Ceres: Organic-Rich Sites of Exogenic Origin?

Abstract

Ceres, the largest object in the asteroid belt, is the only potential ocean world in the inner Solar System. Previous studies identified deposits of aliphatic organics in and around the Ernutet crater, and at small locations at Inamahari and Urvara craters. The origin of organics, either endogenic or exogenic, in these fresh exposures is still under debate. This study addresses the origin of the organics by analyzing their global distribution and geologic context. Our first step involved a global search for organic-rich sites that might have escaped previous detections. We achieved this by using a deep neural network, utilizing spectral redness in the Dawn's Framing Camera multispectral data to identify potential organic-rich sites. The identified sites were further studied by using IR spectrometer data to infer the compositions of materials showing spectral redness. Of the newly identified red-sloped sites, only two can be considered certain to be organic-rich. We also identified sites with spectral redness, but without any signature of organics in their infrared spectra. These sites could be attributed to the aqueous alteration of magnetite into ferric-iron bearing phases. At Ernutet, Inamahari, and Urvara, the organic-rich material is confined to the near surface only. Additionally, the absence of tectonic/volcanic features at these sites makes an endogenic origin questionable. The global rarity of detectable organics also supports this assessment. Consequently, we suggest that organics at these sites were originally delivered by low-velocity, organic-rich impactor(s) from the main belt and subsequently excavated, rather than originating from endogenous processes.