¹A.C.Martin,¹J.P.Emery,^1,2M.J.Loeffler
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114921]
¹Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, AZ 86011, United States of America
²Center for Materials Interfaces in Research and Applications, Northern Arizona University, Flagstaff, AZ 86011, United States of America
Copyright Elsevier

Studies have long utilized laboratory derived spectra to understand the surface composition and other properties of planetary bodies. One variable that is believed to affect remotely acquired spectra in the mid-infrared (MIR; 5–35 μm) is the surface porosity of the airless body, yet there have been few laboratory studies to quantify this effect. Thus, here we report systematic laboratory experiments aimed at quantifying the effect that porosity has on the MIR spectra of silicate regoliths. To simulate the effects of regolith porosity, we mixed olivine powder with KBr powder of the same size range (< 20 μm, 20–45 μm, and 45–63 μm). Olivine was mixed with KBr from 0% – 90% with 10% intervals by weight. Finally, we measured spectra with a Fourier transform infrared (FTIR) spectrometer in the MIR. Our results indicate a transition from a primarily surface scattering regime to a primarily volume scattering regime with increasing regolith porosity. Evidence of the dominating regime transition includes: the primary Christiansen Feature at ~8.8 μm decreases in spectral contrast, and shifts slightly to longer wavelengths as regolith porosity increases, reststrahlen bands in the 10-μm do not shift significantly in wavelength but do decrease in spectral contrast, vibrational bands in the volume scattering regime (i.e., peaks in emissivity) increase in spectral contrast with increasing regolith porosity, and the spectral contrast of 10-μm plateau increases exponentially with increasing regolith porosity. The MIR spectral analysis of asteroids, such as Hektor, suggests a highly porous surface regolith (at least 81% void space) of fine-particulate silicates. These results demonstrate that some asteroids support highly porous regoliths whose spectra are not well-matched by standard (low porosity) laboratory spectra of powders. The spectra presented here enable analysis of both the porosity and mineralogy of olivine-rich, low porosity asteroid surfaces.

^1,2,3E.T.Dunham et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.02.002]
¹Arizona State University (ASU), Center for Meteorite Studies, Tempe, AZ, 85287-1404
²Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287-1404
³The University of California, Los Angeles (UCLA), Department of Earth, Planetary and Space Sciences, Los Angeles CA 90095-1567
Copyright Elsevier

Short-lived radionuclides (SLRs) once present in the solar nebula can be used as probes of the formation environment of our Solar System within the Milky Way Galaxy. The first-formed solids in the Solar System, calcium-, aluminum-rich inclusions (CAIs) in meteorites, record the one-time existence of SLRs such as 10Be and 26Al in the solar nebula. We measured the 10Be–10B isotope systematics in 29 CAIs from several CV3, CO3, CR2, and CH/CB chondrites and show that all except for a FUN CAI record a homogeneous initial 10Be/9Be with a single probability density peak at 10Be/9Be = 7.4 × 10–4. Integrating these data with those of previous studies, we find that most CAIs (81%) for which 10Be–10B isotope systematics have been determined, record a homogeneous initial 10Be/9Be ratio in the early Solar System with a weighted mean 10Be/9Be = (7.1 ± 0.2) × 10–4. This uniform distribution provides evidence that 10Be was predominantly formed in the parent molecular cloud and inherited by the solar nebula. Possible explanations for why unusual CAIs (FUNs, PLACs, those from CH/CBs, and those irradiated on the parent body) recorded a 10Be/9Be ratio outside of 7.1 × 10−4 include the following: 1) They incorporated a component of 10Be that was produced in the nebula by irradiation; 2) they formed after normal CAIs; and 3) they were processed (post-formation) in a way that affected their original 10Be signatures. Given the rarity of these examples, the overall uniformity of initial 10Be/9Be suggests that Solar System 10Be was predominantly inherited from the molecular cloud.