Distant Formation and Differentiation of Outer Main Belt Asteroids and Carbonaceous Chondrite Parent Bodies

Abstract

Volatile compositions of asteroids provide information on the Solar System history and the origins of Earth's volatiles. Visible to near-infrared observations at wavelengths of <2.5 µm have suggested a genetic link between outer main belt asteroids located at 2.5–4 au and carbonaceous chondrite meteorites (CCs) that show isotopic similarities to volatile elements on Earth. However, recent longer wavelength data for large outer main belt asteroids show 3.1 μm absorption features of ammoniated phyllosilicates that are absent in CCs and cannot easily form from materials stable at those present distances. Here, by combining data collected by the AKARI space telescope and hydrological, geochemical, and spectral models of water-rock reactions, we show that the surface materials of asteroids having 3.1 μm absorption features and CCs can originate from different regions of a single, water-rock-differentiated parent body. Ammoniated phyllosilicates form within the water-rich mantles of the differentiated bodies containing NH3 and CO2 under high water-rock ratios (>4) and low temperatures (<70°C). CCs can originate from the rock-dominated cores, that are likely to be preferentially sampled as meteorites by disruption and transport processes. Our results suggest that multiple large main belt asteroids formed beyond the NH3 and CO2 snow lines (currently >10 au) and could be transported to their current locations. Earth's high hydrogen to carbon ratio may be explained by accretion of these water-rich progenitors.