The following was sent out by Gerald Neukum, PI for the HRSC camera on Mars Express:

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Subject : AW: Is There Living Green Algae in the Gusev Crater on Mars?

The color of the greenish patches in Gusev crater in the first image released of this site is not correct. Unfortunately an early version of Gusev slipped through where the color had not been adjusted finally to be really close to true color as the human eye would see it in a non-dusty atmosphere of Mars. The early version of Gusev has been taken out of the web in the meantime and been replaced by a newly computed version with proper colors where the patches are grey to black with a blue tint. This is close to what a human eye would see under normal atmospheric conditions. It is not easy presently to arrive at true colors of the Martian landscape in the images because Mars is very dusty and the scenes were taken at high sun. The scattering properties under these atmospheric conditions are such that the dust in the atmosphere acts as tiny red filters and one looks at the surface as if one observed it in a diffuse reddish glow with somewhat fuzzy appearance. Imagine you would have to judge a color of a cloth in a room with artificial reddish light: very difficult.
The colors in the other picture released so far are rather close to true colors but a slight tint in reddish or bluish direction from non-perfect adjustments of the color histograms cannot be excluded due to the somewhat abnormal "seeing" conditions with large amounts of dust under high sun.
Best regards,
Gerhard Neukum
HRSC Principal Investigator

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The new version of the image of Gusev is available here (German text):

http://www.dlr.de/mars-express/images/230104/bilder_230104.html

Bob Clark