Methods of Water Ice and Water Vapor Detection

LCROSS will use several independent methods for the detection of water ice and water vapor, as well as studying the ejecta environment:

1. Ice: Near-IR spectroscopy of the scattered sunlight absorption (fundamental and overtone) features of water ice in situ
2. Vapor: Near-IR spectra of H₂O vapor (sublimed ice) emission bands (overtone vibration bands at 1.4 and 1.8 um) in situ, and of fundamental bands near 3 um from ground-based 10 m class telescopes
3. Measurement of an extended OH- atmosphere via spectroscopy at the 308 nm OH- band at UV-visible wavelengths along with nearby scattering continuum
4. Spectroscopy covering the 619 nm H2O+ band and adjacent scattering continuum
5. Narrow band imaging at mid-IR wavelengths to follow thermal evolution of plume and newly deposited regolith, which will be affected by water vapor in the ejecta.
   

Combining multiple independent measurement methods greatly increases the likelihood of obtaining a constrained and definitive understanding of the impact event and the amount of water contained in the regolith. Furthermore, many of these measurements, because of their instrument requirements or the timescale of the physical process, are more effectively made either from a platform very near the event (e.g. the S-S/C) or from ground-based telescopes. Combining measurements from the Shepherding Spacecraft (S-S/C) with ground-based observations and (subsequent) mapping by lunar orbiting assets (e.g., LRO and Chandrayaan-1) enhances the overall robustness and effectiveness of the LCROSS. Table 1 lists the various measurement techniques, the time and spatial scale best suited to each observation, and where the measurement can be made. LRO’s UV, IR and topographic mapping of the impact craters and associated ejecta blankets may provide additional information about water ice and other volatiles in the permanently shadowed regolith together with an evaluation of the mechanical properties of lunar regolith in permanent shadow. Similarly, a possible visit to the craters by follow-on missions will provide detail down to the cm scale for further analysis.

Measurement Risk Assessment and Mitigation
Success of the LCROSS mission is subject to some modest risks. The primary risk is the shadowed regolith may be sufficiently non-uniform in terms of its hydrogen content, due to the local cratering history, so that even two impacts may not guarantee results that are definitive with respect to planning for subsequent landed missions.

In analyzing this risk we have used the conservative estimate that only 5% of the excavated crater mass is ejected into the observable plume. Even with these conservative numbers we conclude that if ice deposits exist, the signature will be detectable to both orbital assets and ground-based observers. Figure 1 shows the estimated signal-to-noise (S/N) (for the water component only) for S-S/C and ground-based NIR observations. Observing the Centaur impact, the S-S/C instruments will have excellent S/N (>>10) and will be sensitive to regolith water concentrations as small as 0.1%. Ground based measurements will be most effective at detecting water immediately after the impact when the ejecta plume is at its brightest. The OH may be detectable for hours, and the H₂O/OH exosphere for days.

Figure 1 Significant preliminary analysis has been done to determine how visible the ejecta plume will be to S-S/C and ground based instruments. S/N >> 10 is achievable at the S-S/C for regolith water ice concentrations as low as 0.1%.

It is possible (but highly unlikely) that the impactors may strike sites lacking any appreciable hydrogen/water ice deposit. The issue is one of how to interpret the results if no water is observed in the dust clouds that rise from the impact events. In such a case the neutron and radar mapping data acquired by the LRO will be important in understanding the LCROSS results – in deciding whether the impact sites are in anomalous regions or not. We will be able to identify the location of the impacts from 1) approach imaging by the S-S/C, 2) telemetry data, 3) plume geometry, and 4) LRO observations (using LOLA, LAMP, and Diviner to map topography, UV signature, and possibly the remnant thermal signature of the impact, respectively). This information will allow us to determine precisely where the impacts occurred with respect to neutron spectroscopy maps and radar reflectivity assessments. If the impact sites appear to be properly representative of the region, then two null observations will set significantly stronger limits than known to date. If definitive, then a null event will be important to the way in which future missions will be conducted.

Figure 2 shows the mission success achieved for a combination of possible outcomes. In almost all cases the results obtained by LCROSS will provide decisive information regarding the distribution and composition of water. In all cases, LCROSS will provide valuable knowledge regarding the Lunar Polar regolith properties, environment, and impact processes.

Figure 1.2-4 LCROSS will provide decisive information regarding the distribution and characterization of Lunar Polar hydrogen for almost all possible observation scenarios. In all cases LCROSS will provide valuable information on the Lunar Polar regolith and cratering properties.

Technical Information
Overview | Mission Rationale | Spacecraft and System Description | Instrumentation | Water Detection | Targeting
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