Searching for Seasonal Jets on Mars in CaSSIS and HiRISE Images

EPSC Abstracts Vol. 13, EPSC-DPS2019-388-2, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019. CC Attribution 4.0 license. Searching for Seasonal Jets on Mars in CaSSIS and HiRISE Images Candice Hansen (1), Susan Conway (2), Ganna Portyankina (3), Nicolas Thomas (4), Alfred McEwen (5), Jason Perry (5), Antoine Pommerol (4), Camila Cesar (4). (1) Planetary Science Institute, Arizona, USA (cjhansen@psi.edu), (2) CNRS, Université de Nantes, France, (3) University of Colorado, Boulder, CO, USA (4) University of Bern, Switzerland, (5) University of Arizona, Tucson, Arizona USA Abstract Spring is an active time on Mars as the CO2 seasonal cap sublimates. Stereo images from HiRISE and CaSSIS have been collected to search for the gas jets that produce seasonal fans visible every spring. 1. Introduction Every year in the spring fans appear on Mars’ seasonal polar cap. The generally accepted Kieffer model [1] is that sunlight penetrates the translucent layer of CO2 ice, and warms the surface, which causes basal sublimation of the layer of seasonal ice. Gas trapped below the ice builds up pressure until the overlying ice ruptures and the gas escapes, entraining material from the surface. The particles are carried out, get blown downwind, and fall to the surface in fan-shaped deposits (see Figure 1). Figure 1: Fan-shaped deposits appear on top of the seasonal layer of ice every spring. This image, ESP_011671_0935, was taken at 86.4S / 99.0E, at Ls 196 in a region informally known as Manhattan. Seasonal activity has been studied with the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) for 7 Mars years, and images are largely consistent with the Kieffer model [2, 3, 4]. However, the best proof would be to capture a jet fountaining up with its load of surface dust. In pursuit of this goal we can now deploy the Colour and Stereo Surface Imaging System (CaSSIS) on the ExoMars Trace Gas Orbiter. 2. HiRISE Search for Jets Every spring tens of thousands of fans appear on the seasonal cap. Ideally a stereo pair of HiRISE images would detect the jet of particles shooting above the surface. It turns out to be surprisingly difficult to achieve this seemingly simple objective. The amount of gas released is limited by the supply, and might last just minutes to hours. The closest spacing for HiRISE images is on adjacent orbits, with ~2 hr separation, but only at latitudes poleward of 80o. Given other constraints on sequencing it is typical to get just one of these stereo pairs per week. At the most active time in the spring that means that only 2 – 5 stereo pairs may be scheduled. In the last 7 Mars years of MRO operation only ~23 pairs have been returned. While the stereo data shows very interesting detail on surface morphology no jets have been detected. Another limitation is that the MRO orbit is fixed at a mean local solar time of ~3 pm. It is possible that the jets are more likely to erupt in the morning or midday. If that is the case HiRISE will never be able to image them. The most promising HiRISE image to-date is shown in Figure 2, in Russell Crater dunes. Dust appears to be billowing from one of the linear gullies. At ~54o south latitude these dunes are covered with a seasonal layer of ice every winter, and show typical spring activity with fans and blotches. But is this an example of gas being released and carrying material out in the manner hypothesized in the Kieffer model? Without stereo data to pin down the source and vent structure it is difficult to be certain. are on the order of 70m [6], but this is relevant to the higher latitudes and longer fans. 4. Summary and Conclusions A fair amount of luck is required to catch an eruption underway. The combination of HiRISE and CaSSIS means that almost all latitudes are accessible with stereo. HiRISE has the advantage of higher resolution and larger fans to observe than CaSSIS, but CaSSIS offers more opportunities. It is also possible that jets will never be imaged if the dust load is not opaque enough to be visible in images acquired at relatively low phase angles. The opacity of the jet may just not be enough to be detectable, but we will continue to look. Figure 1: This image of Russell crater dunes, ESP_047078_1255, was taken at 54.3S / 12.9E, Ls 202. A thin cloud of dust might be coming from a rupture in the ice. Acknowledgements We would like to thank our funding source for this work: NASA and the MRO project. 3. CaSSIS Opportunities References The CaSSIS camera [5] has the ability to complete a stereo pair within minutes. The most poleward latitude is limited by the inclination of the TGO orbit to 74o, but the full range of latitudes equatorward of this limit can be imaged in stereo quickly. Furthermore, the TGO orbit evolves through local time, giving us the opportunity to image over the full range of a martian day. [1] Kieffer, H., Cold jets in the Martian polar caps, J. Geophys.Res. 112:E08005 (2007). A particularly opportune time took place from May to July of 2018, when high latitudes were visible at local morning between 6 and 10 local time. At the date of this writing 16 stereo pairs have already been analyzed to look for plumes. As of yet none have been identified, but the mere fact that we have as many stereo pairs to look at as in the entire 7 Mars years of HiRISE observations bodes well. One limitation that works against CaSSIS is its lower resolution (~5m) relative to HiRISE (~1m). Fans are typically shorter at the latitudes CaSSIS observes, which may be related to the gas supply and the thickness of the seasonal ice layer. If this is related to the height the jets achieve, which is likely [6], then it will be more difficult to identify a jet with certainty in the CaSSIS stereo images. Estimates for jet height [2] Hansen, C. J. et al., HiRISE observations of gas sublimation-driven activity in Mars’ southern polar regions: I. Erosion of the surface, Icarus 205:283 (2010). [3] Thomas, N. et al., HiRISE observations of gas sublimation-driven activity in Mars’ southern polar regions: II. Surficial deposits and their origins, Icarus 205:296 (2010). [4] Portyankina, G. et al., HiRISE observations of gas sublimation-driven activity in Mars’ southern polar regions: III. Models of processes involving translucent ice. Icarus 205:311 (2010). [5] Thomas, N. et al., The Colour and Stereo Surface Imaging Systemfor the ExoMars Trace Gas Orbiter, Space Science Reviews 212:1897 (2017). [6] Thomas, N. et al., HiRISE observations of gas sublimation-driven activity in Mars’ southern polar regions: IV. Fluid dynamics models of CO2 jets, Icarus 212:66 (2011).