Faith Vilas

We report the observation of six asteroidal companions in direct images of five asteroids from Hubble Space Telescope (HST). These are (45) Eugenia, (87) Sylvia, (93) Minerva, (107) Camilla (the discovery observation for this companion), and (121) Hermione. Images of the companions are barely above the threshold detectability and appear to have the same colors as the primary body. The primary asteroids are all resolved in the HST images, and their shapes and sizes can be clearly seen after reconstruction. These asteroids generally appear to be darker than expected from data in the literature, which is generally derived from ground-based adaptive optics observations and from IR radiometry (the SIMPS). The asteroids average about 5% smaller than their radiometric diameters, which is an effect of the image reconstruction process used here.

A strategy for planetary exploration using a rover capable of science autonomy is presented. We encoded into a rover a set of driving hypotheses pertaining to the geologic origin of a field site and equipped the rover with the instrumentation needed to measure the observables related to the hypotheses, as well as the software tools to analyze them to a relatively high level of confidence. We investigated the effects of different exploration strategies that make use of rover science autonomy and compared the operational efficiency and science yield of three geological exploration scenarios: (1) standard human-directed exploration, (2) rover-directed exploration, and (3) astronaut/rover collaborative exploration. We show that exploration with a rover capable of science autonomy is operationally more efficient than the human-directed strategy, resulting in higher rates of data collection and hence a greater science yield per command cycle. Additionally, we explored and developed astronaut/rover collaborative exploration strategies and present a basic framework for effective planetary exploration that leverages the expertise of a science team, the efficiency of a science-autonomous rover, and the contextual abilities of astronauts.

Bombardment by solar wind ions is one of the main drivers of space weathering on airless bodies. Here, we simulate the solar-wind-driven spectral alteration of loosely packed olivine powders by irradiation with 1.2 keV helium ions (He⁺). We measured the reflectance spectra of the olivine powder in the ultraviolet–visible–near-infrared (UV–Vis–NIR) wavelength range (0.2–2 μm) as a function of ion fluence. In the Vis–NIR range, we observed spectral darkening, absorption band shallowing, and spectral reddening, in agreement with lunar-style space weathering and previous laboratory studies. In the UV–Vis, spectral darkening was also observed. However, a spectral bluing took place at wavelengths below 400 nm. As the simulated space weathering progressed, the spectral slopes shifted from steep-UV/shallow-NIR slopes to shallow-UV/steep-NIR slopes. Moreover, the change in the UV slope was almost 10 times larger than in the NIR, supporting the hypothesis that the UV spectral slope could be an earlier indicator of space weathering.

This paper presents far-ultraviolet through mid-infrared (0.12–20 μm) reflectance spectra of 27 fine-particulate (μm) terrestrial mineral samples, providing continuous spectra that cover an unusually broad spectral range and are of unusually fine particle size relative to most existing spectral libraries. These spectra of common geologic materials are useful for future applications that study the dust on various planetary bodies. Reflectance spectra were acquired of the samples at multiple laboratories at multiple wavelengths. All of the spectra were compared to one another to observe the general, common spectral characteristics (e.g., slope, band shape, and band depth), and the best segments of the spectra representing the mineral reflectance were scaled and spliced together to form a “Frankenspectrum” for each mineral that best represents the full wavelength range of far-ultraviolet, visible, near-infrared, and middle-infrared wavelengths. These scaled and spliced Frankenspectra, as well as the entire set of individual “original” reflectance spectra from each laboratory, are available in the Planetary Data System Geosciences Node.

Visible/near-infrared narrowband spectroscopy (dispersion per element ∼ 6 Å) was obtained of the Jovian irregular satellites JVI Himalia, JVII Elara, JVIII Pasiphae, JIX Sinope, JX Lysithea, JXI Carme, JXII Ananke, and JXVII Callirrhoe in 2006, 2009, and 2010 using the MMT Observatory Red Channel spectrograph. These spectra sample three prograde (i = 28°), four retrograde (i = 149° and 165°), and one independent satellite. Our results suggest that the prograde cluster satellites represent fragments probing the cluster’s original parent body, with the largest satellite, Himalia, being the core of the parent body, while Elara preserves the geochemical/mineralogical transition between the core and an outer layer of the body, and Lysithea formed farther from the center of the parent body. The spectral signatures suggest that the prograde parent body fragmented in the early stages of aqueous alteration. This supports the change from more organic-rich material at Lysithea to more carbonized material at Himalia, consistent with weathering/processing of a carbon-bearing material at Himalia. At twice the distance from Jupiter, the retrograde cluster anchored by Pasiphae also suggests that Ananke preserves the transition between the core and an outer layer of a parent body. Both Sinope and Carme are similar to D-class asteroids. Bluing/flattening near 0.4–0.5 μm in Carme’s spectrum suggests a carbonized component to Carme’s surface material, consistent with greater levels of weathering/processing. Sinope’s red spectrum is consistent with broadband photometry and does not confirm or negate the proposal that it had a common parent body with the Pasiphae cluster.

The carbonaceous asteroid Ryugu has been explored by the Hayabusa2 spacecraft to elucidate the actual nature of hydrous asteroids. Laboratory analyses revealed that the samples from Ryugu are comparable to unheated CI carbonaceous chondrites; however, reflectance spectra of Ryugu samples and CIs do not coincide. Here, we demonstrate that Ryugu sample spectra are reproduced by heating Orgueil CI chondrite at 300°C under reducing conditions, which caused dehydration of terrestrial weathering products and reduction of iron in phyllosilicates. Terrestrial weathering of CIs accounts for the spectral differences between Ryugu sample and CIs, which is more severe than space weathering that likely explains those between asteroid Ryugu and the collected samples. Previous assignments of CI chondrite parent bodies, i.e., chemically most primitive objects in the solar system, are based on the spectra of CI chondrites. This study indicates that actual spectra of CI parent bodies are much darker and flatter at ultraviolet to visible wavelengths than the spectra of CI chondrites.

Context. Hydrated minerals, such as phyllosilicates, on asteroids can provide constraints on the temperature or compositional distribution of the early Solar System. Previous studies pointed out the possibility that absorption in the near-ultraviolet (NUV, 0.35–0.5 μm) wavelength region is a proxy for hydrated minerals in primitive asteroids. However, the radial distribution of the NUV absorption among primitive asteroids was not revisited after the Eight Color Asteroid Survey (ECAS). Aims. Our objectives are first to evaluate the possibility of using the NUV absorption as diagnostics of hydrated minerals based on the recent datasets of primitive asteroids and hydrated carbonaceous chondrites, and second to investigate the reflectance spectrophotometry of the primitive asteroids in the NUV as functions of heliocentric distance and size. Methods. The NUV and visible reflectance spectrophotometry of more than 9000 primitive asteroids was investigated using two spectrophotometric surveys, ECAS and the Sloan Digital Sky Survey (SDSS), which cover wavelengths down to 0.32 μm and 0.36 μm, respectively. We classified asteroids from the main asteroid belt, the Cybele and Hilda zones, and Jupiter Trojans based on Tholen’s taxonomy and described the statistical distribution of primitive asteroid types. We also examined the relationship of the NUV, 0.7 μm, and 2.7 μm absorptions among primitive asteroids and hydrous carbonaceous chondrites CI and CM. Results. We find strong correlations between the NUV and the OH-band (2.7 μm) absorptions for primitive asteroids and hydrated meteorites, suggesting the NUV absorption can be indicative of hydrated silicates. Moreover, there is a great difference in the NUV absorption between the large asteroids (diameter d > 50 km) and small asteroids (d < 10 km) in the taxonomic distribution. The taxonomic distribution of asteroids differs between the inner main belt and middle-outer main belt. Notably, the C types are dominating large members through the main belt and the F types are dominating small asteroids of the inner main belt. The asteroids beyond the main belt consist mostly of P and D types, although P types are common everywhere in the main belt. The peculiar distribution of F types might indicate a different formation reservoir or a displacement process of F types in the early Solar System. The strongest absorptions of the NUV and 0.7 μm band were observed in G types, which likely comprise CM-like Fe-rich phyllosilicates. On the other hand, according to a recent sample return from an F-type asteroid (162173) Ryugu, the F types with the OH-band at 2.7 μm and the shallow NUV absorption could comprise CI-like Mg-rich phyllosilicates.

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed 17 Ryugu samples measuring 1 to 8 millimeters. Carbon dioxide–bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and calcium- and aluminum-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed through aqueous alteration reactions at low temperature, high pH, and water/rock ratios of <1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate that Ryugu’s parent body formed ~2 million years after the beginning of Solar System formation.

Introduction: The Toolbox for Research and Exploration (TREX) is a NASA SSERVI (Solar System Exploration Research Virtual Institute) node. TREX (trex.psi.edu) aims to develop tools and research methods for the exploration of airless bodies, specifically the Moon, the Martian moons, and near-Earth asteroids. TREX studies are organized into four Themes: (1) Laboratory studies, (2) Lunar studies, (3) Small bodies studies, and (4) Field work. The work presented here is part of Theme 3: Investigations of fine-grained materials on the surface of small bodies. Background: Reflectance spectroscopy is a widely used technique to study planetary bodies in our solar system, partly because its ability to identify compositional characteristics via remote sensing [1]. It has been shown that reflectance spectra of minerals on the surface of airless bodies can exhibit differences in their spectral characteristics, when compared to laboratory measurements of their Earth analogs [2]. This is because minerals exposed to the interplanetary environment are constantly subject to particle and electromagnetic radiation, and micrometeorite bombardment, which gradually alter their chemical and physical properties, a process called space weathering. To accurately interpret reflectance spectra from remote observations, it is key to fully understand how space weathering alters the spectral characteristics of minerals. Laboratory studies are a powerful tool to simulate space-weathering-like processes and unravel the fate of minerals exposed to space-like radiation and micrometeorite impact environments. Such studies give access to additional characterization environments that can further our understanding of micrometeorite- and radiation-material interactions. In this work, we conduct ion irradiation experiments to evaluate the evolution of minerals exposed to a solar wind-like environment Methods: For this work, olivine grains were ground into loose powders (<25 µm) to generate regolith-like samples. The powders were produced in a glovebox under a nitrogen environment (water and oxygen content <1 ppm). This procedure enabled us to generate olivine grains with fresh faces that were not previously exposed to air. The prepared samples were transferred to IGNIS (Ion-Gas-Neutrals Interactions with Surfaces) –a vacuum irradiation and characterization facility­– in a portable sample capsule [3]. Throughout the experiment, from the sample preparation, to transport, irradiation, and characterization, the samples were never exposed to air, preventing any alterations due to interactions with the atmosphere. In IGNIS, the olivine loose powders were exposed to ion beams to simulate mineral exposure to solar wind ions. The irradiations were performed using hydrogen and argon ions, at fluences up to 1x1018 ions cm-2. X-ray photoelectron spectroscopy (XPS) and reflectance spectroscopy were performed in situ to assess the influence of ion bombardment on the surface chemistry and the reflectance spectra characteristics. By performing sample characterization in situ, the evolution of the surfaces under ion bombardment was tracked with high fidelity, by minimizing the influence of air exposure on the sample’s characteristics. Preliminary results: Olivine powders irradiated with 1 keV argon ions, to a fluence of 1x1017 ions cm-2 exhibit a clear alteration of their optical properties. A darkening of the sample was evident by direct inspection of the powders before and after irradiation (Figure 1). The irradiation was carried out in steps; XPS and reflectance spectra were collected between steps, at 1x1016, 2x1016, 5x1016 and 1x1017 ions cm-2. The collected reflectance spectra show a darkening trend of the olivine samples with increasing irradiation fluence (Figure 2). The change in the albedo of the olivine powders with increasing fluence exhibited a wavelength dependence, with a more pronounced darkening towards the UV. The depth of the absorption band at 1.05 µm, characteristic of olivine spectra, were also altered with the ion irradiation. Figure 1. Darkening of olivine powders observed after 1 keV Ar+ irradiation with a dose of 1017ions cm-2   Figure 2 Ultraviolet-visible reflectance spectra of olivine powders irradiated with 1 keV Ar+. The sample darkening is evident by the overall decreased reflectance. References: [1] Cloutis, E.A., et al. (2018) Primitive Meteorites and Asteroids: Physical, Chemical, and Spectroscopic Observations Paving the Way to Exploration, Elsevier, 273-343. [2] Adams, J.B. and Jones, R.L. (1970) Science, 737-739. [4] Jaramillo, C. et al. (2021) AGU Fall meeting, P11A-06

Examination of the opposition geometry properties show that Ryugu’s surface regolith is commensurate with laboratory studies of the photometric behavior of powdered carbonaceous chondrites. The regolith is consistent with a broad grain size distribution that contains a fine-grained component.