EPSC 2020

Authors OSIRIS-REx Asteroid Sample Return Mission

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Download date 26/09/2021 15:50:00

Item License https://creativecommons.org/publicdomain/zero/1.0/

Link to Item http://hdl.handle.net/10150/657174 EPSC2020 SB11: Physical properties of small bodies: observations and techniques

The Global Average Disk-Resolved Photometric Properties of (101955) Bennu

Xiao-Duan Zou (Planetary Science Institute), Jian-Yang Li, Beth Clark, Dathon Golish, Salvatore Ferrone, Amy Simon, Dennis Reuter, Deborah Domingue, Hannah Kaplan, Maria Antonietta Barucci, Sonia Fornasier, Alice Praet, Pedro Henrique Hasselmann, Carina Bennett, Edward Cloutis, Eri Tatsumi, Daniella DellaGiustina and Dante Lauretta OVIRS Data

• OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) Reuter et al. 2018 • A point spectrometer • Heritage from the New Horizons LEISA • Field of view: 4 mrad • Effective spectral range: 0.4 to 4.3 μm • We limit our analysis to the range: 0.4 to 3.7 μm

Zou et al. Photometry of asteroid (101955) Bennu with OVIRS on OSIRIS-REx. ICARUS (submitted) Zou et al. Photometry of asteroid (101955) Bennu with OVIRS on OSIRIS-REx. ICARUS (submitted) Geometry Coverage on Bennu

• 299,702 calibrated spots and selected 70,342 spectra for photometric modeling • Complete coverage over the whole surface of Bennu • Dependence of incidence and emission angle on latitude • Asymmetric coverage between north and south in phase angle • Assume uniform photometric function (other than albedo variations) Zou et al. Photometry of asteroid (101955) Bennu with OVIRS on OSIRIS-REx. ICARUS (submitted) Characteristics of Photometric Data

Scattering geometry RADF vs. scattering vs. Latitude geometry • Density distribution of photometric data points at 0.55 µm • The spots with high incidence and emission angles (i > 70° or e > 70°) are discarded • The lowest phase angle is 5.3° • Strong correlation between (i, e) and latitude • Otherwise good coverage in (i, e, alpha) Photometric Models

Zou et al. Photometry of asteroid (101955) Bennu with OVIRS on OSIRIS-REx. ICARUS (submitted) 푅푀푆 = 푚푒푎푠푢푟푒푑 − 푚표푑푒푙푒푑 2

• RMS is equivalent to the 휒2 commonly defined in least chi-square minimization to quantify model quality • Overall stable RMS over the spectral range from 0.4 to 3.7 µm • Sudden drop in model quality crossing the 2.8 µm near the start of band segment 4 • McEwen, Minnaert, and Akimov models all fit data comparably well

Zou et al. Photometry of asteroid (101955) Bennu with OVIRS on OSIRIS-REx. ICARUS (submitted) • Goodness of fit has to be inspected

Zou et al. Photometry of asteroid (101955) Bennu with OVIRS on OSIRIS-REx. ICARUS (submitted) global map of the photometrically corrected (to 30°, 0°, 30°) OVIRS spots at 0.55 μm Bolometric Bond albedo

Bolometric Bond albedo is the quantity required for Yarkovsky and thermal inertia measurements by the OSIRIS-REx mission. It is the average of the spherical Bond albedo weighted by spectral irradiance of the

Sun, Js(λ). This integrates spherical albedo over 0.39-3.7 wavelengths, λ .

Zou et al. Photometry of asteroid (101955) Bennu with OVIRS on OSIRIS-REx. ICARUS (submitted) Photometric correction factor

The correction factor is applied from phase angle 8° to phase angle 30° for spectra obtained at various latitudes from 0° (equatorial) to 80° (close to the poles). Zou et al. Photometry of asteroid (101955) Bennu with OVIRS on OSIRIS-REx. ICARUS (submitted) Summary and conclusions

• We found that a McEwen model (McEwen, 1986) with an exponential phase function and an exponential polynomial partition function (a total of 7 free parameters) is the best fit for Bennu. • We used this model to correct the OVIRS spectra of Bennu to a standard reference viewing and illumination geometry of (i, e, α) = (30°, 0°, 30°) at visible to infrared wavelengths. From our best-fit model, we produced a bolometric Bond albedo map; values for Bennu’s surface range from 0.021 to 0.027. • In this work, we measured the phase reddening and compared our results to previously visited asteroids and comets, also compared to OCAMS and ground base measurements of Bennu. • Check out all the details in our submitted paper.

Zou et al. Photometry of asteroid (101955) Bennu with OVIRS on OSIRIS-REx. ICARUS (submitted)