Europlanet Science Congress 2020
Virtual meeting
21 September – 9 October 2020
Europlanet Science Congress 2020
Virtual meeting
21 September – 9 October 2020
EPSC Abstracts
Vol. 14, EPSC2020-122, 2020
Europlanet Science Congress 2020
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Modeling first-order scattering processes from OSIRIS-REx color images of the rough surface of asteroid (101955) Bennu

Pedro Henrique Hasselmann1, Sonia Fornasier1,2, Maria Antonietta Barucci1, Alice Praet1, Beth Ellen Clark3, Jian-Yang Li4, Dathon Golish5, Daniella N. DellaGiustina5, Prasanna Deshapriya1, Xiao-Duan Zou4, Mike G. Daly6, Olivier S. Barnouin7, Amy A. Simon8, and Dante S. Lauretta5
Pedro Henrique Hasselmann et al.
  • 1Observatoire de Paris, LESIA, Paris, France (
  • 2Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris, France.
  • 3Department of Physics and Astronomy, Ithaca College, Ithaca, NY, USA.
  • 4Laboratory California Institute of Technology 21, Planetary Science Institute, Tucson, AZ, USA.
  • 5Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA.
  • 6The Centre for Research in Earth and Space Science, York University, Toronto, Ontario, Canada.
  • 7The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA.
  • 8NASA Goddard Space Flight Center, Greenbelt, MD, USA.

Introduction: The OSIRIS-REx mission has revealed a dark, boulder-rich, apparently dust-poor surface of the B-type asteroid (101955) Bennu [1], therefore a challenge for bi-directional reflectance (rF) modeling. With an estimated geometric albedo of 4.5% [2], Bennu is darker than many comets, and its reflectance distribution is dominated by single-scattering processes. The general approach to model a dark asteroid’s bi-directional reflectance distribution is to apply the standard Hapke IMSA model and its shadowing function [3]. However, this can imprecisely describe the roughness slopes for rocky surfaces [4]. Assuming that surfaces are fully diffuse can negate a specular forward-scattering contribution from crystalline components in the regolith [7]. To achieve a more complete photometric modeling of Bennu’s scattering curve, we rely on the radiative transfer semi-numerical model of Van Ginneken et al. [8,9].

Observations: MapCam is an optical imager [10] on-board the OSIRIS-REx spacecraft, equipped with four broadband color filters (60-90 nm wide) centered at 473 (b'), 550 (v'), 698 (w') and 847 (x') nm. We analyzed images acquired during the Equatorial Station (EQ), a campaign of the Detailed Survey mission phase [11], over a full rotation of (101955) Bennu at a nadir spatial resolution of ~33 cm/px. EQ comprehended seven phase angle configurations α =[7.5° ,30° ,45° ,90° ,130°] .

We analyzed the pixel subtended by the mission’s candidate sample collection sites, for which highly precise, laser altimeter-based digital terrain models (DTMs) were available [12,13]. These four candidate sites were called Sandpiper (latitude=-47°; longitude=322°), Osprey (11°; 88°), Nightingale ( 56°; 43°), and Kingfisher (11°; 56°). The varied latitude and longitudes provided the range of observational conditions required for our analysis.

Methodology: The methodology consists of the following steps:

a) NAIF SPICE kernels [14] and the DTMs are ingested into a ray-tracing code for rendering shadowed images. This allows us to discount for the effects of macroscopic shadows, leaving only the sub-facet texture to be modeled. These ancillary images provide the incidence, emergence, phase & azimuth for every facet.

b) We apply Van Ginneken’s model to every facet. Occlusion and shadowing as well as the retro-reflection among the reliefs are taken into account. The model has two major components: the analytical expression for the specular reflection; and the numerically-integrated diffusive reflection. At total, the model has three free parameters: ρ (single-scattering albedo), σ (RMS roughness slope), and g (specular-to-diffuse ratio), plus the three more related to the scattering phase function (bi-lobe Henyey-Greenstein function: c, b1, and b2)

c) Inverse problem: We run the Monte Carlo Markov Chain to sample the multi-parametric space in order to reconstruct a posteriori probability distribution of solutions for every free parameter, i.e., (ρ, σ, g, b1, b2, c), from which the statistics for every solution are estimated.

Results: The MCMC technique reveals some interesting aspects of Bennu's surface (Fig. 1): while the RMS roughness slope of 27+1-5 is in line with what has been obtained for other asteroids using Hapke shadowing function, we are puzzled by the indication of a non-zero specular reflection ratio from the surface (2.6+1−0.8 %). The specular reflection hints at a possible mono-crystalline component.

As for the diffuse rough component, the analysis of the photometric correction of OCAMS images taken at varied phase angles (α) indicates a more complex scenario. Up to α = 90°, the photometric correction is vastly improved by mixing two different solutions for roughness (one with low RMS σ and another with global RMS σ), a bi-modality already perceived from the MCMC a posteriori distributions. We have shown that most of Bennu's brightness variation can be explained by tuning the roughness slope statistical distribution.

Finally, we report a back-scatter phase function for the phase angle range between 7.5°, and 130°, without any expressive spectral trend in the visible range. The MCMC inversion hints at a possible second forward-scatter lobe of at least ~0.2 width. This leads to two possible solutions for the asymmetric factor (ξ (1) = −0.360 ± 0.030 and ξ(2) = −0.444 ± 0.020). We also report a dark global approximate single-scattering albedo at 550 nm from the collective analysis of all site candidates of 4.64+0.08-0.09 % . The single-scattering MapCam four-band colors show a similar spectral trend to the global average OVIRS EQ3 spectrum. The four sites together provide a general description of Bennu's colors.

Fig. 1. Parametric solutions after the MCMC technique for all sample sites together, and the scattering phase function (bottom row) for each MapCam filters.


[1] Lauretta, D. et al. Met.. & Plan. Scie., 50, Issue 4, 834-849, 2015.

[2] DellaGiustina, D.N. et al., Nat. Astron. 3, 241-351, 2019.

[3] Hapke, B. Icarus 59, 41-59,1984.

[4] Shkuratov et al., JQSRT 113, 18, 2431-2456, 2012.

[5] Davidsson, B. et al. Icarus 201, 335–357, 2009.

[6] Labarre, S. et al. Icarus 290, 63-80, 2017.

[7] Bowell, E., et al. Asteroid II, Univ. of Ariz. Press, 1989.

[8] Van Ginneken, B. et al. AO 37, 130-139, 1998.

[9] Goguen, J. et al. Icarus 208, 548–557, 2010.

[10] Rizk, B. et al.. Space Sci. Rev. 214, 26, 2018.

[11] DellaGiustina, D.N. et al., ESS 5, 929, 2018.

[12] Daly, M. G. et al., SSR 212, 899, 2017.

[13] Barnouin, O. S. et al., PLANSS 180, 104764, 2020.

[14] Acton, C. H.. PLANSS 44.1 (1996): 65-70.

[15] Hapke, B. Cambridge Univ. Press, 2ed., 2012.

How to cite: Hasselmann, P. H., Fornasier, S., Barucci, M. A., Praet, A., Clark, B. E., Li, J.-Y., Golish, D., N. DellaGiustina, D., Deshapriya, P., Zou, X.-D., G. Daly, M., S. Barnouin, O., A. Simon, A., and S. Lauretta, D.: Modeling first-order scattering processes from OSIRIS-REx color images of the rough surface of asteroid (101955) Bennu, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-122,, 2020.