Gullies on Mars

{{wide image|ESP 039621 1315gullies.jpg|600px|align-cap=center|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. Location is Phaethontis quadrangle.}}

File:Gully in Phaethontis.jpg-like ridges at their downslope ends, which suggests they have formed at the site of a now-absent flowing ice. Note that they are cut into a mantle, which is much smoother than the rough-textured underlying material. Image taken by Mars Global Surveyor.]]

After being discovered, many hypotheses were put forward to explain the gullies.{{Cite web|url=http://www.psrd.hawaii.edu/Aug03/MartianGullies.html|title = PSRD: Gullied Slopes on Mars}} 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. Calculations showed that the pressure and temperatures were not right 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.{{cite journal|last1=Heldmann|first1=J|title=Observations of martian gullies and constraints on potential formation mechanisms|journal=Icarus|volume=168|issue=2|pages=285–304|date=2004|doi=10.1016/j.icarus.2003.11.024|bibcode=2004Icar..168..285H|url=https://zenodo.org/record/1259029}} Eventually, most hypotheses were narrowed to involve 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.Forget, F. et al. 2006. Planet Mars Story of Another World. Praxis Publishing. Chichester, UK.

Close-up images with HiRISE showed details that support the idea that a fluid was involved. Images show that channels were formed a number of 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 teadrop-shaped islands were common in some channels.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. The following group of pictures of gullies illustrate some of the shapes that lead researchers to think that water was involved in creating at least some of the gullies.

ESP 037506 2285gullychannels.jpg|Gullies on wall of crater, as seen by HiRISE under HiWish program Location is the Mare Acidalium quadrangle.

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. Picture was taken with HiRISE under HiWish program. Location is the Mare Acidalium quadrangle. Note this is an enlargement of a previous image.

ESP 039753 1385gulliespits.jpg|Gullies in crater in Phaethontis quadrangle, as seen by HiRISE under HiWish program

ESP 039753 1385channeldetail.jpg|Close-up of channels in gullies showing that the paths of the channels changed over time. This feature suggest formation by running water with a high load of sediment. Picture was taken with HiRISE under HiWish program. Location is the Mare Acidalium quadrangle. Note this is an enlargement of a previous image in Phaethontis quadrangle.

ESP 039793 1385gullies.jpg|Gullies in crater, as seen by HiRISE under HiWish program. Location is Eridania quadrangle.

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. Note: this is an enlargement of the previous image by HiRISE under HiWish program. Location is Eridania quadrangle.

However, more studies open up other possibilities; a study released in October 2010, proposes that some gullies, the ones on sand dunes, may be produced by a buildup of solid carbon dioxide during cold winter months.[https://www.sciencedaily.com/releases/2010/10/101029135505.htm NASA/Jet Propulsion Laboratory. "Study links fresh Mars gullies to carbon dioxide." ScienceDaily 30 October 2010. 10 March 2011]{{cite journal|last1=Diniega|first1=S.|last2=Byrne|first2=S.|last3=Bridges|first3=N. T.|last4=Dundas|first4=C. M.|last5=McEwen|first5=A. S.|title=Seasonality of present-day Martian dune-gully activity|journal=Geology|volume=38|issue=11|pages=1047–1050|date=2010| doi=10.1130/G31287.1 |bibcode=2010Geo....38.1047D}}

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.

The exact cause/causes of these gullies is still under debate. A study supported formation by the melting of ground ice or snowpack as the chief cause. Over 54,000 CTX images were examined which covered about 85% of the surface of the planet.Harrison, T., G. Osinski, L. Tornabene, E. Jones. 2015. Global documentation of gullies with the Mars Reconnaissance Orbiter Context Camera and implications for their formation. Icarus: 252, 236–254.

Most of the gully alcove heads occur at the same level, just as one would expect if water came out of an aquifer. 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 layers that allow water to flow. They may consist of porous sandstone. 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.[http://www.space.com/scienceastronomy/mars_aquifer_041112.html Mars Gullies Likely Formed By Underground Aquifers. Leonard David, 12 November 2004 (Space.com)] Aquifers are quite common on Earth. A good example is "Weeping Rock" in Zion National Park Utah.Harris, A and E. Tuttle. 1990. Geology of National Parks. Kendall/Hunt Publishing Company. Dubuque, Iowa 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. 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.Foget, F. et al. 2006. Planet Mars Story of Another World. Praxis Publishing. Chichester, UK Examples of gullies coming from different levels is shown below in the image of Lohse Crater and the image of gullies in Ross Crater.

Image: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.

Image: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.

Image:Context for Gullies in Ross crater.jpg|CTX image of part of Ross Crater showing context for next image from HiRISE.

Image:Gullies in Ross Crater.JPG|Gullies in Ross Crater, as seen by HiRISE under the HiWish program. Because the gullies are on the narrow rim of a crater and they start at different heights, this example is not consistent with the model of gullies being caused by aquifers.

ESP 039621 1315gullies2levels.jpg|Gullies in two levels of a crater wall, as seen by HiRISE under HiWish program. Gullies at two levels suggests they were not made with an aquifer, as was first suggested. Location is Phaethontis quadrangle.

Image:Lohse Crater.JPG|Lohse Crater Gullies on Central Peak. Image located in Argyre quadrangle. Having gullies on a central peak runs counter to the idea that they were formed by an aquifer, as was first suggested.

Much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust.{{cite journal|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|issue=E10|pages=23429–23570|date=2001|doi=10.1029/2000JE001455|bibcode=2001JGR...10623429M |s2cid=129376333|doi-access=free}}{{cite journal|pmid=11473309|author1-link=John F. Mustard|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|doi=10.1038/35086515|bibcode=2001Natur.412..411M|s2cid=4409161|access-date=2011-01-18|archive-date=2016-06-10|archive-url=https://web.archive.org/web/20160610025332/http://www.planetary.brown.edu/pdfs/2610.pdf|url-status=dead}}{{cite journal|last1=Carr|first1=Michael H.|title=Mars Global Surveyor observations of Martian fretted terrain|journal=Journal of Geophysical Research|volume=106|issue=E10|pages=23571–23595|date=2001|doi=10.1029/2000JE001316|bibcode=2001JGR...10623571C|s2cid=129715420 }} 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 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.[https://web.archive.org/web/20131224133733/http://www.nbcnews.com/id/15702457 Martian gullies could be scientific gold mines. Leonard David, 11/13/2006.]{{cite journal|pmid=18725636|date=2008|last1=Head|first1=JW|last2=Marchant|first2=DR|last3=Kreslavsky|first3=MA|title=Formation of gullies on Mars: Link to recent climate history and insolation microenvironments implicate surface water flow origin|volume=105|issue=36|pages=13258–63|doi=10.1073/pnas.0803760105|pmc=2734344|journal=PNAS|bibcode=2008PNAS..10513258H |doi-access=free}} Calculations show that a third of a mm of runoff can be produced each day for 50 days of each Martian year even under current conditions.{{cite journal|last1=Clow|first1=G|title=Generation of liquid water on Mars through the melting of a dusty snowpack|journal=Icarus|volume=72|issue=1|pages=93–127|date=1987|bibcode=1987Icar...72...95C|doi=10.1016/0019-1035(87)90123-0 }}{{Cite journal|last=Christensen|first=Philip R.|date=March 2003|title=Formation of recent martian gullies through melting of extensive water-rich snow deposits|url=https://www.nature.com/articles/nature01436|journal=Nature|language=en|volume=422|issue=6927|pages=45–48|doi=10.1038/nature01436|pmid=12594459|bibcode=2003Natur.422...45C|s2cid=4385806|issn=1476-4687}} Because there are few craters on this mantle, the mantle is thought to be 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.[https://web.archive.org/web/20080504022016/http://news.nationalgeographic.com/news/2008/03/080319-mars-gullies_2.html Melting Snow Created Mars Gullies, Expert Says] 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 generously with dust. The atmosphere of Mars contains a great deal of fine dust particles. Water vapor will condense on the particles, then fall down to the ground due to the additional weight of the water coating. When Mars is at its greatest tilt or obliquity, up to 2 cm of ice could be removed from the summer ice cap and be deposited at midlatitudes. This movement of water could last for several thousand years and create a snow layer of up to around 10 meters thick.{{cite journal|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|bibcode=1985Natur.315..559J|date=1985|doi=10.1038/315559a0|issue=6020|s2cid=4312172|url=https://zenodo.org/record/1233025}}{{cite journal|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|issue=E1|pages=1579–1584|bibcode=1995JGR...100.1579J|date=1995|doi=10.1029/94JE02801}} When ice at the top of the mantling layer goes back into the atmosphere, it leaves behind dust, which insulates the remaining ice.{{cite news|author=MLA NASA/Jet Propulsion Laboratory|date=December 18, 2003|title=Mars May Be Emerging From An Ice Age|work=ScienceDaily|access-date=February 19, 2009|url=https://www.sciencedaily.com/releases/2003/12/031218075443.htm }}

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.{{cite journal|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|issue=2|journal=Icarus|bibcode=2007Icar..188..315D|access-date=2010-10-15|archive-date=2017-07-06|archive-url=https://web.archive.org/web/20170706040142/http://planetary.brown.edu/pdfs/3138.pdf|url-status=dead}}

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.{{cite journal|pages=26695–26712|date=2000|doi=10.1029/2000JE001259|last1=Kreslavsky|volume=105|issue=E11|bibcode=2000JGR...10526695K|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|access-date=2011-01-18|archive-date=2021-08-31|archive-url=https://web.archive.org/web/20210831002212/http://www.planetary.brown.edu/pdfs/2447.pdf|url-status=dead}}{{cite journal|last1=Hecht|first1=M|title=Metastability of liquid water on Mars|pages=373–386|date=2002|volume=156|issue=2|doi=10.1006/icar.2001.6794|journal=Icarus|url=http://www.geo.brown.edu/geocourses/geo292/papers/Hecht2002.pdf|bibcode=2002Icar..156..373H}}{{dead link|date=October 2017 |bot=InternetArchiveBot |fix-attempted=yes }} A large study done with several years worth of data from Mars Global Surveyor showed that there is a tendency for gullies to be on poleward facing slopes; these slopes have more shade that would keep snow from melting and allow large snowpacks to accumulate.

In general, 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 areas—pole facing, steep slopes. At a certain time of the year, sunlight will melt snow with the resulting water producing gullies.

Direct evidence for these snowpacks was recently discovered for the first time, showing that this mantle is indeed composed of <~1% dust and ice {{Cite journal|last1=Khuller|first1=A. R.|last2=Christensen|first2=P. R.|title=Evidence of Exposed Dusty Water Ice within Martian Gullies|journal=Journal of Geophysical Research: Planets|year=2021|volume=126|issue=2|language=en|pages=e2020JE006539|doi=10.1029/2020JE006539|bibcode=2021JGRE..12606539R|issn=2169-9100|doi-access=|s2cid=234174382 }} 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.

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.{{cite journal|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=fr }} 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 to lead to erosion.{{cite journal|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|doi=10.1016/j.geomorph.2014.12.007|bibcode=2015Geomo.231..101J|url=https://hal.archives-ouvertes.fr/hal-01483501/file/JouannicGeomorpholgy_HAL.pdf }}{{cite journal|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|pages=1534|bibcode=2001LPI....32.1534C|url=http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1534.pdf}}{{cite web |url=http://www.spaceref.com/16090/news/viewpr.html?pid=7124 |title=Sorry - You Seem to Have Lost Your Way - SpaceRef |access-date=2011-03-10 |url-status=dead |archive-url=https://archive.today/20120910131532/http://www.spaceref.com/16090/news/viewpr.html?pid=7124 |archive-date=2012-09-10 }} However, it is likely that ice deposited in the pores of soil in the ground will diffuse back into the atmosphere rather than melt.{{Cite journal|last=Christensen|first=P. R.|date=2006-06-01|title=Water at the Poles and in Permafrost Regions of Mars|url=http://dx.doi.org/10.2113/gselements.2.3.151|journal=Elements|volume=2|issue=3|pages=151–155|doi=10.2113/gselements.2.3.151|bibcode=2006Eleme...2..151C |issn=1811-5209}} Similar pore ice diffusion was also observed in-situ at the Phoenix landing site {{Cite journal|last1=Smith|first1=P. H.|last2=Tamppari|first2=L. K.|last3=Arvidson|first3=R. E.|last4=Bass|first4=D.|last5=Blaney|first5=D.|author5-link= Diana Blaney |last6=Boynton|first6=W. V.|last7=Carswell|first7=A.|last8=Catling|first8=D. C.|last9=Clark|first9=B. C.|last10=Duck|first10=T.|last11=DeJong|first11=E.|date=2009-07-03|title=H 2 O at the Phoenix Landing Site|url=https://www.science.org/doi/10.1126/science.1172339|journal=Science|language=en|volume=325|issue=5936|pages=58–61|doi=10.1126/science.1172339|pmid=19574383|bibcode=2009Sci...325...58S|s2cid=206519214|issn=0036-8075}}

In support for ground ice being involved, a group of researchers found that gullies are preferentially distributed on areas with some ground ice rather than no ice at all. A large data set of gullies was used in this study.Noblet, A., et al. 2024. A global map of gullied hillslopes on Mars. Icarus. Volume 418. 116147

As soon as gullies were discovered, researchers began to image many gullies over and over, looking for possible changes.{{cite web | url=https://www.uahirise.org/hipod/ESP_032011_1425 | title=HiRISE | HiPOD: 15 Oct 2023 }} By 2006, some changes were found.{{cite journal | 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 | journal = Science | volume = 314 | issue = 5805| pages = 1573–1577 | doi=10.1126/science.1135156 | pmid=17158321| bibcode = 2006Sci...314.1573M | s2cid = 39225477 }} Later, with further analysis it was determined that the changes could have occurred by dry granular flows rather than being driven by flowing water.{{cite journal | last1 = Kolb | display-authors = etal | year = 2010 | title = Investigating gully flow emplacement mechanisms using apex slopes | doi = 10.1016/j.icarus.2010.01.007 | journal = Icarus | volume = 208 | issue = 1| pages = 132–142 | bibcode=2010Icar..208..132K}}{{cite journal | last1 = McEwen | first1 = A. | display-authors = etal | year = 2007 | title = A closer look at water-related geological activity on Mars | journal = Science | volume = 317 | issue = 5845| pages = 1706–1708 | bibcode = 2007Sci...317.1706M| doi = 10.1126/science.1143987 | pmid = 17885125| s2cid = 44822691 }}{{cite journal | last1 = Pelletier | first1 = J. | display-authors = etal | year = 2008 | title = Recent bright gully deposits on Mars wet or dry flow? | journal = Geology | volume = 36 | issue = 3| pages = 211–214 | doi=10.1130/g24346a.1| bibcode = 2008Geo....36..211P}} With continued observations many more changes were found in Gasa Crater and others.{{cite news |title=NASA orbiter finds new gully channel on Mars |url=https://www.sciencedaily.com/releases/2014/03/140322094409.htm |access-date=1 August 2021 |work=ScienceDaily |date=22 March 2014}} 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. So, although today there is little liquid water, present geological/climatic processes could still form gullies.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 Large amounts of water or great changes in climate are not needed. {{Cite journal |last1=Dundas |first1=Colin M. |last2=Conway |first2=Susan J. |last3=Cushing |first3=Glen E. |date=2022 |title=Martian gully activity and the gully sediment transport system |journal=Icarus |volume=386 |pages=115133 |doi=10.1016/j.icarus.2022.115133|doi-access=free |bibcode=2022Icar..38615133D }} However, some gullies in the past may have been aided by weather changes that involved larger amounts of water, perhaps from melted snow.Dundas, C., S. Diniega, C. Hansen, S. Byrne, A. McEwen. 2012. Seasonal activity and morphological changes in martian gullies. Icarus, 220. 124–143. With more repeated observations, more and more changes have been found; since the changes occur in the winter and spring, experts are tending to suspect that gullies were formed from carbon dioxide ice (dry ice). Recent studies describe using the High Resolution Imaging Science Experiment (HiRISE) camera on MRO to examine 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. When dry ice frost changes to a gas, it may lubricate dry material to flow especially on steep slopes.{{cite web |title=NASA Spacecraft Observes Further Evidence of Dry Ice Gullies on Mars |url=http://www.jpl.nasa.gov/news/news.php?release=2014-226 |website=Jet Propulsion Laboratory NASA |archive-url=https://web.archive.org/web/20141108221036/http://www.jpl.nasa.gov/news/news.php?release=2014-226 |archive-date=8 November 2014 |date=10 July 2014}}{{cite web |last1=Dundas |first1=Colin |title=Activity in Martian Gullies (ESP_032078_1420) |url=https://hirise.lpl.arizona.edu/ESP_032078_1420 |website=HiRISE |access-date=1 August 2021 |date=10 July 2014}}{{cite web |last1=Taylor Redd |first1=Nola |title=Gullies on Mars carved by dry ice, not water |url=https://www.space.com/26534-mars-gullies-dry-ice.html |website=Space.com |access-date=1 August 2021 |date=16 July 2014}} 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.{{cite web |last1=Cowing |first1=Keith |title=Frosty gullies on Mars |url=http://spaceref.com/mars/frosty-gullies-on-mars.html |archive-url=https://archive.today/20140815150036/http://spaceref.com/mars/frosty-gullies-on-mars.html |url-status=dead |archive-date=August 15, 2014 |website=SpaceRef |access-date=1 August 2021 |date=14 August 2014 }}

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.Dundas, C., S. Diniega, A. McEwen. 2015. Long-term monitoring of martian gully formation and evolution with MRO/HiRISE. Icarus: 251, 244–263Fergason, R., C. Dundas, R. Anderson. 2015. IN-DEPTH REGIONAL ASSESSMENT OF THERMOPHYSICAL PROPERTIES OF ACTIVE GULLIES ON MARS. 46th Lunar and Planetary Science Conference. 2009.pdf 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.Dundas, C. et al. 2016. HOW WET IS RECENT MARS? INSIGHTS FROM GULLIES AND RSL. 47th Lunar and Planetary Science Conference (2016) 2327.pdf. The timing of gully activity is seasonal and happens during the period when seasonal frost is present and defrosting.M.

Vincendon, M. 2015. Identification of Mars gully activity

types associated with ice composition. JGR:120, 1859–1879.

These observations support a model in which currently active gully formation is driven mainly by seasonal CO₂ frost.{{cite journal | last1 = Raack | first1 = J. | display-authors = etal | year = 2015 | title = Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars| journal = Icarus | volume = 251 | pages = 226–243 | doi=10.1016/j.icarus.2014.03.040 | bibcode=2015Icar..251..226R| s2cid = 120751790 }} Simulations described in a 2015 conference, show that high pressure CO₂ gas trapping in the subsurface can cause debris flows.{{Cite web|url=http://www.uahirise.org/ESP_044327_1375|title=HiRISE | Frosted Gully Slopes in Shadows (ESP_044327_1375)}} The conditions that can lead to this are found in latitudes where gullies occur.C. Pilorget, C., F. Forget. 2015. "CO2 Driven Formation of Gullies on Mars." 46th Lunar and Planetary Science Conference. 2471.pdf This research was described in a later article entitled, "Formation of gullies on Mars by debris flows triggered by CO2 sublimation."{{cite journal | last1 = Pilorget | first1 = C. | last2 = Forget | first2 = F. | year = 2016 | title = Formation of gullies on Mars by debris flows triggered by CO₂ sublimation | url = https://authors.library.caltech.edu/63704/2/ngeo2619-s1.pdf| journal = Nature Geoscience | volume = 9 | issue = 1| pages = 65–69 | doi = 10.1038/ngeo2619 | bibcode = 2016NatGe...9...65P }} 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.{{cite web |title=Gullies on Mars sculpted by dry ice rather than liquid water |url=https://www.sciencedaily.com/releases/2015/12/151222082255.htm |website=ScienceDaily |access-date=1 August 2021 |date=22 December 2015}}

Observations of gullies that are on sand dunes support the idea that current gully changes may be caused by dry ice.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.2014.05.013C.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 Some gullies on sand dunes have even been observed to change noticeably in just one year. Dry ice, or solid carbon dioxide, accumulates in the cold winter and then when it is starting to warm up, changes appear in gullies. 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. 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.{{cite journal | url=https://www.sciencedirect.com/science/article/pii/S0019103518304627 | doi=10.1016/j.icarus.2019.03.034 | title=Present-day development of gully-channel sinuosity by carbon dioxide gas supported flows on Mars | date=2019 | last1=Pasquon | first1=Kelly | last2=Gargani | first2=Julien | last3=Massé | first3=Marion | last4=Vincendon | first4=Mathieu | last5=Conway | first5=Susan J. | last6=Séjourné | first6=Antoine | last7=Jomelli | first7=Vincent | last8=Balme | first8=Matthew R. | last9=Lopez | first9=Simon | last10=Guimpier | first10=Anthony | journal=Icarus | volume=329 | pages=296–313 | bibcode=2019Icar..329..296P }}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

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.

The major issue with the CO₂ frost model is trying to explain the erosion of rocks. 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.{{Cite journal|last1=Khuller|first1=A. R.|last2=Christensen|first2=P. R.|last3=Harrison|first3=T. N.|last4=Diniega|first4=S.|title=The Distribution of Frosts on Mars: Links to Present-Day Gully Activity|url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020JE006577|journal=Journal of Geophysical Research: Planets|year=2021|volume=126|issue=3|language=en|pages=e2020JE006577|doi=10.1029/2020JE006577|bibcode=2021JGRE..12606577K|s2cid=233906372|issn=2169-9100}} Instead, CO₂ frost might only be able to modify pre-existing 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. This research adds evidence that liquid water was not involved with gully formation.{{cite journal|doi=10.1002/2016GL068956 | volume=43 | issue=17 | title=New insights into gully formation on Mars: Constraints from composition as seen by MRO/CRISM | journal=Geophysical Research Letters | pages=8893–8902 | last1 = Núñez | first1 = J. I.| bibcode=2016GeoRL..43.8893N | year=2016 | s2cid=133329690 }}{{cite web |title=Today's Gullies On Mars Are Probably Not Formed by Liquid Water |url=http://spaceref.com/mars/todays-gullies-on-mars-are-probably-not-formed-by-liquid-water.html |website=SpaceRef |access-date=1 August 2021 |date=29 July 2016 }}{{Dead link|date=November 2023 |bot=InternetArchiveBot |fix-attempted=yes }} 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.

Some researchers believe that gully formation may involve both dry ice and liquid water, especially in the past.M.

Vincendon (2015) JGR, 120, 1859–1879.Dundas, C. 2016. Nat. Geosci, 9, 10–11S. J. Conway, J., et al. 2016. MARTIAN GULLY ORIENTATION AND SLOPE USED TO TEST MELTWATER AND CARBON

DIOXIDE HYPOTHESES. 47th Lunar and Planetary Science Conference (2016). 1973.pdf

It is estimated that a few million years ago, the tilt of the axis of Mars was 45 degrees instead of its present 25 degrees.{{cite book| author=Hugh H. Kieffer| title=Mars| url=https://books.google.com/books?id=NoDvAAAAMAAJ| access-date=7 March 2011|date=1992|publisher=University of Arizona Press |isbn=978-0-8165-1257-7}} Its tilt, also called obliquity, varies greatly because its two tiny moons cannot stabilize it, like our relatively large moon does to the Earth.{{cite journal| 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| bibcode=1995JGR...100.1579J| date=1995| issue=E1| doi=10.1029/94JE02801 }} During such periods of high tilt, the summer rays of the sun strike the mid-latitude crater surfaces straight on, thus the surface remains dry.

Image:Marssummersun.jpg|The straight on rays of the sun prevent snow from accumulating in mid-latitude craters when the tilt of Mars is high.

Note that at high tilt, the ice caps at the poles disappear, the atmosphere thickness, and the moisture in the atmosphere goes up. These conditions cause snow and frost to appear on the surface. However, any snow that falls at night and during the cooler parts of the day disappears when the day warms.

Things are quite different as fall approaches, for the pole-facing slopes remain in the shade all day. Shade causes snow to accumulate through the fall and winter seasons.

Image:Mars fallsunt.jpg|Shade on the pole-facing wall of a mid-latitude crater promotes snow accumulation. Note the snow will be grey to black due to dust.

Image:Winterhightiltmars.jpg|By winter a large mass of snow has accumulated in the pole-facing pole of a crater. As the seasons warm, this snow deposit will melt to produce gullies.

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.

There may be sufficient water to produce gullies by erosion. Or, the water may soak into the ground, and later move down as a debris flow. Gullies on Earth formed by this process resemble Martian gullies.

The great changes in the tilt of Mars explain both the strong relationship of gullies to certain latitude bands and the fact that the vast majority of gullies exist on shady, pole-facing slopes. 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.

Research published in January 2015 suggests that these seasonal changes could have happened within the last two million years (between 400,000 and two million years ago) creating conditions suitable for formation of the gullies through ice melt.{{cite web|last1=Source: Brown University|title=Gully patterns document Martian climate cycles|url=http://www.astrobio.net/topic/solar-system/mars/gully-patterns-document-martian-climate-cycles/|website=Astrobiology Magazine |publisher=NASA|date=Jan 29, 2015 |archive-url=https://web.archive.org/web/20201027004908/https://www.astrobio.net/also-in-news/gully-patterns-document-martian-climate-cycles/ |archive-date=2020-10-27 |url-status=usurped}}{{cite journal|last1=Dickson|first1=James L.|last2=Head|first2=James W.|last3=Goudge|first3=Timothy A.|last4=Barbieri|first4=Lindsay|title=Recent climate cycles on Mars: Stratigraphic relationships between multiple generations of gullies and the latitude dependent mantle|journal=Icarus|volume=252|year=2015|pages=83–94|issn=0019-1035|doi=10.1016/j.icarus.2014.12.035|bibcode=2015Icar..252...83D}}

Today, we have noticed small changes in gullies even though it is not possible for liquid water to exist. However, in the past perhaps water was involved. 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. It has done that many times; the last time just 630,000 years ago.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{{cite journal | url=https://www.science.org/doi/10.1126/science.abk2464 | doi=10.1126/science.abk2464 | title=Gullies on Mars could have formed by melting of water ice during periods of high obliquity | year=2023 | last1=Dickson | first1=J. L. | last2=Palumbo | first2=A. M. | last3=Head | first3=J. W. | last4=Kerber | first4=L. | last5=Fassett | first5=C. I. | last6=Kreslavsky | first6=M. A. | journal=Science | volume=380 | issue=6652 | pages=1363–1367 | pmid=37384686 | s2cid=259287608 | doi-access= | bibcode=2023Sci...380.1363D }}

File:Evidence of recent cyclical changes of climate on gully formation on Mars.jpg

{{clear}}

Some steep slopes show other features in addition to gullies. At the base of some gullies there may be curved ridges or depressions. These have been named "spatulate depressions." Along walls, like crater walls, ice often accumulates during certain phases of the Martian climate cycle. When the climate changes, this ice can sublimate into the thin Martian atmosphere. Sublimation is when a substance goes directly from a solid state to a gas state. Dry ice on Earth does this. So when the ice at the base of a steep wall sublimates, a spatulate depression results. Also, more ice from higher up the wall will tend to flow downward. This flow will stretch the surface rocky debris thereby forming transverse crevasses. Such formations have been termed "washboard terrain" because they resemble the old fashioned washboards.Jawin, E, J. Head, D. Marchant. 2018. Transient post-glacial processes on Mars: Geomorphologic evidence for a paraglacial period. Icarus: 309, 187-206 The parts of gullies and some associated features of gullies are shown below in a HiRISE images.

PSP 001842 1395gulliesglacierscracks.jpg|Wide view of crater showing gullies and other features, as seen by HiRISE

File:1842 1395depressions.jpg|Close view of crater labeled with "spatulate depression" and other features, as seen by HiRISE Note: this is an enlargement of the previous image.jawin, E, J. Head, D. Marchant. 2018. Transient post-glacial processes on Mars: Geomorphologic evidence for a paraglacial period. Icarus: 309, 187-206

File:1842 1395washboard.jpg|Close view of crater labeled with "washboard terrain" and other features, as seen by HiRISE Note: this is an enlargement of a previous image. The washboard terrain was formed before the gully apron since the gully apron cuts across the washboard terrain.

ESP 039753 1385gulliespits.jpg|Gullies in crater in Phaethontis quadrangle, as seen by HiRISE under HiWish program Spatulate depressions are visible.

File:ESP 055056 1420gulliesridges.jpg|Gullies in Noachis quadrangle, as seen by HiRISE under HiWish program Spatulate depressions are visible.

ESP 039753 1385channeldetail.jpg|Close-up of channels in gullies showing that the paths of the channels changed over time. This feature suggest formation by running water with a high load of sediment. Picture was taken with HiRISE under HiWish program. Location is the Mare Acidalium quadrangle. Note this is an enlargement of a previous image in Phaethontis quadrangle.

;Phaethontis quadrangle gullies

The Phaethontis quadrangle is the location of many gullies that may be due to recent flowing water. Some are found in the Gorgonum Chaos[http://hirise.lpl.arizona.edu/PSP_004071_1425 Gorgonum Chaos Mesas (HiRISE Image ID: PSP_004071_1425][http://hirise.lpl.arizona.edu/PSP_001948_1425 Gullies on Gorgonum Chaos Mesas (HiRISE Image ID: PSP_001948_1425)] and in many craters near the large craters Copernicus and Newton (Martian crater).[http://hirise.lpl.arizona.edu/PSP_004163_1375 Gullies in Newton Crater (HiRISE Image ID: PSP_004163_1375)]U.S. department of the Interior U.S. Geological Survey, Topographic Map of the Eastern Region of Mars M 15M 0/270 2AT, 1991

Image:Gully in Phaethontis.jpg|Group of gullies on north wall of crater that lies west of the crater Newton (41.3047 degrees south latitude, 192.89 east longitide). Image taken with Mars Global Surveyor under the MOC Public Targeting Program.

Image:Atlantis Chaos.JPG|Atlantis Chaos, as seen by HiRISE. Click on image to see mantle covering and possible gullies. The two images are different parts of the original image. They have different scales.

Image:Close up view of gullies.jpg|Gullies. Notice how channels curve around obstacles, as seen by HiRISE.

Image:Context image for gullies in crater and trough.JPG|MOLA context image for the series of three images to follow of gullies in a trough and nearby crater.

Image:Gullies in trough and crater.jpg|Gullies in a trough and nearby crater, as seen by HiRISE under the HiWish program. Scale bar is 500 meters long.

Image:Gullies in crater under HiWish.JPG|Close-up of gullies in crater, as seen by HiRISE under the HiWish program.

Image:Gullies in trough.JPG|Close-up of gullies in trough, as seen by HiRISE under the HiWish program. These are some of the smaller gullies visible on Mars.

Image:ESP 020012gulliescropped.jpg|Gullies near Newton Crater, as seen by HiRISE, under the HiWish program. Place where there was an old glacier is labeled.

Image:20803 gullies.jpg|HiRISE image, taken under HiWish program, of gullies in a crater in Terra Sirenum.

Image:20803gullies with glacier remains.jpg|Gullies with remains of a former glacier in crater in Terra Sirenum, as seen by HiRISE under HiWish program.

Image:Gullies near Newton Crater.jpg|Gullies near Newton Crater, as seen by HiRISE under the HiWish Program.

Image:Gullies in Terra Sirenum.jpg|Gullies in a crater in Terra Sirenum, as seen by HiRISE under the HiWish Program.

Image:21845gulliespatt.jpg|Close-up of gully showing multiple channels and patterned ground, as seen by HiRISE under the HiWish program.

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

;Eridania quadrangle gullies

File:ESP 082970 1465gullieswide 01.jpg|Wide view of gullies in a crater, as seen by HiRISE under HiWish program The black strip is where data were not gathered. This image was named HiRISE Picture of the Day for June 25, 2024.

File:ESP 082970 1465gullieswide 02.jpg|Gullies in a crater, as seen by HiRISE under HiWish program This image was named HiRISE Picture of the Day for June 25, 2024.

File:ESP 082970 1465gullies 03.jpg|Gully alcoves as seen by HiRISE under HiWish program This image was named HiRISE Picture of the Day for June 25, 2024.

File:ESP 082970 1465gullyaprons 04.jpg|Gully aprons as seen by HiRISE under HiWish program This image was named HiRISE Picture of the Day for June 25, 2024.

File:ESP 082970 1465gullies 05.jpg|Gully alcoves as seen by HiRISE under HiWish program This image was named HiRISE Picture of the Day for June 25, 2024.

File:ESP 082970 1465gulliesclose 06.jpg|Gully alcove as seen by HiRISE under HiWish program This image was named HiRISE Picture of the Day for June 25, 2024.

Image:Gullies and tongue-shaped glacier.jpg|Gullies in a crater in Eridania, north of the large crater Kepler. Also, features that may be remains of old glaciers are present. One, to the right, has the shape of a tongue. Image taken with Mars Global Surveyor under the MOC Public Targeting Program.

Image:ESP 020330gulliesandmantlelayers.jpg|HiRISE image showing gullies. The scale bar is 500 meters. Picture taken under the HiWish program.

Image:24325mantleandgullies.jpg|Gullies and layers in mantle on a wall, as seen by HiRISE under HiWish program. Location is Eridania quadrangle.

Image:25090gullies.jpg|Gullies, as seen by HiRISE under HiWish program.

Image:2509gulliesclosenew.jpg|Close-up of some gullies from previous image, as seen by HiRISE under HiWish program.

Image:2509gullyfan.jpg|Close-up of apron on one of the gullies from previous image. Image was taken by HiRISE, under the HiWish program

ESP 040993 1450gullies.jpg|Gullies on two different levels in crater, as seen by HiRISE under HiWish program

ESP 048364 1410gullieslayers.jpg|Crater with gullies, as seen by HiRISE under HiWish program

ESP 048062 1425gulliesridges.jpg|Crater with gullies, as seen by HiRISE under HiWish program

File:48364 1410 1kgullies 01.jpg|Gullies, as seen by HiRISE Picture is about 1 km across.

;Argyre quadrangle gullies

Image:Jezza Crater.JPG|Jezza Crater, as seen by HiRISE. North wall (at top) has gullies. Dark lines are dust devil tracks. Scale bar is 500 meters long.

Image:24808multilevelgullies.jpg|Scene in Argyre quadrangle with gullies, alluvival fans, and hollows, as seen by HiRISE under HiWish program. Enlargements of parts of this image are below.

Image:24808multiplefans.jpg|Several levels of alluvial fans, as seen by HiRISE under HiWish program. Locations of these fans are indicated in the previous image.

File:ESP 028896 1355gullies.jpg|Gullies in Nereidum Montes, as seen by HiRISE under HiWish program.

ESP 040974 1395gullies.jpg|Wide view of gullies in Arkhangelsky Crater, as seen by HiRISE under HiWish program

40974 1395gullies.jpg|Close-up of small channels in gullies in Arkhangelsky Crater, as seen by HiRISE under HiWish program Patterned ground in the shape of polygons can be seen to the right. Note: this is an enlargement of the previous image from Arkhangelsky Crater.

40974 1395gully.jpg|Close-up of a gully showing a channel going across the apron, as seen by HiRISE under HiWish program. Note: this is an enlargement of a previous image from Arkhangelsky Crater.

ESP 047395 1415gulliesridges.jpg|Gullies in crater, as seen by HiRISE under HiWish program

47395 1415gullycurvedchannels.jpg|Close view of gullies from previous image The channels are quite curved. Because channels of gullies often form curves, it was thought that they were made by flowing water. Today, it is thought that they could be produced with chunks of dry ice. The image is from HiRISE under HiWish program.

ESP 047528 1355gullies.jpg|Gullies on two sides of a mound, as seen by HiRISE under HiWish program

;Thaumasia quadrangle gullies

Image:Gulliesthaumasal.jpg|Group of gullies, as seen by HiRISE under the HiWish program.

Image:Multiple channels in 21461.jpg|Enlargement of part of previous image showing smaller gullies inside larger ones. Water probably flowed in these gullies more than once.

;Mare Acidalium quadrangle gullies

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.

Image:ESP 024951gulliesandflow.jpg|Gullies and massive flow of material, as seen by HiRISE under HiWish program. Gullies are enlarged in next two images. Location is Bamberg crater.

Image:24951gulliesclose.jpg|Close up view of some gullies, as seen by HiRISE under the HiWish program.

Image:24951gullyclose.jpg|Close up view of another gully in same HiRISE picture. Picture taken under HiWish program.

Image:26574gullies.jpg|Gullies, as seen by HiRISE under HiWish program.

File:ESP 027707 2195gullies.jpg|Gullies in a crater, as seen by HiRISE under HiWish program.

File:27707gulliesclose.jpg|Close-up of gullies in a crater from previous image. Image taken by HiRISE under HiWish program.

ESP 037506 2285gullychannels.jpg|Gullies on wall of crater, as seen by HiRISE under HiWish program Location is the Mare Acidalium quadrangle.

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. Picture was taken with HiRISE under HiWish program. Location is the Mare Acidalium quadrangle. Note this is an enlargement of a previous image.

;Arcadia quadrangle gullies

Image:ESP 025771variousgullies.jpg|A variety of gullies originating at different levels are visible in this HiRISE image that was taken under the HiWish program.

Image:25771gullybenches.jpg|This enlargement of a small part of the previous image shows terraces along a gully channel. The terraces were created when a new channel cut through the old surface. This means that the gully was not in a single event. Water must have flowed more than once in this location.

File:ESP 028290 2285gullies.jpg|Gullies in a crater. Some seem to be young, others are well developed. Picture was taken by HiRISE under the HiWish program.

ESP 044707 2285gullies.jpg|Gullies along mesa wall in North Tempe Terra, as seen by HiRISE under HiWish program

44707 2285apron.jpg|Close view of gully apron, as seen by HiRISE under HiWish program Note this is an enlargement of the previous image.

44707 2285alcove.jpg|Close view of gully alcove, as seen by HiRISE under HiWish program Note this is an enlargement of a previous image.

ESP 044852 2285gullies1.jpg|Gullies on wall of mesa, as seen by HiRISE under HiWish program

;Diacria quadrangle gullies

Image:26420gullieswide.jpg|Wide view of group of gullies, as seen by HiRISE under HiWish program. Note that part of this image is enlarged in the following image.

Image:26420gulliesclose.jpg|Close-up of gullies, as seen by HiRISE under HiWish program.

;Noachis quadrangle gullies

ESP 037793 1445noachisgullies.jpg|Gullies on the wall of a crater, as seen by HiRISE under HiWish program

Image:Close-up of Asimov Crater.JPG|Gullies on mound in Asimov Crater, as seen by HiRISE.

File:75313 1420gullies1k.jpg|Gullies in crater, as seen by HiRISE under HiWish program

ESP 048159 1310gullies.jpg|Wide view of gullies and ridges in crater, as seen by HiRISE under HiWish program

48159 1310gullychannels.jpg|Close view of gully channels, as seen by HiRISE under HiWish program Channels make some tight curves.

48159 1310gullychannelsclose.jpg|Close view of gully channels, as seen by HiRISE under HiWish program Arrows point to small channel within larger channels.

;Casius quadrangle gullies

ESP 041866 2290gulliescasius.jpg|Gullies in crater, as seen by HiRISE under HiWish program

File:Close view of gully.jpg|Close view of gully in crater, as seen by HiRISE

44410 2195glacier.jpg|Glacier and gullies, as seen by HiRISE under HiWish program Some researchers suggest that gullies come after glaciers. Location is Casius quadrangle.

;Ismenius Lacus quadrangle gullies

ESP 045561 2310gully.jpg|Wide view of a gully on a steep slope, as seen by HiRISE under HiWish program

45561 2310gullies.jpg|Closer view of previous image of a gully, as seen by HiRISE under HiWish program

45561 2310gulliesclose.jpg|Close view of channel in gully showing streamlined forms, as seen by HiRISE under HiWish program

Image:Lyot Mars Crater Gullies.jpg|Lyot Crater Gullies, as seen by HiRISE.

ESP 044122 2335gullies.jpg|Gullies in crater, as seen by HiRISE under HiWish program

ESP 045917 2220gulliespyramids.jpg|Gullies, as seen by HiRISE under HiWish program

45917 2220gulliesclose.jpg|Close view of gullies, as seen by HiRISE under HiWish program

45917 2220gulliespolygons.jpg|Close view of gullies, as seen by HiRISE under HiWish program

;Iapygia quadrangle gullies

ESP 047603 1510gullies.jpg|Gullies in crater, as seen by HiRISE under HiWish program

;Hellas quadrangle gullies

ESP 048881 1415gullies.jpg|Gullies in crater, as seen by HiRISE under HiWish program

48881 1415polygons.jpg|Close view of gullies in crater, as seen by HiRISE under HiWish program Polygons are visible in this close view.

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

File:ESP 084659 1355 gullies cropped 01.jpg|Wide view of gullies

File:ESP 084659 1355 gullies cropped 02.jpg|Close view of gully alcoves Picture is about 1 km across.

File:ESP 084659 1355 gullies cropped 03.jpg|Close view of gully alcoves Picture is about 1 km across.

File:ESP 084659 1355 gullies cropped 04.jpg|Close view of gully channels Picture is about 1 km across.

Gullies are found on some dunes. These are somewhat different than gullies in other places, like the walls of craters. Gullies on dunes seem to keep the same width for a long distance and often just end with a pit, instead of an apron. They are often just a few meters across with raised banks along the sides.{{Cite web|url=https://scitechdaily.com/linear-gullies-on-mars-caused-by-sliding-dry-ice/|title = Linear Gullies on Mars Caused by Sliding Dry-Ice|date = 12 June 2013}}Dundas, C., et al. 2012. Seasonal activity and morphological changes in martian gullies. Icarus: 220, 124-143. Many of these gullies are found on dunes in Russell (Martian crater). In the winter dry ice accumulates on the dunes and then in the spring dark spots appear and dark-toned streaks grow downhill. After the dry ice is gone, new channels are visible. These gullies may be caused by blocks of dry ice moving down the steep slope or perhaps from dry ice starts the sand moving.{{Cite web|url=https://www.uahirise.org/ESP_051770_1345|title = HiRISE | Squiggles in Hellas Planitia (ESP_051770_1345)}}McEwen, A., et al. 2017. Mars The Pristine Beauty of the Red Planet. University of Arizona Press. Tucson. In the thin atmosphere of mars, dry ice will expel carbon dioxide with vigor.{{Cite web|url=https://www.nasa.gov/home/hqnews/2013/jun/HQ_13-180_Mars_Dry_Ice_Gullies.html#.WXDOT4WcGUk|title=Marks on Martian Dunes May Reveal Tracks of Dry Ice Sleds|date=24 June 2013}}

ESP 020217 1255dunechannels.jpg|Wide view of dunes in Russell Crater, as seen by HiRISE Many narrow gullies are visible.

20217 1255dunechannelsclose.jpg|Close view of the end of gullies in Russell Crater, as seen by HiRISE Note: These type of gullies do not usually end with an apron. The location is Noachis quadrangle.

20217 1255dunechannelsclosetop.jpg|Close view of the end of gullies in Russell Crater, as seen by HiRISE

ESP 020217 1255dunesclosecolor.jpg|Close, color view of the end of gullies in Russell Crater, as seen by HiRISE

ESP 051770 1345dunegullies.jpg|Gullies on dunes, as seen by HiRISE

• {{annotated link|Argyre quadrangle}}
• {{annotated link|Climate of Mars}}
• {{annotated link|Eridania quadrangle}}
• {{annotated link|Geology of Mars}}
• {{annotated link|Phaethontis quadrangle}}
• {{annotated link|Seasonal flows on warm Martian slopes|Recurring slope lineae on Mars}}
• {{annotated link|Thaumasia quadrangle}}
• {{annotated link|Water on Mars}}

{{Reflist|30em}}

{{Commons category|Gullies on Mars}}

• [https://www.youtube.com/watch?v=mNXBfz1iVzc] Video demonstrates how dry ice can form gullies on dunes
• {{APOD |date=21 February 2003|title=Melting Snow and the Gullies of Mars}}
• {{APOD |date=23 June 2003|title=The Gullies of Mars}}
• {{APOD |date=17 June 2013|title=Dry Ice Sled Streaks on Mars}}
• [http://www.psrd.hawaii.edu/Aug03/MartianGullies.html] Gives a general review of many of the theories involving the origin of gullies.
• {{cite journal|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|issue=2|journal=Icarus|bibcode=2007Icar..188..315D|access-date=2010-10-15|archive-date=2017-07-06|archive-url=https://web.archive.org/web/20170706040142/http://planetary.brown.edu/pdfs/3138.pdf|url-status=dead}} Gives a good review of the history of the discovery of gullies.

{{Mars}}

{{Geography of Mars}}

Category:Geology of Mars