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Mysterious White Rock Fingers on Mars

What caused this unusual white rock formation on Mars? Intrigued by the possibility
that they could be salt deposits left over as an ancient lakebed dried-up, detailed
studies of these fingers now indicate that this is not correct. The light material
appears to have eroded away from the surrounding area, indicating a very low-density
composition, possibly consistent with volcanic ash or windblown dust. The stark
contrast between the rocks and the surrounding sand is compounded by the sand's
unusual darkness. This picture was taken from the Mars Express spacecraft currently
orbiting Mars. Planetary scientist Emily Lakdawalla, among others, has followed her
curiosity about this unusual Martian landform into a fascinating investigation that is
eloquently described in the Planetary Society Weblog. The mysterious white rock spans
about 15 kilometers across inside a larger crater that spans about 100 kilometers.