Spectroscopy of the LCROSS Ejecta Plume from Keck, Gemini, and NASA IRTF Observatories

We will observe the LCROSS impact event with three complementary ground-based instruments, chosen specifically to achieve LCROSS Mission Science Goals, as follows: - Keck+NIRSPEC will acquire high-resolution spectra (R = 25,000) of non-resonant fluorescent water vapor emission lines between 3380 cm-1 and 3530 cm-1. - NASA IRTF+SpeX will acquire a wide contiguous spectral range (2 - 4 µm, although saturation is possible longward of 3.4 µm). This range is expected to characterize the shape of the non-H2O-ice continuum as a function of ejecta grain size and mineralogical composition. SpeX will also sample the H2O-ice fundamental band at 3.0 µm. - Gemini+NIFS will acquire infrared spectra (1.9 - 2.3 µm) over a 3"x3" (6 km x 6 km) field of view, encompassing the entire ejecta plume for the first 30 seconds after impact and resolving the dense core of the plume (where the highest column of H2O-ice would be seen). The height dependence is expected to be diagnostic of the size distribution, since smaller particles will have faster post velocities and be lofted higher. Together, these three data sets will look for water, both as vapor (NIRSPEC) and as ice grains (SpeX and NIFS); characterize the grain size and mineralogy of the impacted regolith; and possibly reveal non-water hydrocarbons or hydrated mineral constituents. Below, Flux predictions are given. Predictions for "dry" ejecta plume models (Goldstein, D.B. et al. 2008, AIP 1084, 1061) for Mg-Fe pyroxene composition, and grain column densities from 2e5 to 1e7 m-2, for radii a=35 µm, averaged over the central 6 x 6 km (3"x3") region. In general, fluxes scale with number densities. If 2e5 m-2 of a=35 µm grains disaggregate to yield 1e10 m-2 of a=1 µm grains, then the ejecta plume will be as bright as for 1e7 m-2 of 35 µm grains but spectra probably will reveal mineral features. Separate thermal and scattered fluxes are shown for the a=35 µm, N=1e7 m-2 case. The spectrum of a V=5 A0V star is shown for comparison (assuming 1" seeing).