Catching the LCROSS Impact Excavation of Lunar Dust+Ice at K-band with NIFS

The LCROSS Mission will hit the floor of a permanently shadowed crater on the Moon in 2009B, and impact excavate 6000 kg of lunar regolith to look for 60 kg of water. We propose to observe the temporal and spatial evolution of the impact plume using NIFS with its Integral Field Unit. The ejecta curtain evolves very fast, but NIFS can catch the dense core of the ejecta curtain in the first 30 seconds after impact, and measure the spreading plume over 3 minutes as the ejecta overfills the 3 arcsec x 3 arcsec FOV (or 6 km x 6 km on the moon). We take advantage of the spatial sampling of NIFS and the grain size-dependence of the spectral slope in the NIFS 2 um region to assess the height-sorted (velocity-sorted) particle size distribution. The LCROSS shepherding spacecraft can detect water ice at a concentration of 1% (tau_ice[2 µm]=0.001) in the same wavelength range as NIFS. However, NIFS spectral data cube from 1.9 to 2.3 microns uniquely enables the de-convolution of the particle size distribution and the surface density distribution from the observed surface brightness, providing a more critical assessment of the ejected mass and hence the ice concentration. Only NIFS+ Gemini-N can catch the side-on view of the plume's spatial and spectral evolution with time. NIFS data complements our team's observations of the 2.95 µm non-resonant fluorescent water vapor lines with Keck II+NIRSPEC and the 3 µm ice fundmental band with IRTF+SpeX.