Measuring Pickup Ions to Characterize the Lunar Surface and Exosphere

Details Measuring Pickup Ions to Characterize the Lunar Surface and Exosphere Measuring Pickup Ions to Characterize the Lunar Surface and Exosphere

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsh51b0271c&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #SH51B-0271

Physics

7514 Energetic Particles (2114), 2152 Pickup Ions, 2411 Electric Fields (2712)

Scientific paper

It has been known for some time that measurement of the ion components, born from neutral exospheres imbedded in the solar wind, can be used to determine the composition and structure of the parent neutral exospheres (Hartle et al., 1973, Hartle and Thomas, 1974, Luhmann, 1996). The ion pickup process has been observed and verified for more than two decades, including pickup ions born from cometary comas, exospheres of Venus, Mars and Titan, and interstellar gases. Several observations (Mall, et al., 1998 and Hilchenbach et al., 1992) of lunar pickup ions have been reported from passing spacecraft including observations of metallic elements that were presumably sputtered from the lunar surface. The ions so formed, primarily by photoionization, electron impact and charge exchange, are picked up and accelerated by the motional electric field E = -V × B, where V is the plasma bulk velocity and B the magnetic field. The unique orbital characteristics of pickup ions make it possible to infer important details about their sources. For a given ion mass, energy, and incoming direction, the ion trajectory can be mapped back to a point where the velocity vanishes at the cusp of a cycloid. When the gyroradius is much greater than the neutral source scale height (most cases), this cusp point is, it can be safely assumed, the source point. This also requires that the source velocity is much less than pickup acceleration integrated from the source point to spacecraft (again, true in most cases). This makes it possible to derive the neutral exosphere density at that point, assuming the ionization rate is known. When this measurement scheme is carried out on numerous orbits of a mission, it will be possible to derive neutral exosphere densities of all those species whose pickup ions can be measured. With the exception of H+, ion gyroradii are much greater than their source gas scale heights for typical solar wind conditions. Then, for a given ion mass, a spectrometer in lunar orbit will measure ions produced in the exosphere at increasing distances from the spacecraft as it sweeps from low to high energies, until the surface source is reached. Then, sweeping to even higher energies, the spectrometer will measure exospheric ions from greater distances until the ion flux becomes undetectable as the source point passes through more than one neutral scale height. As will be shown, there are several advantages to this approach, including the ability to determine the neutral exosphere below the spacecraft orbit (from the surface to the orbit) instead of just along the spacecraft track and the ability to map surface mineralogy and composition through sputtered ions.

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