The Opitical Maturity of Ejecta from Large Rayed Craters: Preliminary Results and Implications

Details The Opitical Maturity of Ejecta from Large Rayed Craters: Preliminary Results and Implications The Opitical Maturity of Ejecta from Large Rayed Craters: Preliminary Results and Implications

Jan 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...19g&link_type=abstract

Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 19

Mathematics

Logic

Cratering, Ejecta, Geochronology, Lunar Craters, Lunar Maria, Selenology, Moon, Lunar Geology, Radiometers, Stratigraphy

Scientific paper

Shoemaker argued that several lines of evidence suggest a significant increase in the production of large craters on the Earth late in geological time (about 200-300 Ma). We have conducted a preliminary test of this hypothesis via a study of large recent craters on the Moon, and have concluded that the lunar record does not support this idea. The relatively recent (<1 Ga) flux of asteroids and comets forming craters on the Earth and Moon may be accurately recorded by lunar craters whose ejecta are optically immature, and therefore distinguishable from the overall mature background soils. Lucey et al. have developed a methodology for extracting an optical maturity (OMAT) parameter from multispectral images. We have generated a preliminary dataset of average radial profiles of OMAT values around large craters (about 20 km diameter) that have been suggested or suspected of belonging to the Copernican timestratigraphic system. Our survey covers the Moon from +/- 60 deg latitude. The average OMAT profile for each large crater is generated from nearside and farside OMAT mosaics. The profiles clearly fall into three bins based on their relative age. Young craters' profiles are characterized by high OMAT values near the rim and very steep dropoffs in OMAT value over a long distance away from the crater. Old craters whose profiles over their ejecta are essentially flat sloped and indistinguishable from background; their OMAT values at the rim are very low. Intermediate craters in this bin are between young and old; they have moderate OMAT values at the rim, and their profiles, while somewhat flat, are distinguishable from background some distance away from the crater rim. Profiles for a sample of large craters show a range of possible values and slopes; clearly the trend is a continuum. These suggested bins are based on the absolute ages of a few lunar craters, such as Tycho and Copernicus. Tycho and Copernicus are approximately the same size, but their OMAT profiles are quite different. Tycho is clearly young, with a high OMAT value at the crater rim and a steep dropoff in OMAT values as one follows the ejecta away from the crater. Copernican ejecta appears to be almost entirely mature, but there may be some suggestion of more immature ejecta within 2 crater diameters of the rim. Copernicus is therefore either old, or is on the old side of intermediate. Copernicus has been radiometrically dated at about 810 Ma, so the upper limit of this technique to identify and relatively date large craters must fall at less than about 800 Ma. Tycho is about 100 Ma. Craters with ejecta both much less and more mature than these craters have been identified. The density of large craters classified as younger than Copernicus is at least 40% less than the density of farside craters with bright rays, which was suggested to represent a population of craters younger than Copernicus. Furthermore, these craters have a "flatter" size-frequency distribution (relatively more large craters) than do the farside rayed craters or other lunar craters younger than 3.2 Ga. We suspect that there is an age-size bias to our classification, such that the OMAT profiles of smaller craters look like that of larger but older craters. Nevertheless, two large nearside craters previously mapped as Copernican, Eudoxus (67 km), and Aristillus (55 km), which are close in size to our "calibration" craters (Copernicus 92 km and Tycho - 86 km), are now known to have optically mature ejecta. Thus it is likely that the density of large craters younger than Copernicus must be reduced from the estimates in, and the case for a modest increase in the cratering rate (in the past 800 m.y. versus the previous 2.4 b.y.) has been weakened. However, if Copernican ejecta are completely mature, then there may well be additional large craters younger than 800 Ma, and the cratering rate over the past 800 m.y. may have increased. Our preliminary investigation also shows no evidence for a change in the cratering rate since Tycho (about 109Ma) compared with the cratering rate since Copernicus (about 810 Ma). There are two additional results from this preliminary study that are relevant to lunar stratigraphy based on our analyses of Aristillus, Autolycus and Lichtenberg Craters. We find optically mature ejecta around the crater Lichtenberg (20-km diameter). Marc lavas are emplaced over the bright ejecta and rays of Lichtenberg; this has been cited as evidence for Copernican-age mare volcanism. Our data indicate that this superposition does not require an unusually young age for the mare lavas, based on the relative age of the crater. Both Aristillus and Autolycus have been morphologically classified as rayed, due to the nature of their ejecta; Autolycus has been radiometrically dated at about 2 Ga and Aristillus at 1.3 Ga . These ages seemed in possible contradiction with the supposition that rayed craters are all relatively recent. The OMAT profiles for both of these craters, though, are otherwise indistinguishable from OMAT background values and are very flat. The profiles indicate that the maturity of the ejecta from these craters is much greater than the ejecta of Copernicus, which is consistent with their radiometric ages. It is therefore clear that the presence of rays alone do not imply that a large crater is near or younger than the age of Copernicus. Since these craters possess mature ejecta, and are classified as old, this does not support the suggestion by that the cratering rate during the Copernican was about 2x lower than that during the Eratosthenian.

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