Possible Atmospheric Water Vapor Contribution from Martian Swiss Cheese Terrain

Mars's south polar residual cap (SPRC) is a several-meters-thick CO2 ice cap with a variety of features, including quasi-circular depressions known as "Swiss cheese" that may expose underlying water ice. Swiss cheese pits have been suggested as a source for the observation of unusually high water vapor during the southern summer of Mars Year (MY) 8 (1969). To evaluate this hypothesis, we map the current extent of Swiss cheese pits to estimate the contribution to atmospheric water vapor from sublimation from the pits. We use data from the Mars Reconnaissance Orbiter Context Camera to map individual features and use the Mars Climate Sounder to obtain surface temperatures to estimate areas of exposed water ice to infer the amount of water vapor sublimated under typical south polar summer atmospheric conditions. We find that there is a negligible impact on atmospheric water vapor from sublimation with the current coverage and temperatures of Swiss cheese terrain (0.2% of the SPRC at an average of &sim;161 K). At current typical temperatures, complete removal of residual CO2 from 99% of the SPRC would be required to sublimate enough water vapor to reproduce the MY 8 observation. However, a modest increase in temperature (&sim;20 K) could lead to a dramatic increase in sublimation rate, such that only water ice over 2.6% of the SPRC area would recreate the MY 8 observation. &gt;180 K surface water ice has been observed on Mars, but such temperatures are likely transient at the south pole over the past &sim;30 Mars years. &nbsp;</p

Three Years of ACB Phase Function Observations from the Mars Science Laboratory: Interannual and Diurnal Variability and Constraints on Ice Crystal Habit

We examine 3 yr of phase-function observations of water-ice clouds taken during the Aphelion Cloud Belt season by the Mars Science Laboratory (MSL). We derive lower-bound single-scattering phase functions for Mars years (MYs) 34, 35, and 36, over a range of scattering angles from 45° to 155°, expanding on the MY 34 phase function previously derived from MSL observations using the same method. We also modify the procedure used for MY 34 to make use of cloud opacity values derived from other MSL observations, often taken in conjunction with the phase-function observations. From these, we see little variability, both interannually and diurnally in the phase function at Gale Crater. We use our derived phase functions to attempt to constrain a dominant ice-crystal geometry by fitting a two-term Henyey–Greenstein function. In comparing to HG functions of Martian dust and modeled water-ice crystals, we see agreement especially with droxtal water-ice crystals, dust at Gale crater, and irregular volcanic glasses. This could be indicative of crystals composed of some irregular shape