| Lunar chronology can provide a necessary time scale for establishing and perfecting the geological evolution history of the moon,and is an important content of lunar geology research.In this paper,the formation time of impact craters is the most common geomorphic feature on the moon,studied by using a variety of remote sensing data.We determined the formation age of craters around the Chang’e-5 landing site,which constrains the geological history of the region.At the same time,the maturity profile evolution of ejecta materials of craters is analyzed,and the possibility of determining the age of craters by maturity is explored.China’s Chang ’E-5 mission has successfully returned 1,731 grams of lunar regolith samples from the northeastern of the Oceanus Procellarum.The CE-5 landing site is an Eratosthenian-aged mare unit,one of the youngest mare units.Remote sensing analysis shows that the Eratosthenian-aged mare units where the CE-5 landed on,might have been contaminated by distant ejecta delivered by several craters,such as Pythagoras,Sharp B,Harpalus,Copernicus,and other craters.These ejecta materials,the same as local young mare basalts,would serve as new calibration points for lunar chronology curve.Among above craters,the absolute model ages(AMAs)of Pythagoras,Sharp B and Harpalus are not well constrained.Here we used the Crater Size Frequency distribution(CSFD)method to yield their formation ages and the crater density values(N(1)/(km-2)).Using high resolution images,craters on the selected counting areas of their floors,ejecta blankets were identified.Our results indicate that Pythagoras is an Eratosthenian-age crater with an AMA of 2.13±0.16 Ga and a N(1)=1.79 ×10-3 km-2.The Sharp B and Harpalus are both Copernican.The AMA of Sharp B crater is 1.01±0.12 Ga with a N(1)of 8.46 × 10-4 km-2 while the AMA of Harpalus is 1.10± 0.04 Ga with a N(1)of 9.22 × 10-4 km-2.The ejecta from Sharp B and Harpalus as well as local mare basalt of the CE-5 site may be served as calibration points for lunar chronology curve,which will fill the age gaps of lunar chronology curve.Based on the estimated ages,we interpreted that Pythagoras ejecta might be beneath the young CE-5 mare unit,while the Sharp B and Harpalus ejecta should overlie the surface of the CE-5 site.At the same time,this study also proposes a new method to constrain the age of impact craters based on the evolution of OMAT profile.Crater rays are formed during a cratering event as target material is ballistically ejected to distances of many crater radii forming narrow,generally high albedo,approximately linear features extending outward from the crater.The formation and degradation of lunar crater ray is one of the hot topics in lunar science.In this paper,the current understanding of crater ray origin,their different types on lunar surface,and the processes related with degradation of lunar crater ray systems are summarized.After the formation of the crater rays,the fresh and bright ejecta materials around the crater gradually mature under the joint action of a variety of geological processes,and reflectivity gradually disappears in accordance with the background.The OMAT profiles of young craters with various ages were compared.It is found that the average OMAT profile of crater ray could be fitted with the power law function;and that the shape of the OMAT profiles is strongly dependent on the exposure time on lunar surface as well as the crater age.This suggests that by using young crater with ray and known age as the reference,an empirical equation between the shape parameter and age could be built.This study provides a new method for dating young lunar craters. |
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