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INVESTIGATING SURFACE COLOR VARIEGATION ON NEAR …on Bennu, which has a blue slope (Fig. 2). Figure...
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INVESTIGATING SURFACE COLOR VARIEGATION ON NEAR-EARTH ASTEROID BENNU USING OSIRIS-REX MAPCAM DATA. L. Le Corre 1 , D. DellaGiustina 2 , K.J. Becker 2 , D. Golish 2 , C. Bennett 2 , V. Reddy 2 , M.R.M. Izawa 3 , P. Smith 2 , B. Rizk 2 , C. d’Aubigny 2 , D. Lauretta 2 , B.E. Clark 4 , H. Campins 5 , S. Fornasier 6 , and the OSIRIS-REx Science Team, 1 Planetary Science Institute ([email protected]), 2 University of Arizona, Lunar and Plan- etary Laboratory, 3 Institute for Planetary Materials, 4 Ithaca College, 5 University of Central Florida, 6 LESIA, Observa- toire de Paris. Introduction: The OSIRIS-REx spacecraft arrived at near-Earth asteroid Bennu on December 3, 2018. During the Approach phase of the mission, the onboard MapCam camera, part of the OSIRIS-REx Camera Suite (OCAMS), acquired images of the asteroid using four narrow bandpass filters with a pixel scale up to 12 m/pixel. MapCam has four color filters—b´ (473 nm), v (550 nm), w (698 nm), and x (847 nm)—and a pan- chromatic filter [1]. Using the color images from No- vember 4 to 9, 2018, we generated a color mosaic of the surface of Bennu. Our goal is to investigate surface color variegation, quantify trends in color parameters (spectral slope, band depth), and map the distribution of potential absorption features. Bennu Before OSIRIS-REx: Ground-based photo- metric measurements by [2] and spectra from [3] showed that Bennu is a B-type asteroid. Clark et al. [3] showed that Bennu’s disk-integrated spectrum has a blue spectral slope and is essentially featureless. They proposed CI or CM carbonaceous chondrites as proba- ble meteorite analogs for the asteroid. Data Processing: Calibrated reflectance data (I/F) was corrected for viewing and illumination effects to adjust the brightness of the images acquired during dif- ferent days with different viewing conditions. This al- lows us to build a more homogenous image mosaic without too much discrepancy between frames (i.e., a seamless mosaic). Color Analysis: Various color parameters were computed such as band depth, color ratios, and slopes using unprojected multi-band cubes and with the repro- jected global mosaic of Approach images (10 m/pixel at best). Color sets. Color sets consist of four color filter im- ages acquired close to each other in time. Images were aligned to their backplanes to reduce the pointing errors and were then centered on Bennu to create a unprojected color cube with four bands (Fig.1). The most striking color variations are associated with a very dark outcrop, dark boulders or clusters of dark boulders that can be resolved in the approach phase images (Fig.1). The average geometric albedo of Bennu is 0.0435, and we estimate the albedo of the dark out- crop to be 0.034 in the panchromatic filter. We can also compute other parameters such as ratio of b´/x (473 nm/847 nm) to characterize these dark surface features. They seem to generally have a much redder spectral slope between b´ and x than the typical average terrain on Bennu, which has a blue slope (Fig. 2). Figure 1. RGB color image of Bennu (left) created us- ing MapCam filters w, v, and b´ as the red, green, and blue channels. Color ratio image (right) from b´/v. Color spectra. The dark terrain also stands out in other color ratios, for example b´ /v, because they have a deeper 550-nm band depth than the average terrain on Bennu (Fig. 2). Figure 2. (Top) Average global color spectra of Bennu (red) and dark boulder (black). (Bottom) Same spectra normalized at 550 nm. Error bars are computed with one standard deviation. 2794.pdf 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
Transcript of INVESTIGATING SURFACE COLOR VARIEGATION ON NEAR …on Bennu, which has a blue slope (Fig. 2). Figure...
INVESTIGATING SURFACE COLOR VARIEGATION ON NEAR-EARTH ASTEROID BENNU USING OSIRIS-REX MAPCAM DATA. L. Le Corre1, D. DellaGiustina2, K.J. Becker2, D. Golish2, C. Bennett2, V. Reddy2, M.R.M. Izawa3, P. Smith2, B. Rizk2, C. d’Aubigny2, D. Lauretta2, B.E. Clark4, H. Campins5, S. Fornasier6, and the OSIRIS-REx Science Team, 1Planetary Science Institute ([email protected]), 2University of Arizona, Lunar and Plan-etary Laboratory, 3Institute for Planetary Materials, 4Ithaca College, 5University of Central Florida, 6LESIA, Observa-toire de Paris.
Introduction: The OSIRIS-REx spacecraft arrived
at near-Earth asteroid Bennu on December 3, 2018. During the Approach phase of the mission, the onboard MapCam camera, part of the OSIRIS-REx Camera Suite (OCAMS), acquired images of the asteroid using four narrow bandpass filters with a pixel scale up to 12 m/pixel. MapCam has four color filters—b´ (473 nm), v (550 nm), w (698 nm), and x (847 nm)—and a pan-chromatic filter [1]. Using the color images from No-vember 4 to 9, 2018, we generated a color mosaic of the surface of Bennu. Our goal is to investigate surface color variegation, quantify trends in color parameters (spectral slope, band depth), and map the distribution of potential absorption features.
Bennu Before OSIRIS-REx: Ground-based photo-metric measurements by [2] and spectra from [3] showed that Bennu is a B-type asteroid. Clark et al. [3] showed that Bennu’s disk-integrated spectrum has a blue spectral slope and is essentially featureless. They proposed CI or CM carbonaceous chondrites as proba-ble meteorite analogs for the asteroid.
Data Processing: Calibrated reflectance data (I/F) was corrected for viewing and illumination effects to adjust the brightness of the images acquired during dif-ferent days with different viewing conditions. This al-lows us to build a more homogenous image mosaic without too much discrepancy between frames (i.e., a seamless mosaic).
Color Analysis: Various color parameters were computed such as band depth, color ratios, and slopes using unprojected multi-band cubes and with the repro-jected global mosaic of Approach images (10 m/pixel at best).
Color sets. Color sets consist of four color filter im-ages acquired close to each other in time. Images were aligned to their backplanes to reduce the pointing errors and were then centered on Bennu to create a unprojected color cube with four bands (Fig.1).
The most striking color variations are associated with a very dark outcrop, dark boulders or clusters of dark boulders that can be resolved in the approach phase images (Fig.1). The average geometric albedo of Bennu is 0.0435, and we estimate the albedo of the dark out-crop to be 0.034 in the panchromatic filter. We can also compute other parameters such as ratio of b´/x (473 nm/847 nm) to characterize these dark surface features.
They seem to generally have a much redder spectral slope between b´ and x than the typical average terrain on Bennu, which has a blue slope (Fig. 2).
Figure 1. RGB color image of Bennu (left) created us-ing MapCam filters w, v, and b´ as the red, green, and blue channels. Color ratio image (right) from b´/v.
Color spectra. The dark terrain also stands out in
other color ratios, for example b´/v, because they have a deeper 550-nm band depth than the average terrain on Bennu (Fig. 2).
Figure 2. (Top) Average global color spectra of Bennu (red) and dark boulder (black). (Bottom) Same spectra normalized at 550 nm. Error bars are computed with one standard deviation.
2794.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
Absorption features. The location of the largest dark spot is indicated on the PolyCam Approach map (Fig. 3). We used images from the Approach phase to com-pute the distribution of the 550-nm band depth (Fig. 4) as well as the 700-nm band depth for this dark outcrop, using a standard formula to calculate band depths as de-scribed in [4]. It allows us to visualize the distribution of the areas with deepest band depth, confirming that the dark terrains (Fig. 5) exhibit the deepest 550-nm absorp-tion band depth (Fig. 4). At the Approach resolution and with only a preliminary photometric model, it is difficult to identify further surface features with a pronounced 550-nm absorption band in the map. In addition, a better photometric correction is required to study more accu-rately the distribution of surface units with this spectral signature. The 700-nm absorption was not detected.
Nature of the 550-nm Absorption Feature: CM and CI chondrites have been proposed as analogs for Bennu. The mineralogy of these meteorites reflects ex-tensive aqueous processing and includes phyllosilicates, carbonates, and Fe oxides, notably magnetite. Magnet-ite reflectance spectra exhibit a local minimum in reflec-tance centered near ~550 nm [6], as do the related min-erals ferrihydrite and maghemite [3]. Magnetite is fur-ther characterized by a low overall reflectance and has a blue spectral slope shortward of 1000 nm.
References: [1] Rizk, B. et al. (2018), Space Sci-ence Reviews, 214, 26. [2] Hergenrother, C. W. et al. (2013), Icarus 226, 663–670. [3] Clark, B. E. et al. (2011), Icarus 216(2), 462–475. [4] DellaGiustina, D. et al. (2019), Earth and Space Science 5(12), 929-949. [5] Cloutis, E. et al. (2011), Icarus 212, 180–209. [6] Izawa, M. et al. (2019), Icarus 319, 525-539.
Acknowledgments: This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Pro-gram.
Figure 3. Panchromatic global map of Bennu from the PolyCam imager, showing the location of the dark out-crop indicated by a white circle. Images used for this mosaic were acquired during the Approach phase.
Figure 4. 550 nm-band depth image of Bennu created using MapCam Approach phase data from November 8, 2018. The darkest spot, indicated by white arrow, and identified as an possible outcrop, has a deep 500-nm feature compared to the global average. Image has been smoothed.
Figure 5. v-band image of Bennu acquired during Ap-proach phase (November 8). The dark outcrop is visible as the largest low reflectance unit (indicated by the white arrow). This outcrop is surrounded by smaller dark boulders also showing a relatively deep 550-nm band depth (Fig. 4). Image has been smoothed.
2794.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
