Pickup Ion Mass Spectrometry for Surface Bounded Exospheres and Composition Mapping of Lunar and Planetary Surfaces.

摘要

Many of the small to medium sized objects in the solar system can be characterized as having surface bounded exospheres, or atmospheres so tenuous that scale lengths for inter-particle collisions are much larger than the dimensions of the objects. The atmospheres of these objects are the product of their surfaces, both the surface composition and the interactions that occur on them and also their interiors when gases escape from there. Thus by studying surface bounded exospheres it is possible to develop insight into the composition and processes that are taking place on the surface and interiors of these objects. The Moon and Mercury are two examples of planetary bodies with surface bounded exospheres that have been studied through spectroscopic observations of sodium, potassium, and, on the moon, mass spectrometric measurements of lunar gases such as argon and helium. The creation of these ions and the action of acceleration in the solar wind derived motional electric field (pickup ions) provides a highly sensitive method for studying these exospheric objects, both their surfaces and atmospheres through ion mass spectrometry. Because of the extreme sensitivity of ion mass spectrometry and the fact that the acceleration separates the ion population from background thermal ions, it is possible to characterize exospheric atmospheres to a level that cannot be achieved by other commonly used methods. For example we anticipate that measurements of the lunar atmosphere from an orbiting spacecraft will dramatically lower the threshold concentration by which water in the atmosphere can be detected. We will discuss here how this measurement can be made, what sensitivity is required, how to relate the observation of atmospheric ions to the source of these ions and how an elemental map of the surface can be constructed using these data. Here we concentrate on the Earth's moon since it is largely unmagnetized resulting in relatively simple trajectories for the ions found there. Also, it is accessible, well studied, and finally, because the Moon is a focus of a renewed exploration initiative to the Moon and Mars. A pickup ion instrument on a spacecraft with a low lunar orbit will be capable of measurements of both the lunar atmosphere and surface.