Planetary boundary layer and circulation dynamics at Gale Crater, Mars

Fonseca, R. M., Zorzano Mier, M. P., Martín Torres, J. 2018. Planetary boundary layer and circulation dynamics at Gale Crater, Mars. Icarus 302, 537-559 DOI: 10.1016/j.icarus.2017.11.036

The Mars implementation of the Planet Weather Research and Forecasting (PlanetWRF) model, MarsWRF, is used here to simulate the atmospheric conditions at Gale Crater for different seasons during a period coincident with the Curiosity rover operations. The model is first evaluated with the existing single-point observations from the Rover Environmental Monitoring Station (REMS), and is then used to provide a larger scale interpretation of these unique measurements as well as to give complementary information where there are gaps in the measurements.

The variability of the planetary boundary layer depth may be a driver of the changes in the local dust and trace gas content within the crater. Our results show that the average time when the PBL height is deeper than the crater rim increases and decreases with the same rate and pattern as Curiosity’s observations of the line-of-sight of dust within the crater and that the season when maximal (minimal) mixing is produced is L-s 225 degrees-315 degrees (L-s 90 degrees-110 degrees). Thus the diurnal and seasonal variability of the PBL depth seems to be the driver of the changes in the local dust content within the crater. A comparison with the available methane measurements suggests that changes in the PBL depth may also be one of the factors that accounts for the observed variability, with the model results pointing towards a local source to the north of the MSL site.

The interaction between regional and local flows at Gale Crater is also investigated assuming that the meridional wind, the dynamically important component of the horizontal wind at Gale, anomalies with respect to the daily mean can be approximated by a sinusoidal function as they typically oscillate between positive (south to north) and negative (north to south) values that correspond to upslope/downslope or downslope/upslope regimes along the crater rim and Mount Sharp slopes and the dichotomy boundary. The smallest magnitudes are found in the northern crater floor in a region that comprises Bradbury Landing, in particular at Ls 90 when they are less than 1 m s(-1), indicating very little lateral mixing with outside air. The largest amplitudes occur in the south-western portions of the crater where they can exceed 20 m s-1. Should the slope flows along the crater rims interact with the dichotomy boundary flow, which is more likely at L-s 270 degrees and very unlikely at L-s 90 degrees, they are likely to interact constructively for a few hours from late evening to nighttime (similar to 17-23 LMST) and from pre-dawn to early morning (similar to 5-11 LMST) hours at the norther crater rim and destructively at night (similar to 22-23 LMST) and in the morning (similar to 10-11 LMST) at the southern crater rim.

We conclude that a better understanding of the PBL and circulation dynamics has important implications for the variability of the concentration of dust, non-condensable and trace gases at the bottom of other craters on Mars as mixing with outside air can be achieved vertically, through changes in the PBL depth, and laterally, by the transport of air into and out of the crater. (C) 2017 The Authors. Published by Elsevier Inc.

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