In a region of the South Pole known informally as “Ithaca,” numerous fans of dark frost form every spring. HiRISE collected a time lapse series of these images, starting at Ls = 185 and culminating at Ls = 294. Pronounced “el-sub-es” Ls is the way we measure time on Mars: at Ls = 180 the Sun passes the equator on its way south; at Ls = 270 it reaches its maximum subsolar latitude and summer begins.

In the earliest image fans are dark, but small narrow bright streaks can be detected. In the next image, acquired at Ls = 187, just 106 hours later, dramatic differences are apparent. The dark fans are larger and the bright fans are more pronounced and easily detectable. The third image in the sequence shows no bright fans at all.

We believe that the bright streaks are fine frost condensed from the gas exiting the vent. The conditions must be just right for the bright frost to condense.

Written by: Candy Hansen  (12 December 2007)