ArticlePublisher preview available

The unexpected surface of asteroid (101955) Bennu

April 2019
Nature 568(7750)

April 2019
568(7750)

DOI:10.1038/s41586-019-1033-6

Authors:

Dani DellaGiustina

The University of Arizona

Show all 30 authorsHide

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.

The global mosaic of Bennu, projected onto a sinusoidal map that preserves area The PolyCam images were photometrically corrected to mimic imaging conditions with phase, emission and incidence angles of 0°. The map has a pixel scale of 1.2 m per pixel. Images were taken on 25 November 2018.

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Areas used for the calculation of the albedo variation in Fig. 1d Blue and orange outlines represent dark and bright clasts, respectively.

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Timeline of the various observations made during the Approach phase The figure shows the key parameters affecting imaging conditions as a function of range to the asteroid and calendar date.

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Schematic of Preliminary Survey, showing passes over the north pole, equator, and south pole Each trajectory leg lasts two days. The observations consist of MapCam mosaics made far from Bennu, both on the inbound and outbound legs from the closest approach, OLA observations made near the closest approach, both inbound and outbound, and additional MapCam mosaics made soon after the OLA observations but on the outbound legs of the polar flybys only. The time of closest approach to the pole was set at a nominal 17:00 utc for all flybys.

…

Range of albedo on the surface of Bennu a, Histogram showing the normal albedo distribution of Bennu’s surface based on low-phase-angle images acquired by the PolyCam imager⁹ on 25 November 2018 (total number of pixels used as input to the histogram, 694,633). The axis along the top of the plot gives values for the same data when corrected to standard laboratory conditions (30° phase, 0° emission, 30° incidence) to enable direct comparison with the meteorite record. b–e, PolyCam images acquired on 1 and 2 December 2018 highlight the range of albedo heterogeneity on Bennu. b, One of the darkest boulders (about 3.3% normal albedo), perched on the surface of the asteroid (phase angle 51°, 0.32 m per pixel). c, A 30-m boulder that defines the prime meridian and has a near-average albedo of about 4% (phase angle 49°, 0.32 m per pixel). d, A boulder includes a clast that is 33% brighter than its host matrix (phase angle 33°, 0.43 m per pixel; see Methods). e, The brightest object identified thus far on Bennu (phase angle 34°, 0.42 m per pixel). Source data

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LETTER https://doi.org/10.1038/s41586-019-1033-6

The unexpected surface of asteroid (101955) Bennu

D. S. Lauretta1,12, D. N. DellaGiustina1,12, C. A. Bennett1, D. R. Golish1, K. J. Becker1, S. S. Balram-Knutson1, O. S. Barnouin2,

T. L. Becker1, W. F. Bottke3, W. V. Boynton1, H. Campins4, B. E. Clark5, H. C. Connolly Jr6, C. Y. Drouet d’Aubigny1, J. P. Dworkin7,

J. P. Emery8, H. L. Enos1, V. E. Hamilton3, C. W. Hergenrother1, E. S. Howell1, M. R. M. Izawa9, H. H. Kaplan3, M. C. Nolan1,

B. Rizk1, H. L. Roper1, D. J. Scheeres10, P. H. Smith1, K. J. Walsh3, C. W. V. Wolner1 & The OSIRIS-REx Team11

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 chondrites

. 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 processes

. 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 ratios

—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 centimetres

. 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.

Measurements from the OSIRIS-REx spacecraft’s approach to

and initial survey of Bennu identified spectral features, constrained

the shape, rotation period and mass, characterized the photometric

properties, described the global thermal inertia and revealed the sur

ficial characteristics of the asteroid. These data allow us to evaluate

the Design Reference Asteroid (DRA), a document that we created to

inform mission design on the basis of telescopic observations

. The

DRA ‘scorecard’ (Table1) tracks how our pre-encounter knowledge

matches reality.

Bennu’s global properties largely match those determined by the

pre-encounter astronomical campaign. In disk-integrated observations,

the visible-to-near-infrared spectrum has a blue (negative) slope

, con-

firming the B-type taxonomy. At longer wavelengths, a 2.7-µm spectral

absorption band is present, consistent with the presence of hydrated

silicates. Thermal emission spectra are similar to those of CM carbo-

naceous chondrites and contain a spectral feature at 23µm, which is

also consistent with phyllosilicates. Thus, OSIRIS-REx spectral data

support the affinity with hydrated carbonaceous chondrites indicated

by ground-based observations3.

Bennu’s physical properties are also consistent with findings from the

astronomical campaign (Table1). Bennu exhibits the expected spin-

ning-top shape

, and its rotation period, obliquity and rotation pole

are within the 1σ (σ, standard deviation) uncertainties of the ground-

based values. Its shape and topography indicate low levels of internal

shear strength or cohesion. A mass determination from a radio science

experiment

yields a density of 1,190±13kgm

−3

. The low density

of Bennu is consistent with a rubble-pile structure containing 50%

macroporosity, assuming a particle density characteristic of CM

chondrites. Bennu thus appears to be a microgravity aggregate.

At 100 million to 1 billion years old, Bennu’s surface is older than

expected according to dynamical models of rubble-pile evolution, but

shows overprinting from more recent activity

. High-standing north–

south ridges extend from pole to pole16, dominating the topography

and apparently directing the flow of surface material. Recent surface

processes are evident in the deficiency of small craters, infill of large

craters and surface mass wasting16,18. Fractured boulders have mor-

phologies that suggest the influence of impact or thermal processes.

Measurements by the OSIRIS-REx Camera Suite (OCAMS) con-

firm that Bennu is one of the darkest objects in the Solar System, with

a global geometric albedo of 4.4%19,20. This finding is in agreement

with pre-encounter measurements

and consistent with CI and CM

chondrites3.

However, Bennu’s surface displays an unexpected degree of albedo

heterogeneity (Fig.1). The ratio of reflected to incident flux (I/F)

of Bennu’s surface at a solar phase angle of 0° (Fig.1a) ranges from

3.3%±0.2% in the dark regions (Fig.1b) to a maximum of ≥15%

within discrete boulders of 2–3m (Fig.1e). The majority of large

(≥30m) boulders have an albedo similar to the global average (Fig.1c).

This wide range of albedo may confound the spacecraft guidance

lidar system, requiring reassessment of the approach to sample-site

targeting4.

The darkest material is concentrated in a large outcrop in Bennu’s

southern (−Z) hemisphere (Fig.2) and in a subset of boulders perched

on the surface (Fig.1b). Such material is also present in diffuse blan-

keting units that are not linked to distinct morphometric features19.

Some instances show spectral absorption at 0.55µm, which has not

previously been reported for any dark asteroid21. Magnetite (Fe3O4)

is the most likely source of this spectral feature22. This interpretation

is consistent with the emissivity spectra obtained by the OSIRIS-REx

Thermal Emission Spectrometer (OTES), which show features at 18µm

and 29µm that may be due to magnetite15.

The detection of magnetite on the surface of Bennu supports an

affinity with both CI and some (rare) CM chondrites

23,24

. Magnetite

in these meteorites is thought to be the product of aqueous alteration

within a parent asteroid25. If so, then the darker regions of Bennu,

where magnetite appears to be concentrated, may bear fresher material

than the brighter regions. This interpretation is consistent with some

1Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA. 2The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA. 3Southwest Research Institute, Boulder, CO,

USA. 4Department of Physics, University of Central Florida, Orlando, FL, USA. 5Department of Physics and Astronomy, Ithaca College, Ithaca, NY, USA. 6Department of Geology, Rowan University,

Glassboro, NJ, USA. 7NASA Goddard Space Flight Center, Greenbelt, MD, USA. 8Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, USA. 9Institute for Planetary

Materials, Okayama University–Misasa, Misasa, Japan. 10Smead Department of Aerospace Engineering, University of Colorado, Boulder, CO, USA. 11A list of authors and their affiliations appears at

the end of the paper. 12These authors contributed equally: D. S. Lauretta, D. N. DellaGiustina. *e-mail:

lauretta@orex.lpl.arizona.edu

4 APRIL 2019 | VOL 568 | NATURE | 55

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Nature and Origins of Meteoritic Breccias

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The Action of Water

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Jul 2006

Adrian J. Brearley

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The OSIRIS-REx Asteroid Sample Return Mission is the third mission in National Aeronautics and Space Administration (NASA)'s New Frontiers Program and is the first U.S. mission to return samples from an asteroid to Earth. The most important decision ahead of the OSIRIS-REx team is the selection of a prime sample-site on the surface of asteroid (101955) Bennu. Mission success hinges on identifying a site that is safe and has regolith that can readily be ingested by the spacecraft's sampling mechanism. To inform this mission-critical decision, the surface of Bennu is mapped using the OSIRIS-REx Camera Suite and the images are used to develop several foundational data products. Acquiring the necessary inputs to these data products requires observational strategies that are defined specifically to overcome the challenges associated with mapping a small irregular body. We present these strategies in the context of assessing candidate sample sites at Bennu according to a framework of decisions regarding the relative safety, sampleability, and scientific value across the asteroid's surface. To create data products that aid these assessments, we describe the best practices developed by the OSIRIS-REx team for image-based mapping of irregular small bodies. We emphasize the importance of using 3-D shape models and the ability to work in body-fixed rectangular coordinates when dealing with planetary surfaces that cannot be uniquely addressed by body-fixed latitude and longitude.

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Article

Oct 2018

Magnetite (Fe³⁺(Fe²⁺Fe³⁺)2O4) is ubiquitous in Earth and planetary materials, forming in igneous, metamorphic, and sedimentary settings, sometimes influenced by microbiology. Magnetite can be used to study many and varied planetary processes, such as the oxidation state of magmas, paleomagnetism, water–rock interactions such as serpentinization, alteration and metamorphism occurring on meteorite parent bodies, and for astrobiology. The spectral reflectance signature of magnetite in the ultraviolet, visible, and near-infrared is somewhat unusual compared to common planetary materials, suggesting that remote detection and characterization of magnetite should be possible. Here we present a systematic investigation of the reflectance spectral properties of magnetite using natural and synthetic samples. We investigate the effects of chemical substitutions, grain size variations, and mixtures with other phases in order to better constrain remote spectral searches for, and interpretation of, magnetite-bearing lithologies. Magnetite is characterized by high extinction over the entire wavelength range considered here, and therefore surface scattering dominates over volume scattering. Magnetite reflectance spectra are strongly influenced by the presence of delocalized electrons above the Verwey transition temperature (∼120 K), leading to metal-like scattering behavior, that is, high extinction, surface scattering dominant, and a general increase in reflectance with increasing wavelength, i.e. “red-sloped and featureless”. Superimposed upon the metal-like reflectance are local reflectance maxima which we ascribe to Fresnel reflectance peaks corresponding to Fe–O oxygen–metal charge transfer processes (∼0.27 and ∼0.39 µm) and Fe-related field–d orbital transitions (∼0.65 µm). We also find a systematic shift in the wavelength position of the 0.65 µm Fresnel peak with increasing chemical impurity in magnetite. Magnetite reflectance spectra are most similar to those of titanomagnetite and wüstite, and unlike those of other Fe–(Ti) oxides, such as ilmenite, hæmatite, ulvospinel, maghemite, pseudobrookite, and armalcolite.

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