On Mars's Atmospheric Sputtering After MAVEN's First Martian Year of Measurements

Abstract

Mars likely lost a significant part of its atmosphere to space during its history. The sputtering of the atmosphere, by precipitating planetary heavy pickup ions accelerated by the solar wind, is one of the processes that could have significantly contributed to this atmospheric escape, in particular since the cessation of its global magnetic field, 4.0–4.1 Gyr ago. We present a 2 year baseline analysis of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of the precipitating flux. We use this measurement to model the expected escape rate and exospheric structure induced by this precipitation. We conclude that sputtering signatures in the dayside exosphere will be difficult to identify by MAVEN, and the induced atmospheric escape of O atoms remains orders of magnitude smaller than the expected rate induced by dissociative recombination of O2+ in Mars's ionosphere. On the contrary, deep in the nightside, Mars's sputtering might be the main source of the nonthermal part of the exospheric density profiles of species with mass larger or equal to Ar.

Variability of Precipitating Ion Fluxes During the September 2017 Event at Mars

“…In order to reconstruct the precipitating flux, we used the method developed by Leblanc et al () and Leblanc et al (, ). We focused on a range in altitude between 200 and 350 km, close enough to the Martian exobase so that any ion within this range of altitude with velocity at least than 75° with direction respect to the local nadir direction has a very large probability to impact Mars' atmosphere.…”

First In Situ Evidence of Mars Nonthermal Exosphere

“…HELIOSARES has been successfully used to characterize the oxygen component of the exosphere that was measured by MAVEN at different seasons and at various SZAs (Leblanc et al, ). More recently, the model was used to calculate the expected increase of the exospheric density induced by pickup ion sputtering assuming the typical precipitating flux that was measured by MAVEN (Leblanc et al, ). The study showed that deep in the nightside, the nonthermal component of Ar is dominated by particles ejected by pickup ion sputtering.…”

“…In the HELIOSARES model, three main sources of exospheric Ar can be identified: the Ar atoms ejected into the exosphere by collision with nonthermal O that were produced by the dissociative recombination of O 2 + (Figure b); the thermal component, which corresponds to the thermal expansion of atmospheric Ar (Figure c); and the sputtered component, which corresponds to Ar atoms that are ejected by pickup ions sputtering the atmosphere (Figure d). One should note that in the case of this simulation, we simulated O + precipitating particles assuming the average dayside and nightside differential energy distribution of the precipitating particles that was observed by MAVEN/SWIA from 2015 to 2017 (Leblanc et al, ). …”

“…Leblanc et al () simulated the density profile induced by the expected two main sources of nonthermal exospheric components. Their model computes the hot Ar density profiles at high altitudes produced indirectly by photo‐chemical reactions or directly by sputtering.…”

“…As explained in the present paper, the data obtained using the original NGIMS mode of measurement cannot be used above 250 km without averaging many months of data. Leblanc et al (2018) simulated the density profile induced by the expected two main sources of nonthermal exospheric components. Their model computes the hot Ar density profiles at high altitudes produced indirectly by photo-chemical reactions or directly by sputtering.…”