Hydrogen escape from Mars enhanced by deep convection in dust storms

• Published in: Nature astronomy Vol. 2; no. 2; pp. 126 - 132
• Main Authors: Heavens, Nicholas G., Kleinböhl, Armin, Chaffin, Michael S., Halekas, Jasper S., Kass, David M., Hayne, Paul O., McCleese, Daniel J., Piqueux, Sylvain, Shirley, James H., Schofield, John T.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01-02-2018; Nature Publishing Group
• Subjects: 704/445/823; 704/445/824; 704/445/845; Astronomy; Astrophysics and Cosmology; Atmosphere; Atmospheric chemistry; Atmospheric water; Convection; Dust; Dust storms; Hydrogen; Letter; Mars; Physics; Physics and Astronomy; Seasonal distribution; Seasonal variations; Surface water; Ultraviolet radiation; Vertical distribution; Water content; Water loss; Water transport; Water vapor
• ISSN: 2397-3366
• DOI: 10.1038/s41550-017-0353-4

Present-day water loss from Mars provides insight into Mars’s past habitability
1
–
3
. Its main mechanism is thought to be Jeans escape of a steady hydrogen reservoir sourced from odd-oxygen reactions with near-surface water vapour
2
,
4
,
5
. The observed escape rate, however, is strongly variable and correlates poorly with solar extreme-ultraviolet radiation flux
6
–
8
, which was predicted to modulate escape
9
. This variability has recently been attributed to hydrogen sourced from photolysed middle atmospheric water vapour
10
, whose vertical and seasonal distribution is only partly characterized and understood
11
–
13
. Here, we report multi-annual observational estimates of water content and dust and water transport to the middle atmosphere from Mars Climate Sounder data. We provide strong evidence that the transport of water vapour and ice to the middle atmosphere by deep convection in Martian dust storms can enhance hydrogen escape. Planet-encircling dust storms can raise the effective hygropause (where water content rapidly decreases to effectively zero) from 50 to 80 km above the areoid (the reference equipotential surface). Smaller dust storms contribute to an annual mode in water content at 40
−
50 km that may explain seasonal variability in escape. Our results imply that Martian atmospheric chemistry and evolution can be strongly affected by the meteorology of the lower and middle atmosphere of Mars.
Mars Climate Sounder’s multi-annual observations of the vertical distribution of water and dust in the Martian atmosphere show that deep convection from dust storms transports water from the lower to the middle atmosphere, enhancing water loss to space.