An Investigation of Trajectories of Atoms in Mercury's Exosphere
Abstract
Mercury's neutral exosphere consists of atoms or molecules ejected from the surface that are on individual trajectories that may re-impact the surface if there is insufficient energy to escape the gravity of the planet. This is an investigation of how the radiation pressure, orbital acceleration of the planet, and planetary rotation combine together to produce complicated trajectories. Because of Mercury's non-zero eccentricity the planet is not in uniform circular motion, which leads to radial and tangential accelerations that vary throughout the Mercury year. Besides radiation pressure the trajectory of an exospheric atom is affected by the planet accelerating during the time of flight of the atom that 1) causes the atom's position with respect to the ejection point to vary in a manner that is different than if the planet were not accelerating and 2) causes the planet-atom distance to vary in a manner that is different than for a typical ballistic trajectory resulting in variation of the gravitational force that the planet exerts on the atom. These effects are small but persistent and affect where the atom re-impacts the surface, which may lead to asymmetrical distributions of atoms in the surface regolith and exosphere.Preliminary results from simulations of ejected atoms that include 1) radiation pressure that varies with the atom's velocity due to Doppler shifting, 2) radial and tangential accelerations of the planet, and 3) the variation of the planet's gravity on the atom with distance above the planet show that atoms ejected at low energies normal to the surface from the subsolar point re-impact on the dusk side hemisphere of the planet. However atoms ejected at high energies normal to the surface from the subsolar point re-impact on the dawn side hemisphere of the planet. A fraction of atoms ejected normal to the surface from the dawn terminator within an energy range that results in the atom re-impacting and sticking to the night side surface behind the dawn terminator are moved back to the dawn terminator due to planetary rotation. Conversely atoms ejected from the dusk terminator that re-impact and stick to the night side surface behind that terminator are rotated further behind the dusk terminator.
- Publication:
-
AAS/Division for Planetary Sciences Meeting Abstracts #48
- Pub Date:
- October 2016
- Bibcode:
- 2016DPS....4810801B