Effect of the Lateral Exospheric Transport on the Horizontal Hydrogen Distribution Near the Exobase of Mars

Abstract

We simulate the hydrogen density near the exobase of Mars, using the 3‐D Martian Global Circulation Model of Laboratoire de Météorologie Dynamique, coupled to an exospheric ballistic model to compute the downward ballistic flux. The simulated hydrogen distribution near the exobase obtained at two different seasons—Ls = 180° and Ls = 270°—is close to Zero Net Ballistic Flux equilibrium. In other words, the hydrogen density near the exobase adjusts to have a balance between the local upward ballistic and the downward ballistic flux due to a short lateral migration time in the exosphere compared to the vertical diffusion time. This equilibrium leads to a hydrogen density n near the exobase directly controlled by the exospheric temperature T by the relation nT5/2 = constant. This relation could be used to extend 1‐D hydrogen exospheric model of Mars used to derive the hydrogen density and escape flux at Mars from Lyman‐α observations to 3‐D model based on observed or modeled exospheric temperature near the exobase, without increasing the number of free parameters.

Seasonal Variability of Deuterium in the Upper Atmosphere of Mars

“…At the exobase, the D density was considered to be a free parameter with a range of 6 values discussed in the next section. A mathematical formulation ( nT 5/2 = constant, where n is the density of a light species and T is its temperature) first derived by Hodges & Johnson () and recently verified by Yelle et al () for H atoms, was used to determine the SZA variability of D densities at the exobase. Below the assumed homopause at 120 km (Mahaffy et al, ; Nagy et al, ), a linear profile for D was used for simplicity, similar to that described in Bhattacharyya, Clarke, Bertaux, et al () and that was consistent with a diffusion‐generated atmosphere.…”

Estimate of the D/H Ratio in the Martian Upper Atmosphere from the Low Spectral Resolution Mode of MAVEN/IUVS

“…The derived range of D/H at 80 km is between 0.6 and 3.7 × 10 −3 respectively, lower than the derived D/H ratio during period 1, but in the range of the D/H ratio measured during period 2 and in Martian water vapor. The derived hydrogen density at 200 km, between 0.6 and 4.5 × 10 6 cm −3 , is larger than the simulated hydrogen density (Chaufray et al., 2018) which could be attributed to a larger amount of water vapor at high altitudes during this season (Chaufray et al., 2021).…”

“…The derived exobase hydrogen and deuterium densities are lower than those derived from period 1 or period 4 (see Section 6.4). This variation could be due to SZA variations with larger H and D densities near the terminator than at lower SZA, as suggested by 3D simulations (Chaufray, Bextaux et al, 2015; Chaufray, Gonzalez‐Galindo et al., 2015, Chaufray et al., 2018).…”

“…The derived D/H ratio at 80 km is between 1.3–3.6 × 10 −3 , which is larger than the D/H ratio measured in the water vapor (9 ± 4 × 10 −4 ) near the surface of Mars (Owen et al., 1988). Other effects due to HDO and H 2 O condensation and photochemistry (Bertaux & Montmessin 2001; Krasnopolsky 2002), and exospheric ballistic transport (Chaufray et al., 2018), are not included in our simple models and should be included to better estimate the full vertical D/H profile. Assuming a constant D/H ratio below 120 km to roughly mimic the effect of photochemistry on the density profile would lead to an even larger derived D/H ratio at 80 km.…”

“…Light species, such as hydrogen (H) and helium (He) are expected to be more abundant at night reaching peak abundance in the early morning during the southern summer season (Chaufray, Bextaux et al, 2015, 2018; Elrod et al., 2017). It is likely that a global enhancement of deuterium (D) will also occur on the nightside and that D will reach its maximum abundance in the early morning.…”