The THEMIS image is actually in the caldera of Alba Patera, which you would expect for a geothermal hot spot.
This MGS image near the same area shows two dark streaks at the bottom:
MOC narrow-angle image R16-00268
North middle-latitude crater in Alba Patera caldera.
http://www.msss.com/moc_gallery/r16_.../R1600268.html
They both have the feature of widening from a central origin point. I consider this characteristic of material being vented and then being spread aerially by wind. If they arose simply from a dust deposit, you would expect the streaks to only extend straight out from the deposit with the width staying the same as the width of the deposit. Alba Patera is known to have had recent lava flows. Dark streaks whatever their cause would have to be geologically recent. Due to dust cover we would actually expect dark streaks to only have a lifetime measured in millenia or less.
There are many other dark streaks in the accompanying context image which also have the feature of widening from a central point. The famous "slope dark streaks" near the equator also have this feature, but those are due clearly to gravity-induced flow down a slope. The dark streaks in the Alba Patera region have the appearance of wind carried flow, as well as being far from the equator. However, the slopes directions do need to be checked by MOLA measurements to be sure of this.
But particular evidence these streaks are wind-developed as well as to the possibility of venting is provided by the upper of the two steaks in image R1600268. This streak appears not to originate from a nearby crater, but slightly above the crater in the image. But by enlarging the image you see this streak actually does originate from the crater. What happens is the streak is directed upwards in the image, then horizontally to the right. This could be due to a venting directing the vented material in one direction initially, then the prevailing wind directing it to the right afterwards.
I argued as well this characteristic of a wind-developed streak widening from the origin point as indicative of a venting here:
Newsgroups: sci.astro, alt.sci.planetary, sci.geo.geology
From: rgregorycl...@yahoo.com (Robert Clark)
Date: 13 Feb 2004 07:03:49 -0800
Subject: Re: Active volcanic vent on Mars?
http://groups.google.com/group/sci.g...98bf4b3a4d15ec
Another released THEMIS image in the Alba Patera area, though on the flanks not the caldera, also shows streaks:
Alba Patera Windstreaks (Released 1 June 2005)
http://themis.asu.edu/zoom-20050601A.html
The accompanying caption discusses an unusual feature of these streaks, the dark streaks seem to split apart in two in the lee of a crater. The caption ascribes this to a vortex in the lee. However, similar features near craters at the south pole have been ascribed to venting of CO2 by Mars scientists:
Newsgroups: sci.astro, alt.sci.planetary, sci.geo.geology
From: rgregorycl...@yahoo.com (Robert Clark)
Date: 13 Mar 2004 20:31:20 -0800
Subject: Re: Active volcanic vent on Mars?
http://groups.google.com/group/sci.g...da137b7c3474c4
It would be interesting to find out by experiments which if either of these explanations can reproduce this effect.
Another image of the Alba patera caldera might show indications of wide-spread venting:
MOC Image m0300999 - Browse Page.
http://ida.wr.usgs.gov/html/m03009/m0300999.html
The image shows a highly pockmarked surface that could indicate gas release: rather than simply being a highly cratered surface, the holes here have the appearance of being due to surface or subsurface processes.
However, another interpretation of this surface is that it is karst terrain. Karst terrain is due to acidic water dissolution of evaporitic deposits such as carbonate and sulfate:
A KARST PRIMER.
http://csweb.winona.edu/semnwrb/file...t/sprkarst.htm
Karst in Alba Patera has been proposed before. See the caption here:
THEMIS: Image Detail: Alba Patera.
http://www.marstoday.com/news/viewsr.html?pid=5278
This image also has the appearance of karst terrain:
Relay 16 Test traverse in western caldera of Alba Patera
http://www.msss.com/moc_gallery/e01_.../E0502944.html
This dissolution of the surface material causing a pockmarked surface is also described here:
SIMPLE NON-FLUVIAL MODELS OF PLANETARY SURFACE MODIFICATION, WITH
APPLICATION TO MARS. A. D. Howard1, 1Department of Environmental Sciences, P.O. Box 400123, University
of Virginia, Charlottesville, VA 22904-4123, *****@virginia.edu
http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1054.pdf
A related phenomenon proposed to exist in Alba Patera is thermokarst, similar to karst except it occurs in permafrost:
Possible thermokarst and alas formation in Utopia Planitia, Mars
Richard J. Soare,1 J.M. Wan Bun Tseung1 and Claude Peloquin.1 Dept. of Geography, Planning and Environment,
Concordia University, 1455 De Maisonneuve W., Montreal, Canada, H3G 1M8. E-mail: ******@colba.net
http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1103.pdf
If thermokarst, we would expect the age to young as well perhaps less than millenia, in connection to the relative youth of permafrost which arises and disappears with periodic ice ages.
Bob Clark