Image of the Mare Australe Quadrangle (MC-30). The region includes theSouth Polar ice cap. The central part is mainly a permanent residual ice cap surrounded by layered and troughed terrain which is, in turn, encircled by heavily cratered terrain.
Around the southern ice cap is a surface, called theDorsa Argentea Formation that may be an old ice-rich deposit. It contains a group of sinuous, branched ridges that resembles eskers that form when streams are under glaciers.[5] The formation often contains pits: two major locations are namedCavi Angusti andCavi Sisyphi. The pits have steep sides and an irregular shape. They are up to 50 km across and 1 km deep.[6]
The quadrangle also containsAngustus Labyrinthus, a formation of intersecting valley or ridges, nicknamed the "Inca City".[7] Researchers were surprised to see parts of the surface having a Swiss-cheese appearance. Also, some areas showed strange spider-shaped forms, which were determined to be caused by carbon dioxide gas blowing dust around at certain times of the year.
Some craters in Mare Australe show gullies.Martian gullies are small, incised networks of narrow channels and their associated downslopesediment deposits, found on the planet ofMars. They are named for their resemblance to terrestrialgullies. First discovered on images fromMars Global Surveyor, they occur on steep slopes, especially on the walls of craters. Usually, each gully has adendriticalcove at its head, afan-shapedapron at its base, and a single thread of incisedchannel linking the two, giving the whole gully an hourglass shape.[8] They are believed to be relatively young because they have few, if any craters. A subclass of gullies is also found cut into the faces of sand dunes which themselves considered to be quite young. 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 believed that the processes carving the gullies involve liquid water. However, this remains a topic of active research.As soon as gullies were discovered,[8] researchers began to image many gullies over and over, looking for possible changes. By 2006, some changes were found.[9] Later, with further analysis it was determined that the changes could have occurred by dry granular flows rather than being driven by flowing water.[10][11][12] With continued observations many more changes were found in Gasa Crater and others.[13]With more repeated observations, more and more changes have been found; since the changes occur in the winter and spring, experts are tending to believe that gullies were formed from dry ice. 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.[14][15][16] In some years frost, perhaps as thick as 1 meter.
South polar layered deposits (SPLD) are the permanent, underlying geological structure, primarily made of water ice with dust.[17] In contrast, the Mars ice cap is a seasonal layer of carbon dioxide (CO2) frost that forms on top of the SPLD in the winter. The SPLD is much thicker, contains layered deposits that have accumulated over millions of years, and includes buried CO2 ice. The ice cap is a thin, transient CO2 coating that sublimates in the summer.[18][19]
Research based on slight changes in the orbits of spacecraft around Mars over 16 years found that when one hemisphere experiences winter, approximately 3 trillion to 4 trillion tons of carbon dioxide freezes out of the atmosphere onto the northern and southern polar caps. This represents 12 to 16 percent of the mass of the entire Martian atmosphere. These observation support predictions from the Mars Global Reference Atmospheric Model—2010.[20][21]
Scientists reported in July 2018, the discovery of a lake of liquid water under the southern ice cap. The measurements were gathered with the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board the European Space Agency's orbiting Mars Express spacecraft. Radar reflections showed a bright spot in the ice layers that analysis later showed that it had to be a lake of liquid water. It is believed that the water remains liquid, even at the temperature of -68 degrees Celsius because there is likely much dissolved salt that lowers the freezing point. The lake is about 20 kilometers across and at least 10 centimeters deep It could contain 10 billion liters of liquid water.[22][23] There could very well be many small bodies of water under the ice cap; however, they are difficult to detect with MARSIS. Also, the raw date coverage needed for these detections is limited—only a few percent of the area has a full set of data.[23]
During the winter, much frost accumulates. It freezes out directly onto the surface of the permanent polar cap, which is made of water ice covered with layers of dust and sand. The deposit begins as a layer of dustyCO 2 frost. Over the winter, it recrystallizes and becomes denser. The dust and sand particles caught in the frost slowly sink. By the time temperatures rise in the spring, the frost layer has become a slab of semi-transparent ice about 3 feet thick, lying on a substrate of dark sand and dust. This dark material absorbs light and causes the ice to sublimate (turn directly into a gas) below the surface. Eventually much gas accumulates and becomes pressurized. When it finds a weak spot, the gas escapes and blows out the dust.[24] Speeds can reach 100 miles per hour.[25] Dark channels can sometimes be seen; they are called "spiders".[26][27][28][29][30][31] The surface appears covered with dark spots when this process is occurring.[25][32] The official name for spiders is "araneiforms".[33] These features can be seen in some of the pictures below.
Star burst Channels caused by escaping gas, as seen byHiRISE. Star burst channels, also called spiders, may be about 500 meters in diameter and 1 meter deep.
Spider on the second Martian day of spring, as seen by HiRISE
Wide view of plumes, as seen by HiRISE underHiWish program. Many of the plumes show spiders when enlarged.
Plumes and spiders, as seen by HiRISE under HiWish program
Wide view of crater that contains examples of spiders, as seen by HiRISE under HiWish program. Close views of some of the spider formations are shown in following images.
Close view of spider formations, as seen by HiRISE under HiWish program. The polygon shapes are channels caused by pressurized carbon dioxide going through cracks. During the winter season, a slab of dry ice forms on the ground surface. The surface contains cracks in the shape of polygons. During certain times, CO2 under the ice becomes pressurized from sunlight penetrating the slab of dry ice. As the gas moves around the channels become larger.
As the temperature warms and more sunlight becomes available in the spring, frost starts to disappear. This process begins with the appearance of dark spots. By the time the temperature rises to the melting point of water ice, all ice is gone. The process was first followed with repeated images by the Mars Global Surveyor.[34][35] With the much greater resolution of HiRISE, it was seen that many spots had the shape of fans. Some of these spots and fans are visible in the pictures below. Thousands of fans were examined by a Citizen Science project. Nearly all (96%) of the fans measured in this study have a length below 100 m. The average fan length is at 33.1 m. The three largest fans lengths of 373 m, 368 m and 361 m and were all in the same region.[36]
Wide view of defrosting, as seen by HiRISE under HiWish program. Gas and dark dust is probably being blown out of ground and then carried by winds toward the Northwest.
Close view of defrosting, as seen by HiRISE under HiWish program. Gas and dark dust is probably being blown out of ground and then carried by winds toward the Northwest.
Wide view of spring defrosting. The dark spots are where the ice has disappeared, thus revealing the dark surface beneath the ice. Picture taken with HiRISE under HiWish program.
Close view of spring defrosting. The dark spots are where the ice has disappeared, thus revealing the dark surface beneath the ice. Picture taken with HiRISE under HiWish program.
Many areas on Mars, including Eridania, experience the passage of giantdust devils. A thin coating of fine bright dust covers most of the Martian surface. When a dust devil goes by it blows away the coating and exposes the underlying dark surface.
Dust devils occur when the sun warms up the air near a flat, dry surface. The warm air then rises quickly through the cooler air and begins spinning while moving ahead. This spinning, moving cell may pick up dust and sand then leave behind a clean surface.[37]
Dust devils have been seen from the ground and high overhead from orbit. They have even blown the dust off of thesolar panels of the twoRovers on Mars, thereby greatly extending their lives.[38] The twin Rovers were designed to last for 3 months, instead they lasted more than six years, and one is still going after 8 years. The pattern of the tracks have been shown to change every few months.[39]
A study that combined data from theHigh Resolution Stereo Camera (HRSC) and theMars Orbiter Camera (MOC) found that some large dust devils on Mars have a diameter of 700 meters and last at least 26 minutes.[40]
Dust Devil, as shown from HiRISE. Dust devil is moving to the upper left, leaving a dark track to the lower right. The shadow of the dust devil is to the upper left of the dust devil.
Weinbaum (crater), as seen by CTX camera (on Mars Reconnaissance Orbiter)
Weinbaum crater region showingdust devil tracks, as seen by CTX camera (on Mars Reconnaissance Orbiter). Field of picture is just outside rim of crater and is an enlargement of the previous image of Weinbaum crater.
Western side ofMitchel crater, as seen by CTX camera (on Mars Reconnaissance Orbiter)
Dust devil tracks in Mitchell crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of a previous image of west side of Mitchel crater.
Schmidt (Martian crater), as seen by CTX camera (on Mars Reconnaissance Orbiter). Arrows indicate the north and south rims of crater.
Dunes anddust devil tracks in Schmidt crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Narrow, dark lines are dust devil tracks. Note: this is an enlargement of the previous image of Schmidt crater.
Dust devil tracks and layers, as seen by HiRISE under HiWish program
TheDorsa Argentea Formation (DAF) is thought to be a large system ofeskers that were under an ancient ice cap in the south polar region of Mars.[41][42][43][44] This large polar ice sheet is believed to have covered about 1.5 million square kilometers. That area is twice the area of the state ofTexas.[45][circular reference][46] The ice sheet formed near the boundary of theNoachian-Hesperian era and receded in the earlyHesperian era.[44][47] A thick ice sheet was able to be formed more easily in the south polar region than in the North pole because the south pole is higher in altitude.[48][49][50][51][52] There may have been much more water available in the Martian atmosphere when the ice sheet developed.[53]
This group of ridges extends from 270–100 E and 70–90 S, around the south pole of Mars. It sits under the Late Amazonian South Polar Layered Deposits (SPLD). The amount of these ridges is huge, one study studied seven different ridge systems which contained almost 4,000 ridges that had a total length 51,000 km.[54]
Most eskers are thought to be formed inside ice-walled tunnels by streams which flowed within and under glaciers. After the retaining ice walls melted away, stream deposits remained as long winding ridges.
MARSIS radar data suggest that significant areas of layered, potentially ice-rich parts of the Dorsa Argentea Formation remain today.[55][56][57]
Ridges, believed to be eskers of the Dorsa Argentea Formation, as seen by Mars Global Surveyor wide angle MOC. White arrows point to the ridges.
A team of researchers used an early Mars global climate model together with the University of Maine Ice Sheet Model to determine how the eskers formed. They concluded that to get enough a high enough temperature in the Martian atmosphere to form the ice sheet, a greenhouse gas in addition to a thicker carbon dioxide atmosphere was needed to warm the surface near the poles by at least 20 degrees C. Also, to produce the shape of the ice sheet, at least part of the Tharsis volcanoes needed to be present.[58]
Strong evidence for a one time ancient ocean was found from data gathered from the north and south poles. In March 2015, a team of scientists published results showing that this region was highly enriched with deuterium, heavy hydrogen, by seven times as much as the Earth. This means that Mars has lost a volume of water 6.5 times what is stored in today's polar caps. The water for a time would have formed an ocean in the low-lying Mare Boreum. The amount of water could have covered the planet about 140 meters, but was probably in an ocean that in places would be almost 1 mile deep.
This international team used ESO’s Very Large Telescope, along with instruments at the W. M. Keck Observatory and the NASA Infrared Telescope Facility, to map out different forms of water in Mars’s atmosphere over a six-year period.[59][60]
Many places on Mars show rocks arranged in layers. The study of layering on Mars greatly expanded when the Mars Global Surveyor sent back images.[61][62] Rock can form layers in a variety of ways. Volcanoes, wind, or water can produce layers.[63][64][65]A detailed discussion of layering with many Martian examples can be found in Sedimentary Geology of Mars.[66] A paper by Grotzinger and Milliken discusses the role of water and wind in forming layers of sedimentary rocks.[67] Since craters are low spots on the landscape material can more easily accumulate there and may resist erosion longer than other places.
Close-up of Layers in wall ofMcMurdo Crater, as seen by HiRISE
Dust devil tracks in Smith Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous image of Smith Crater.
Lau Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Curved ridges are probablyeskers which formed under glaciers.
Close up of eskers in Lau Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). The dark lines are dust devil tracks. Note: this is an enlargement of the previous image of Lau Crater.
Heaviside Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter).
Gullies in a crater that is on the floor of Heaviside Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous image of Heaviside Crater.
Liais (crater), as seen by CTX camera (on Mars Reconnaissance Orbiter)
Layers is Liais crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous image of Liais crater.
East side ofSouth crater, as seen by CTX camera (on Mars Reconnaissance Orbiter)
Layers in southern part of mound in South crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous image of east side of South crater.
Layers just outside the rim of South crater in the form of "Swiss cheese", as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of a previous image of east side of South crater.
Western side of South crater, as seen by CTX camera (on Mars Reconnaissance Orbiter)
Layers in mound in western side of South crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of a previous image of west side of South crater.
Layers just outside rim of western side of South crater in the form of "Swiss cheese", as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of a previous image of west side of South crater.
Layers in Rayleigh crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous image of Rayleigh crater.
West side ofMain crater, as seen by CTX camera (on Mars Reconnaissance Orbiter)
Dust trails in Main crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Arrow indicates wind direction. In the spring, as the temperature rises, dry ice turns into a pressurized gas, and then blows through a weak spot and carries with it dust. If there is a wind, the dust is deposited in an elongated form as in this image.
Dunes and defrosting spots in Richardson Crater, as seen by HiRISE. It is thought that films of liquid water sometimes exist in and around the dark spots.
Jeans crater showing layers and dark spots from frost disappearing, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous image of Jeans crater.
Playfair (Martian crater), as seen by CTX camera (on Mars Reconnaissance Orbiter). Dark dots are places where frost has disappeared.
Defrosting going on in Playfair crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Dark areas are where frost has disappeared thereby showing dark ground. Note: this is an enlargement of the previous image of Playfair crater.
Gullies in Lyell crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous image of Lyell crater.
Defrosting taking place in Lyell crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Dark areas are where frost has disappeared thereby showing dark ground. Note: this is an enlargement of a previous image of Lyell crater.
West side ofJoly (crater), as seen by CTX camera (on Mars Reconnaissance Orbiter)
Dark spots and "spiders" in Joly crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). "Spiders" appear as fuzzy spots; they are where dark dust is accumulating in channels below clear slabs of frozen carbon dioxide. Note: this is an enlargement of the previous image of Joly crater.
Eskers in Joly crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Eskers are the ridges in the image; they are formed by streams running under a glacier.
Reynolds crater showing streaks from defrosting, as seen by CTX camera (on Mars Reconnaissance Orbiter). Layers are also visible. Note: this is an enlargement of the previous image of Reynolds crater. Streaks are caused by pressurized carbon dioxide blowing out dust that is blown by the wind into streaks.
Reynolds crater showing layers and dark spots from defrosting, as seen by CTX camera (on Mars Reconnaissance Orbiter). Area is covered by frost, when the frost disappears the underlying dark ground is exposed. Layers are also visible. Note: this is an enlargement of a previous image of Reynolds crater.
Much of the surface of Mars shows the ground arranged into polygons of various sizes. Sometimes, during the right season the lows areas of polygons are full of frost. When this happens, the shapes are enhanced forming beautiful views.
Still closer view of polygons, as seen by HiRISE under the HiWish program
Close view of polygons with frost in the low parts, as seen by HiRISE under the HiWish program. Circular shapes are also visible.
Parts of Mare Australe display pits that make the surface look like Swiss cheese.[68][69][70][71] These pits are in a 1-10 meter thick layer of dry ice that is sitting on a much larger water ice cap. Pits have been observed to begin with small areas along faint fractures. The circular pits have steep walls that work to focus sunlight, thereby increasing erosion. For a pit to develop a steep wall of about 10 cm and a length of over 5 meters in necessary.[72]
^Davies, M.E.; Batson, R.M.; Wu, S.S.C. (1992). "Geodesy and Cartography". In Kieffer, H.H.; Jakosky, B.M.; Snyder, C.W.; et al. (eds.).Mars. Tucson: University of Arizona Press.ISBN978-0-8165-1257-7.
^Patrick Moore and Robin Rees, ed.Patrick Moore's Data Book of Astronomy (Cambridge University Press, 2011), p. 130.
^Hartmann, W. 2003. A Traveler's Guide to Mars. Workman Publishing. NY NY.
^abMalin, M., Edgett, K. 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2335.
^Malin, M., K. Edgett, L. Posiolova, S. McColley, E. Dobrea. 2006. Present-day impact cratering rate and contemporary gully activity on Mars. Science 314, 1573_1577.
^Kolb, et al. 2010. Investigating gully flow emplacement mechanisms using apex slopes. Icarus 2008, 132-142.
^McEwen, A. et al. 2007. A closer look at water-related geological activity on Mars. Science 317, 1706-1708.
^Pelletier, J., et al. 2008. Recent bright gully deposits on Mars wet or dry flow? Geology 36, 211-214.
^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
^Kieffer H, Christensen P, Titus T. 2006 Aug 17. CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar ice cap. Nature: 442(7104):793-6.
^Portyankina, G., et al. 2017. Present-day erosion of Martian polar terrain by the seasonal CO2 jets. Icarus: 282, 93-103.>
^Portyankina, G., et al. 2019. How Martian araneiforms get their shapes: morphological analysis and diffusion-limited aggregation model for polar surface erosion Icarus.https://doi.org/10.1016/j.icarus.2019.02.032
^SPRING DEFROSTING OF MARTIAN POLAR REGIONS: MARS GLOBAL SURVEYOR MOC AND TES MONITORING OF THE RICHARDSON CRATER DUNE FIELD, 1999–2000. K. S. Edgett, K. D. Supulver, and M. C. Malin, Malin Space Science Systems, P.O. Box 910148, San Diego, CA 92191-0148, USA.
^K.-Michael Aye, K., et al. PROBING THE MARTIAN SOUTH POLAR WINDS BY MAPPING CO2 JET DEPOSITS. 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083). 2841.pdf
^Reiss, D. et al. 2011. Multitemporal observations of identical active dust devils on Mars with High Resolution Stereo Camera (HRSC) and Mars Orbiter Camera (MOC). Icarus. 215:358-369.
^Allen, C. 1979. Volcano-ice interactions on Mars. J. Geophys. Res.: Solid Earth (1978–2012), 84 (B14), 8048-8059.
^Kargel, J., R. Strom. 1992. Ancient glaciation on mars. Geology, 20 (1), 3-7.
^abHead, J, S. Pratt. 2001. Extensive Hesperian-aged south polar ice sheet on Mars: Evidence for massive melting and retreat, and lateral flow and pending of meltwater. J. Geophys. Res.-Planet, 106 (E6), 12275-12299.
^Fastook, J., et al. 2012. Early Mars climate near the Noachian-Hesperian boundary: Independent evidence for cold conditions from basal melting of the south polar ice sheet (Dorsa Argentea Formation) and implications for valley network formationIcarus: 219,. 25-40.
^Wordsworth, R. et al. 2013. Global modelling of the early martian climate under a denser CO2 atmosphere: Water cycle and ice evolutionIcarus, 222 (1), 1-19
^Forget, F. et al. 2013. 3D modelling of the early martian climate under a denser CO2 atmosphere: Temperatures and CO2 ice cloudsIcarus, 222 (1), 81-99
^Mischna, M, et al. 2013. Effects of obliquity and water vapor/trace gas greenhouses in the early martian climateJ. Geophys. Res.-Planet, 118 (3), 560-576
^Urata, R. O. Toon. 2013. Simulations of the Martian hydrologic cycle with a general circulation model: Implications for the ancient martian climateIcarus, 226 (1), 229-250
^Wordsworth, R. 2016. The Climate of Early MarsAnnu. Rev. Earth Planet. Sci. 44, 381-408.
^Carr, M., J. Head. 2015. Martian surface/near-surface water inventory: Sources, sinks, and changes with timeGeophys. Res. Lett., 42, pp. 1-7 10.1002/2014GL062464.
^Kress, A., J. Head. 2015. Late Noachian and early Hesperian ridge systems in the south circumpolar Dorsa Argentea Formation, Mars: Evidence for two stages of melting of an extensive late Noachian ice sheet. Planetary and Space Science: 109-110, 1-20
^Plaut, J., Ivanov, A., Safaeinili, A., Milkovich, S., Picardi, G., Seu, R., Phillips, R. 2007a. Radar sounding of subsurface layers in the South Polar plains of Mars:correlation with the Dorsa Argentea formation. Lunar Planet. Sci. XXXVIII(abstract 2144).
^Head, J., Marchant, D. 2006. Regional polar glaciation in the hesperian period of the history of Mars. The South Circumpolar Dorsa Argentea Formation as an Ancient Ice Sheet Remnant. Fourth Mars Polar Science Conference. Davos, Switzerland.
^Head, J., Marchant, D., Forget, F. 2007. Regional polar glaciation in the hesperian period of the history of Mars: the south circumpolar Dorsa Argentea formation as an ancient ice sheet remnant. Seventh International Conference on Mars. Pasadena, CA (abstract 3115).
^Scanlon, K., et al. 2018. The Dorsa Argentea Formation and the Noachian-Hesperian climate transition. Icarus: 299, 339-363.
^. Villanueva, L., Mumma, R. Novak, H. Käufl, P. Hartogh,T. Encrenaz, A. Tokunaga, A. Khayat, M. Smith. Strong water isotopic anomalies in the martian atmosphere: Probing current and ancient reservoirs. Science, 2015 DOI: 10.1126/science.aaa3630
^Grotzinger, J. and R. Milliken (eds.). 2012. Sedimentary Geology of Mars. SEPM.
^Grotzinger, J.P. and R.E. Milliken (2012). The sedimentary rock record of Mars: Distribution, origins, and global stratigraphy, SEPM Special Publication, 102, 1-48.
^Thomas, P., M. Malin, P. James, B. Cantor, R. Williams, P. GieraschSouth polar residual cap of Mars: features, stratigraphy, and changesIcarus, 174 (2 SPEC. ISS.). 2005. pp. 535–559.http://doi.org/10.1016/j.icarus.2004.07.028
^Thomas, P., P. James, W. Calvin, R. Haberle, M. Malin. 2009. Residual south polar cap of Mars: stratigraphy, history, and implications of recent changesIcarus: 203, 352–375http://doi.org/10.1016/j.icarus.2009.05.014
^Thomas, P., W.Calvin, P. Gierasch, R. Haberle, P. James, S. Sholes. 2013. Time scales of erosion and deposition recorded in the residual south polar cap of marsIcarus: 225: 923–932http://doi.org/10.1016/j.icarus.2012.08.038
^Thomas, P., W. Calvin, B. Cantor, R. Haberle, P. James, S. Lee. 2016. Mass balance of Mars’ residual south polar cap from CTX images and other dataIcarus: 268, 118–130http://doi.org/10.1016/j.icarus.2015.12.038
^Buhler, Peter, Andrew Ingersoll, Bethany Ehlmann, Caleb Fassett, James Head. 2017. How the martian residual south polar cap develops quasi-circular and heart-shaped pits, troughs, and moats. Icarus: 286, 69-9.
