Production mechanisms for faint but possibly detectable coronae about asteroids

Asteroidal surfaces are exposed to the same processes which create observable exospheres about such refractory, volatile-free surfaces as those of the Moon and Mercury. Are there modest coronae about asteroids? We have chosen to examine this problem for two species, sodium and water vapor. Sodium was chosen because NaD emission is easily observed from the Earths surface and emissions are observed in the exospheres of both the Moon and Mercury. In addition, the spatial distribution of emission in the sodium D lines about an asteroid would be a direct measure of the velocity distribution of the sodium source, which cannot be measured directly in the case of Mercury or the Moon. This provides a new technique for assessing the possible water content of asteroids. There is now very good evidence that water is present in hydrates and other aqueous alteration products. A detection of an hydroxyl corona about one of these asteroids would be proof of the presence of these aqueous bi-products ; the amount of the water production would a direct measure of the water content of the asteroidal regolith. Water-rich asteroids are of course a potential resource and hold important clues to the evolution of the planets. We have calculated the production of sodium and water (hydroxyl) coronae about asteroids due to just two production mechanisms : sputtering and impact vaporization. We find that for realistic assumptions an asteroid with perihelion near 2 AU would have a small (about 3R) OH corona, while the likely sodium coronae would be about half as bright. These emissions will be extremely difficult to detect from the ground, but they may be detectable from space.