Abstract. Mercury's sodium atmosphere is known to be highly variable both temporally and spatially. During a week-long period from November 13 to 20, 1997, the total sodium content of the Hermean atmosphere increased by a factor of 3, and the distribution varied daily. We demonstrate a mechanism whereby these rapid variations could be due to solar wind-magnetosphere interactions. We assume that photon-stimulated desorption and meteoritic vaporization are the active source processes on the first (quietest) day of our observations. Increased ion sputtering results whenever the magnetosphere opens in response to a southward interplanetary magnetic field (IMF) or unusually large solar wind dynamic pressure. The solar wind dynamic pressure at Mercury as inferred by heliospheric radial tomography increased by a factor of 20 during this week, while the solar EUV flux measured by the Solar EUV Monitor (SEM) instrument on board the Solar and Heliospheric Observatory (SOHO) increased by 20%. While impact vaporization provides roughly 25% of the source, it is uniformly distributed and varies very little during the week. The variations seen in our data are not related to Caloris basin, which remained in the field of view during the entire week of observations. We conclude that increased ion sputtering resulting from ions entering the cusp regions is the probable mechanism leading to large rapid increases in the sodium content of the exosphere. While both the magnitude and distribution of the observed sodium can be reproduced by our model, in situ measurements of the solar wind density and velocity, the magnitude and direction of the interplanetary magnetic field, and Mercury's magnetic moments are required to confirm the results.

1] We examined the thermal IR spectra of 35 rock samples and their fine-powder equivalents to better understand the relationship between spectral features, particle size, and composition. The Christiansen features of solid crystalline rocks differ from those of their powdered samples, and the magnitude of this difference changes systematically with rock type. Both the Christiansen feature (CF) and the transparency feature (TF) in spectra of particulate igneous rocks are useful in rock type determination. They correlate better with the SCFM chemical index of rock type than with other numerical rock type indices that we considered. Major rock type divisions (acidic, intermediate, basic, and ultrabasic) could be distinguished using both CF and TF, but some overlap occurred if one or the other spectral feature was used alone. Spectral contrast decreases as rocks become more basic, probably because of increasing opaque mineral content. Likewise, metallic iron, formed as a result of space weathering in an airless environment, appears to be the most likely cause of reduced spectral contrast in mature lunar soils. Thus reststrahlen features should be difficult to detect in remote-sensing measurements of such bodies as the Moon and Mercury. Moreover, the vacuum environment of an airless planetary surface appears to enhance the CF, but it may eliminate the TF, depending critically on particle size. The Martian pressure and radiative environment, on the other hand, appears to enhance the TF, which should be sought in spectra of finely particulate surface areas on Mars. Additional environmental simulation spectral experiments are needed to more accurately define the roles of pressure, particle size, opacity, and insolation angle on apparent emissivity.

No abstract

The spectrum of Mercury at the Fraunhofer sodium D lines shows strong emission features that are attributed to resonant scattering of sunlight from sodium vapor in the atmosphere of the planet. The total column abundance of sodium was estimated to be 8.1 x 10(11) atoms per square centimeter, which corresponds to a surface density at the subsolar point of about 1.5 x 10(5) atoms per cubic centimeter. The most abundant atmospheric species found by the Mariner 10 mission to Mercury was helium, with a surface density of 4.5 x 10(3) atoms per cubic centimeter. It now appears that sodium vapor is a major constituent of Mercury's atmosphere.

It has been speculated that the composition of the exosphere is related to the composition of Mercury's crustal materials. If this relationship is true, then inferences regarding the bulk chemistry of the planet might be made from a thorough exospheric study. The most vexing of all unsolved problems is the uncertainty in the source of each component. Historically, it has been believed that H and He come primarily from the solar wind (Goldstein, B.E., et al. in J. Geophys. Res. 86:5485-5499, 1981), Na and K come from volatilized materials partitioned between Mercury's crust and meteoritic impactors (Hunten, D.M., et al. in Mercury, pp. 562-612, 1988; Morgan, T.H., et al. in Icarus 74:156-170, 1988; Killen, R.M., et al. in Icarus 171:1-19, 2004b). The processes that eject atoms and molecules into the exosphere of Mercury are generally considered to be thermal vaporization, photon-stimulated desorption (PSD), impact vaporization, and ion sputtering. Each of these processes has its own temporal and spatial dependence. The exosphere is strongly influenced by Mercury's highly elliptical orbit and rapid orbital speed. As a consequence the surface undergoes large R. Killen ( )