The unexpected surface of asteroid (101955) Bennu
Nature Springer Nature 568:7750 (2019) 55-60
Abstract:
NASA'S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at the near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth1. Bennu is a low-albedo B-type asteroid2 that has been linked to organic-rich hydrated carbonaceous chondrites3. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Therefore, the primary mission objective is to return a sample of Bennu to Earth that is pristine-that is, not affected by these processes4. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu's global properties, support the selection of a sampling site and document that site at a sub-centimetre scale5-11. Here we consider early OSIRIS-REx observations of Bennu to understand how the asteroid's properties compare to pre-encounter expectations and to assess the prospects for sample return. The bulk composition of Bennu appears to be hydrated and volatile-rich, as expected. However, in contrast to pre-encounter modelling of Bennu's thermal inertia12 and radar polarization ratios13-which indicated a generally smooth surface covered by centimetre-scale particles-resolved imaging reveals an unexpected surficial diversity. The albedo, texture, particle size and roughness are beyond the spacecraft design specifications. On the basis of our pre-encounter knowledge, we developed a sampling strategy to target 50-metre-diameter patches of loose regolith with grain sizes smaller than two centimetres4. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 metres in extent, the sampling of which poses a substantial challenge to mission success.Remote-sensing characterization of major Solar System bodies with the Twinkle space telescope
Journal of Astronomical Telescopes Instruments and Systems SPIE 5:1 (2019) 014006
Abstract:
Remote-sensing observations of Solar System objects with a space telescope offer a key method of understanding celestial bodies and contributing to planetary formation and evolution theories. The capabilities of Twinkle, a space telescope in a low Earth orbit with a 0.45-m mirror, to acquire spectroscopic data of Solar System targets in the visible and infrared are assessed. Twinkle is a general observatory that provides on-demand observations of a wide variety of targets within wavelength ranges that are currently not accessible using other space telescopes or that are accessible only to oversubscribed observatories in the short-term future. We determine the periods for which numerous Solar System objects could be observed and find that Solar System objects are regularly observable. The photon flux of major bodies is determined for comparison to the sensitivity and saturation limits of Twinkle's instrumentation and we find that the satellite's capability varies across the three spectral bands (0.4 to 1, 1.3 to 2.42, and 2.42 to 4.5 μm). We find that for a number of targets, including the outer planets, their large moons, and bright asteroids, the model created predicts that with short exposure times, high-resolution spectra (R ~ 250, λ < 2.42 μm; R ~ 60, λ > 2.42 μm) could be obtained with signal-to-noise ratio (SNR) of > 100 with exposure times of <300 s. For other targets (e.g., Phobos), an SNR > 10 would be achievable in 300 s (or less) for spectra at Twinkle's native resolution. Fainter or smaller targets (e.g., Pluto) may require multiple observations if resolution or data quality cannot be sacrificed. Objects such as the outer dwarf planet Eris are deemed too small, faint or distant for Twinkle to obtain photometric or spectroscopic data of reasonable quality (SNR > 10) without requiring large amounts of observation time. Despite this, the Solar System is found to be permeated with targets that could be readily observed by Twinkle.Modeling the angular dependence of emissivity of randomly rough surfaces
Journal of Geophysical Research American Geophysical Union 124:2 (2019) 585-601
Abstract:
Directional emissivity (DE) describes how the emissivity of an isothermal surface changes with viewing angle across thermal infrared wavelengths. The Oxford Space Environment Goniometer (OSEG) is a novel instrument that has been specifically designed to measure the DE of regolith materials derived from planetary surfaces. The DE of Nextel high emissivity black paint was previously measured by the OSEG and showed that the measured emissivity decreases with increasing emission angle, from an emissivity of 0.97 ± 0.01 at 0° emission angle to an emissivity of 0.89± 0.01 at 71° emission angle. The Nextel target measured was isothermal (<0.1 K surface temperature variation) and the observed change in emissivity was due to Fresnel related effects and was not due to non-isothermal effects. Here we apply several increasingly complex modelling techniques to model the measured DE of Nextel black paint. The modelling techniques used here include the Hapke DE model, the Fresnel equations, a multiple slope Fresnel model and a Monte Carlo ray-tracing model. It was found that only the Monte Carlo raytracing model could accurately fit the OSEG measured Nextel data. We show that this is because the Monte Carlo ray-tracing model is the only model that fully accounts for the surface roughness of the Nextel surface by including multiple scattering effects.SEIS: Insight's Seismic Experiment for Internal Structure of Mars
SPACE SCIENCE REVIEWS 215:1 (2019) UNSP 12
Analysis of gaseous ammonia (NH3) absorption in the visible spectrum of Jupiter - Update
Icarus Elsevier 321 (2018) 572-582