C‐Complex Asteroids: UV‐Visible Spectral Characteristics and Implications for Space Weathering Effects

Abstract

Space weathering effects on the rocky S-class asteroids are well understood. However, on the low-albedo C-complex asteroids, such as spacecraft targets Bennu and Ryugu, the situation is more complicated, especially due to a lack of spectral features throughout the visible-near infrared spectral region. Here we show, through a combination of observational data and laboratory data of carbonaceous chondrites, phyllosilicates, and mixtures, that the UV-visible spectral region is a diagnostic regime for studying space weathering effects on C-complex asteroids. We show that space-weathering-produced opaque constituents, such as graphitized carbons, darken mixtures with phyllosilicates and produce a bluing the UV-visible spectrum, consistent with what is seen on the asteroids compared with carbonaceous chondrites. Furthermore, we demonstrate that the UV upturns in the spectra of Bennu and Ryugu are consistent with the presence of graphitized carbon on those surfaces, the result of surface processing. Plain Language SummaryWe suggest that the relatively weak UV absorptions of C-complex asteroids compared with carbonaceous chondrite meteorites could be due to the increased presence of carbonized species on the asteroid surfaces, the result of space weathering processes. In fact, carbonized materials could be the source of the UV-visible spectra of Ryugu and Bennu, which appear to show UV upturns rather than UV absorptions. Though iron-rich and carbon-rich opaques have similar spectral effects in the UV-VIS, many laboratory experiments indicate that carbonaceous materials are the products of space weathering processes and thus carbonaceous materials could dominate spectral trends. Furthermore, we show that carbons may exhibit spectral features that change with processing and thus can help us to understand exposure age of some low-albedo targets.

Variations in color and reflectance on the surface of asteroid (101955) Bennu

“…However, contrary to the bluing and darkening predicted from the production of graphitized carbon and magnetite, the formation of nanophase iron in low-albedo asteroids such as Bennu may have a reddening and brightening effect, owing to the higher reflectance of nanophase iron than that of the primitive materials found in carbonaceous meteorites (59). Space weathering trends observed on Bennu corroborate earlier studies (13,44) that found that primitive asteroids, though dark relative to the asteroid population, are brighter in the UV than their primitive meteorite counterparts-that is, they are spectrally bluer at shorter wavelengths. Ion bombardment of primitive low-albedo meteorites in the laboratory leads to spectral bluing and brightening (20).…”

“…This absorption feature on Bennu has previously been attributed to magnetite (5), which is also detected in thermal emissivity spectra from OTES (7). However, this absorption feature could also result from graphitized carbon (44), and both magnetite and graphite are associated with space weathering (discussed further below).…”

“…Dark, optically opaque minerals (hereafter, opaques), such as nanophase iron, graphitized carbon, sulfides, and magnetite, are commonly produced by space weathering of primitive materials (19,19,23,44,57). Laboratory studies have shown that phyllosilicates intimately mixed with certain carbon species (including graphite) and magnetite can lead to bluer and darker spectral slopes in the visible wavelengths (44) (fig.…”

Spectral effects of varying texture and composition in two‐component “mudpie” simulations: Insights for asteroid (101955) Bennu

“…Continuous reflectance spectra of Bennu from the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS; Reuter et al 2018) at visible (VIS; 0.4-0.7 µm) and near-infrared (NIR; 0.7-4.3 µm) wavelengths reveal an absorption feature at 2.74 µm indicative of aqueous alteration of Mg-bearing phyllosilicates (Hamilton et al 2019;Hendrix and Vilas 2019). Similar 2.7-2.8 µm features are observed in CI and CM carbonaceous meteorites (Takir et al 2013), as well as in Fe-and Mgrich phyllosilicates such as saponite (McKee and Moore 1979;Barber 1981;Cloutis et al 2011aCloutis et al , 2011bHoward et al 2015;Donaldson Hanna et al 2018).…”

“…2018) at visible (VIS; 0.4–0.7 µm) and near‐infrared (NIR; 0.7–4.3 µm) wavelengths reveal an absorption feature at 2.74 µm indicative of aqueous alteration of Mg‐bearing phyllosilicates (Hamilton et al. 2019; Hendrix and Vilas 2019). Similar 2.7–2.8 µm features are observed in CI and CM carbonaceous meteorites (Takir et al.…”