Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) north polar springtime recession mapping: First 3 Mars years of observations

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We report on mapping of the north polar region of Mars using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument. We have observed 3 Mars years (28–30) of late winter and spring recessions (Ls = 304°–92°). Our investigations have led to the following observations. (1) We classify the retreat of the north polar seasonal cap into “presublimation,” “early spring,” “asymmetric” and “stable” periods according to the prevalent H2O ice grain size distributions. (2) During the early spring, the signatures of CO2 ice at the edge of the cap are obscured by H2O ice, which increases the apparent size of the H2O ice annulus around the seasonal CO2 cap at this time. At around Ls = 25°, this process changes into an asymmetrical distribution of H2O deposition, covering CO2 signatures more rapidly in the longitude range from 90 to 210°E. (3) We detect signatures of “pure” CO2 ice in extremely limited locations (in Lomonosov Crater) even in midwinter. H2O ice signatures appear everywhere in the retreating CO2 seasonal cap, in contrast with the south polar seasonal cap. (4) We find that average H2O ice grain sizes continuously increase from northern midwinter to the end of springtime; this is the inverse of the behavior of CO2 ice grain sizes in the southern springtime.