View source for 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.
<ref name="Malin, M. 2000"> last1 = Malin | first1 = M. | last2 = Edgett | first2 = K. | year = 2000 | title = Evidence for recent groundwater seepage and surface runoff on Mars | url = | journal = Science | volume = 288 | issue = | pages = 2330–2335 | doi=10.1126/science.288.5475.2330 | pmid=10875910| </ref>
They are estimated to be relatively young because they have few, if any, craters. 

[[File:50858 1435gullies.jpg|600pxr|Image of gullies with main parts labeled.  The main parts of a Martian gully are alcove, channel, and apron.  Since there are no craters on this gully, it is thought to be rather young.  Picture was taken by HiRISE under HiWish program.]]

<gallery class="center"  widths="380px" heights="360px">
File:Close view of gullies ESP 080430 2310 01.jpg|Gullies on crater wall  The bright apron is a bit unusual.
File:Close view of gullies ESP 080430 2310 02.jpg|Gully on crater wall  The bright apron is a bit unusual.

File:ESP 084896 1355 small gullies 02.jpg|Gullies, as seen by HiRISE.  The gullies range from very samll to large, as such they may represent different stages in the formation of gullies.  The colored strip is about 1 km wide.
File:ESP 084896 1355 small gullies 03.jpg|Small gully  This gully may be in its initial state of formation.  
File:ESP 084896 1355 small gullies 04.jpg|Gully, as seen by HiRISE
</gallery>

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. 

<ref>cite journal|last1=Edgett|first1=K.|display-authors=etal|date=2003|title=Polar-and middle-latitude martian gullies: A view from MGS MOC after 2 Mars years in the mapping orbit|journal=Lunar Planet. Sci.|volume=34|at=Abstract 1038|url=http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1038.pdf|bibcode=2003LPI....34.1038E</ref> 
Others have found that the greater number of gullies are found on poleward facing slopes, especially from 30° to 44° S.<ref>Dickson, J; Head, J; Kreslavsky, M (2007). "Martian gullies in the southern mid-latitudes of Mars: Evidence for climate-controlled formation of young fluvial features based upon local and global topography" (PDF). Icarus. 188: 315–323.</ref>
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.
<ref>name="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.<ref>Heldmann, J; Carlsson, E; Johansson, H; Mellon, M; Toon, O (2007). "Observations of martian gullies and constraints on potential formation mechanismsII. The northern hemisphere". Icarus. 188: 324–344.</ref>
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.
<ref>Harrison, T., G. Osinski1, and L. Tornabene.  2014. GLOBAL DOCUMENTATION OF GULLIES WITH THE MARS RECONNAISSANCE ORBITER CONTEXT CAMERA (CTX) AND IMPLICATIONS FOR THEIR FORMATION.  45th Lunar and Planetary Science Conference. pdf</ref>

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 gullies involve liquid water.<ref>https://www.uahirise.org/hipod/ESP_014074_1445</ref>   Because the gullies are so young, this would suggest that liquid water has been present on Mars in its very recent geological past, consequently adding to the possibility of living forms on the present surface.

After being discovered, many hypotheses were put forward to explain the gullies.
<ref>http://www.psrd.hawaii.edu/Aug03/MartianGullies.html</ref>
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.
<ref>Musselwhite, C., et al.  2001.  Liquid CO2 Breakout and the formation of recent small gullies on Mars.  Lunar and Planetary Science XXXII.  1030.pdf</ref>  <ref> Stewart, S.  2001. Lunar and Planetary Science XXXII.   17820.pdf</ref>   
<ref>Stewart, S.  2001.   Surface runoff features on Mars:  Testing the carbon dioxide formation hypothesis.  Lunar and Planetary Science XXXII.   1780.pdf</ref>
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.
<ref name="Heldmann 2004">last1=Heldmann|first1=J|title=Observations of martian gullies and constraints on potential formation mechanisms|journal=Icarus|volume=168|pages=285–304|date=2004|doi=10.1016/j.icarus.2003.11.024|</ref>
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.<ref>https://www.hou.usra.edu/meetings/lpsc2019/pdf/3060.pdf</ref> <ref>Khuller1, A.,  P. R. Christensen.  2019. EVIDENCE OF WATER-RICH SNOW DEPOSITS WITHIN MARTIAN GULLIES.   50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132). 3060.pdf</ref> <ref> Heldmann, J (2004). "Observations of martian gullies and constraints on potential formation mechanisms". Icarus. 168: 285–304. </ref>  <ref>Forget, F. et al. 2006. Planet Mars Story of Another World. Praxis Publishing. Chichester, UK.</ref>
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.  On the Earth, running water is the cause of streamlined forms.<ref>Head, J.,  D. Marchant, M. Kreslavsky.  2008. Formation of gullies on Mars: Link to recent climate history and insolation microenvironments implicate surface water flow origin.  PNAS: 105 (36), 13258–13263.</ref>


<gallery class="center"  widths="380px" heights="360px">
File:26420gulliesclose.jpg|Streamlined features in gullies thought to have formed by running water
File:Multiple channels in 21461.jpg| Smaller gullies inside larger ones   Water may have flowed in these gullies more than once.
File:ESP 039793 1385channeldetails.jpg|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.
</gallery>

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.


<gallery class="center"  widths="380px" heights="360px">
File:45752 1410gullies.jpg|Gullies    The location is the Phaethontis quadrangle.
File:46386 1420gullies.jpg|Gullies, as seen by HiRISE   Location is the Phaethontis quadrangle.
File:ESP 037506 2285gullychannelsclose.jpg|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. Location is the Mare Acidalium quadrangle.  

File:Close view of gully in Phaethontis 01.jpg|Gullies on crater wall, as seen by HiRISE under HiWish program
File:Close view of gullies in Phaethontis quadrangle 02.jpg|Close view of Gullies on crater wall
</gallery>

===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.
<ref > Heldmann, J (2004). "Observations of martian gullies and constraints on potential formation mechanisms". Icarus. 168: 285–304.</ref>
One variation of this model is that rising hot magma could have melted ice in the ground and caused water to flow in aquifers. 
<ref>http://www.space.com/scienceastronomy/mars_aquifer_041112.html Mars Gullies Likely Formed By Underground Aquifers. Leonard David, 12 November 2004 (Space.com)</ref> 
Aquifers are quite common on Earth. A good example is "Weeping Rock" in Zion National Park Utah.
<ref>Harris, A and E. Tuttle. 1990. Geology of National Parks. Kendall/Hunt Publishing Company. Dubuque, Iowa</ref>
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, one kind of gully seems to be present on sand dunes.
<ref>Reiss, D, R. Jaumann.  2003.  Recent debris flows on Mars:  Seasonal observations of the Russell Crater dune field.  Geophysical Research letters:  30, 54</ref> 

[[File:ESP 051770 1345dunegullies.jpg |thumb|300px|left| Gullies on dunes   Some gullies  on sand dunes appear each Martian year.   It is difficult to conceive of an aquifer causing gullies on dunes.]]

[[File:ESP 054026 1300gulliesdunes.jpg|thumb|300px|center|Gullies on Dunes in Matara Crater, as seen by HiRISE]]

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.
<ref>Foget, F. et al. 2006. Planet Mars Story of Another World. Praxis Publishing. Chichester, UK</ref> Examples of gullies coming from different levels are shown below in the image of Lohse Crater and the image of gullies in Ross Crater.


<gallery class="center"  widths="380px" heights="360px">
File:Wide view of gully on hill.jpg|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.
File:Gully on mound.JPG|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.
File:47528 1355gulliesmound.jpg|Gullies on 2 sides of a mound.  This arrangement is difficult to explain with aquifers.
File:ESP 039621 1315gullies2levels.jpg|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.
</gallery>

===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.
<ref>last1=Malin|first1=Michael C.|last2=Edgett|first2=Kenneth S.|title=Mars Global Surveyor Mars Orbiter Camera: Interplanetary cruise through primary mission|journal=Journal of Geophysical Research|volume=106|pages=23429–23570|date=2001|doi=10.1029/2000JE001455</ref> <ref>|pmid=11473309|last1=Mustard|first1=JF|date=2001|pages=411–4|issue=6845|last2=Cooper|volume=412|first2=CD|journal=Nature|last3=Rifkin|first3=MK|title=Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice.|url=http://www.planetary.brown.edu/pdfs/2610.pdf</ref> <ref>last1=Carr|first1=Michael H.|title=Mars Global Surveyor observations of Martian fretted terrain|journal=Journal of Geophysical Research|volume=106|pages=23571–23595|date=2001|doi=10.1029/2000JE001316|</ref>
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.<ref>http://www.nbcnews.com/id/15702457/ns/technology_and_science-space/t/martian-gullies-could-be-scientific-gold-mines/#.WxVAOUxFzIU</ref> <ref>Head, JW; Marchant, DR; Kreslavsky, MA (2008). "Formation of gullies on Mars: Link to recent climate history and insolation microenvironments implicate surface water flow origin". PNAS. 105 (36): 13258–63.</ref>
Indeed, calculations show that a third of a mm of runoff can be produced through the melting of a dusty snowpack each day for 50 days of each Martian year even under current conditions.
<ref>last1=Clow|first1=G|title=Generation of liquid water on Mars through the melting of a dusty snowpack|journal=Icarus|volume=72|pages=93–127|date=1987|</ref> 
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.
<ref>last1=Christensen|first1=PR|title=Formation of recent martian gullies through melting of extensive water-rich snow deposits.|journal=Nature|volume=422|issue=6927|pages=45–8|date=2003|pmid=12594459|doi=10.1038/nature01436 </ref>
The ice-rich mantle may be the result of climate changes.
<ref>http://news.nationalgeographic.com/news/2008/03/080319-mars-gullies_2.html Melting Snow Created Mars Gullies, Expert Says</ref>
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.
<ref name="ReferenceA">last1=Jakosky|first1=Bruce M.|last2=Carr|first2=Michael H.|title=Possible precipitation of ice at low latitudes of Mars during periods of high obliquity|journal=Nature|volume=315|pages=559–561|date=1985|doi=10.1038/315559a0|issue=6020</ref> <ref>|last1=Jakosky|first1=Bruce M.|last2=Henderson|first2=Bradley G.|last3=Mellon|first3=Michael T.|title=Chaotic obliquity and the nature of the Martian climate|journal=Journal of Geophysical Research|volume=100|pages=1579–1584|date=1995|</ref>
When ice at the top of the mantling layer goes back into the atmosphere, it leaves behind dust, which insulates the remaining ice. 


<gallery class="center"  widths="380px" heights="360px">
45917 2220gulliesmantle.jpg|Close view that displays the thickness of the mantle, as seen by HiRISE under HiWish program
46444 2225mantle.jpg|Mantle
</gallery>

<ref>author=MLA NASA/Jet Propulsion Laboratory|date=December 18, 2003|title=Mars May Be Emerging From An Ice Age|work=ScienceDaily|accessdate=February 19, 2009|url=https://www.sciencedaily.com/releases/2003/12/</ref>
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.
<ref name="2007Icar..188..315D">last1=Dickson|first1=J|last2=Head|first2=J|last3=Kreslavsky|first3=M|title=Martian gullies in the southern mid-latitudes of Mars: Evidence for climate-controlled formation of young fluvial features based upon local and global topography|doi=10.1016/j.icarus.2006.11.020|url=http://www.planetary.brown.edu/pdfs/3138.pdf|date=2007|pages=315–323|volume=188|journal=Icarus|format=PDF</ref>
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.
<ref>pages=26695–26712|date=2000|doi=10.1029/2000JE001259|last1=Kreslavsky|volume=105|first1=Mikhail A.|journal=Journal of Geophysical Research|last2=Head|first2=James W.|title=Kilometer-scale roughness of Mars: Results from MOLA data analysis|url=http://www.planetary.brown.edu/pdfs/2447.pdf|</ref> <ref>last1=Hecht|first1=M|title=Metastability of liquid water on Mars|pages=373–386|date=2002|volume=156|doi=10.1006/icar.2001.6794|journal=Icarus|url=http://www.geo.brown.edu/geocourses/geo292/papers/Hecht2002.pdf|format=PDF|</ref>
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.

A related idea is that buried snow deposits may be uncovered, melt, and help to form gullies.  Evidence for snow deposits being exposed has been observed and reported.<ref>Kuller, A., P. Christensen. 2019. EVIDENCE OF WATER-RICH SNOW DEPOSITS WITHIN MARTIAN GULLIES. 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132). 3060.pdf</ref> <ref>https://www.hou.usra.edu/meetings/lpsc2019/pdf/3060.pdf</ref>

===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.
<ref>last1=Peulvast|first1=J.P.|date=1988|title=Mouvements verticaux et genèse du bourrelet Est-groenlandais. dans la région de Scoresby Sund|journal=Physio Géo|volume=18|pages=87–105|language=French </ref> 
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.
<ref>author1=Jouannic G. |author2=J. Gargani |author3=S. Conway |author4=F. Costard |author5=M. Balme |author6=M. Patel |author7=M. Massé |author8=C. Marmo |author9=V. Jomelli |author10=G. Ori |date=2015|title= Laboratory simulation of debris flows over a sand dune : Insights into gully-formation (Mars)|journal=Geomorphology|volume=231|pages=101–115|url=http://www.sciencedirect.com/science/article/pii/S0169555X14005972|doi=10.1016/j.geomorph.2014.12.007|</ref> <ref>last1=Costard|first1=F.|display-authors=etal|date=2001|title=Debris Flows on Mars: Analogy with Terrestrial Periglacial Environment and Climatic Implications|journal=Lunar and Planetary Science|volume=XXXII||url=http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1534.pdf|format=PDF </ref> <ref>{{cite web |url=http://www.spaceref.com/16090/news/viewpr.html?pid=7124 |title=Archived copy |accessdate=2011-03-10 |deadurl=yes |archiveurl=https://archive.is/20120910131532/http://www.spaceref.com/16090/news/viewpr.html?pid=7124 |</ref>

==Dry ice makes gullies today==

[[File:ESP 032011 1425newgullies.jpg|600pxr|Changes in gullies, as seen by HiRISE  This shows that gullies are forming today, even though liquid water can not exist on the surface today.]]

Changes in gullies, as seen by HiRISE  This shows that gullies are forming today, even though liquid water can not exist on the surface today


As soon as gullies were discovered,
<ref name="Malin, M. 2000"> last1 = Malin | first1 = M. | last2 = Edgett | first2 = K. | year = 2000 | title = Evidence for recent groundwater seepage and surface runoff on Mars | url = | journal = Science | volume = 288 | issue = | pages = 2330–2335 | doi=10.1126/science.288.5475.2330 | pmid=10875910| </ref>
researchers began to image many gullies over and over, looking for possible changes.<ref> https://www.uahirise.org/hipod/ESP_032011_1425</ref>  By 2006, some changes were found.
<ref> last1 = Malin | first1 = M. | last2 = Edgett | first2 = K. | last3 = Posiolova | first3 = L. | last4 = McColley | first4 = S. | last5 = Dobrea | first5 = E. | year = 2006 | title = Present-day impact cratering rate and contemporary gully activity on Mars | url = | journal = Science | volume = 314 | issue = | pages = 1573–1577 | doi=10.1126/science.1135156 | pmid=17158321| </ref>   Liquid water can not exist on Mars today to carve gullies, yet images showed that new gullies were forming.<ref>https://www.uahirise.org/ESP_039701_1095</ref>  <ref>https://www.uahirise.org/ESP_032011_1425</ref>  There must be other mechanisms going on today.
Later, analysis revealed that the changes could have occurred by dry granular flows rather than being driven by flowing water.
<ref>| last1 = Kolb | display-authors = et al.  | year = 2010 | title = Investigating gully flow emplacement mechanisms using apex slopes | doi = 10.1016/j.icarus.2010.01.007 | journal = Icarus | volume = 208 | issue = | pages = 132–142 | </ref> <ref> last1 = McEwen | first1 = A. | display-authors = et al.   | year = 2007 | title = A closer look at water-related geological activity on Mars | url = | journal = Science | volume = 317 | issue = | pages = 1706–1708 | </ref> <ref>| last1 = Pelletier | first1 = J. | display-authors = et al.   | year = 2008 | title = Recent bright gully deposits on Mars wet or dry flow? | url = | journal = Geology | volume = 36 | issue = | pages = 211–214 | doi=10.1130/g24346a.1| </ref>
Changes were found in Gasa Crater and other craters.
<ref>NASA/Jet Propulsion Laboratory. "NASA orbiter finds new gully channel on Mars." ScienceDaily. ScienceDaily, 22 March 2014. www.sciencedaily.com/releases/2014/03/140322094409.htm </ref>
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.
<ref>Dundas, C., S.
Diniega, and A. McEwen.  2014.  LONG-TERM MONITORING OF MARTIAN GULLY ACTIVITY WITH HIRISE.  45th Lunar and Planetary Science Conference.  2204.pdf</ref>  
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.
<ref>Dundas, C. et al.  2016.  HOW WET IS RECENT MARS? INSIGHTS FROM GULLIES AND RSL. 47th Lunar and Planetary Science Conference (2016) 2327.pdf.</ref> 
The timing of gully activity is seasonal and happens during the period when seasonal frost is present and defrosting.
<ref>Vincendon, M.  2015.  JGR:120, 1859–1879.</ref>
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.
<ref>Dundas, C., S. Diniega, C. Hansen, S. Byrne, A. McEwen.  2012.  Seasonal activity and morphological changes in martian gullies.  Icarus, 220.  124–143.</ref>  
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.  <ref>http://www.jpl.nasa.gov/news/news.php?release=2014-226</ref> <ref>http://www.skyandtelescope.com/astronomy-news/martian-gullies-triggered-by-exploding-dry-ice-122320158/</ref> <ref>http://hirise.lpl.arizona.edu/ESP_032078_1420</ref>  <ref>http://www.space.com/26534-mars-gullies-dry-ice.html</ref>  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. 
<ref>http://spaceref.com/mars/frosty-gullies-on-mars.html</ref>
These observations support a model in which currently active gully formation is driven mainly by seasonal CO<sub>2</sub> frost.
<ref>Dundas, C., S. Diniega, A. McEwen. 2015. Long-term monitoring of martian gully formation and evolution with MRO/HiRISE. Icarus: 251, 244–263</ref>
<ref> last1 = Raack | first1 = J. | display-authors = etal   | year = 2015 | title =  Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars| url = | journal = Icarus | volume = 251 | issue = | pages = 226–243 | doi=10.1016/j.icarus.2014.03.040 | </ref>
Simulations described in a 2015 conference, show that high pressure CO<sub>2</sub> gas trapping in the subsurface can cause debris flows.
<ref>http://www.uahirise.org/ESP_044327_1375</ref>  
The conditions that can lead to this are found in latitudes where gullies occur.<ref>C. Pilorget, C., F. Forget.  2015.  "CO2 Driven Formation of Gullies on Mars."  46th Lunar and Planetary Science Conference.  2471.pdf</ref>  
This research was described in a later article entitled, "Formation of gullies on Mars by debris flows triggered by CO2 sublimation."
<ref>| last1 = Pilorget | first1 = C. | last2 = Forget | first2 = F. | year = 2016 | title = Formation of gullies on Mars by debris flows triggered by CO<sub>2</sub> sublimation | url = | journal = Nature Geoscience | volume = 9 | issue = | pages = 65–69 | doi = 10.1038/ngeo2619 | </ref> 
In the model, CO<sub>2</sub> ice accumulates in the cold winter.
<ref>Schorghofer, N., K. Edgett.  2005.  Seasonal surface frost at low latitudes on Mars.  Icarus:  180, 321-334.</ref>
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 CO<sub>2</sub> 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]] (CO<sub>2</sub> 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.<ref> Raack, J., et al.  2020.  Present-day gully activity in Sisyphi Cavi, Mars – Flow-like features and block movements.  Icarus.  350.   https://doi.org/10.1016/j.icarus.2020.113899. </ref>   Perhaps in the past, water was also involved.<ref name="NASA-20140710">last=Harrington |first=J.D. |last2=Webster |first2=Guy |title=RELEASE 14-191 – NASA Spacecraft Observes Further Evidence of Dry Ice Gullies on Mars |url=http://www.nasa.gov/press/2014/july/nasa-spacecraft-observes-further-evidence-of-dry-ice-gullies-on-mars |date=July 10, 2014 |work=NASA</ref>
<ref>CNRS. "Gullies on Mars sculpted by dry ice rather than liquid water." ScienceDaily. ScienceDaily, 22 December 2015. www.sciencedaily.com/releases/2015/12/151222082255.htm </ref> <ref>http://www.skyandtelescope.com/astronomy-news/martian-gullies-triggered-by-exploding-dry-ice-122320158</ref> <ref>https://www.uahirise.org/ESP_067299_1435</ref> <ref>Dickson, J., et al.  2021.  THE ELEVATION DISTRIBUTION OF MID-LATITUDE GULLIES ON MARS AS A TEST OF CO2 AND
H2O FORMATION AND MODIFICATION PROCESSES.  52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2426.pdf</ref>

As more pictures are examined and analyzed opinions on whether water was involved are shifting.  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.  It has done that many times; the last time just 630,000 years ago.  When the tilt increases to 35 degrees, both the pressure and temperature are such that ice in the ground will melt.  Places that are too high in altitude, and thus have too low a pressure, do not display gullies.  Gullies are found on many places with steep slopes.  However, the Thaumasia Highlands have many steep slopes, but very, very few gullies.  The rather high elevation of the Thaumasia area keeps the pressure low--too low for liquid water to form.<ref>Dickson, J., et al.  2023.  Gullies on Mars could have formed by melting of water ice during periods of high obliquity.  Science.  Vol 380, Issue 6652.  pp. 1363-1367.  DOI: 10.1126/science.abk246</ref> <ref>https://www.science.org/doi/10.1126/science.abk2464</ref>

In support of water being involved is the fact that many gully alcoves have a greater volume than the aprons.  The material that was in the alcove may have contained much water ice that disappeared into the atmosphere.<ref>Gulick, V. and N. Glines.  2021.  STUDIES OF MARTIAN GULLY SYSTEMS AND THEIR POTENTIAL PALEOENVIRONMENTAL
SETTINGS. 52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548).  2773.pdf.</ref> <ref>Gulick et al. 2017 LPSC #1970</ref> <ref>Huang, H, and V. Gulick.  2023.  MORPHOLOGIC ANALYSIS OF MARTIAN GULLIES IN FOUR HIGH-NORTHERN LATITUDE
CRATERS.  54th Lunar and Planetary Science Conference 2023 (LPI Contrib. No. 2806).  1863.pdf</ref>  Addition evidence that water is involved is that some gullies occur on slopes that are not steep enough for a dry flow, but would be steep enough if water was involved.<ref>Huang, R., et al.  2021.  SLOPE ANALYSIS OF MARTIAN GULLIES IN THREE HIGH-NORTHERN LATITUDE CRATERS.  52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548).  2625.pdf.</ref> Also, some gullies erode over 40 meters into bedrock, which so a long period of erosion is required--maybe more than dry ice could do.<ref> Huang, R., and V. Gulick.   2023.  MORPHOLOGIC ANALYSIS OF MARTIAN GULLIES IN FOUR HIGH-NORTHERN LATITUDECRATERS.  54th Lunar and Planetary Science Conference 2023 (LPI Contrib. No. 2806).  1863.pdf.</ref>

Some small patches of light-toned material have been observed in gullies.  These have gotten smaller over time. 
We have been looking at gullies for many years now, looking for such changes.  There is some spectral evidence that the material contains water. The authors of the study suggest that several factors may be involved in gully formation.<ref>A. Khuller, P. Christensen.  2021.  Evidence of exposed dusty water ice within Martian gullies
J Geophys Res Planets, 126. 10.1029/2020JE006539</ref> 

In summary of our present understanding of gullies it can be said:  A number of studies have demonstrated that gullies are being modified on present day Mars. <ref>C.M. Dundas, A.S. McEwen, S. Diniega, C.J. Hansen, S. Byrne, J.N. McElwaine.  The formation of gullies on Mars today.  Geol. Soc. London Spec. Publ., 467 (2019), pp. 67-94, 10.1144/SP467.5</ref> <ref>C.M. Dundas, S. Diniega, C.J. Hansen, S. Byrne, A.S. McEwen.  Seasonal activity and morphological changes in Martian gullies.  Icarus, 220 (2012), pp. 124-143, 10.1016/j.icarus.2012.04.005 </ref> <ref>.M. Dundas, S. Diniega, A.S. McEwen.  Long-term monitoring of Martian gully formation and evolution with MRO/HiRISE.  Icarus, 251 (2015), pp. 244-263, 10.1016/j.icarus.2014.05.013</ref> <ref>J. Raack, S.J. Conway, T. Heyer, V.T. Bickel, M. Philippe, H. Hiesinger, A. Johnsson, M. Massé.  Present-day gully activity in Sisyphi Cavi, Mars - flow-like features and block movements.  Icarus, 350 (2020), 10.1016/j.icarus.2020.113899. article #113899</Ref>  Today, liquid water cannot exist on the Red planet because the both the pressure and the temperature is too low.  Further evidence that water is not involved is that minerals are not changed by gully formation.<ref>J.I. Núñez, O.S. Barnouin, S.L. Murchie, F.P. Seelos, J.A. McGovern, K.D. Seelos, D.L. Buczkowski.  New insights into gully formation on Mars: constraints from composition as seen by MRO/CRISM.  Geophys. Res. Lett., 43 (2016), pp. 8893-8902, 10.1002/2016GL068956</ref>  For many years, many believed that gullies had to be made with liquid water.  So, researchers have proposed other mechanisms that could account for gully formation without liquid water.<ref>S.J. Conway, T. de Haas, T.N. Harrison.  Martian gullies: a comprehensive review of observations, mechanisms and insights from Earth analogues.  Geol. Soc. London Spec. Publ., 467 (2019), pp. 7-66, 10.1144/SP467.14</ref>   Most involve dry ice (solid carbon dioxide) accumulating during cold seasons and then changing to a gas in the spring.  The gas coming off could start material moving down slopes.  The gas mixed with sand and other debris could act like water to erode channels.  Also, pieces of dry ice could easily side down due to the lubricating effect of gas coming off the dry ice. 



<gallery class="center"  widths="380px" heights="360px">
File:Channels on Southern Sand Dunes with ice blocks visible ESP 079809 1325.jpg|Channels on sand dunes, as seen by HiRISE.  Arrows show chunks of ice that moved down to enlarge gullies.

</gallery>

Some gullies on sand dunes have been observed to change noticeably in just one year.  The changes occur when dry ice has accumulated and has started to disappear.  It is thought that dry ice could be causing a flow in the sand as it sublimates; the carbon dioxide gas released would speed up the flow.<ref>C.M. Dundas, S. Diniega, A.S. McEwen.  2014.  Long-term monitoring of martian gully formation and evolution with MRO/HiRISE.  Icarus, 251.  pp. 244-263, 10.1016/j.icarus.2014.05.013</ref> <ref>C.M. Dundas, A.S. McEwen, S. Diniega, C.J. Hansen, S. Byrne, J.N. McElwaine.  2017.  The formation of gullies on Mars today.  Geol. Soc. Lond. Spec. Publ., 46710.1144/SP467.5</ref>  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.  Each year there were changes. The changes were in the length of the gully, the curviness of the gully, and changes in the volume both the alcove and the apron.  The alcove lost material, while the apron gained.  In just one Martian year, the apron went from a length of 800 meters to a length of nearly 940 meters.  Moreover, the changes happened just as the dry ice began to sublimate.<ref>https://www-sciencedirect-com.wikipedialibrary.idm.oclc.org/science/article/pii/S0019103518304627</ref> <ref>Pasquon, K., et al.  2019.  Present-day development of gully-channel sinuosity by carbon dioxide gas supported flows on Mars.  
Icarus.  Volume 329.  Pages 296-313</ref>

However, one wonders if these processes could account for the formation of all the gullies.  Maybe, liquid water was sometimes necessary, especially to move large boulders.  A study of over 700 sites, published in 2022 in Icarus, concluded that liquid water would not have been needed.  During the duration of the study many large boulders were moved—one being 5 meters across.  Many types of changes were seen in gullies.  Some channels were extended, new channels were formed, and other channels were filled with new debris.<ref> Dundas, C., et al.  2022.  Martian gully activity and the gully sediment transport system.  Icarus.  (in press) </ref> <ref> https://www-sciencedirect-com.wikipedialibrary.idm.oclc.org/science/article/pii/S0019103522002408</ref> <ref>Dundas, et al.  2023.   Martian gully activity and the gully sediment transport system.  Icarus.  Volume 386, 1 November 2022, 115133</ref> <ref>https://www.sciencedirect.com/science/article/pii/S0019103522002408#bb0145</ref> 
Perhaps, some water was involved in the past, but all the gullies seen today could have been made without water.

==References==

{{Reflist|colwidth=30em}}

==See also==


*[[High Resolution Imaging Science Experiment (HiRISE)]]
*[[Martian features that are signs of water ice]]
*[[Sublimation]]
*[[Sublimation landscapes on Mars]]
*[[Water]]
*[[What Mars Actually Looks Like!]]

==External links==
* [[https://www.youtube.com/watch?v=mNXBfz1iVzc]] Video demonstrates how dry ice can form gullies on dunes
*[[https://www.youtube.com/watch?v=B1UU8XSMHmM Pictures of gullies on dunes]]
*[[https://www.youtube.com/watch?v=jZpJqlzCRpw Demonstration of dry ice moving down dune ]]
*https://www.hou.usra.edu/meetings/lpsc2022/pdf/1928.pdf  Map of gullies on Mars