Seasonal Variability of Deuterium in the Upper Atmosphere of Mars

Abstract

Measurements by multiple Mars Atmosphere and Volatile Evolution mission instruments, obtained between November 2014 and November 2017, are analyzed to produce deuterium properties in the upper atmosphere of Mars. We show here, for the first time, the seasonal distribution and variability of D densities, temperatures, and estimated Jeans escape rates at the exobase (200 km). Within the data constraints, it is found that the variations in D properties are similar for the northern and southern hemispheres, and peak near southern summer solstice. Trends in the D Lyman-α brightness, temperature, density, and escape rate are increasing during the beginning of the dust storm season, peak near southern summer solstice, and decrease toward the end of the dust storm season. This suggests that seasonal drivers at Mars cause deuterium in the upper atmosphere to become globally enhanced when Mars is closest to the Sun and during the Martian dust season when water is provided to the upper atmosphere by subsurface, hydrological, and dust storm dynamics.Plain Language Summary Water escape at Mars can be examined by analyzing the properties of present day hydrogen and its isotope deuterium. The ratio of the abundance of these two atoms at the upper atmosphere of Mars can provide limitations to the preferential escape of one over the other and can provide information on the present rate of escape of water from the upper atmosphere. The results from this work can be paired with estimates of what early Mars and the Sun were like in order to provide estimates for the early water content on ancient Mars. In this paper, the most comprehensive set of observations of deuterium from Mars are analyzed, and the resulting properties of deuterium in the upper atmosphere of Mars are presented. Key Points:• Deuterium (D) emissions from the limb of Mars are analyzed to produce atomic abundances and escape rate estimates in the upper atmosphere • D property variations are pronounced. Brightness and densities increase with Mars' proximity to the Sun and peak near southern summer solstice • Variations of deuterium brightness, densities, and escape rates suggest a seasonally varying D/H ratio at MarsSupporting Information:• Supporting Information S1

Two-dimensional model for the martian exosphere: Applications to hydrogen and deuterium Lyman α observations

“…The CO2 density at 80 km was determined using a neutral atmosphere model which utilized a volume mixing ratio at 80 km, consistent with the relative abundances for CO2 found in the Mars Climate Database for the temperature profile derived from NGIMS measurements as described in section 2.1 [Forget et al, 1999;Lewis et al, 1999]. The model also accounted for molecular and eddy diffusion for interactions of CO2 molecules with other neutral species in the martian atmosphere in order to obtain the CO2 density at 80 km [Matta et al, 2013;Mayyasi et al, 2019]. This modeling procedure was only applied at aphelion and perihelion where the exobase temperature and its variation with SZA had been well established through NGIMS observations.…”

“…Given a temperature at SZA = 0˚, the curve depicted by the red line is then used to determine the variation of temperature with SZA. Figure 1 below, which is an adaptation of figure 2 from Mayyasi et al, [2019], has been included in this paper in order to present a more robust discussion of the models which make extensive use of this temperature trend. The exobase temperature at SZA = 0˚ for thermal H and D at Mars for a particular solar longitude (Ls) along Mars' orbit is pre-determined by fitting the MAVEN-NGIMS derived temperatures for the lower thermosphere at perihelion and aphelion with a sine wave given by…”

“…1 was then utilized to approximate the exobase temperature at the sub-solar point [Mayyasi et al, 2019]. The resulting aphelion and perihelion temperatures at the exobase altitude of 200 km and SZA = 0° for Mars were determined to be 216 ± 39 K and 255 ± 29 K respectively [Mayyasi et al, 2019]. These temperatures are only representative of exospheric populations of D and H that are in thermal equilibrium with the bulk atmosphere (i.e.…”

“…Lyman α emission from deuterium at 1215.33 Å, the heavier isotope of hydrogen, on the other hand, has only recently been studied in detail to determine the D/H ratio at Mars [Clarke et al, 2017;Mayyasi et al, 2017b;2019]. The deuterium to hydrogen ratio is a good indicator of a planet's water loss in the past through atmospheric escape.…”

“…However, all these measurements were confined to the lower atmosphere (below an altitude of ~60 km) and were indirect estimates of the D/H ratio through HDO and H2O observations, with the assumption that HDO and H2O are the major sources of D and H atoms in the martian exosphere. Observations of martian exospheric deuterium Lyman α emission (1215.33 Å) with the IUVS-echelle instrument onboard MAVEN over two Mars years, revealed that deuterium exhibits large seasonal variations like hydrogen, with its peak close to southern summer solstice [Clarke et al, 2017;Mayyasi et al, 2017b;2019]. This might result in a different D/H ratio for the upper atmosphere.…”