Insights into the Nature of Mercury's Surface-bound Exosphere: Results from the Three MESSENGER Flybys
Abstract
The composition and structure on Mercury's surface-bounded exosphere are controlled by inter-actions among the surface, magnetosphere, solar wind, and sunlight. Prior to the MESSENGER flybys the exosphere was known to contain H and He, observed by Mariner 10, as well as Na, K, and Ca, observed from ground-based telescopes. When species are liberated from the surface with sufficient energy, those with strong resonance lines in the visible can be accelerated by solar radiation pressure in some circumstances to form an anti-sunward tail. During the three MESSENGER flybys (M1 on January 14, 2008, M2 on October 6, 2008, and M3 on September 30, 2009), the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) mapped Na, Ca, and Mg in the planet's anti-sunward tail and also detected Ca+ in a narrow region approximately 1-2 Mercury radii above the anti-sunward surface. After observed radiances are converted to column densities, the values for Ca+ and Ca have the same order of magnitude. Because the residence time for Ca+ is much less than that for Ca, it is not possible to produce the Ca+ locally by ionization of Ca, and it is more likely that Ca+ is delivered by transport to the region where it was observed. Na radiances, observed in the tail approximately 2500 km above the anti-sunward surface, were comparable during M1 and M2 but were a factor of ∼20 smaller during M3. This difference is consistent with a factor of two reduction in solar radiation pressure for M3 caused by Mercury's change in position in its highly eccentric orbit around the Sun. Whereas a Na atom released in the anti-sunward direction with as little as 0.23 electron volts (eV) of kinetic energy (anti-sunward velocity of 1.4 km s-1 ) could escape down the tail during M1 and M2, an ejection energy of 0.75 eV was required for escape during M3. If the Na source rates were comparable for all the flybys, the combined observations suggest that a significant fraction of Na seen in the tail during M1 and M2 was released from the surface by low-energy processes. In contrast to Na, Ca and Mg tail-region emissions were comparable for M2 and M3 (tail-region observations of these species were not scheduled during M1). These were first detected with statistical significance starting approximately 22,000 km behind the planet. Unlike Na, radiation pressure is small compared to gravity for Ca and negligible for Mg, and these atoms must be released anti-sunward with energies greater than 5 eV and 2.5 eV, respectively, in order to reach such a distance. If they originate from ion sputtering or impact vaporization, they must come from the high-energy tails of these source distributions. Altitude profiles of Na above the north and south poles, obtained during M3, revealed the presence of a two-component structure for Na with e-folding heights (He) of ∼ 200 km and 490 km corresponding to kinetic energies of 0.18 and 0.73 eV, respectively. The Ca profiles above both poles exhibited a single-component distribution with He ∼ 1750 km (2.8 eV). Whereas the Mg profile above the south pole had a single-component distribution with He ∼ 2150 km, the north-pole profile showed evidence for a two-component structure consistent with a second source concentrated near 500 km in altitude. Azimuthal scans with the MASCS line of sight oriented perpendicular to the planet-Sun line were acquired while the spacecraft was in shadow. These also reveal differing spatial distri-butions of Na, Ca, and Mg close to the planet. Na exhibits north-south enhancements that differed in relative strength between encounters. In contrast, Ca shows enhancements in the dawn hemisphere near the equator. Mg appears to be more uniformly distributed than either Na or Ca but exhibits some weak evidence for structure. The Na north-south enhancements are correlated with the distribution of solar wind plasma reaching Mercury's surface, suggest-ing that ion sputtering is a source of Na near the poles. High-energy processes are required to produce the Ca and Mg observed in the tail region and above the poles. One hypothesis is that Ca and Mg are released from the surface as molecules that are subsequently photo-dissociated. While the Na distributions observed by MASCS are consistent with ground-based observations, the differences in spatial distributions for the chemically similar elements Ca and Mg were unexpected and remain unexplained.
- Publication:
-
38th COSPAR Scientific Assembly
- Pub Date:
- 2010
- Bibcode:
- 2010cosp...38..762M