E. A. Cloutis

Abstract During its approach to asteroid (101955) Bennu, NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft surveyed Bennu’s immediate environment, photometric properties, and rotation state. Discovery of a dusty environment, a natural satellite, or unexpected asteroid characteristics would have had consequences for the mission’s safety and observation strategy. Here we show that spacecraft observations during this period were highly sensitive to satellites (sub-meter scale) but reveal none, although later navigational images indicate that further investigation is needed. We constrain average dust production in September 2018 from Bennu’s surface to an upper limit of 150 g s –1 averaged over 34 min. Bennu’s disk-integrated photometric phase function validates measurements from the pre-encounter astronomical campaign. We demonstrate that Bennu’s rotation rate is accelerating continuously at 3.63 ± 0.52 × 10 –6 degrees day –2 , likely due to the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, with evolutionary implications.

Sedimentary rocks at Yellowknife Bay (Gale crater) on Mars include mudstone sampled by the Curiosity rover. The samples, John Klein and Cumberland, contain detrital basaltic minerals, calcium sulfates, iron oxide or hydroxides, iron sulfides, amorphous material, and trioctahedral smectites. The John Klein smectite has basal spacing of ~10 angstroms, indicating little interlayer hydration. The Cumberland smectite has basal spacing at both ~13.2 and ~10 angstroms. The larger spacing suggests a partially chloritized interlayer or interlayer magnesium or calcium facilitating H2O retention. Basaltic minerals in the mudstone are similar to those in nearby eolian deposits. However, the mudstone has far less Fe-forsterite, possibly lost with formation of smectite plus magnetite. Late Noachian/Early Hesperian or younger age indicates that clay mineral formation on Mars extended beyond Noachian time.

Pyroxene reflectance and transmittance spectra have been examined in a search for systematic relationships between spectral features and compositional variations and to assess the applicability of reflectance Spectroscopy to pyroxene geothermometry. Orthopyroxenes containing up to ∼11 % Wollastonite show a positive correlation between Fe 2+ content and wavelength positions of the major absorption bands. Aluminum‐rich orthopyroxenes display absorption bands at lower than expected wavelengths. Spectral‐compositional relationships are more complex for clinopyroxenes, showing both positive and negative correlations between band positions and major cation abundances. These relationships are further complicated by the presence of significant amounts of other transition series elements such as Ti and Cr and by the presence of exsolved phases and compositional zonations. Even small‐scale exsolutions (<1 μm wide) seem to be sufficient to significantly alter spectral properties. Contours of the wavelength positions of band minima projected onto the pyroxene tetralateral generally exhibit concave downward shapes. The orientations of the contours relative to pyroxene geotherms are such as to effectively preclude the use of Spectroscopy to significantly constrain the temperatures of formation of pyroxenes.

Spectral reflectance measurements of characterized (phase abundance, particle size) mixtures of olivine and orthopyroxene were utilized to define the correlations between spectral and albedo parameters of such assemblages and their mineralogical or textural properties. Thirty‐three different spectral parameters falling into three general classes (relative or ratioed, absolute or albedo, and wavelength) were investigated for empirical sensitivity to one or more of the mixture properties. Theoretical considerations and previous experimental observations were utilized to understand their functional relationships. The ratio of areas for the 1‐ and 2‐μm absorption bands is shown to be a sensitive indicator of the olivine‐orthopyroxene abundance and is very nearly independent of particle size and mineral composition. In conjunction with an abundance determination, the wavelength position of the 1‐μm absorption feature can be utilized to determine the molar iron contents of the olivine and orthopyroxene phases. This calibration is insensitive to particle size but will produce systematic deviations if the phases have significantly different iron contents or if more than a few percent of a clinopyroxene component is present. The spectral albedo in the 0.6‐ to 0.7‐μm region is relatively insensitive to phase abundance and can be used to constrain particle size if phase composition has been determined.