Nature Astronomy (2021) https://doi.org/10.1038/s41550-021-01425-w

M. S. Chaffin, D. M. Kass, S. Aoki, A. A. Fedorova, J. Deighan, K. Connour, N. G. Heavens, A. Kleinböhl, S. K. Jain, J.-Y. Chaufray, M. Mayyasi, J. T. Clarke, A. I. F. Stewart, J. S. Evans, M. H. Stevens, W. E. McClintock, M. M. J. Crismani, G. M. Holsclaw, F. Lefevre, D. Y. Lo, F. Montmessin, N. M. Schneider, B. Jakosky, G. Villanueva, G. Liuzzi, F. Daerden, I. R. Thomas, J.-J. Lopez-Moreno, M. R. Patel, G. Bellucci, B. Ristic, J. T. Erwin, A. C. Vandaele, A. Trokhimovskiy, O. I. Korablev

Mars has lost most of its initial water to space as atomic hydrogen and oxygen. Spacecraft measurements have determined that present-day hydrogen escape undergoes large variations with season that are inconsistent with long-standing explanations. The cause is incompletely understood, with likely contributions from seasonal changes in atmospheric circulation, dust activity and solar extreme ultraviolet input. Although some modelling and indirect observational evidence suggest that dust activity can explain the seasonal trend, no previous study has been able to unambiguously distinguish seasonal from dust-driven forcing. Here we present synoptic measurements of dust, temperature, ice, water and hydrogen on Mars during a regional dust event, demonstrating that individual dust events can boost planetary H loss by a factor of five to ten. This regional storm occurred in the declining phase of the known seasonal trend, establishing that dust forcing can override this trend to drive enhanced escape. Because similar regional storms occur in most Mars years, these storms may be responsible for a large fraction of Martian water loss and represent an important driver of Mars atmospheric evolution.

From bottom to top: dust optical depth (τ_dust) observed by the MCS induces a large change in mid-atmosphere temperatures and intensifies interhemispheric circulation, inhibiting ice condensation and lowering ice optical depth (τ_ice). IUVS observes equatorial clouds capping the Tharsis volcanoes before and after but not during the event. TGO observes the water that would have condensed into clouds at higher altitudes during the event, peaking ~1 week after the beginning of the event. IUVS observes hydrogen increase in brightness by ~50% as a result of this event, consistent with an increase in H loss by a factor of several. Because this event occurred well after southern summer solstice and perihelion, we can conclude that the increase in H loss is controlled by dust dynamics rather than by seasonal changes.