A strong seasonal dependence in the Martian hydrogen exosphere

Abstract : Hubble Space Telescope (HST) and Mars Express (MEX) observed unexpected rapid changes in the martian hydrogen exosphere involving a decrease in scattered Lyman α intensity in Fall 2007(solar longitude, Ls = 331°-345°). These changes detected were speculated to be a combination of seasonal variation and/or dust storms and lower atmospheric dynamics. Here we present HST observations of Mars in 2014 over a broad range of heliocentric distances and seasons (Ls = 138°-232°) which indicate a factor of ~3.5 change in martian Lyman α brightness associated with a factor ~5.4 variation of hydrogen escape flux in the absence of global dust storms and significant solar variability. We thus conclude that seasonal effects have a strong influence on the hydrogen exosphere, which in turn has major implications for the processes that control water supply to the martian upper atmosphere and the history of water escape from Mars.