Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p11a1316g&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P11A-1316
Other
[0328] Atmospheric Composition And Structure / Exosphere, [3367] Atmospheric Processes / Theoretical Modeling, [5421] Planetary Sciences: Solid Surface Planets / Interactions With Particles And Fields, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
A survey of the source processes that are thought to be responsible for replenishing the Hermean exosphere is presented, including ion sputtering, impact vaporization, photon stimulated desorption, and thermal desorption. Once released the oxygen particles are affected by other mechanisms, including gravity, photoionization, photodissociation, and to some extent also by solar radiation acceleration. A model, entitled Hermean Exosphere Model of Oxygen (HEMO) is proposed. This model will include the aforementioned processes, as well as incorporate an existing magnetosphere model to simulate how ions are guided toward the surface to cause sputtering as well as how released ions or newly-formed ions, following photoionization, have their trajectories altered by interacting with the intrinsic magnetic field at Mercury. The sole in situ data of oxygen, from Mariner 10 as re-examined by Shemansky, suggests a near-ground density of 4.4 E4 /cm^3. In this same paper, the sodium near-ground density is given as 1.7 - 3.8 E4 /cm^3. Oxygen should be more abundant than sodium in Mercury’s exosphere, so where are the measurements of these large oxygen abundances [Shemansky, 1988]? The authors of the original Mariner 10 results admitted that the data on oxygen was uncertain due to poor signal to noise ratios and an undetermined scale height [Broadfoot et al., 1976]. The HEMO simulations aim to illuminate the question of why it is difficult to detect oxygen, when it should be more abundant than sodium, given that oxygen is expected to account for 60% of the Hermean crust [Killen et al., 2005]. The Hermean regolith will be treated as analogous to the Lunar regolith, with some variations in abundances of specific species such as iron and titanium. HEMO will be for several sets of conditions, including different regolith species abundances, different orbital locations of Mercury, and both quiet and active Sun conditions. For each set of conditions, the model will be run several times, each time keeping track of the trajectories of neutral and ionized oxygen particles. The results will then be averaged to yield expected densities of oxygen in Mercury’s exosphere, and comparison of this average density to specific simulation runs with nearly the same exospheric density can help to build a sense of intuition as to the relative importance of the various source processes. Finally, the results of the HEMO model can be used to aid in the interpretation of future oxygen data sets to be gathered by the MESSENGER and BepiColombo missions to Mercury.
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