A strong seasonal dependence in the Martian hydrogen exosphere

Hubble Space Telescope and Mars Express observed unexpected rapid changes in the Martian hydrogen exosphere involving a decrease in scattered Lyman alpha intensity in fall 2007 (solar longitude, L-s = 331 degrees-345 degrees). These changes detected were speculated to be a combination of seasonal variation and/or dust storms and lower atmospheric dynamics. Here we present Hubble Space Telescope observations of Mars in 2014 over a broad range of heliocentric distances and seasons (L-s = 138 degrees-232 degrees) which indicate a factor of similar to 3.5 change in Martian Lyman alpha brightness associated with a factor of similar to 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.