Io so che tutte le foto marziane in HD compaiono in una pagina come questa, con un numero PIA:

The PDS-product color is given the same minimum and maximum value, in calibrated units of reflectance (I/F), for each color bandpass. The result is that the colors are relatively bland, but they are also consistent from place to place on Mars. (The PDS label (.LBL file) provides the information for converting the DN value to I/F: I/F = DN*SCALING_FACTOR + OFFSET.) It isn't natural color, as seen by normal human eyes, because the IR, RED, and BG channels are displayed in red, green, and blue colors. For the Extras products, each color band is individually stretched to maximize contrast, so the colors are enhanced differently for each image based on the color and brightness of each scene. Scenes with dark shadows and bright sunlit slopes or with both bright and dark materials are stretched less, so the colors are less enhanced than is the case over bland scenes.

In spite of the variable level of color enhancement for the Extras products, we can make some generalizations to better understand what the stretched color images are showing. Dust (or indurated dust) is generally the reddest material present and looks reddish in the RGB color and yellow in the IRB color. Coarser-grained materials (sand and rocks) are generally bluer (or sometimes purplish in IRB color) but also relatively dark, except where coated by dust. Frost and ice are also relatively blue, but bright, and often concentrated at the poles or on pole-facing slopes. Some bedrock is also relatively bright and blue, but not as much as frost or ice, and it has distinctive morphologies. The IR and RED bandpasses are often highly correlated so the IR provides little new information, and the RGB color does a better job of showing the RED vs. blue-green color variations by moving these differences into the warmer colors. Also, one of the four channels of IR data has special problems and may be excluded from the RDR products or have anomalies, whereas the RGB color always covers the full color swath, unless some data was lost in transmission to Earth. The IR channels do provide unique information in some small but important areas on Mars, particularly when the mineral olivine or a diversity of minerals altered by water are present. The best way to understand what the colors indicate about composition is to compare them to mineral maps derived from the 18 m/pixel CRISM data (see http:/ /crism.jhuapl.edu/). HiRISE color images enable extrapolation of the CRISM results to smaller spatial scales.