Dust devil track survey at Elysium Planitia, Mars: Implications for the InSight landing sites

Abstract

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) robotic lander is scheduled to land in Elysium Planitia on Mars in September 2016. InSight will perform the first comprehensive surface-based geophysical investigation including seismic measurements. Knowledge about encounter rates of dust devils with the InSight lander are important for two main reasons: (1) dust devils will affect the scientific measurements, i.e., wind-induced seismic noise, and (2) the power-supply of the InSight lander and instruments is provided by solar arrays and previous landers and rovers on Mars were affected by a steady decline in electrical power output due to atmospheric dust deposition on the solar panels. Long term science operations were only made possible by dust clearing events of the solar arrays caused by wind gusts and dust devils. In this study we analyzed dust devil tracks (DDTs) at the final InSight landing site region in Elysium Planitia. Formation of DDTs is caused by the removal of a layer of dust by passing dust devils, hence in principle the same process as clearing of dust from solar panels. We mapped the number, size (width and length), and orientation of DDTs in repeat observations using High Resolution Imaging Science Experiment (HiRISE) images covering the exact same surface area acquired within a relatively short time span (<90martian days). In total, we analyzed 557 newly formed dust devil tracks in 8 study areas. DDTs are morphologically relatively straight with a low mean sinuosity of 1.03 and only reach maximum widths of 30m. The mean DDT width is 4m, indicating that the dust devil size population is dominated by small dust devils with a diameter <10m. The size–frequency distribution of DDTs follows a −2 power law. The mean lengths of DDTs are 0.62km and 1.23km for complete (tracks which are visible from their start to end point) and incomplete DDTs (tracks running across the HiRISE footprint), respectively. The alignment of DDTs in combination with Mars Climate Database (MCD) predicted wind directions imply that dust devils are moving from SE to NW until early northern autumn with a reversal to NW–SE directions of movement at LS=200° consistent with the seasonal reversal in direction of the Hadley circulation. DDT formation rates vary between 0.002 and 0.08ddtkm−2sol−1. DDT area formation rates using the measured DDT widths, lengths, and formation rates are in the range of 0.0003–0.00006km2km−2sol−1, implying that a given spot on the surface may be cleared of dust only once between ∼3000 and 16,000sols (i.e. every ∼5–24Mars years). Measured DDT formation rates were used to find a scaling factor to the seasonal DDA index, and then integrated over the year to estimate a mean annual DDT formation rate of 0.046ddtkm−2sol−1. This translates into a solar panel clearing recurrence interval estimate of ∼11Mars years using the mean annual DDT formation rate, and the mean DDT width and length from all measured DDTs. Due to several uncertainties this solar panel clearing recurrence interval for the InSight landing should be seen as an upper limit estimate.