Fan-shaped deposits on top of the seasonal polar caps are routinely observed by HiRISE to emerge in spring and fade in summer [Hansen et al. 2010]. These fans are widely accepted to be a result of CO2 jet eruptions [Kieffer, 2007], and therefore fan lengths, sizes, and shapes provide information about wind directions and strengths at the times such eruptions occur. Seasonal monitoring observations reveal how this process evolves throughout a season and over multiple martian years, and thus inform us about martian climate. Fans were outlined by citizen scientists in the frame of the Planet Four (P4) project at 27 regions of interest (ROIs) around the southern polar region. The analysis of P4 data successfully produced information on wind directions and speeds at those ROIs for the spring season of two martian years (MY 29 and 30) [Aye et al., 2019]. The seasonal fan markings change considerably from one HiRISE image to another at most of these ROIs, and thus consequent temporal variations in wind speeds and directions can be derived. We have, for the first time, compared observed wind directions and speeds derived from P4 with near-surface winds predicted by a mesoscale atmospheric model (MRAMS [Rafkin and Michaels, 2019]) at the same ROIs. At most ROIs P4 winds are broadly consistent with the winds from MRAMS, but the detailed level of consistency varies between different ROIs. This indicates strong influences of local topography and nearby larger-scale features on the local wind patterns. The spatial resolution of HiRISE images (and thus P4 data) is superior to that of any atmospheric model that can be run in a realistic amount of time. Some ROIs thus present additional challenges to the atmospheric simulations because of their smaller-scale topographical features that would deflect near-surface winds. Nevertheless, the fact that the P4 observations and MRAMS winds match at most ROIs supports the hypothesis that fan formation is greatly controlled by the wind, and not just due to ballistic trajectories of material exiting an angled vent. At the same time, the fact that at other ROIs the model cannot satisfactorily match the P4 data even where the topography is generally unremarkable indicates that the model may be missing some important physical process/effect and more work still needs to be done to better understand these CO2 jet phenomena. In summary, HiRISE and consequently P4 data provide new unique near-surface wind data points for both validating atmospheric modeling and quantitatively investigating Mars’ polar seasonal processes.
Aye, K. M. et al., 2019, Icarus, 319, 558
Hansen C. J. et al., 2010, Icarus, 205, 283
Kieffer, H. H. 2007, JGR, 112, 674E08005
Rafkin, S. and Michaels, T., 2019, Atmosphere, 10 (12), 747.