Search for Candidate Exogenous Material on Bennu using MapCam and PolyCam Images

<p><strong>Introduction:</strong> Exogenous material has been reported on asteroids (101955) Bennu and (162173) Ryugu.  The albedo, color, and spectra of six bright boulders on Bennu show that they are distinct from the rest of Bennu’s comparatively dark surface and are likely basaltic material from asteroid (4) Vesta (DellaGiustina et al. 2019; 2020).  Bright boulders identified on asteroid (162173) Ryugu show absorptions near 1 µm but not near 2 µm, suggesting olivine-rich anhydrous silicates (Tatsumi et al., 2019; 2020).</p><p><strong>Methods: </strong>In this work, we used images of Bennu obtained by the MapCam and PolyCam instruments (Rizk et al. 2018; Golish et al. 2020) on NASA’s OSIRIS-REx spacecraft (Lauretta et al. 2017) to search for more potentially exogeneous material on the surface of Bennu.  </p><p><strong>Results: </strong>We identified approximately 50 bright boulders on Bennu with significant x-band (0.85 µm) absorptions, including the six bright boulders previously described in DellaGiustina et al. (2020).  These bright boulders are distributed across Bennu’s surface, concentrated in terrains with larger than average particle size.  The boulders exhibit three main morphologies (homogeneous, heterogenous, and breccia) and three spectral types (reflectance peak at 0.55 µm, flat, and reflectance peak at 0.7 µm). There is some correlation between the morphology and the spectra. More specifically, three of the four boulders with reflectance peaks at 0.55 µm are in the breccia category and one is in the heterogeneous category, whereas all the boulders with reflectance peaking at 0.7 µm fall in the homogeneous and heterogenous categories. Bright boulders with reflectance peaking at 0.55 µm have lower normal albedo than the other two spectral groups, suggesting a different composition and possibly a different origin. An initial comparison with Ryugu (Tatsumi et al. 2019) indicates that Bennu is more abundant in possible exogeneous material.  </p><p><strong>Discussion: </strong>Further analysis of MapCam and PolyCam images has revealed a wider diversity of potentially exogenous lithologies on Bennu, expanding on the findings of DellaGiustina et al. (2020).  We continue our study of this diversity of material on Bennu’s surface to constrain the asteroid’s origin, evolution, and collisional history (e.g., Ballouz et al. 2020).</p><p><strong>References: </strong>DellaGiustina D. et al. (2019) Nat. Astron. 3, 341-351. DellaGiustina D. et al. (2020) Nat. Astron, in revision. Lauretta D. S. et al. (2017) Space Sci. Rev. 212, 925–984. Tatsumi E. et al. (2019) LPI Contribution No. 2132, id.1753. Rizk B. et al. (2018) Space Sci. Rev. 214, 26. Golish D.R. et al. (2020) Space Sci. Rev. 216, 12. Ballouz R.-L. et al. 2020, EPSC abstract.</p>

Mechanisms involved in the production of differently colored feathers in the structurally-colored Swallow Tanager (Tersina viridis; Aves: Thraupidae)

ABSTRACTNon-iridescent, structurally-based coloration in birds originates from the feather’s internal nanostructure (the keratin spongy matrix), but the presence of melanin and the characteristics of the barb’s cortex can affect the resulting color. Here we investigate how this nanostructure is regulated and combined with other elements in differently-colored plumage patches. To do so, we investigated the association between light reflectance and the morphology of feathers from the back and belly plumage patches of male Swallow Tanagers (Tersina viridis), which look greenish-blue and white, respectively. Both plumage patches have a reflectance peak around 550 nm, but the reflectance spectrum is much less saturated in the belly. The barbs of both types of feathers have similar spongy matrices at their tips which produce similar reflectance spectra. However, the color of the belly feather barbs changes from light green at the tip to white closer to the rachis. These barbs lack pigments and their morphology changes considerably: the spongy matrix is reduced, being almost hollow, and has a different shape towards the rachis. Instead, we observed deposition of melanin underneath the spongy matrix in the back feathers which had a much saturated coloration that was consistent along the barbs’ length. Overall, our results suggest that the color differences between the white and greenish-blue plumage are mostly due to the differential deposition of melanin and a reduction of the spongy matrix in some parts of the belly feather barbs, and not a result of changes in the periodicity of the spongy matrix.

FVI—A Floating Vegetation Index Formed with Three Near-IR Channels in the 1.0–1.24 μm Spectral Range for the Detection of Vegetation Floating over Water Surfaces

Through the analysis of hyperspectral imaging data collected over water surfaces covered by floating vegetation, such as Sargassum and algae, we observed that the spectra commonly contain a reflectance peak centered near 1.07 μm. This peak results from the competing effects between the well-known vegetation reflectance plateau in the 0.81–1.3 μm spectral range and the absorption effects above 0.75 μm by liquid water within the vegetation and in the surrounding water bodies. In this article, we propose a new index, namely the floating vegetation index (FVI), for the hyperspectral remote sensing of vegetation over surface layers of oceans and inland lakes. In the formulation of the FVI, one channel centered near 1.0 μm and another 1.24 μm are used to form a linear baseline. The reflectance value of the third channel centered at the 1.07-μm reflectance peak above the baseline is defined as the FVI. Hyperspectral imaging data acquired with the AVIRIS (Airborne Visible Infrared Imaging Spectrometer) instrument over the Gulf of Mexico and over salt ponds near Moffett Field in southern portions of the San Francisco Bay were used to demonstrate the success in detecting Sargassum and floating algae with this index. It is expected that the use of this index for the global detection of floating vegetation from hyperspectral imaging data to be acquired with future satellite sensors will result in improved detection and therefore enhanced capability in estimating primary production, a measure of how much carbon is fixed per unit area per day by oceans and inland lakes.

Effect of graphene oxide inclusion on the optical reflection of a silica photonic crystal film

In this study, the inclusion of graphene oxide in silica photonic crystals was found to affect optical reflectance intensity and reflectance peak broadening.

Optics of the ultraviolet reflecting scales of a jumping spider

The jumping spider Cosmophasis umbratica from Singapore is strongly sexually dimorphic. The males, but not the females, reflect ultraviolet as well as green–orange light. The scales responsible for this are composed of a chitin–air–chitin sandwich in which the chitin layers are three-quarters of a wavelength thick and the air gap a quarter wavelength (where λ =600 nm, the peak wavelength of the principal reflection maximum). It is shown that this configuration produces a second reflectance peak at approximately 385 nm, accounting for the observed reflection in the ultraviolet. Other scales have a similar thickness of chitin but lack the air gap and thus produce a dull purple reflection. This novel mechanism provides the spiders with two colour signals, both of which are important in mating displays.