> Nick Hoffman described how to read the THEMIS brightness images here:

> How to do it with NASAVIEW.
> Posted by Nick Hoffman on 8/7/2005 6:02:09 AM
> In Reply to: re: Martian high north latitude hot spots? posted by
> Robert Clark on 8/6/2005 5:06:41 AM
> http://habitablezone.com/space/messages/390308.html

> He discussed this image:

> THEMIS Image Data Page for I02026005.
> CENTER_LATITUDE = 39.1376
> CENTER_LONGITUDE = 251.959
> MINIMUM_BRIGHTNESS_TEMPERATURE = 302.213
> MAXIMUM_BRIGHTNESS_TEMPERATURE = 322.192
> http://themis-data.asu.edu/img/I02026005

> He observed the hottest areas were in the Sun-facing walls in the
> image, which argues against the maximum temperature being due to a hot
> spot.
> But a key fact about this image is that even the minimum temperature is
> quite hot for this latitude. Do a search for instance on
> http://themis-data.asu.edu . Click the Advanced button, then search on
> the IR images in the latitude range 37 to 41. You'll get a couple of
> thousand images. The highest maximal temperatures you'll see will be
> overwhelmingly in the 250's. There are only perhaps 2 or 3 in the
> 300's. So this particular image even has it's minimal temperature 40 to
> 50 degrees hotter than most other maximum temperatures at that
> latitude.
> In reading some articles about the brightness temperatures it turns out
> they are affected by albedo as well as surface temperature. So you
> might guess this is an unusually bright area. But the corresponding
> visual image does not give that impression:

> THEMIS Image Data Page for V02026006.
> http://themis-data.asu.edu/img/V02026006

> It would be interesting to find out if current or past Mars Global
> Surveyor TES imaging also shows this to be an unusually warm area.

> Bob Clark

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_r21/images/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.geo.geology/msg/4d98bf4b3a4d15ec

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.geo.geology/msg/efda137b7c3474c4

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/files/general/sprkarst/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_e06/images/E05/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 be 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