Secular Climate Change on Mars: An Update

The stability of the South Polar Residual Cap (SPRC) has been in question since Leighton & Murray (1966) theorized that the cap is predominantly CO2 ice in solid-vapor equilibrium with the atmosphere. In 2001, Malin et al. reported a net loss of cap mass that Blackburn et al. (2010) calculated would sublime the SPRC by the end of the decade. Also in 2010, Haberle & Kahre analyzed Phoenix and VL2 pressure data to quantify the net loss of CO2 from the SPRC since the time of the Viking Missions. Though their loss estimates were consistent with Malin et al. (2001), the unclear accuracy of the pressure sensor and limited data available from Phoenix rendered their study inconclusive. This study modifies the process Haberle & Kahre (2010) use to quantify the change in atmospheric mass since VL2 and is improved by the known accuracy and stability of the MSL pressure sensor and its longer data set (two complete Mars Years). Modifications include excluding warm up errors in the MSL data, correcting for rover elevation changes, binning the data into hourly bins for daily averages, and excluding periods when both MSL and VL2 data are not simultaneously present when calculating annual means. An ensemble of Ames GCM simulations are used to define the offset that accounts for dynamical and physical differences between MSL and VL2. From these calculations we find an estimated net loss of 5 Pascals per Mars Decade of atmospheric CO2 , which is comparable to the year 2 MSL sensor accuracy ( 4 Pa). Given this, and the uncertain accuracy of the VL2 sensor, we see no compelling evidence for secular climate change. This result is consistent with Thomas et al's (2016) recent refinements in actual cap loss rates. However, since the Ames GCM runs at fairly coarse horizontal resolution we plan to use the higher-resolution GFDL FMS/FV3 GCM capable of resolving Gale Crater and its circulation to re-calculate the offset and obtain a more accurate loss/gain rate in the near future.