Galli A., Wurz P., Barabash S., Grigoriev A., Gunell H., Lundin R., Holmstrom M., Fedorov A.
Space Science Reviews
Summary: We present measurements of energetic hydrogen and oxygen atoms (ENAs) on the nightside of Mars detected by the neutral particle detector (NPD) of ASPERA-3 on Mars Express. We focus on the observations for which the field-of-view of NPD was directed at the nightside of Mars or at the region around the limb, thus monitoring the flow of ENAs towards the nightside of the planet. We derive energy spectra and total fluxes, and have compiled maps of hydrogen ENA outflow. The hydrogen ENA intensities reach 105 cm-2 sr-1 s-1, but no oxygen ENA signals above the detection threshold of 104 cm-2 sr-1 s-1 are observed. These intensities are considerably lower than most theoretical predictions. We explain the discrepancy as due to an overestimation of the charge-exchange processes in the models for which too high an exospheric density was assumed. Recent UV limb emission measurements (Galli et al., this issue) point to a hydrogen exobase density of 1010 m-3 and a very hot hydrogen component, whereas the models were based on a hydrogen exobase density of 1012 m-3 and a temperature of 200 K predicted by Krasnopolsky and Gladstone (1996). Finally, we estimate the global atmospheric loss rate of hydrogen and oxygen due to the production of ENAs. © Springer Science+Business Media, Inc. 2007.