Members of the Pathfinder imaging team noticed a bluish tint on some surfaces of rocks at the Pathfinder site:

Photographs show red, white and blue rocks on Mars
http://www.floridatoday.com/space/explore/stories/1997b/072997c.htm
"In panorama photos taken by Pathfinder of its landing site, there is a striking two-color appearance on dozens of rocks, Smith said.
"You divide the scene into two parts - the red part and the blue part,' he said. 'There is a very clear difference between the red side and the blue side.'
"Smith said that the blue side is on the east side of the rocks, suggesting that the sometime-ferocious wind storms on Mars may have scoured off a layer, uncovering the color. The west side of the rocks, protected from the wind, are red, probably from layers of Martian soil, he said.
"The strong wind comes through and it could have sandblasted the one side of these rocks," he said. "

This is discussed further in a report given at the 31st LPSC:

Blue Mars
http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1938.pdf

Note the bluish tint on the wind-blown side would be consistent with endolithic "blue-green algae", that is, cyanobacteria, known to exist under surface on rocks in desert environments and in Antarctica:

Search For Martian Life Will Need Good Vibrations
"University of Bradford PhD student Emma Newton is playing a part in helping NASA's planned exploration of Mars which many people hope will lead to discovering life on the planet.
"Emma works in the Department of Chemical and Forensic Sciences studying lichens and cyanobacteria (also known as blue-green algae) for the purpose of understanding the survival strategies adopted by life in the Antarctic.
"Emma said:'whereas some microbial communities live on the rock surface and use sunscreen pigments to protect themselves, others can live within the rocks themselves. These are known as endolithic and it can be pretty difficult to get a look at them. Raman spectroscopy makes it possible for us to examine these communities without damaging either the microbes or their habitat."
http://www.spacedaily.com/news/mars-life-01i.html

The wind on Mars could have removed the dust covering to reveal the endolithic microbial communities underneath.

Also in a post to sci.astro copied below I discuss the fact that Viking imaging scientists also observed blue-green tints on surfaces at the Viking sites. I should note though that the scientist quoted here Edwin Strickland disagrees with Gil Levin's suggestion that such tints could be indications of lichen on Mars. Strickland noted that searches for chlorophyll were negative.
However, Barry DiGregorio in his book _Mars: the Living Planet_ noted that the spectrographic signature of chlorophyll in lichen can be obscured.

The report by Strickland can be found at:

Soil Stratiography and Rock Coatings Observed in
Color Enhanced Viking Lander Images,
by Edwin Strickland,
Proceedings of Lunar and Planetary Science 10, Volume 3, pp. 1192-1194 (1979),
http://adsabs.harvard.edu/cgi-bin/bib_query?1979LPI....10.1192S

Bob Clark

_______________________________________________________
From: Robert Clark (rgclark@my-deja.com)
Subject: Re: Algae on Mars
Newsgroups: alt.sci.planetary, sci.astro, alt.life-mars, sci.bio.misc
Date: 2000/05/14

The paper cited below by Edwin Strickland, "Soil Stratiography and
Rock Coatings Observed in Color Enhanced Viking Lander Images",
Proceedings of Lunar and Planetary Science 10, Volume 3, pp. 1192-1194
(1979), is now available on NASA's Astrophysical Data service,
http://adsabs.harvard.edu/abstract_service.html?nosetcookie=1.
Search under Strickland, E. for this article as well as other
articles by Strickland examining the colors of Mars.

*********************************************
Forum: alt.sci.planetary
Subject: Re: Algae on Mars
Date: 01/30/2000
Author: Robert Clark <rgclark@my-deja.com>

In article <38932465.6A5A213A@louisville.edu>,
"J. Scott Miller" <Scott.Miller@louisville.edu> wrote:
> Bruce Sterling Woodcock wrote:
> >
> > Firstly, I already posted the URL, but here it is again:
> >
> > http://www.biospherics.com/mars/spie/spiehtml.htm
> >
> > Secondly, I'm not talking about sand changing from light to dark from
> > orbit. I'm talking about rocks changing from red to green in front of
> > Viking's camera.
> >
> > Bruce
>
> I have seen the Viking imagery and have seen no such changes in the rocks. I
> have just read the page again and all I see is someone grasping at straws,
> pulling in unrelated material to make a case for something that others simply
> don't see, others, I might add, that probably know more about the topic than
> the individual writing the paper you keep providing a link for. In other
> words, someone who knows nothing about what is going on is trying to tell
> those that do that they are blind. The scenario is often posted here in
> sci.astro, and routinely that individual is shown to be incorrect in their
> underlying assumptions. This would seem to be the case again.
>
> --
> J. Scott Miller, Program Coordinator Scott.Miller@louisville.edu
> Gheens Science Center and Rauch Planetarium
> http://www.louisville.edu/planetarium
> University of Louisville
>

Barry Digregorio in his book _Mars: the Living Planet_,
http://www.amazon.com/exec/obidos/ASIN/1883319587/marsbugstheelect,
discusses that members of the Viking imaging team also noted greenish-
blue patches on rocks at the Viking landing sites after Gil Levin
brought them to their attention:

"Subtle color variations of the martian surface materails were
enhanced in eight Viking Lander color images processed at the U.S.
Geological survey's Flagstaff, Arizona, Image Processing Facility ...
color artifacts due to Lander cameras are minor and do not hamper image
anlalyis.[See Color Plate 15.] Two tests demonstrate that colors in the
enhanced images represent real features of the martian scene.
Ovelapping images taken with separate cameras reveal the same relative
colors. Two images of the same scene taken at different times of day
show that colors of sunlit materials do not depend strongly on
illumination ... five distinct soil colors have been recognized at the
Viking 1 site ... a thin layer of intermediate brightness "green-blue"
soil is present as smooth patches that fill shallow dips and hollows in
the duricrust ... Rocks at the Viking 1, most probably bedrock and many
other loose rocks near the lander, have very light surfaces, with a
green ratio greater thn any other surface material. They appear bright
green in enhanced images ... At the Viking 2 site, only one color
population of rocks appear to be present. Green colored surfaces are
abundant on rocks of all textures."
Edwin Strickland, "Soil Stratiography and Rock Coatings Observed in
Color Enhanced Viking Lander Images", Proceedings of Lunar and
Planetary Science 10, Volume 3, pp. 1192-1194 (1979), as cited on page
184 of _Mars: the Living Planet_.

However, it appears from subsequent discussions of Levin with members
of the Viking imaging team cited by DiGregorio that they did not
investigate the color changes on rocks noted by Levin, though they may
have investigated color changes in the soil itself.

Perhaps Mr. Digregorio can provide the NASA reference numbers for the
images that Levin claims show the color variations with time.

_______________________________________________

"In order for a scientific revolution to occur,
most scientists have to be wrong"
-- Bob Clark
_______________________________________________

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