NASA’S Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth [ 1 ]. Bennu is a low-albedo B-type asteroid [ 2 ] that has been linked to organic-rich hydrated carbonaceous chondrites [ 3 ]. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Thus, the primary mission objective is to return a sample of Bennu to Earth that is pristine, i.e., not affected by these processes [ 4 ]. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu’s global properties; support selection of a sampling site; and document that site at sub-centimeter scales [ 5 - 11 ]. Here we consider early observations to understand how Bennu’s properties compare to pre-encounter expectations and 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 modeling of Bennu’s thermal inertia [ 12 ] and radar polarization ratios [ 13 ]—which indicated a generally smooth surface covered by centimeter-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-m-diameter patches of loose regolith with grain sizes less than 2 cm [ 4 ]. We observe only a small number of apparently hazard-free regions, on the order of 5 to 20 meters in extent, the sampling of which poses a substantial challenge to mission success.

1 2 It has long been suggested that hydrothermal systems might have provided habitats for the origin 3 and evolution of early life on Earth, and possibly other planets such as Mars. In this contribution 4 we show that most impact events that result in the formation of complex impact craters (i.e., >2-5 4 and >5-10 km diameter on Earth and Mars, respectively) are potentially capable of generating 6 a hydrothermal system. Consideration of the impact cratering record on Earth suggests that the 7 presence of an impact crater lake is critical for determining the longevity and size of the 8 hydrothermal system. We show that there are six main locations within and around impact 9 craters on Earth where impact-generated hydrothermal deposits can form: 1) crater-fill impact 10 melt rocks and melt-bearing breccias; 2) interior of central uplifts; 3) outer margin of central 11 uplifts; 4) impact ejecta deposits; 5) crater rim region; and 6) post-impact crater lake sediments. 12We suggest that these six locations are applicable to Mars as well. Evidence for impact-13 generated hydrothermal alteration ranges from discrete vugs and veins to pervasive alteration 14 depending on the setting and nature of the system. A variety of hydrothermal minerals have been 15 documented in terrestrial impact structures and these can be grouped into three broad categories: 16(1) hydrothermally-altered target-rock assemblages; (2) primary hydrothermal minerals 17 precipitated from solutions; and (3) secondary assemblages formed by the alteration of primary 18 hydrothermal minerals. Target lithology and the origin of the hydrothermal fluids strongly 19 influences the hydrothermal mineral assemblages formed in these post-impact hydrothermal 20systems. There is a growing body of evidence for impact-generated hydrothermal activity on 21 Mars; although further detailed studies using high-resolution imagery and multispectral 22 information are required. Such studies have only been done in detail for a handful of Martian 23 4 craters. The best example so far is from Toro Crater (Marzo et al., 2010). We also present new 1 evidence for impact-generated hydrothermal deposits within an unnamed ~32-km diameter crater 2 ~ 350 km away from Toro and within the larger Holden Crater. Synthesizing observations of 3 impact craters on Earth and Mars, we suggest that if there was life on Mars early in its history, 4 then hydrothermal deposits associated with impact craters may provide the best, and most 5 numerous, opportunities for finding preserved evidence for life on Mars. Moreover, 6hydrothermally altered and precipitated rocks can provide nutrients and habitats for life long 7 after hydrothermal activity has ceased. 8 5 1

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. CitationHamilton, V.E., et al., "Evidence for widespread hydrated minerals on asteroid (101955) Bennu." Nature astronomy 3, 4 (2019): p.

Christensen, P. R.; Drouet d'Aubigny, C. Y.; Hamilton, V. E.; Reuter, D. C.; Rizk, B.; Simon, A. A.; Asphaug, E.; Bandfield, J. L.; Barnouin, O. S.; Barucci, M. A.; Bierhaus, E. B.; Binzel, R. P.; Bottke, W. F.; Bowles, N. E.; Campins, H.; Clark, B. C.; Clark, B. E.; Connolly, H. C.; Daly, M. G.; Leon, J. de; Delbo', M.; Deshapriya, J. D. P.; Elder, C. M.; Fornasier, S.; Hergenrother, C. W.; Howell, E. S.; Jawin, E. R.; Kaplan, H. H.; Kareta, T. R.; Le Corre, L.; Li, J.-Y.; Licandro, J.; Lim, L. F.; Michel, P.; Molaro, J.; Nolan, M. C.; Pajola, M.; Popescu, M.; Garcia, J. L. Rizos; Ryan, A.; Schwartz, S. R.; Shultz, N.; Siegler, M. A.; Smith, P. H.; Tatsumi, E.; Thomas, C. A.; Walsh, K. J.; Wolner, C. W. V.; Zou, X.-D. and Lauretta, D. S. (2019). Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis. Nature Astronomy, 3 pp. 341-351. For guidance on citations see FAQs.Length of main text: 2956 words Length of methods: 3605 words Length of legends: 565 words Number of references: 53 main text, 69 including methods and supplementary information (refs 67 to 69 are cited in the SI only) , we show that asteroid (101955) Bennu's surface is globally rough, dense with boulders and low in albedo. The number of boulders is surprising given Bennu's moderate thermal inertia, suggesting that simple models linking thermal inertia to particle size do not adequately capture the complexity relating these properties. At the same time, we find evidence for a wide range of particle sizes with distinct albedo characteristics. Our findings imply that ages of Bennu's surface particles span from the disruption of the asteroid's parent body (boulders) to recent in situ production (micron-scale particles).

The Open University's repository of research publications and other research outputs The dynamic geophysical environment of (101955) Bennu based on OSIRIS-REx measurements

The dwarf planet (1) Ceres, the largest object in the main asteroid belt with a mean diameter of about 950 kilometres, is located at a mean distance from the Sun of about 2.8 astronomical units (one astronomical unit is the Earth-Sun distance). Thermal evolution models suggest that it is a differentiated body with potential geological activity. Unlike on the icy satellites of Jupiter and Saturn, where tidal forces are responsible for spewing briny water into space, no tidal forces are acting on Ceres. In the absence of such forces, most objects in the main asteroid belt are expected to be geologically inert. The recent discovery of water vapour absorption near Ceres and previous detection of bound water and OH near and on Ceres (refs 5-7) have raised interest in the possible presence of surface ice. Here we report the presence of localized bright areas on Ceres from an orbiting imager. These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible. Of particular interest is a bright pit on the floor of crater Occator that exhibits probable sublimation of water ice, producing haze clouds inside the crater that appear and disappear with a diurnal rhythm. Slow-moving condensed-ice or dust particles may explain this haze. We conclude that Ceres must have accreted material from beyond the 'snow line', which is the distance from the Sun at which water molecules condense.

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.

No abstract