¹C.Liu et al. (>10)
Astronomy & Astrophysics 658, A67 Link to Article [DOI https://doi.org/10.1051/0004-6361/202141398]
¹Shandong Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai 264209, PR China
Reproduced with permission (C)ESO

Context. Chang’e-4 (CE-4) provides unprecedented information about lunar materials exposed by the South Pole-Aitken (SPA) basin. Diverse results have been obtained from previous interpretations of CE-4 visible and near-infrared (VNIR) spectra. Some studies suggest that materials at the CE-4 landing site are dominated by olivine and orthopyroxene, but others argue that only a small amount of olivine should be exposed at the CE-4 landing site.

Aims. Laboratory spectroscopy studies using the Engineering Model of CE-4 Visible and Near-infrared Imaging Spectrometer (VNIS) are critical in constraining the accurate mineral proportions and composition of soils and boulders at the CE-4 landing site.

Methods. VNIR spectra of nine lunar analogs – prepared by mixing orthopyroxene (OPX), clinopyroxene (CPX), olivine (OL), and plagioclase – were acquired using the CE-4 VNIS Engineering Model. The spectral indices model and modified Gaussian model were developed to estimate CPX/(CPX+OPX) and OL/(OL+CPX+OPX) and are applicable to the in situ spectra acquired by the Yutu-2 VNIS spectrometer.

Results. The lunar rocks and regolith at the CE-4 landing site excavated by the Finsen impact are CPX-rich with limited OL (CPX:OPX:OL = 56:29:17). The mineral chemistries of the four lunar rocks show Mid-Ca, Fe pyroxene, and Mid-Mg OL (Fo60−79), providing critical constraints for mineral compositions in the SPA compositional anomaly. These rocks exhibit high M1 intensity ratios, indicating that they were crystallized at a high temperature (980–1300 °C) and a rapid-cooling magmatic system produced by impact melt differentiation or volcanic resurfacing events.

^1,10P. Zhang (张鹏飞) et al. (>10)
Astronomy & Astrophysics 659, A78 Link to Article [DOI https://doi.org/10.1051/0004-6361/202142590]
¹State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, PR China<
¹⁰CNSA Macau Center for Space Exploration and Science, Macau, PR China
Reproduced with permission (C)ESO

Context. Space weathering (SW) is crucial to improve the understanding of the evolution of optical characteristics on airless bodies. The classical view based on research of the Moon suggests that SW decreases albedo (darkening) and steepens spectral slope (reddening) in visible to near-infrared (VIS-NIR) wavelengths, producing nanophase iron (npFe0). However, this conclusion is not perfectly applicable to asteroids.

Aims. In this study, we focus on investigating the space weathering spectral alteration effects (SWSAE) and the causes of spectral alteration on various types of asteroids after long-term continuous micrometeoroid bombardments.

Methods. We used a pulsed laser to irradiate eight meteorites at the same energy, namely, of 28 mJ, in ten shots, including ordinary chondrites (OCs), aubrite (Aub), enstatite chondrites (ECs), CO, CV, and CM carbonaceous chondrites. Then we measured and compared the virgin and irradiated VIS-NIR reflectance spectra of these meteorites. We further surveyed the causes of spectral alteration through a scanning electron microscope and transmission electron microscope.

Results. Three different SWSAE are shown: (1) darkening and reddening on OCs, Aub, CO, and CV chondrites; (2) brightening and reddening on ECs; (3) brightening and bluing on CM chondrite. After irradiation, npFe0 and nanophase iron-nickel particles were respectively found in CV and CO chondrites; thick amorphous layers without any nanophase particles were found in Aub; melting and sputtering of metal were observed in ECs; a great deal of vesicles or bubbles without any nanophase particles were found in CM chondrite.

Conclusions. The long-term SW via micrometeoroid bombardments can spectrally remodel asteroid surfaces in different ways: darken and redden anhydrous silicate asteroids (e.g., S-, E-, and K-types); brighten and redden metal-rich M-type objects. The SWSAE of volatiles-rich carbonaceous asteroids (e.g., Ch-, Cgh-, and D-types) is related to SW degree: darkening and bluing at low degree then brightening and continue bluing as the SW degree increases. The various spectral units on Ryugu, Bennu, and Phobos can be created by the heterogeneity of the degree of SW.