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.
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MCS-derived and IUVS radiance data shown in Fig. 1 are available to the public on the Planetary Data System Planetary Atmospheres Node (MCS, https://atmos.nmsu.edu/data_and_services/atmospheres_data/MARS/mcs.html; IUVS, https://atmos.nmsu.edu/data_and_services/atmospheres_data/MAVEN/maven_iuvs.html). NOMAD and ACS data are available on the Planetary Science Archive of the European Space Agency at https://archives.esac.esa.int/psa. Complete datasets of retrieved water abundances from NOMAD are available on the BIRA-IASB data repository at http://repository.aeronomie.be/?doi=10.18758/71021054. ACS water data are available at http://exomars.cosmos.ru/ACS_Results_stormy_water_vREzUd4pxG.
Figure 1 results from plotting the accessible datasets described previously. The data reduction procedures to produce these data for the MCS, IUVS, NOMAD and ACS datasets are described in the Methods and references therein.
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This research was supported by NASA through the MAVEN and MRO projects. IUVS data products were produced using the RMACC Summit supercomputer, which is supported by the National Science Foundation (award nos. ACI-1532235 and ACI-1532236), by the University of Colorado Boulder and by Colorado State University. The Summit supercomputer is a joint effort of the University of Colorado Boulder and Colorado State University. M.M.J.C. is supported by the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, which is administered by the Universities Space Research Association under contract with NASA. A.K. acknowledges support from the NASA Mars Data Analysis Program (80NM0018F0719). Work at the Jet Propulsion Laboratory, California Institute of Technology, is performed under contract with NASA. ExoMars is a space mission of the European Space Agency and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (BIRA-IASB), assisted by co-principal investigator teams from Spain (IAA-CSIC), Italy (INAF-IAPS) and the United Kingdom (Open University). For this project we acknowledge funding by the Belgian Science Policy Office, with financial and contractual coordination by the European Space Agency Prodex Office (PEA 4000103401 and 4000121493); by the Spanish MICINN through its Plan Nacional; by European funds under grant nos. PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER); by the United Kingdom Space Agency through grant nos. ST/R005761/1, ST/P001262/1, ST/R001405/1 and ST/S00145X/1; and by the Italian Space Agency through grant no. 2018-2-HH.0. The IAA-CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). This work was supported by the Belgian Fonds de la Recherche Scientifique-FNRS under grant nos. 30442502 (ET_HOME) and T.0171.16 (CRAMIC) and by the Belgian Science Policy Office BrainBe SCOOP Project. S.A. is ‘Chargé de Recherches’ at the F.R.S.-FNRS. NOMAD’s United States investigators are supported by NASA. Science operations of ACS on TGO are funded by Roscosmos and the ESA. IKI affiliates acknowledge support from the Ministry of Science and Higher Education of the Russian Federation. F.M. acknowledges funding from the CNES and ANR (PRCI, CE31 AAPG2019-MCUBE project).
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Claire Newman and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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MAVEN orbit numbers and Earth Year/Month dates are shown on the bottom axis, with Mars Year and Ls on the top axis. The blue/brown and green curves show the location of MAVEN’s apoapsis in solar zenith angle (SZA) and local time, and in geographic latitude, respectively. The tan curve shows the latitude of the subsolar point. Intermittent spacecraft activities such as MAVEN Deep Dips (DD-X), Earth-Sun-Mars conjunctions, and the recent aerobraking campaigns are also indicated. IUVS Stellar occultation campaigns are indicated with the star icons. Time periods useful for IUVS H loss measurements occur when apoapsis SZA and latitude are both close to zero and occur relatively rarely in the dataset. Unfortunately these time periods are relatively rare in the dataset.
Points show the Mars surface geometry of the point along the observation line of sight with minimum ray height. For TGO, occultations are made in both hemispheres, but Fig. 1 shows only Southern Hemisphere data. MCS observations are made at 3 PM across all latitudes, and zonally averaged across longitudes, and so are not shown here. Both TGO and IUVS observing geometry evolved with time over the period of the study, with minimal impact on the conclusion that regional storms can make possible large amounts of high-altitude water that subsequently increases coronal H abundances and loss rates.
From top to bottom, water retrieved in the Northern Hemisphere by ACS and NOMAD; MCS water retrievals using the methods of Heavens et al. 2018; and Southern Hemisphere water retrievals from ACS and NOMAD. Data presented in the text comes from the Southern Hemisphere, from ACS before Ls 330, and from NOMAD afterward. Color scales are unique to each instrument for clarity, but because these schemes are perceptual the perceived darkness in each panel is a trustworthy indicator of the water abundance retrieved. ACS retrieved abundances are higher than NOMAD abundances and display larger variations; MCS retrievals are higher than both and limited in altitude range.
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Chaffin, M.S., Kass, D.M., Aoki, S. et al. Martian water loss to space enhanced by regional dust storms. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01425-w