Physics – Geophysics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p23a1626w&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P23A-1626
Physics
Geophysics
[0933] Exploration Geophysics / Remote Sensing, [1160] Geochronology / Planetary And Lunar Geochronology, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering
Scientific paper
Copernicus Crater (D ~ 95 km) is one of the largest young primary craters on the lunar surface. We present radar mosaics of the Copernicus region that reveal numerous small secondary craters with ejecta blankets of blocky material extending in “tails” downrange from the parent crater. Individual secondary craters are more apparent in the radar circular polarization ratio images than in Clementine 750 nm images of similar resolution. A capability to distinguish between primary and secondary craters at large distances from the parent crater will improve the reliability of the age dating of small areas on the Moon and, possibly, other solar system bodies. Wells et al. (2010) investigated a group of otherwise unremarkable craters near the lunar south pole with parallel, asymmetric ejecta blankets visible in radar CPR but not optical images. The radar CPR is sensitive to blockiness on the radar wavelength, with low CPR indicative of smooth surfaces and high CPR of rough surfaces. The rough ejecta blankets seen by Wells et al (2010) were elongated downrange from Tycho crater, and the group of small craters was deemed to be a population of Tycho secondaries. Similar ejecta blankets are associated with numerous small craters in the Copernicus radar mosaics. These 12.8-cm radar images were obtained with the Arecibo and Green Bank telescopes operating in a radar bi-static mode as part of a project to map the lunar near-side at 80 m/pix (Campbell et al., 2010). Regions of high CPR are overlain on a contrast-enhanced depolarized radar image to aid in the identification of craters with associated CPR features. Using the presence of these high-CPR ejecta blankets as an indicator, the size and location of secondary craters present in the 160 m/pix mosaic are being measured. Several thousand potential secondary craters have been marked for investigation. To date, 157 secondary craters with D>3 km have been cataloged in a 1.47 million square km area surrounding Copernicus crater. 202 secondary craters with D>2.3 km in a 256 thousand square km region to the north of Copernicus have also been investigated. The size frequency distributions of these populations have steep power-law slopes of b= -6.7 and b= -5.2, respectively. These values are steeper than the canonical secondary power-law slope of b= -3 or b= -4. Ongoing debate over the reliability of ages derived from counts of craters with small diameters depends heavily on the relative dominance of small primary and secondary craters on the Moon. Continued investigation of large populations of secondary craters, like those of Copernicus and Tycho, will help to resolve this issue. Future work will focus on greater spatial coverage at higher resolution for Tycho, Copernicus, and other bright-rayed near-side craters such as Kepler, Aristarchus, and Aristillus. References: Campbell et al (2010), Icarus 208, 2, p 565-573. Wells et al (2010), JGR 115, doi:10.1029/2009JE003491
Anderson Rachel
Campbell Bruce A.
Campbell Don B.
Carter Lynn Marie
Wells Kassandra S.
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