Potentially habitable flow-like features from Martian dry ice geyser dune spots: Difference between revisions

These features near the Martian polar regions are associated with the [[Geyser (Mars)|Martian Geysers]]. Before these geysers were well understood, there was a lot of speculation about what they might be. The seasonal patterns they form resemble trees and vegetation, and in 2001 looking at the Mars Global Surveyor images, Arthur C. Clarke called them "Banyan trees"<ref name=Foulke2001>Nicole Foulke, [https://www.popsci.com/military-aviation-space/article/2001-12/banyan-trees-mars The Banyan trees of Mars], Popular science e-mail interview with Arthur C. Clarke, December 17, 2001</ref>, saying, only half joking "I'm now convinced that Mars is inhabited by a race of demented landscape gardeners,"<ref name=ClarkeSmithsonian2001>Arthur C. Clarke, speaking by teleophone for the [http://www.martianspiders.com/Sir%20Arthur%20C_%20Clarke%20at%20the%20Smithsonian,%20June%202001.htm Wernher von Braun Memorial Lecture], Smithsonian institute's National Air and Space Museum, June 6, 2001 - reported by John C. Sherwood</ref>, and a team of Hungarian scientists proposed that they might be the result of spreading colonies of overwintering photosynthetic microbial life. <ref name=Andras/>

Most of the patterns that form here are probably due to dry ice effects. Subsurface layers of dry ice are heated by the sun through the solid state greenhouse effect, and erupt as CO2 gas. The dark streaks and spots are thought to be debris from the geysers.

However, later in the year dark streaks gradually extend down the slopes from the dark spots, sometimes at a rate of meters per day. These are the "flow-like features" that may be associated with liquid brines. There are streaks in both hemispheres but the details of how they form differ.

In the Southern hemisphere, both of the current models for this part of the process involve liquid water<ref name="MartínezRenno2013DarkDuneSpots"/>. In one of these models the features from initially as subsurface melt water, fresh water that forms at 0 °C below snow-ice packs, in solid state greenhouse effects - they are optically thin in visible light but opaque to thermal infrared, so trapping heat in a solid state greenhouse effect familiar in similar situations in Antarctica<ref name="MartínezRenno2013SubsurfaceMeltWater>{{cite journal|url=https://link.springer.com/article/10.1007%2Fs11214-012-9956-3/fulltext.html|last1=Martínez|first1=G. M.|last2=Renno|first2=N. O.|title=Water and Brines on Mars: Current Evidence and Implications for MSL section 2.2.2 Subsurface Melt Water|url=https://link.springer.com/article/10.1007%2Fs11214-012-9956-3#Sec6|journal=Space Science Reviews|volume=175|issue=1-4|year=2013|pages=29–51|issn=0038-6308|doi=10.1007/s11214-012-9956-3}}</ref>. The other model involves thin layers of ULI water (undercooled liquid water)<ref name="MartínezRenno2013ULIWater>{{cite journal|url=https://link.springer.com/article/10.1007%2Fs11214-012-9956-3/fulltext.html|last1=Martínez|first1=G. M.|last2=Renno|first2=N. O.|title=Water and Brines on Mars: Current Evidence and Implications for MSL section 2.2.1 Undercooled Liquid Interfacial Water|url=https://link.springer.com/article/10.1007%2Fs11214-012-9956-3#Sec5|journal=Space Science Reviews|volume=175|issue=1-4|year=2013|pages=29–51|issn=0038-6308|doi=10.1007/s11214-012-9956-3}}</ref> which form on the surface of solar heated grains, then flows downwards, supplying several litres of water per day to the features. In both cases they then pick out salts which let them remain liquid in the cold near surface conditions as they flow down the slopes.

The northern hemisphere flow like features form at much lower surface temperatures and if they involve brines the temperatures are around -90 °C, too low in temperature to be habitable for Earth life, at least. Also, though most of the models for the northern hemisphere features involve water, they can also be explained with dry ice and cascading dust. <ref name="MartínezRenno2013DarkDuneSpots"/>

[[File:Seasonal processes in the Northern polar dunes with Flow Like Features.gif|thumb|Seasonal processes in the Northern polar dunes with Flow Like Features. Time differences between the images are 22 days and 12 days. The final picture shows a long feature that formed new between the two images, and its length is 60 meters so it grew at a rate of at least 5 meters per day.<br><br>These features form at a much lower temperature than the southern hemisphere flow like features, at -90°C average surface temperature on kilometers scale - though the dark features are expected to be considerably warmer, and the subsurface is also expected to be heated by the solid state greenhouse effect through surface layers of dry ice (similarly to the proposed models for the Martian Geysers).<br><br>They progress through a sequence of changes, first wind blown, and then seepage features associated with the dune spots, and then finally, dark seepage features appear all along the dune crest as in this sequence. These images show the growth of the seepage features.<ref name=Kereszturi/>]]

The flow like features in the northern hemisphere polar ice cap form at average surface temperatures of around 150°K - 180°K, i.e. up to -90&nbsp;°C approximately.

The flows start as wind-blown features but then are followed by seepage features which increase at between 0.3 meters and 7 meters a day.<ref name="MartínezRenno2013DarkDuneSpots"/><ref name=Kereszturi>Kereszturi, A., et al. [https://www.researchgate.net/profile/Szaniszlo_Berczi/publication/222062822_Indications_of_brine_related_local_seepage_phenomena_on_the_northern_hemisphere_of_Mars/links/0fcfd509b9bbd37ff0000000.pdf "Indications of brine related local seepage phenomena on the northern hemisphere of Mars."] Icarus 207.1 (2010): 149-164.</ref>

The models suggest that subsurface melt water layers, and interfacial water could form with surface temperatures as low as 180°K (-90&nbsp;°C). Salts in contact with them could then form liquid brines.<ref name=Kereszturi/><ref name="MartínezRenno2013"/>

An alternative mechanism for the Northern hemisphere involves dry ice and sand cascading down the slope but most of the models involve liquid brines for the seepage stages of the features.<ref name="MartínezRenno2013DarkDuneSpots"/>

For details see the Dark Dune Spots section of Nilton Renno's paper<ref name="MartínezRenno2013DarkDuneSpots"/> which also has images of the two types of feature as they progress through the season.