Click for full image.
The image to the right, cropped to post here, is a captioned photo from the high resolution camera on Mars Reconnaissance orbiter and released today. From the caption:
The black spots [recent impacts] form because the craters exposed cleaner materials in the subsurface beneath the bright, dusty surface.
Our image is also interesting because the surface has a criss-cross pattern formed by wind activity. Bright ripples that are oriented from the upper right to the lower left are perpendicular to the wind flow. In contrast, outcrops that have been eroded by the wind are oriented perpendicular to the ripples to produce the criss-cross pattern we now observe.
The overview map below might also help explain this criss-cross pattern.
The caption notes that this image, indicated by the red cross, is located near the large meandering canyon dubbed Mangala Valles. What must also be realized is its proximity to both Mars’ giant volcanoes as well as the planet’s largest volcanic ash deposit, dubbed the Medusae Fossae Formation.
The darker northwest-to-southeast ridges appear to be ash deposits that are being eroded away by the wind, thus explaining their orientation parallel to the prevailing wind direction. If you go the Medusae Fossae link above, you will see from the cool image there the resemblance these ridges have to that eroding ash deposit.
The lighter northeast-to-southwest ridges appear to be the underlying bedrock, their orientation caused not by wind but by a more fundamental geological process related to the complex formation of Mangala Valles itself, as discussed in this paper [pdf]
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