View source for Astronomy:Seasonal flows on warm Martian slopes

{{Short description|Surface features on Mars}}
[[File:Oblique View of Warm Season Flows in Newton Crater.jpg|thumb|Reprojected view of warm-season flows in [[Astronomy:Newton (Martian crater)|Newton Crater]] ]]
'''Seasonal flows on warm Martian slopes''' (also called '''recurring slope lineae''', '''recurrent slope lineae''' and '''RSL''')<ref name="RSL">{{cite book |last1=Kirby |first1=Runyon |last2=Ojha |first2=Lujendra   |date=August 18, 2014 |doi=10.1007/978-1-4614-9213-9_352-1 |chapter=Recurring Slope Lineae |title=Encyclopedia of Planetary Landforms |isbn=978-1-4614-9213-9 |pages=1 }}</ref><ref name="NYT-20151005-kc" /> are thought to be salty water flows occurring during the warmest months on [[Astronomy:Mars|Mars]], or alternatively, dry grains that "flow" downslope of at least 27 degrees.

The flows are narrow (0.5 to 5 meters) and exhibit relatively dark markings on steep {{Nowrap|(25° to 40°)}} slopes, appear and incrementally grow during warm [[Astronomy:Climate of Mars#Seasons|seasons]] and fade in cold seasons. Liquid [[Earth:Brine|brine]]s near the surface have been proposed to explain this activity,<ref name="Ojha NG2015">{{cite journal |title=Spectral evidence for hydrated salts in recurring slope lineae on Mars |journal=Nature Geoscience |volume=8 |issue=11 |pages=829 |date=28 September 2015 |last1=Ojha |first1=Lujendra |last2=Wilhelm |first2=Mary Beth |last3=Murchie |first3=Scott L. |last4=McEwen |first4=Alfred S. |doi=10.1038/ngeo2546 |display-authors=0|bibcode=2015NatGe...8..829O |s2cid=59152931 }}</ref> or interactions between sulfates and chlorine salts that interact under to produce landslides.<ref>{{Cite journal|last1=Bishop|first1=J. L.|last2=Yeşilbaş|first2=M.|last3=Hinman|first3=N. W.|last4=Burton|first4=Z. F. M.|last5=Englert|first5=P. A. J.|last6=Toner|first6=J. D.|last7=McEwen|first7=A. S.|last8=Gulick|first8=V. C.|last9=Gibson|first9=E. K.|last10=Koeberl|first10=C.|date=2021|title=Martian subsurface cryosalt expansion and collapse as trigger for landslides|journal=Science Advances|language=en|volume=7|issue=6|pages=eabe4459|doi=10.1126/sciadv.abe4459|issn=2375-2548|pmc=7857681|pmid=33536216}}</ref>

==Overview==
Research indicates that in the past there was [[Astronomy:Water on Mars|liquid water flowing on the surface of Mars]],<ref>{{cite web |url=http://www.space.com/scienceastronomy/flashback-water-on-mars-announced-10-years-ago-100622.html |title=Flashback: Water on Mars Announced 10 Years Ago| publisher=SPACE.com| date=June 22, 2000| access-date=December 19, 2010}}</ref><ref name='Willson 2018'>{{cite web|url=https://www.space.com/8642-flashback-water-mars-announced-10-years.html |title=Flashback: Water on Mars Announced 10 Years Ago| publisher=SPACE.com| date=June 22, 2010| access-date=May 13, 2018}}</ref><ref>{{cite web| url=https://science.nasa.gov/headlines/y2001/ast05jan_1.htm| title=Science@NASA, The Case of the Missing Mars Water| access-date=March 7, 2009| url-status=dead| archive-url=https://web.archive.org/web/20090327234049/https://science.nasa.gov/headlines/y2001/ast05jan_1.htm| archive-date=March 27, 2009}}</ref> creating large areas similar to Earth's oceans.<ref>{{Cite book|isbn = 978-0-312-24551-1|title = Mapping Mars: Science, Imagination, and the Birth of a World|last1 = Morton|first1 = Oliver|date = 2002-10-04| publisher=Picador |url-access = registration|url = https://archive.org/details/mappingmarsscien00mort_0}}</ref><ref>{{cite web|url=http://www.psrd.hawaii.edu/July03/MartianSea.html|title=PSRD: Ancient Floodwaters and Seas on Mars|publisher=Psrd.hawaii.edu|date=July 16, 2003|access-date=December 19, 2010}}</ref><ref>{{cite web|url=http://www.spaceref.com/news/viewpr.html?pid=26947 |title=Gamma-Ray Evidence Suggests Ancient Mars Had Oceans &#124; SpaceRef|publisher=SpaceRef |date=November 17, 2008| access-date=December 19, 2010}}</ref><ref name="2003JGRE..108.5042C">{{cite journal| last1=Carr| first1=M.| last2=Head| first2=J.| date=2003| title=Oceans on Mars: An assessment of the observational evidence and possible fate| journal=Journal of Geophysical Research| volume=108| issue=E5| page=5042| bibcode=2003JGRE..108.5042C| doi=10.1029/2002JE001963| s2cid=16367611| doi-access=free}}</ref> However, the question remains as to where the water has gone.<ref>{{cite web| url=http://www.adlerplanetarium.org/cyberspace/planets/mars/water.html|title=Water on Mars: Where is it All?| access-date=March 7, 2009| archive-url=https://web.archive.org/web/20071203120931/http://www.adlerplanetarium.org/cyberspace/planets/mars/water.html| archive-date=December 3, 2007}}</ref>

The ''[[Astronomy:Mars Reconnaissance Orbiter|Mars Reconnaissance Orbiter]]'' (MRO) is a multipurpose spacecraft launched in 2005 designed to conduct reconnaissance and [[Astronomy:Exploration of Mars|exploration of Mars]] from orbit.<ref name=uk>{{cite news| title=Nasa Find Potential Signs Of Flowing Water On Mars| url=http://www.huffingtonpost.co.uk/2011/08/04/nasa-to-announce-new-find_n_918444.html?| newspaper=Huffpost UK| date=August 4, 2011| access-date=August 5, 2011}}</ref> The spacecraft is managed by the [[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] (JPL).<ref name=jpl>{{cite web| url=http://www.nasa.gov/mission_pages/MRO/news/mro20110804.html| title=NASA Spacecraft Data Suggest Water Flowing on Mars| publisher=Jet Propulsion Laboratory, Pasadena, California| access-date=March 31, 2012| archive-date=March 4, 2016| archive-url=https://web.archive.org/web/20160304075750/http://www.nasa.gov/mission_pages/MRO/news/mro20110804.html| url-status=dead}}</ref> The HiRISE camera onboard the MRO is at the forefront of the ongoing RSL studies as it helps chart the features with images of closely monitored sites typically taken every few weeks.<ref name="David 2015">{{cite news |last=David |first=Leonard |url=http://www.space.com/30618-mars-water-streaks-rsl-mystery.html |title=Mars' Mysterious Dark Streaks Spur Exploration Debate |work=Space.com |date=23 September 2015 |access-date=2015-09-25 }}</ref> The 2001 ''Mars Odyssey'' orbiter has been using [[Physics:Spectrometer|spectrometer]]s and a thermal imager for over 16 years to detect evidence of past or present [[Astronomy:Water on Mars|water]] and ice.<ref name="Jack Wilson">Equatorial locations of water on Mars: Improved resolution maps based on Mars Odyssey Neutron Spectrometer data (PDF). Jack T. Wilson, Vincent R. Eke, Richard J. Massey, Richard C. Elphic, William C. Feldman, Sylvestre Maurice, Luıs F. A. Teodoroe. ''Icarus'',  299, 148-160. January 2018. Quote: "Finally, we find that the sites of recurring slope lineae (RSL) do not correlate with subsurface hydration. This implies that RSL are not fed by large, near-subsurface aquifers, but are instead the result of either small (<120 km diameter) aquifers, deliquescence of perchlorate and chlorate salts or dry, granular flows."</ref><ref name="NASA JPL">{{cite web | url=http://mars.jpl.nasa.gov/odyssey/mission/science/goals/ | title=Mars Odyssey Goals | work=NASA JPL }}</ref> It has detected none at the RSL.<ref name='Jack Wilson'/> On October 5, 2015, possible RSL were reported on [[Astronomy:Mount Sharp|Mount Sharp]] near the [[Engineering:Curiosity (rover)|''Curiosity'' rover]].<ref name="NYT-20151005-kc">{{cite news |last=Chang |first=Kenneth |title=Mars Is Pretty Clean. Her Job at NASA Is to Keep It That Way. |url=https://www.nytimes.com/2015/10/06/science/mars-catharine-conley-nasa-planetary-protection-officer.html |date=5 October 2015 |work={{wipe|New York Times}} |access-date=6 October 2015 }}</ref>

==Features==
[[File:Warm Season Flows on Slope in Newton Crater (animated).gif|thumb|350px|Warm season flows on slope in [[Astronomy:Newton (Martian crater)|Newton Crater]] (video-gif)]]

Distinctive properties of recurring slope lineae (RSL) include slow incremental growth, formation on warm slopes in warm seasons, and annual fading and recurrence,<ref name="Dundas2015">{{cite conference |last1=Dundas |first1=C. M. |last2=McEwen |first2=A. S. |title=NEW CONSTRAINTS ON THE LOCATIONS, TIMING AND CONDITIONS FOR RECURRING SLOPE |url=http://www.hou.usra.edu/meetings/lpsc2015/pdf/2327.pdf |conference=46th Lunar and Planetary Science Conference (2015) |publisher=Lunar and Planetary Institute |date=March 16–20, 2015}}</ref> showing a strong correlation with solar heating.<ref name="Dundas2015"/> RSL extend down slope from [[Earth:Bedrock|bedrock]] outcrops often following small gullies about {{Convert|0.5|to|5|m|sp=us}} wide, with lengths up to hundreds of meters, and some of the locations display more than 1,000 individual flows.<ref name="WRD-20140218" /><ref name=mag/> RSL advance rates are highest at the beginning of each season, followed by much slower lengthening.<ref name="McEwen EPSC2015"/> RSL appear and lengthen in the late southern spring and summer from 48°S to 32°S latitudes that favor equator-facing slopes, which are times and places with peak surface temperatures from {{Nowrap|−23 °C to 27 °C}}. Active RSL also occur in equatorial regions (0–15°S), most commonly in the [[Astronomy:Valles Marineris|Valles Marineris]] troughs.<ref name="McEwen EPSC2015"/><ref>Stillman, D., et al.  2016.  Characteristics of the Numerous and Widespread Recurring Slope Lineae (RSL) in Valles Marineris, Mars.  Icarus:  285,  195-210.</ref>

Researchers surveyed flow-marked slopes with the ''[[Astronomy:Mars Reconnaissance Orbiter|Mars Reconnaissance Orbiter]]''{{'s}} [[Astronomy:Compact Reconnaissance Imaging Spectrometer for Mars|CRISM]] and although there is no spectrographic evidence for actual water,<ref name="WRD-20140218" /> the instrument has directly imaged perchlorate salts thought to be dissolved in water brines in the subsurface.<ref name="Ojha NG2015" /> This may indicate the water quickly evaporates upon reaching the surface, leaving only the salts. The cause of the surface darkening and lightening is poorly understood: a flow initiated by salty water (brine) could rearrange grains or change surface roughness in a way that darkens the appearance, but the way the features brighten again when temperatures drop is harder to explain.<ref name=jpl/><ref name=nasa/> However, in November 2018, it was announced that CRISM had fabricated some additional pixels representing the minerals alunite, kieserite, serpentine and perchlorate.<ref name="SA-20181122">{{citation |last=Mcrae |first=Mike |title=One of NASA's Mars Exploration Tools Has a Glitch That Created The Illusion of Water |url=https://www.sciencealert.com/nasa-s-mars-reconnaissance-orbiter-has-a-glitch-that-created-the-illusion-of-water |date=22 November 2018 |work=ScienceAlert.com |access-date=22 November 2018 }}</ref><ref name='Grossman 2018'>[https://www.sciencenews.org/article/mars-reconnaissance-orbiter-glitch-liquid-water An orbiter glitch may mean some signs of liquid water on Mars aren't real]. Lisa Grossman, ''Science News'', 21 November 2018.</ref> The instrument team found that some false positives were caused by a filtering step when the detector switches from a high luminosity area to shadows.<ref name="SA-20181122"/> Reportedly, 0.05% of the pixels were indicating perchlorate, now known to be a false high estimate by this instrument.<ref name="SA-20181122"/> Reduced content of salts on the slopes  reduces the chances of the presence of brines.<ref name='Grossman 2018'/>

==Hypotheses==
A number of different hypotheses for RSL formation have been proposed. The seasonality, [[Earth:Latitude|latitude]] distribution, and brightness changes strongly indicate a [[Chemistry:Volatility|volatile]] material—such as water or liquid {{chem|C|O|2}}—is involved. One hypothesis is that RSL could form by rapid heating of nocturnal frost<ref name="Dundas2015"/> in agreement with experimental results.<ref>{{cite journal | last1=Massé | first1=M. | last2=Conway |first2=S. | last3=Gargani |first3=J. | last4=Patel |first4=M.R. | last5=Pasquon |first5=K. | last6=Mc Ewen |first6=A. | last7=Carpy |first7=S. | last8=Chevrier |first8=V. | last9=Balme |first9=M.R. | last10=Ojha |first10=L. | last11=Vincendon |first11=M. | last12=Poulet |first12=F. | last13=Costard |first13=F. | last14=Jouannic |first14=G. | title=Transport processes induced by metastable boiling water under Martian surface conditions | journal=Nature Geoscience | volume=9 | pages=425–428 | date=2016 | issue=6 | doi=10.1038/ngeo2706 | url=https://hal.archives-ouvertes.fr/hal-01414502/file/MasseEtalNatGeo2015_HAL.pdf }}</ref> Another one proposes flows of carbon dioxide, but the settings in which the flows occur are too warm for [[Chemistry:Dry ice|carbon-dioxide frost]] ({{chem|C|O|2}}), and at some sites is too cold for pure water.<ref name="Dundas2015"/> Other hypotheses include dry granular flows, but no entirely dry process can explain seasonal flows that progressively grow over weeks and months.<ref name="McEwen EPSC2015"/> Cornice avalanches is another hypothesis.  The idea is that wind collects snow or frost just past the peak of a mountain and then this becomes an avalanche after it warms up. <!--[http://corniceavalanchesonmars.blogspot.com/]--> Seasonal melting of shallow ice would explain the RSL observations, but it would be difficult to replenish such ice annually.<ref name="McEwen EPSC2015"/> However, as of 2015, direct observations of seasonal deposition of soluble salts strongly suggest that RSL involve [[Earth:Brine|brine]] (hydrated salts).<ref name="Ojha NG2015" />

===Brines===

The leading hypothesis involves the flow of [[Earth:Brine|brine]]s (very salty water).<ref name="Ojha NG2015" /><ref name="WRD-20140218">{{cite magazine |last=Mann |first=Adam |title=Strange Dark Streaks on Mars Get More and More Mysterious|url=https://www.wired.com/wiredscience/2014/02/flowing-lineae-water-mars/ |date=February 18, 2014 |magazine=Wired  |access-date=February 18, 2014 }}</ref><ref name="mag">{{Cite web |last=Kerr |first=Richard A. |date=August 4, 2011 |title=Is Mars Weeping Salty Tears? |url=https://www.science.org/content/article/mars-weeping-salty-tears |access-date=July 14, 2023 |website=Science}}</ref><ref name=voanews>{{cite web| url=http://www.voanews.com/english/news/science-technology/NASA-Finds-Possible-Signs-of-Flowing-Water-on-Mars-126807133.html| title=NASA Finds Possible Signs of Flowing Water on Mars| publisher=voanews.com| access-date=August 5, 2011}}</ref><ref name="NASA-20131210">{{cite web |last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |title=NASA Mars Spacecraft Reveals a More Dynamic Red Planet |url=http://www.jpl.nasa.gov/news/news.php?release=2013-361&1#1 |date=December 10, 2013 |work=[[Organization:NASA|NASA]] |access-date=December 10, 2013 }}</ref><ref name="Wall-Brines 2015">{{cite news |last=Wall |first=Mike |url=http://www.space.com/30673-water-flows-on-mars-discovery.html?adbid=10153086098981466&adbpl=fb&adbpr=17610706465 |title=Salty Water Flows on Mars Today, Boosting Odds for Life |work=Space.com |date=28 September 2015 |access-date=2015-09-28 }}</ref> Salt deposits over much of Mars indicate that brine was abundant in Mars's past.<ref name=jpl/><ref name=nasa>{{cite web| url=http://www.nasa.gov/mission_pages/MRO/news/mro20110804.html| title=NASA Spacecraft Data Suggest Water Flowing on Mars| publisher=NASA| access-date=July 5, 2011| archive-date=March 4, 2016| archive-url=https://web.archive.org/web/20160304075750/http://www.nasa.gov/mission_pages/MRO/news/mro20110804.html| url-status=dead}}</ref> Salinity lowers the [[Physics:Freezing point|freezing point]] of water to sustain a liquid flow. Less saline water would freeze at the observed temperatures.<ref name=jpl/> Thermal infrared data from the [[Engineering:Thermal Emission Imaging System|Thermal Emission Imaging System]] (THEMIS) on board the ''[[Astronomy:2001 Mars Odyssey|2001 Mars Odyssey]]'' orbiter, have allowed the temperature conditions under which RSL form to be constrained. While a small number of RSL are visible at temperatures above the freezing point of water, most are not, and many appear at temperatures as low as {{convert|-43|C|K}}. Some scientists think that under these cold conditions, a brine of [[Chemistry:Iron(III) sulfate|iron(III) sulfate]] (Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>) or [[Chemistry:Calcium chloride|calcium chloride]] ({{chem|Ca|Cl|2}}) is the most likely mode of RSL formation.<ref name="Mitchell 2015">{{cite conference |last1=Mitchell |first1=J. |last2=Christensen |first2=P. |title=RECURRING SLOPE LINEAE AND THE PRESENCE OF CHLORIDES IN THE SOUTHERN HEMISPHERE OF MARS |url=http://www.hou.usra.edu/meetings/lpsc2015/pdf/1624.pdf |conference=46th Lunar and Planetary Science Conference (2015) |publisher=Lunar and Planetary Institute |date=March 16–20, 2015 }}</ref>  Another team of scientists, using the [[Astronomy:Compact Reconnaissance Imaging Spectrometer for Mars|CRISM]] instrument onboard MRO, reported that the evidence for hydrated salts is most consistent with the spectral absorption features of [[Chemistry:Magnesium perchlorate|magnesium perchlorate]] (Mg(ClO<sub>4</sub>)<sub>2</sub>), [[Chemistry:Magnesium chloride|magnesium chloride]] (MgCl<sub>2</sub>(H<sub>2</sub>O)<sub>x</sub>) and [[Chemistry:Sodium perchlorate|sodium perchlorate]] ({{chem|Na|Cl|O|4}}).<ref name="Ojha NG2015"/><ref name="Wall-Brines 2015"/>

Experiments and calculations demonstrated that recurring slope lineae could be produced by the deliquescence and rehydration of hydrous chlorides and oxychlorine salts.  However, under present Martian atmospheric conditions there is not enough water to complete this process.<ref>{{cite journal | last1=Wang | first1=A. | last2=Ling | first2=Z. C. | last3=Yan | first3=Y. C. | last4=McEwen | first4=A. S. | last5=Mellon | first5=M. T. | last6=Smith | first6=M. D. | last7=Jolliff | first7=B. L. | last8=Head | first8=J. | title=Atmosphere - Surface H<sub>2</sub>O Exchange to Sustain the Recurring Slope Lineae (RSL) on Mars | journal=Lunar and Planetary Science Conference | volume=48 | issue=1964 | pages=2351 | date=2017-03-24 | bibcode=2017LPI....48.2351W }}</ref>

These observations are the closest scientists have come to finding evidence of liquid water on the planet's surface today.<ref name=jpl/><ref name=nasa/> Frozen water, however, has been detected near the surface in many middle to high-latitude regions. Purported droplets of brine also appeared on struts of the Phoenix Mars Lander in 2008.<ref name="droplets">{{cite journal |last1=Renno |first1=Nilton |last2=Bos |first2=Brent |display-authors=0 |title=Possible physical and thermodynamical evidence for liquid water at the Phoenix landing site |journal=J. Geophys. Res. Planets |date=14 October 2009 |volume=114 |issue=E1 |doi=10.1029/2009JE003362 |ref=phoenix-droplets|hdl=2027.42/95444 |url=https://deepblue.lib.umich.edu/bitstream/2027.42/95444/1/jgre2665.pdf |bibcode=2009JGRE..114.0E03R |hdl-access=free }}</ref>

===Source of water===

Liquid brine flows near the surface might explain this activity, but the exact source of the water and the mechanism behind its motion are not understood.<ref>{{cite journal| last1=McEwen| first1=Alfred.S.| last2=Ojha| first2=Lujendra | last3=Dundas| first3=Colin M.| date=June 17, 2011| title=Seasonal Flows on Warm Martian Slopes| journal=Science| volume=333| issue=6043| pages=740–743| doi=10.1126/science.1204816| issn=0036-8075| bibcode = 2011Sci...333..740M| pmid=21817049| s2cid=10460581}}</ref><ref name=hirise>{{cite web| url=http://hirise.lpl.arizona.edu/sim/science-2011-aug-4.php| title=Seasonal Flows on Warm Martian Slopes| publisher=hirise.lpl.arizona.edu| access-date=August 5, 2011}}</ref>  A hypothesis proposes that the needed water could originate in the seasonal oscillations of near-surface adsorbed water provided by the [[Astronomy:Atmosphere of Mars#Water vapor|atmosphere]]; [[Chemistry:Perchlorate|perchlorate]]s and other salts known to be present on the surface are able to attract and hold water molecules from the surrounding environment ([[Physics:Hygroscopic|hygroscopic]] salts),<ref name="McEwen EPSC2015"/> but the dryness of the Martian air is a challenge. Water vapor must be efficiently trapped over very small areas, and seasonal variation in the atmospheric column abundance of water vapor does not match the RSL activity over active locations.<ref name="Dundas2015"/><ref name="McEwen EPSC2015">{{cite conference |last1=McEwen |first1=A. |last2=Chojnacki |first2=M. |last3=Dundas |first3=C. |last4=L. Ojha |first4=L. |title=Recurring Slope Lineae on Mars: Atmospheric Origin? |url=http://meetingorganizer.copernicus.org/EPSC2015/EPSC2015-786-1.pdf |conference=European Planetary Science Congress 2015 |publisher=EPSC Abstracts |place=France |date= 28 September 2015 }}</ref>

Deeper [[Astronomy:Groundwater on Mars|groundwater]] may exist and could reach the surface at springs or seeps,<ref>{{cite journal | last1 = Levy | first1 = Joseph | year = 2012 | title = Hydrological characteristics of recurrent slope lineae on Mars: Evidence for liquid flow through regolith and comparisons with Antarctic terrestrial analogs | journal = Icarus | volume = 219 | issue = 1| pages = 1–4 | doi = 10.1016/j.icarus.2012.02.016 }}</ref><ref name="Torres 2015">{{cite journal |title=Transient liquid water and water activity at Gale crater on Mars |journal=Nature Geoscience |volume=8 |issue=5 |pages=357 |date= 13 April 2015 |last1=Martín-Torres |first1=F. Javier |last2=Zorzano |first2=María-Paz |last3=Valentín-Serrano |first3=Patricia |last4=Harri |first4=Ari-Matti |last5=Genzer |first5=Maria |doi=10.1038/ngeo2412 |bibcode=2015NatGe...8..357M }}</ref> but this cannot explain the wide distribution of RSL, extending from the tops of ridges and peaks.<ref name="McEwen EPSC2015"/> Also, there are apparent RSL on equatorial dunes composed of permeable sand, unlikely to be a groundwater source.<ref name="McEwen EPSC2015"/>

An analysis of near-subsurface data from the ''Mars Odyssey'' neutron spectrometer revealed that the RSL sites do not contain any more water than found at anywhere else at similar latitudes. The authors concluded that RSL are not supplied by large, near-surface briny aquifers. It is still possible with this data that water vapor is from deeply buried ice, from the atmosphere, or from small deeply buried aquifers.<ref name='Jack Wilson'/>

===Dry sand flows===
Dry granular flow was proposed since the first observations of RSLs but this interpretation was ruled out because of the seasonality of the process. The first proposition of a seasonal triggering in a dry context was published in March 2017 using a Knudsen pump effect.<ref name='Scmhidt Knudsen pump 2017'>{{cite journal | doi = 10.1038/ngeo2917 | volume=10 | title=Formation of recurring slope lineae on Mars by rarefied gas-triggered granular flows | year=2017 | journal=Nature Geoscience | pages=270–273 | last1 = Schmidt | first1 = Frédéric | last2 = Andrieu | first2 = François | last3 = Costard | first3 = François | last4 = Kocifaj | first4 = Miroslav | last5 = Meresescu | first5 = Alina G.| issue=4 | arxiv = 1802.05018 | s2cid=55016186 }}</ref> The authors demonstrated that the RSLs stopped at an angle of 28° in Garni crater, in agreement with dry granular avalanche. In addition, the authors pointed out several limitations of the wet hypothesis, such as the fact that the detection of water was only indirect (salt detection but not water). This theory pushed back the dry flow theory. 
Research published in November 2017 concludes that the observations are best explained by dry flow processes,<ref name='dry flows 2017'>[https://www.jpl.nasa.gov/news/news.php?feature=7005 Recurring Martian Streaks: Flowing Sand, Not Water?]. ''JPL News'', NASA. 20 November 2017.</ref><ref name='Dundas 2017'>{{cite journal | doi = 10.1038/s41561-017-0012-5 | volume=10 | title=Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water | year=2017 | journal=Nature Geoscience | pages=903–907 | last1 = Dundas | first1 = Colin M. | last2 = McEwen | first2 = Alfred S. | last3 = Chojnacki | first3 = Matthew | last4 = Milazzo | first4 = Moses P. | last5 = Byrne | first5 = Shane | last6 = McElwaine | first6 = Jim N. | last7 = Urso | first7 = Anna| issue=12 | hdl = 10150/627918 | s2cid=24606098 | hdl-access = free }}</ref><ref name='Raack 2017'>[https://www.space.com/38790-mars-mystery-land-formation-with-little-water.html A Mars Mystery: How Did Land Form Without Much Water?].  Jan Raack. ''Space.com''. 21 November 2017.</ref> and remark that there is no actual spectrographic evidence for water.<ref name='Dundas 2017'/><ref name="WRD-20140218" /> Their research shows RSL exist only on slopes steeper than 27 degrees, enough for dry grains to descend the way they do on faces of active dunes.<ref name='dry flows 2017'/> The RSL do not flow onto shallower than 27 degree slopes, which is inconsistent with models for water.<ref name='Dundas 2017'/> A 2016 report also cast doubt on possible sources of underground water at RSL sites,<ref>[https://www.jpl.nasa.gov/news/news.php?feature=6562 Mars Canyons Study Adds Clues about Possible Water]. ''JPL News, NASA. 7 July 2016.</ref> but the new research article acknowledged that hydrated salts could draw some humidity from the atmosphere and seasonal changes in hydration of salt-containing grains might result in some trigger mechanism for RSL grainflows, such as expansion, contraction, or release of some water, that would change the cohesion of grains and cause them to fall or "flow" downslope.<ref name='dry flows 2017'/> Furthermore, neutron spectrometer data by the ''Mars Odyssey'' orbiter obtained over one decade, was published in December 2017, and shows no evidence of water (hydrogenated regolith) at the active sites, so its authors also support the hypotheses of either short-lived atmospheric water vapour deliquescence, or dry granular flows. Nevertheless, the footprint of this instrument (~100&nbsp;km) is much larger than the RSLs (~100m).<ref name='Jack Wilson'/>

==Habitability and planetary protection ==

These features form on Sun-facing slopes at times of the year when the local temperatures reach above the melting point for ice. The streaks grow in spring, widen in late summer and then fade away in autumn. Since these features could involve water in some form, and even though this water could still be too cold or too salty for life, the corresponding areas are currently treated as potentially habitable. Hence they are categorized in [[Astronomy:Planetary protection|planetary protection]] recommendations as "Uncertain Regions, to be treated as [[Astronomy:Planetary protection#Mars special regions|Special Regions]]" (i.e. a region on the Mars surface where Earth life could potentially survive).<ref name="RummelBeaty2014">{{cite journal|last1=Rummel|first1=John D.|last2=Beaty|first2=David W.|last3=Jones|first3=Melissa A.|last4=Bakermans|first4=Corien|last5=Barlow|first5=Nadine G.|last6=Boston|first6=Penelope J.|last7=Chevrier|first7=Vincent F.|last8=Clark|first8=Benton C.|last9=de Vera|first9=Jean-Pierre P.|last10=Gough|first10=Raina V.|last11=Hallsworth|first11=John E.|last12=Head|first12=James W.|last13=Hipkin|first13=Victoria J.|last14=Kieft|first14=Thomas L.|last15=McEwen|first15=Alfred S.|last16=Mellon|first16=Michael T.|last17=Mikucki|first17=Jill A.|last18=Nicholson|first18=Wayne L.|last19=Omelon|first19=Christopher R.|last20=Peterson|first20=Ronald|last21=Roden|first21=Eric E.|last22=Sherwood Lollar|first22=Barbara|last23=Tanaka|first23=Kenneth L.|last24=Viola|first24=Donna|last25=Wray|first25=James J.|title=A New Analysis of liquid "Special Regions": Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2)|journal=Astrobiology|volume=14|issue=11|year=2014|pages=887–968|issn=1531-1074|doi=10.1089/ast.2014.1227|pmid=25401393|url=https://www.researchgate.net/profile/David_Beaty/publication/268444482_A_new_analysis_of_Mars_Special_Regions_findings_of_the_second_MEPAG_Special_Regions_Science_Analysis_Group_SR-SAG2/links/547c9b0b0cf27ed9786229dd.pdf<!--|page=902-->|bibcode=2014AsBio..14..887R}}</ref>

While the wet flows hypothesis has lost some ground since 2015,<ref name="SA-20181122"/><ref name='Grossman 2018'/><ref name='dry flows 2017'/><ref name='Dundas 2017'/> these regions are still amongst the most favoured candidate sites to support Earth bacteria brought by contaminated landers. Some recurring slope lineae are in reach of the [[Engineering:Curiosity (rover)|''Curiosity'' rover]] but planetary protection rules have prevented close exploration by the rover.<ref name="NYT-20151005-kc" /><ref name="divert-Curiosity">{{cite journal |last1=Witze |first1=Alexandra |title=Mars contamination fear could divert Curiosity rover |journal=Nature |date=7 September 2016 |volume=537 |issue=7619 |pages=145–146 |doi=10.1038/537145a |pmid=27604926 |doi-access=free }}</ref> This has led to some debate about whether these rules should be loosened.<ref name="fairen-2017">{{cite journal |last1=Fairen |first1=Alberto G. |last2=Parro |first2=Victor |last3=Schulze-Makuch |first3=Dirk |last4=Whyte |first4=Lyle |title=Searching for Life on Mars Before It Is Too Late |journal=Astrobiology |date=1 October 2017 |volume=17 |issue=10 |pages=962–970 |doi=10.1089/ast.2017.1703 |pmid=28885042 |pmc=5655416 }}</ref><ref name="Schulze-2017">{{cite web |last1=Schulze-Makuch |first1=Dirk |title=It's Time to Loosen Planetary Protection Rules for Mars |url=https://www.airspacemag.com/daily-planet/its-time-loosen-planetary-protection-rules-mars-180964845/ |website=Air and Space magazine |publisher=Smithsonian |access-date=3 January 2019}}</ref>

==Recurrent slope lineae near equator==
<gallery class="center" widths="190px" heights="180px" >

Viking image of Mars with arrow showing location of seasonal flows.jpg|Image of disk of Mars taken by Viking.  Arrow shows location of recurrent slope lineae in following HiRISE images.
49955 1665rslcontextmap.jpg|Labeled map of features near to Coprates Chasma.  Arrow shows location of recurrent slope lineae in following HiRISE images.

ESP 049955 1665rslbox.jpg|Wide view of part of Valles Marineris, as seen by HiRISE under HiWish program  Box shows location of recurrent slope lineae that are enlarged in next image.
49955 1665rslcolorarrows.jpg|Close, color view of recurrent slope lineae, as seen by HiRISE under HiWish program  Arrows point to some of the recurrent slope lineae.  Fan may have been built up by past recurrent slope lineae.

49955 1665rsldrawing6.jpg|Recurrent slope lineae elongate when the slopes are at their warmest.  Near the equator, RSL elongate on northern slopes in the northern summer and on the southern slopes in the southern summer. 
</gallery>

==Gallery==

<gallery  widths="200px" heights="180px" >
File:PIA14470 Ice, Salt and Warm-Season Flows on Mars.jpg|Ice (white), salt (red) and warm-season flows (blue) on Mars
File:Dark Flows in Newton Crater Extending During Summer (animated).gif|Dark flows in Newton Crater extending during summer (video-gif).
File:Warm Season Flows on Slope in Horowitz Crater (animated).gif|Warm season flows on slope in Horowitz Crater (video-gif).
File:PIA19805-SeasonalFlows-CopratesChasma-VallesMarineris-20150721.jpg|Seasonal flows on [[Astronomy:Coprates Chasma|Coprates Chasma]] in [[Astronomy:Valles Marineris|Valles Marineris]].
</gallery>

==See also==

{{colbegin|colwidth=30em}}
* [[Astronomy:Climate of Mars|Climate of Mars]] 
* Evolution of water on Mars and Earth 
* [[Astronomy:Geology of Mars|Geology of Mars]]
* Gullies on Mars
* [[Astronomy:Life on Mars|Life on Mars]]
* Mars Ocean Hypothesis 
* [[Astronomy:Outflow channels|Outflow channels]]
* [[Astronomy:Planetary protection|Planetary protection]]
* [[Astronomy:Water on Mars|Water on Mars]]
{{colend}}

==References==
{{Reflist|30em}}

==External links==
*[http://www.nasa.gov/mission_pages/MRO/multimedia/gallery/gallery-index.html NASA Picture Gallery] on Seasonal flows on warm Martian slopes.
* Stillman, D., et al.  2017.  Characteristics of the numerous and widespread recurring slope lineae (RSL) in Valles Marineris, Mars.  Icarus.  Volume 285.  Pages 195-210
{{Geography of Mars}}

[[Category:Surface features of Mars]]
[[Category:Water on Mars]]

{{Sourceattribution|Seasonal flows on warm Martian slopes}}