were common in some channels.

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  Streamlined features in gullies thought to have formed by running water

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  Smaller gullies inside larger ones Water may have flowed in these gullies more than once.

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  Close-up of gullies in crater showing channels within larger valleys and curves in channels. These characteristics suggest they were made by flowing water. Location is Eridania quadrangle.

The following group of pictures of gullies illustrates some of the shapes that lead researchers to think that water was involved in creating at least some of the gullies.

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  Gullies The location is the Phaethontis quadrangle.

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  Gullies, as seen by HiRISE Location is the Phaethontis quadrangle.

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  Close-up of gully channels, as seen by HiRISE under HiWish program. This image shows many streamlined forms and some benches along a channel. These features suggest formation by running water. Benches are usually formed when the water level goes down a bit and stays at that level for a time. Picture was taken with HiRISE under HiWish program. Location is the Mare Acidalium quadrangle.

Aquifers

One of the earliest ideas attempting to explain gully formation was that water came out of aquifers. Most of the gully alcove heads occur at the same level, just as one would expect if water came out of aquifers. The aquifer layer would be perched on top of another layer that prevents water from going down (in geological terms it would be called impermeable). Because water in an aquifer is prevented from going down, the only direction the trapped water can flow is horizontally. Eventually, water could flow out onto the surface when the aquifer reaches a break—like a crater wall. The resulting flow of water could erode the wall to create gullies. Various measurements and calculations show that liquid water could exist in aquifers at the usual depths where gullies begin.

One variation of this model is that rising hot magma could have melted ice in the ground and caused water to flow in aquifers. 

Aquifers are quite common on Earth. A good example is "Weeping Rock" in Zion National Park Utah.

However, the idea that aquifers formed the gullies does not explain the ones found on isolated peaks, like knobs and the central peaks of craters. Also, a type of gully seems to be present on sand dunes.

Gullies on dunes Some gullies on sand dunes appear each Martian year. It is difficult to conceive of an aquifer causing gullies on dunes.

Aquifers need a wide collecting area which is not present on sand dunes or on isolated slopes. Even though most of the original gullies that were seen seemed to come from the same layer in the slope, some exceptions to this pattern have been found. Examples of gullies coming from different levels is shown below in the image of Lohse Crater and the image of gullies in Ross Crater.

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  CTX image of the next image showing a wide view of the area. Since the hill is isolated it would be difficult for an aquifer to develop. Rectangle shows the approximate location of the next image.

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  Gully on mound as seen by Mars Global Surveyor, under the MOC Public Targeting Program. Images of gullies on isolated peaks, like this one, are difficult to explain with the theory of water coming from aquifers because aquifers need large collecting areas.

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  Gullies on 2 sides of a mound. This arrangement is difficult to explain with aquifers.

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  Gullies in two levels of a crater wall, as seen by HiRISE under HiWish program. Gullies at two levels suggest they were not made with an aquifer, as was first suggested. Location is Phaethontis quadrangle.

Snowpacks

The main basis for the snowpack hypothesis for gully formation is that much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust.

This ice-rich mantle, a few yards thick, soothes the land. The mantle may be like a glacier, and under certain conditions the ice that is mixed in the mantle could melt and flow down the slopes and make gullies.

Because there are few craters on this mantle, the mantle is relatively young. An excellent view of this mantle is shown below in the picture of the Ptolemaeus Crater Rim, as seen by HiRISE. The ice-rich mantle may be the result of climate changes.

Changes in Mars's orbit and tilt cause significant changes in the distribution of water ice from polar regions down to latitudes equivalent to Texas. During certain climate periods water vapor leaves polar ice and enters the atmosphere. The water comes back to ground at lower latitudes as deposits of frost or snow mixed with dust. This movement of water could last for several thousand years and create a snow layer of up to around 10 meters thick.

When ice at the top of the mantling layer goes back into the atmosphere, it leaves behind dust, which insulates the remaining ice.

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  Close view that displays the thickness of the mantle, as seen by HiRISE under HiWish program

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  Mantle

When the slopes, orientations, and elevations of thousands of gullies were compared, clear patterns emerged from the data. Measurements of altitudes and slopes of gullies support the idea that snowpacks or glaciers are associated with gullies. Steeper slopes have more shade which would preserve snow. Higher elevations have far fewer gullies because ice would tend to sublimate more in the thin air of the higher altitude. For example, Thaumasia quadrangle is heavily cratered with many steep slopes. 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. In summary, it is now estimated that during periods of high obliquity, the ice caps will melt causing higher temperature, pressure, and moisture. The moisture will then accumulate as snow in midlatitudes, especially in the more shaded area. At a certain time of the year, sunlight will melt snow with the resulting water producing gullies.

Melting of ground ice

The third theory might be possible since climate changes may be enough to simply allow ice in 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. 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. Small amounts of liquid water from melted ground ice could be enough.

Dry ice makes gullies today

As soon as gullies were discovered,

researchers began to image many gullies over and over, looking for possible changes.  By 2006, some changes were found.

Later, analysis revealed that the changes could have occurred by dry granular flows rather than being driven by flowing water.

Changes were found in Gasa Crater and other craters.

Channels widened by 0.5 to 1 m; meter sized boulders moved; and hundreds of cubic meters of material moved.  It was calculated that gullies could be formed under present conditions with as little as 1 event in 50–500 years.  Although today there is little liquid water, present geological/climatic processes could still form gullies.

Sinuous channels which were thought to need liquid water for their formation have even been seen to form over just a few years when liquid water cannot exist.

The timing of gully activity is seasonal and happens during the period when seasonal frost is present and defrosting.

Observations with HiRISE show widespread activity in southern hemisphere gullies, especially in those that appear fresh. Significant channel incision and large-scale mass movements have been seen. Neither large amounts of water or great changes in climate were not needed. But, some gullies in the past may have been aided by weather changes that involved larger amounts of water, perhaps from melted snow. Repeated observations, showed that changes occur in the winter and spring. Studies with the High Resolution Imaging Science Experiment (HiRISE) camera on MRO examined gullies at 356 sites, starting in 2006. Thirty-eight of the sites showed active gully formation. 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. Some scientists tended to suspect that gullies were formed from carbon dioxide ice (dry ice). When dry ice frost changes to a gas, it may lubricate dry material to flow especially on steep slopes. In some years frost, perhaps as thick as 1 meter, triggers avalanches. This frost contains mostly dry ice, but also has tiny amounts of water ice. These observations support a model in which currently active gully formation is driven mainly by seasonal CO₂ frost.

Simulations described in a 2015 conference, show that high pressure CO2 gas trapping in the subsurface can cause debris flows.

The conditions that can lead to this are found in latitudes where gullies occur. This research was described in a later article entitled, "Formation of gullies on Mars by debris flows triggered by CO2 sublimation."

In the model, CO2 ice accumulates in the cold winter.

It piles up on a frozen permafrost layer that consists of ice-cemented dirt.  When the higher intensity sunlight of spring begins, light penetrates the translucent dry ice layer, consequently warming the ground.  The CO2 ice absorbs heat and sublimates—that is changes directly from a solid to a gas.  This gas builds up pressure because it is trapped between the ice and the frozen ground.  Eventually, pressure builds up enough to explode through the ice taking with it soil particles.  The dirt particles mix with the pressurized gas and act as a fluid that can flow down the slope and carve gullies.

On July 10, 2014, NASA reported that gullies on the surface of Mars were mostly formed by the seasonal freezing of carbon dioxide (CO₂ ice or 'dry ice'), and not by that of liquid water as thought earlier. So, the current thought is that gullies can be formed today by chunks of dry ice moving down steep slopes today. Perhaps in the past, water was also involved.

See also

• Martian features that are signs of water ice
• Water
• Sublimation
• Sublimation landscapes on Mars
• What Mars Actually Looks Like!

External links

• [[1]] Video demonstrates how dry ice can form gullies on dunes
• [Pictures of gullies on dunes]
• [Demonstration of dry ice moving down dune ]