Water and Water Ions in the Martian Thermosphere/Ionosphere

Abstract:

We present here the first model of the Martian thermosphere/ionosphere in which we predict the density profiles of water vapor and water ions in the altitude range 80 to 400 km. The model is based on data from the MAVEN spacecraft, including the NGIMS, LPW, EUVM and STATIC. The model includes 28 ions, of which 14 are protonated species, and 10 minor neutral species. The protonated species include H₂O⁺ and H₃O⁺, and the minor neutral species include H₂O. These species are coupled to each other and to the background species by over 400 reactions. The peaks of the density profiles of water ions and OH⁺ are in fairly good agreement with the values measured by the NGIMS instrument. The density profiles of water vapor are computed with zero flux boundary conditions at the top and bottom of the model, which shows that water can be made in situ via ion chemistry, somewhat like in the interstelllar medium. It appears, however, that the predicted densities may be too small to account for the densities of H₃O⁺ that are seen at low altitudes beyond the terminator. On the other hand, the densities of water vapor must be small enough so that HCO⁺ is not destroyed completely by proton transfer. The actual amount of water in the thermosphere is bracketed by these two requirements. The computed water vapor densities at the lower boundary are smaller than some of those at the upper boundaries of middle atmosphere models. We tentatively predict that there must be some influx of water from below. We also compute the H_n ⁺ species, where n=(1-3), and convincingly demonstrate the the mass-2 ion is H_2⁺ and not D⁺, as it is on Venus.