Explanation for the Increase in High‐Altitude Water on Mars Observed by NOMAD During the 2018 Global Dust Storm

Abstract

The Nadir and Occultation for MArs Discovery (NOMAD) instrument on board ExoMars Trace Gas Orbiter measured a large increase in water vapor at altitudes in the range of 40-100 km during the 2018 global dust storm on Mars. Using a three-dimensional general circulation model, we examine the mechanism responsible for the enhancement of water vapor in the upper atmosphere. Experiments with different prescribed vertical profiles of dust show that when more dust is present higher in the atmosphere, the temperature increases, and the amount of water ascending over the tropics is not limited by saturation until reaching heights of 70-100 km. The warmer temperatures allow more water to ascend to the mesosphere. Photochemical simulations show a strong increase in high-altitude atomic hydrogen following the high-altitude water vapor increase by a few days. Plain Language Summary The ExoMars Trace Gas Orbiter (TGO) is currently in orbit aroundMars measuring the composition of the atmosphere. TGO was able to make observations before, during, and after a large planet-encircling dust storm that occurred from June to August 2018. The TGO measurements provide the first opportunity to observe how water vapor is distributed with height in the atmosphere during a global-scale dust event. It was found that there was a large increase in water vapor very high (40-100 km) in the atmosphere during the dust storm. Using a three-dimensional numerical model of the Mars atmosphere, we found that when dust from the storm is transported up to levels above~40 km, it warms the atmosphere due to solar absorption, and this in turn prevents ice clouds from forming at heights of 40-60 km and allows more water vapor to ascend to greater heights in the atmosphere. This is of interest in terms of the planetary evolution, as water molecules at greater heights are more readily dissociated by sunlight and lost from the atmosphere. This is an important factor for understanding the changes that have occurred since the period when surface features on Mars indicate that liquid water was present.

Martian Water Ice Clouds During the 2018 Global Dust Storm as Observed by the ACS‐MIR Channel Onboard the Trace Gas Orbiter

“…Moreover, peak cloud altitudes are slightly higher during the onset of the GDS than reported in non‐GDS years. Indeed, models (Neary et al, ) showed that GDSs are expected to slightly increase the average and maximum altitude of water ice clouds, which is consistent with our observations. The sharp increase in water ice cloud altitude observed during the GDS indeed confirms that this water ice cloud increase is directly connected to dust storm activity.…”

“…We note that the above behavior appears to be relatively uniform around the planet (i.e., both zonally and meridionally). Specifically, in the southern hemisphere, even though the observed latitude varies from 74 S to 5 S between and , one does not see a change in both the haze top altitude and the maximal altitude of the water ice clouds; in agreement with the simulations presented in Neary et al (, Figure ). We nonetheless identify a temporal trend likely related to the progressive decay of the dust storm (Guzewich et al, ): From to , the lower altitude of the detected water ice clouds in the southern hemisphere decreases from 68 to 54 km (cf.…”

“…The large-scale dust storm probably facilitates the formation of high-altitude water ice clouds through increase in the altitude of both water vapor and condensation nuclei. This increase of water ice altitude during the GDS is indeed consistent with the recently reported increase of water vapor at higher altitudes Neary et al, 2019). Note.…”

“…Recent studies have notably reveal how more precise vertical representation of water-related atmospheric phenomena may impact water cycle modeling The MY 28 dust storm has been shown to have profoundly influenced the water vapor atmospheric distribution, elevating water well above the troposphere (>60 km) while pushing up the hygropause altitude (Heavens et al, 2018). A rapid increase of the H 2 O and HDO abundances at altitudes between 40 and 80 km was observed during MY 34 storm and recently reproduced in a GCM simulation (Neary et al, 2019). This large augmentation of the high-altitude water content is believed to boost hydrogen, or nearly equivalently, water escape from Mars Fedorova et al, 2020;Heavens et al, 2018).…”

“…However, the formation of clouds impacts the ability of water (or hydrogen) to escape as it confines (in theory) a significant fraction of water below the cloud level. Thus, understanding the extent to which high-altitude water vapor condenses to form clouds during GDSs is important in assessing the possible propagation pathways of water vapor up to the exobase (Neary et al, 2019). Because water vapor can exist in a supersaturated state in the Martian atmosphere (Maltagliati et al, 2011) and has been observed in a large amount during the MY 34 GDS (Fedorova et al, 2020), simply supposing that the freezing point can be used to set the limits where water is free to be mobilized, is likely to be problematic.…”