Scalloped topography

{{short description|Depressions in topography at mid-latitudes of Mars}}

Scalloped topography is common in the mid-latitudes of Mars, between 45° and 60° north and south. It is particularly prominent in the region of Utopia Planitia,{{cite journal |last1=Lefort |first1=A. |last2=Russell |first2=P. |last3=Thomas |first3=N. |last4=McEwen |first4=A.S. |last5=Dundas |first5=C.M. |last6=Kirk |first6=R.L. |year=2009 |title=HiRISE observations of periglacial landforms in Utopia Planitia |url=http://www.agu.org/pubs/crossref/2009/2008JE003264.shtml |journal=Journal of Geophysical Research |volume=114 |issue=E4 |page=E04005 |doi=10.1029/2008JE003264 |bibcode=2009JGRE..114.4005L|doi-access=free }}Morgenstern A, Hauber E, Reiss D, van Gasselt S, Grosse G, Schirrmeister L (2007): Deposition and degradation of a volatile-rich layer in Utopia Planitia, and implications for climate history on Mars. Journal of Geophysical Research: Planets 112, E06010. in the northern hemisphere, and in the region of Peneus and Amphitrites Paterae{{cite journal |last1=Lefort |first1=A. |last2=Russell |first2=P. |last3=Thomas |first3=N. |year=2009 |title=Scalloped terrains in the Peneus and Amphitrites Paterae region of Mars as observed by HiRISE |journal=Icarus |volume= 205 |issue=1 |pages= 259–268 |doi=10.1016/j.icarus.2009.06.005 |bibcode=2010Icar..205..259L}}Zanetti, M., Hiesinger, H., Reiss, D., Hauber, E. and Neukum, G. (2009), [http://www.lpi.usra.edu/meetings/lpsc2009/pdf/2178.pdf "Scalloped Depression Development on Malea Planum and the Southern Wall of the Hellas Basin, Mars"], 40th Lunar and Planetary Science Conference, abstract 2178 in the southern hemisphere. Such topography consists of shallow, rimless depressions with scalloped edges, commonly referred to as "scalloped depressions" or simply "scallops". Scalloped depressions can be isolated or clustered and sometimes seem to coalesce. A typical scalloped depression displays a gentle equator-facing slope and a steeper pole-facing scarp.{{Cite web |url=http://www.uahirise.org/ESP_038821_1235 |title=HiRISE | Pitted Landforms in Southern Hellas Planitia (ESP_038821_1235)}} This topographic asymmetry is probably due to differences in insolation. Scalloped depressions are believed to form from the removal of subsurface material, possibly interstitial ice, by sublimation (direct transition of a material from the solid to the gas phase with no intermediate liquid stage). This process may still be happening at present.{{cite web|url=http://hiroc.lpl.arizona.edu/images/PSP/diafotizo.php?ID=PSP_002296_1215|title=Scalloped Topography in Peneus Patera Crater|publisher=HiRISE Operations Center|date=2007-02-28|access-date=2014-11-24}} This topography may be of great importance for future colonization of Mars because it may point to deposits of pure ice.{{cite journal |last1=Dundas |first1=C. |last2=Bryrne |first2=S. |last3=McEwen |first3=A. |year=2015 |title=Modeling the development of martian sublimation thermokarst landforms |url=https://zenodo.org/record/1259051 |journal=Icarus |volume=262 |pages=154–169 |doi=10.1016/j.icarus.2015.07.033 |bibcode=2015Icar..262..154D}}

A study published in Icarus, found that the landforms of scalloped topography can be made by the subsurface loss of water ice by sublimation under current Martian climate conditions over periods of tens of thousands of Mars years. Scalloped depressions are thought to begin with a small trigger like a small impact, local darkening, erosion, or cracks from thermal contraction. Cracks are common in ice-rich ground on the Earth. Their model predicts that these scalloped depression will develop when the ground has large amounts of pure ice, up to many tens of meters in depth. So, scalloped features can serve as markers for large deposits of pure ice. Ice in and around scalloped topography is not just in the pore spaces of the ground it is excess ice, probably 99% pure as was found by the Phoenix mission.{{cite journal |last1=Smith |first1=P. |display-authors=etal |year=2009 |title=H₂O at the Phoenix landing site |journal=Science |volume=325 |issue=5936 |pages=58–61 |bibcode=2009Sci...325...58S |doi=10.1126/science.1172339 |pmid=19574383|s2cid=206519214 }}{{cite journal |last1=Mellon |first1=M. |display-authors=etal |year=2009 |title=Ground ice at the Phoenix landing site: Stability state and origin |doi=10.1029/2009JE003417 |journal=J. Geophys. Res. |volume= 114 |issue=53 |pages=E00E07 |bibcode=2009JGRE..114.0E07M|doi-access=free }}{{cite journal |last1=Cull |first1=S |display-authors=etal |year=2010 |title=Compositions of subsurface ices at the Mars Phoenix landing site |doi= 10.1029/2010GL045372 |bibcode=2010GeoRL..3724203C |journal=Geophys. Res. Lett. |volume=37 |issue=24 |page=L24203 |url=https://repository.brynmawr.edu/cgi/viewcontent.cgi?article=1010&context=geo_pubs|doi-access=free }} The shallow Subsurface Radar (SHARAD), aboard the Mars Reconnaissance Orbiter can detect ice-rich layers only when thicker than 10–20 meters over wide areas;{{cite journal |last1=Seu |first1=R. |display-authors=etal |year=2007 |title=SHARAD sounding radar on the Mars Reconnaissance Orbiter |doi=10.1029/2006JE002745 |journal=J. Geophys. Res. |volume= 112 |issue=E5 |pages=E05S05 |bibcode=2007JGRE..112.5S05S|doi-access=free }} it has discovered ice in the region of scalloped topography.Stuurman, C., et al. 2016. SHARAD reflectors in Utopia Planitia, SHARAD detection and characterization of subsurface water ice deposits in Utopia Planitia, Mars. Geophysical Research Letters, Volume 43, Issue 18, 28 September 2016, Pages 9484–9491.

The details on the formation of scalloped topography still being worked out. One study, published in 2016 in Icarus proposes a five step process.

1. Major changes in the planet’s tilt change the climate. This climate change causes an icy mantle to form.
2. Various conditions cause the mantle to thaw or evaporate.
3. Meltwater moves in the ground, at least to the depth of the scalloped depressions.
4. Freezing and thawing of the ice produces masses of ice (ice lenses).
5. With another tilt change the climate changes and masses of ice sublimate, resulting in scalloped depressions.Soare, R., et al. 2016. Ice-rich (periglacial) vs icy (glacial) depressions in the Argyre region, Mars: a proposed cold-climate dichotomy of landforms: 282, 70-83.

In Utopia Planitia, a series of curvilinear ridges parallel to the scarp are etched on the floor of large scalloped depressions, possibly representing different stages of scarp erosion. Recently, other researchers have advanced an idea that the ridges represent the tops of layers.{{cite journal |last1=Sejourne |first1=A. |display-authors=etal |year=2012 |title=Evidence of an eolian ice-rich and stratified permafrost in Utopia Planitia, Mars |journal=Icarus |volume=60 |issue=1 |pages=248–254 |doi=10.1016/j.pss.2011.09.004 |bibcode=2012P&SS...60..248S}} Sometimes the surface around scalloped terrain or scalloped topography displays "patterned ground", characterized by a regular pattern of polygonal fractures. These patterns indicate that the surface has undergone stress, perhaps caused by subsidence, desiccation, or thermal contraction.{{cite web|url=http://hiroc.lpl.arizona.edu/images/PSP/diafotizo.php?ID=PSP_002439_2265|title=Scalloped Depressions with Layers in the Northern Plains|publisher=HiRISE Operations Center|date=2007-02-28|access-date=2014-11-24}} Such patterns are common in periglacial areas on Earth. Scalloped terrains in Utopia Planitia display polygonal features of different sizes: small (about 5–10 m across) on the scarp, and larger (30–50 m across) on the surrounding terrains. These scale differences may indicate local difference in ground ice concentrations.