Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u21a0034v&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #U21A-0034
Other
0328 Exosphere, 5405 Atmospheres (0343, 1060), 5408 Aurorae And Airglow, 5464 Remote Sensing, 6235 Mercury
Scientific paper
Because of the difficulties of observing H Lyman α at Mercury from Earth orbit, observations of this emission by the Ultraviolet and Visible Spectrometer (UVVS) component of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument onboard MESSENGER represent the first chance in over thirty years to revisit the neutral H exosphere of Mercury that was first explored by the Mariner 10 Ultraviolet Spectrometer. The Mariner 10 observations revealed two interesting features of the H distribution in Mercury's exosphere. The first was a "bump" near 200 km altitude that may have been an instrumental artifact but has otherwise defied explanation. The second was a two-component nature to the distribution, which has been ascribed to a "cold" (~ 110 K) population of H atoms near the surface and a "warm" (~ 420 K) population at higher altitudes. However, issues exist with this explanation, and the true origin of this two-component H distribution remains one of the biggest questions in understanding Mercury's exosphere. During the first MESSENGER flyby of Mercury, the UVVS conducted observations of H Lyman α at both the tail and dayside regions of Mercury. While the tail-region observations revealed no H signal above the interplanetary background, the dayside observations showed an exospheric distribution consistent with the Mariner 10 "warm" component. Spacecraft priorities precluded observations of the H distribution at the low-altitude regions of the Mariner 10 "cold" distribution and "bump." During the second flyby, however, observations of the low-altitude H distribution are a high priority, and a dedicated, high- spatial-resolution H Lyman α observational sequence will occur radially from the surface to roughly 800 km altitude. Dayside observations of H will also be conducted at higher altitudes but will again be subject to spacecraft constraints, and additional H observations will be carried out in the tail region. We present an analysis of the H Lyman α observations from MESSENGER's second flyby and a synthesis of observations from both flybys in an attempt to answer the two outstanding questions: "What is the nature of the two-component H distribution in the exosphere?," and "If it exists, what is the origin of the 'bump' near 200 km altitude?" The UVVS data will be compared and contrasted with the Mariner 10 data to address these questions as well as to reveal changes that have occurred over the intervening 33 years and between the first and second MESSENGER flybys. We note that the UVVS data represent only a glimpse at this aspect of the complicated Mercury exospheric system. A full understanding of the exosphere, and of the H Lyman α emission in particular, will require many observations spanning a variety of observing conditions, as will be provided by the UVVS during the orbital phase of the MESSENGER mission.
Bradley Eric Todd
Izenberg Noam R.
Killen Rosemary Margaret
Kochte M. C.
Lankton Mark R.
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