The hydrogen atoms present in the Martian exosphere resonantly scatter solar Lyman-alpha photons (121.56 nm which is in the UV) that can be imaged by the Advanced Camera for Surveys (ACS) onboard HST.
From an observing campaign back in 2007 it was found that the brightness of the Martian hydrogen exosphere decreased by ~40% within a period of 4 weeks.
This meant that the hydrogen escape rate changed dramatically during this period. Such an event defied earlier theoretical predictions, which assume the hydrogen atoms to be in a steady state with a constant escape rate.
From this dataset it was difficult to figure out if such a dramatic change was brought about by a global dust storm at Mars, which occurred in June 2007.
More HST observations in 2014 showed us that the hydrogen exosphere is influenced by seasonal changes, especially when Mars is approaching perihelion.
We use a radiative transfer model developed at BU to analyze the HST observations of Mars because the Lyman-alpha emission from the Martian hydrogen exosphere is optically thick, i.e. the solar photons undergo multiple scattering before reaching the observer. Upo analysis of the HST data we have discovered almost an order magnitude change in hydrogen escape rate from Mars from seasonal variations. The data also indicate the presence of a population of high-energy H atoms that varies seasonally. The most recent HST observing campaign in 2016-2017 has shown that the peak increase in seasonal H escape occurs closer to Martian southern summer solstice rather than at perihelion, when Mars is closest to the Sun, with the Lyman-alpha flux at its maximum. This tells us that the processes influencing the production and escape of H are much more complicated than was thought before. We are currently in a unique position to study these processes due to a bevy of instruments gathering data on the Martian atmosphere and HST is one such prime instrument as it is the only instrument to be far away from Mars to image the highly extended hydrogen exosphere of Mars.