The LCROSS cratering experiment

Science

Volumn 330, Issue 6003, 2010, Pages 468-472

  Schultz, Peter H ^a   Hermalyn, Brendan ^a   Colaprete, Anthony ^b   Ennico, Kimberly ^b   Shirley, Mark ^b   Marshall, William S ^b,c  

^a BROWN UNIVERSITY   (United States)

^b NASA AMES RESEARCH CENTER   (United States)

^c UNIVERSITIES SPACE RESEARCH ASSOCIATION   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

VOLATILE ORGANIC COMPOUND; HYDROXIDE;

CRATER; LUNAR CRUST; SATELLITE MISSION; SPACECRAFT; SURFACE LAYER; WAVELENGTH;

ARTICLE; ENVIRONMENTAL MONITORING; INFRARED SPECTROSCOPY; LUNAR CRATER OBSERVATION AND SENSING SATELLITE; PLUME; PRIORITY JOURNAL; SPACE FLIGHT; SPECTRAL SENSITIVITY; SURFACE PROPERTY; TELECOMMUNICATION; THERMAL ANALYSIS; MICROCLIMATE; MOON;

EXTRATERRESTRIAL ENVIRONMENT; HYDROXIDES; MOON;

EID: 77958172267     PISSN: 00368075     EISSN: 10959203     Source Type: Journal    
DOI: 10.1126/science.1187454     Document Type: Article

References (30)

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7. -6 (ratio of luminous flux to impact energy) for the LCROSS impact speed with initial temperatures of <2500 K (9).
8. The supporting online material (SOM) text and (6) provide more detailed discussion about the VSP spectra.
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14. 4) tanks] remained with other volatile components in the Centaur (insulating foam and paint), contributing an additional 35 kg (6). The impact speed would not fully vaporize this material, except scoured surfaces directly in contact with the regolith during penetration. Even then, only a small fraction of this component would have been ejected at (or excited to) sufficient speeds to reach sunlight.
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16. Estimates for the ejected mass covered a wide range depending on models and assumptions. Extrapolations from late-stage scaling relations (for 45° ejection angles) predicted that only about one projectile mass (∼2000 kg or about 1% of the total ejected mass) would reach an altitude of 2 km (21, 26), which is about an order of magnitude less than estimates from smooth-particle-hydrocode models (20). For standard ejection models (23), one projectile mass is probably an overestimate because such extrapolations do not include the effect of early time-energy losses (compressible target) and the nature of the impactor (hollow). A high-angle plume, however, substantially offsets this reduction. Because energy and momentum partitioning still limits the maximum ejected mass, not much more than two to three projectile masses (4000 to 6000 kg) should have reached sunlight at ∼0.8 km. Derivations of the total ejecta mass in sunlight from LCROSS VSP and Lunar Reconnaissance Orbiter (LRO) LAMP observations are consistent with this prediction (6, 26). It also should be noted that scattering in the near infrared was five times less than in the visible, which is about a factor of two times less than expected or modeled (6).
17. The expanding annulus of ejecta arises from the evolution of ballistic ejecta. With time, ejection velocities systematically decrease, launch positions increase in distance from the point of impact, and ejection angles remain nearly constant. The evolution of ballistic ejecta in a given direction produce an inclined thin sheet called the ejecta curtain that advances across the surface away from the crater rim. In all directions, this curtain resembles an inverted cone that expands with time around the impact point. It represents the locus of individual ejecta ballistic particles at a given time, as revealed in laboratory experiments (19) and models (20).
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21. P. H. Schultz et al., Eos Trans. AGU 90, abstr. no. U22A-08 (2009).
22. The VSP radiance flattened as its narrow field of view subsampled the sunlit ejecta cloud (6). After about 180 s, the Centaur crater (and ejecta in the foreground) was no longer directly centered, and the VSP and NSP instruments no longer had the remaining high-angle component in their fields of view (Fig. 3B). Consequently, radiance levels dropped rapidly during final approach. Nevertheless, still-returning ejecta still in sunlight continued to liberate adsorbed volatiles, which detached as an expanding, tenuous vapor cloud. Ejecta and volatiles within this narrow high-angle plume would have been difficult to detect from the Earth. Even though some ejecta did reach sufficient altitudes, the low column density and limited extent (as viewed from the side) made telescopic observations difficult.
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25. 2O in the regolith could come from solar wind processes (3-5), other sources include recent gas release from the lunar interior (1, 15, 27) and cometary or asteroid impacts. Although the high impact speeds of cometary impacts generate gas expansion speeds exceeding the lunar escape velocity, recent models predict that a small fraction could be retained (28). Such contributions to polar cold traps from infrequent cometary sources require a replenishment rate that exceeds the loss rate caused by the continuous "gardening" by small impacts. It is also possible, however, that a cometary impact (or impacts) recently resupplied the current inventory, which will gradually disappear until the next volatile-rich collision. Two other processes can also temporarily sequester volatiles within the regolith. First, not all volatiles released (or generated) by small cometary impacts escape as expanding vapor; rather, a fraction will be injected into the regolith below the surface before migrating upward. Second, impact-trapped pockets of hydrous phases (concentrations as high as 20 weight percent) occur in both natural and experimental impact glasses (29). Such high concentrations are possible because of the high solubility of glass at high pressures and temperatures, combined with rapid quenching. In both cases, slow diffusion and subsequent impact-gardening would gradually release the entrained volatiles and allow their migration to cold traps.
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29. R. S. Harris et al., LPI contribution no. 1360, abstr. no. 8051, Saint-Hubert, Canada, 22 to 26 September 2007.
30. We thank the LCROSS Project and NASA's Exploration Systems Mission Directorate (ESMD) and NASA Science Mission Directorate (SMD; Planetary Geology and Geophysics) for support. We are also very grateful to the NASA Ames Vertical Gun Range technical team and the Thermophysics Facilities Branch at NASA Ames for their continued support in experiments. B.H. was supported through a NASA Rhode Island Space Grant fellowship for part of this study.