A bright streak in an image taken in September 2005 (right) represents new sediment deposits laid down sometime since a previous image was taken in August 1999 (left). Both shots feature the same section of an unnamed crater in Terra Sirenum, an area in Mars's southern hemisphere.

Many scientists theorize that liquid water briefly flowing across the red planet's surface is the only logical explanation for the streak. But other experts counter that the deposits are just as likely to be the result of a liquid carbon dioxide stream.

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