¹J.F.Pernet-Fisher,¹K.H.Joy,¹J.D.Gilmour
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113977]
¹Department of Earth and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
Copyright Elsevier

Lunar ferroan anorthosites are the ideal samples for investigating primitive volatile systematics. Not only are these lithologies thought to be direct crystallization products of the Lunar Magma Ocean (LMO), but many samples display short (T38 < 5 Myr) cosmic ray exposure (CRE) ages, minimizing the effects of cosmic ray spallation reactions. Here we report noble gas (He, Ne, Ar, Kr, Xe) abundances and isotope systematics for nine ferroan anorthosites (FAN) collected during the Apollo 16 mission and one anorthosite sample collected during the Apollo 15 mission. The CRE ages calculated for these samples range from T38 ~ 0.13 to ~226 Myr, indicating that not all anorthosites were emplaced at the lunar surface at the same time.

In general, He-Ne-Ar-Kr-Xe isotope systematics can be accounted for by variable contributions from cosmogenic spallation reactions and solar-wind implantation. The Xe isotope systematics of lunar anorthosites offer our best chance of resolving primitive Xe components on the Moon. Three of the samples investigated here (60,515, 65,325, 60,025) display a Xe isotope signature within error of terrestrial air. These samples have likely been comprised by anomalously adsorbed terrestrial air, as was also recognized by early Xe isotope studies of lunar anorthosites (e.g., Niemeyer and Leich, 1976). The three samples that have the shortest CRE ages (69,955, 60,135, 60,015) display ratios of heavy Xe isotopes (134Xe and 136Xe) over lighter isotopes (130Xe and 132Xe) that are lower than air and solar wind. Mixing modeling for these three samples suggests that such signatures can be accounted for by the addition of up to ~30% cometary Xe (based on the reported Xe isotope composition of comet 67P/Churyumov-Gerasimenko; Marty et al., 2017) to mixtures of adsorbed terrestrial air and Solar Wind. One sample (60135) displays lower than solar 136Xe/132Xe from gases extracted in an intermediate temperature heating step, indicating that such a component may have only been superficially implanted. However, two other samples (69,955, 60,015) display heavy Xe isotope ratios deficits only in the highest temperature gas extraction steps, indicating that this component is hosted within the plagioclase crystal structure. It is not clear how a cometary component was introduced into the lunar crust. In one scenario, cometary Xe was mixed directly into the LMO during periods of high impact bombardment (such as the Late Veneer) prior to the formation of the lunar crust before ~4.2 Ga. Alternatively, cometary Xe may have been directly implanted into plagioclase crystals via diffusion as a result of micrometeorite impacts over geological time in the near surface lunar environment.

¹A. Galiano et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113959]
¹INAF-IAPS, Rome, Italy
Copyright Elsevier

The Near-Earth Asteroid 162,173 Ryugu (1999 JU3) was investigated by the JAXA Hayabusa2 mission from June 2018 to November 2019. The data acquired by NIRS3 spectrometer revealed a dark surface with a positive near-infrared spectral slope. In this work we investigated the spectral slope variations across the Ryugu surface, providing information about physical/chemical properties of the surface.

We analysed the calibrated, thermally and photometrically corrected NIRS3 data, and we evaluated the spectral slope between 1.9 μm and 2.5 μm, whose values extend from 0.11 to 0.28 and the mean value corresponds to 0.163±0.022. Starting from the mean value of slope and moving in step of 1 standard deviation (0.022), we defined 9 “slope families”, the Low-Red-Slope families (LR1, LR2 and LR3) and the High-Red-Sloped families (HR1, HR2, HR3, HR4, HR5, HR6). The mean values of some spectral parameters were estimated for each family, such as the reflectance factor at 1.9 μm, the spectral slope, the depth of bands at 2.7 μm and at 2.8 μm. A progressive spectral reddening, darkening and weakening/narrowing of OH bands is observed moving from the LR families to the HR families.

We concluded that the spectral variability observed among families is the result of the thermal metamorphism experienced by Ryugu after the catastrophic disruption of its parent body and space weathering processes that occurred on airless bodies as Ryugu, such as impact cratering and solar wind irradiation. As a consequence, the HR1, LR1, LR2 and LR3 families, corresponding to equatorial ridge and crater rims, are the less altered regions on Ryugu surface, which experienced the minor alteration and OH devolatilization; the HR2, HR3, HR4, HR5 families, coincident with floors and walls of impact craters, are the most altered areas, result of the three processes occurring on Ryugu. The strong reddening of the HR6 family (coincident with Ejima Saxum) is likely due to the fine-sized material covering the large boulder.