Martian Oxygen and Hydrogen Upper Atmospheres Responding to Solar and Dust Storm Drivers: Hisaki Space Telescope Observations

We present variations of oxygen 130.4 nm and hydrogen Ly-β airglow of the Martian upper atmosphere observed by the Hisaki spacecraft in the Earth orbit. In 5-year intermittent observations covering various Mars seasons, the 130.4 nm brightness varied from ∼700 to ∼1,200 Rayleigh, correlated with the solar 130.4 nm flux. The Ly-β brightness, on the other hand, varied from ∼50 to ∼260 Rayleigh, correlated with the column dust optical depth rather than the solar Ly-β flux. This suggests that the global oxygen column density in the upper atmosphere was relatively stable over the observations while the hydrogen column density was highly variable, controlled by dust storms. Although the observations were made in different Mars Years, we suggest that the hydrogen column density increased by at least 2–5 times from non-dusty to dusty seasons. The source of the hydrogen atoms is likely water vapor transported from the lower atmosphere which subsequently photodissociate into hydrogen atoms. The amount of oxygen atoms dissociated from water vapor is small compared with those dissociated from the main CO₂ atmosphere. Large brightness of both emissions was detected during the comet siding spring (CSS) approach. We suggest that a large solar 130.4 nm flux caused the large 130.4 nm brightness and an effect of the CSS approach was small. For Ly-β, a regional dust storm during the CSS approach likely caused the large brightness. Although it is possible that the hydrogen atoms are transported from the comet, we leave the effect of CSS on the Ly-β brightness uncertain.