Physics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..600g&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Physics
Scientific paper
ABSTRACT Introduction Recent work [1-3] using Earth-based radar measurements has shown that large impacts on the Moon produce a distinct facies of highly comminuted ejecta depleted in fragments >10 cm in diameter, forming concentric radar-dark haloes around the source craters and representing a mantling layer on the order of 10m thick. We have also recently observed similar haloes of fine ejecta surrounding Martian craters >5 km in diameter, characterized by micron- to mm-sized particles. Preliminary measurements suggest that the lunar and Martian fine ejecta haloes are geometrically similar: that is, they scale in much the same way with respect to their source craters. This implies that a) the comminution process and emplacement of ejecta on the two planets occur in similar ways; and b) like the Martian case, the lunar crater haloes also have a thin mantling layer of very fine particles, which cannot be detected using Earth-based radar. Because of their extensive spatial coverage and high resolution, LRO DIVINER measurements will provide the first opportunity to bridge this gap and to more completely characterize the rock size distribution represented in lunar continuous ejecta. Radar remote sensing of fine-grained ejecta haloes Fine-grained lunar ejecta haloes were first observed [4], and later studied in detail [1-3], using Earth-based delay-Doppler radar imagery at 70-cm wavelength. Recent observations in two circular polarizations have been made using the Arecibo telescope in Puerto Rico and the Greenbank telescope in West Virginia in a bistatic geometry [5; Campbell et al., IEEE]. In general, the radar backscatter of the lunar regolith is comprised of echoes from the surface, rocks suspended within the fine-grained matrix, and a possible basal regolith interface. The relative importance of each of these components varies with radar wavelength and is a function of surface roughness, surface and volume rock populations, the depth and dielectric properties of the matrix, and the roughness of the basal substrate. Scattering from surface topography and rocks dominates the echo at short wavelengths, whereas longer wavelength echoes are also sensitive to larger surface blocks and those buried within the regolith. Scattering from surface rocks is most efficient for rocks ranging from 1/10 to 10 wavelengths, and incident energy penetrates the target material to a depth determined by the radar wavelength and the loss tangent of the material. Thus, the echo at a particular wavelength represents the depth-integrated properties over the radar path length. The polarization of reflected radar signals yields additional information about the physical properties of the regolith. A circularly polarized wave reflected from the lunar surface consists of opposite-sense (OC) and same-sense (SC) components. Variations in these two components have been used to constrain the changes in rock population between the ejecta haloes and the surrounding terrain required to produce the observed signature. Analysis of the radar observations and comparisons with mineral maps derived from Clementine multispectral reflectance data and with Lunar Orbiter photographs indicated that the ejecta haloes, which appear dark at 70-cm, indicate a mantling layer of highly comminuted ejecta, <10m thick, that is depleted in wavelengthscale (>10 cm) scatterers [1]. These characteristic radardark annuli appear outside the rough, blocky, radar-bright material located near the crater rims. The haloes typically extend up to 3 crater radii beyond the edges of the blocky ejecta, are commonly correlated with radial, soft ridged ejecta visible in photographs, and .are nearly ubiquitous on the nearside for craters larger than 10km in diameter (Fig. 1). Fine-grained ejecta haloes on Mars Using THEMIS IR data, we have found 50 Martian craters >5km in diameter with haloes of low nighttime temperature material , suggesting small, unconsolidated particles (Fig 2). Comparison of nighttime IR images with Viking visible-wavelength mosaics and MOC narrow angle images show that for Bonestell, the low-temperature halo corresponds with a deposit of higher-albedo ejecta with a soft hummocky surface. The digitate margins of the halo match those of the hummocky ejecta. For Mojave, the Viking mosaic shows a pattern of radial petals of ejecta (suggesting fluidization) that broadly corresponds to the low-nighttime temperature halo, but additional fine structure is visible in the higher-resolution THEMIS mosaic. The unnamed crater also displays a low-temperature halo with a digitate to serrated, wispy margin, but Viking images show only faint corresponding brightness variations. The radial pattern of these low-temperature Martian haloes, and their association with ridge-forming ejecta, support their identification as fine-grained ejecta features analogous to those found on the Moon. In addition, preliminary work suggests that the scaling of the martian haloes relative to their source craters is similar in form to that for the lunar haloes. The key point relating observations of comminuted ejecta on Mars to those on the Moon is that the presence of a very fine ejecta fraction in association with Martian craters, and the similarity between the Martian and lunar ejecta haloes, suggest a common fine-fraction particle size distribution for ejecta on both planets. That is, we predict that lunar ejecta blankets are mantled by a layer of comminuted particles with some statistical size distribution ranging at least from several μm up to 10cm. Unlike the lunar radar-detected haloes, however, the thermal IR signature of the Martian material arises from radiation from the top 50 μm of regolith material, and the particle size range represented by the Martian halo material, inferred from calculated thermal inertia [6, 7] to be <10 microns to 10mm in diameter, is very different from that of the lunar haloes. The Earth-based radar technique used thus far to observe the lunar ejecta is insensitive to the small end of this distribution. The LRO DIVINER experiment will allow detection of the finest fraction of this population. Integration of DIVINER thermal IR observations with existing radar observations for lunar crater ejecta will result in a deeper understanding of the physical properties of the upper meters of lunar ejecta blankets than has been possible to date. This characterization will contribute to our understanding of the physics of cratering and ejecta emplacement, to constraining the physical environment at the surface for purposes of hazard mapping, and to understanding how the regolith evolves. References [1] Ghent, R. R., D. W. Leverington, B. A. Campbell, B. R. Hawke, and D. B. Campbell (2005), JGR-E, 110, E02005, doi:10.1029/2004JE002366. [2] Thompson, T. W., Campbell, B. A., Ghent, R. R., Hawke, B. R., and Leverington, D. W. (2006), JGR-E, v. 111, E06S14, doi:10.1029/2005JE002566. [3] Ghent, R.R., B.A. Campbell, B. R. Hawke, and D.B. Campbell (2007), Geology, 36(5), 343-346. [4] Thompson, T.W. (1974), The Moon 10, 51-85. [5] Campbell, B.A.; Campbell, D.B.; Margot, J.L.; Ghent, R.R.; Nolan, M.; Chandler, J.; Carter, L.M.; Stacy, N.J.S. (2007), IEEE Trans. Geosci. Rem. Sens., 45(12), 4032-4042. [6] Presley, M. A., and Christensen, P. R. (1997), JGR-E, 102(E3), 6551-6566. [7] Presley M. A., R. A. Craddock (2006), JGR-E, 111, E09013, doi:10.1029/2006JE002706.
Campbell Bruce A.
Ghent Rebecca R.
Pithawala T.
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