Infrared Spectroscopy of Large, Low‐Albedo Asteroids: Are Ceres and Themis Archetypes or Outliers?

Abstract

Low‐albedo, hydrated objects dominate the list of the largest asteroids. These objects have varied spectral shapes in the 3‐μm region, where diagnostic absorptions due to volatile species are found. Dawn's visit to Ceres has extended the view shaped by ground‐based observing and shown that world to be a complex one, potentially still experiencing geological activity. We present 33 observations from 2.2 to 4.0 μm of eight large (D > 200 km) asteroids from the C spectral complex, with spectra inconsistent with the hydrated minerals we see in meteorites. We characterize their absorption band characteristics via polynomial and Gaussian fits to test their spectral similarity to Ceres, the asteroid 24 Themis (thought to be covered in ice frost), and the asteroid 51 Nemausa (spectrally similar to the CM meteorites). We confirm most of the observations are inconsistent with what is seen in meteorites and require additional absorbers. We find clusters in band centers that correspond to Ceres‐ and Themis‐like spectra, but no hiatus in the distribution suitable for use to simply distinguish between them. We also find a range of band centers in the spectra that approaches what is seen on Comet 67P. Finally, variation is seen between observations for some objects, with the variation on 324 Bamberga consistent with hemispheric‐level difference in composition. Given the ubiquity of objects with 3‐μm spectra unlike what we see in meteorites, and the similarity of those spectra to the published spectra of Ceres and Themis, these objects appear much more to be archetypes than outliers.

“Water” abundance at the surface of C-complex main-belt asteroids

“…The 3-µm region of Ceres is distinct from most C-complex asteroids, since signatures of ammonia have been observed, suggesting an outer Solar System signature (DeSanctis et al, 2015) and a connection to comets (Poch et al, 2020). Only a few asteroids share this peculiar signature (Takir et al, 2013;Rivkin et al, 2019).…”

“…The black line is y=x. The two Tagish Lake measurements correspond to two different lithologies Rivkin et al (2019).…”

Ceres: Astrobiological Target and Possible Ocean World

“…This suggests Ceres is more akin to other icy moons and dwarf planets of the outer solar system. Ceres is likely not unique in the main belt: at least one body, 10 Hygiea, and shares similar spectral properties (e.g., Vernazza et al, 2017;Rivkin et al, 2019) and density (*2000 g/cm 3 ) (Vernazza et al, 2019).…”

“…Its mean radius and bulk density, respectively, 470 km and 2162 kg m -3 , are intermediate between Enceladus (252 km and 1611 kg m -3 ) and Europa (1560 km and 3014 kg m -3 ), and comparable with those of many other icy moons and icy dwarf planets in the solar system. Ceres shares spectral similarities with C-type asteroids, in particular with Hygiea (Rivkin et al, 2019) and 324 Bamberga (Takir and Emery, 2012). However, as per its outstanding properties, Ceres is more akin to icy satellites than to asteroids, in particular based on its large bulk water content and large size.…”

A model of the 3-μm hydration band with Exponentially Modified Gaussian (EMG) profiles: Application to hydrated chondrites and asteroids

“…Limitations of the model In case of ground-based observations of small bodies, the detection of the sharp low wavelength fall of the 3-µm band is impossible due to the atmospheric absorption between 2500 and 2850 nm (Rivkin et al, 2019). The left wing and the minimum of reflectance of the first component are missing from the spectrum, while its right wing Figure 14 presents the modeled spectrum of (121) Hermione using the complete data, and on a partial spectrum simulating atmospheric window for ground based observations.…”