MARCI and MOC observations of the atmosphere and surface cap in the north polar region of Mars

We used MGS-MOC and MRO-MARCI daily mapping images of the North Polar Region of Mars from 16 August 2005 (L-s = 270 degrees) to 21 May 2009 (L-s = 270 degrees), covering portions of three consecutive martian years (MY 27-MY 29), to observe the seasonal behavior of the polar ice cap and atmospheric phenomena. The rate of cap regression was similar in MY 28 and MY 29, but was advanced by 3.5 degrees of L-s (similar to 7-8 sols) in MY 29. The spatial and temporal behaviors of dust and condensate clouds were similar in the two years and generally in accord with prior years. Dust storms (>100 km(2)) were observed in all seasons, with peak activity occurring at L-s = 10-20 degrees from 50 degrees N to 70 degrees N and at L-s = 135-140 degrees from 70 degrees N to 90 degrees N. The most active quadrant was 0-90 degrees W in MY 28, shifting to 180-270 degrees W in MY 29. The majority of regional storms in both years developed in longitudes from 10 degrees W to 60 degrees W. During late summer the larger storms obscure the North Polar Region in a cloud of dust that transitions to north polar hood condensate clouds around autumnal equinox. Changes in the distribution of perennial ice deposits, especially in Olympia Planum, were observed between the 2 years, with the MY 29 ice distribution being the most extensive observed to date. Modeling suggests that the small, bright ice patches on the residual cap are not the result of slope or elevation effects. Rather we suggest that they are the result of local meteorological effects on ice deposition. The annual darkening and brightening of peripheral areas of the residual cap around summer solstice can be explained by the sublimation of a brighter frost layer revealing an underlying darker, ice rich layer that itself either sublimes to reveal brighter material below or acts as a cold trap, attracting condensation of water vapor that brightens the surface. An alternative explanation invokes transport and deposition of dust on the surface from the cap interior, and later removal of that dust. The decrease in cap albedo and accompanying increase in near surface atmospheric stability may be related to the annual minimum of polar storm activity near northern summer solstice. (C) 2010 Elsevier Inc. All rights reserved.