Presentation #304.06 in the session “Understanding Distant Activity”.
The Oort Cloud is a solar system construct, formed of billion to trillions of icy planetesimals that failed to aggregate onto and become part of one of the giant planets — but were instead scattered out onto nearly hyperbolic, barely bound, multi-million year orbits. Recent work (Stern 2003; Dones+ 2004, 2015; Brasser & Morbidelli 2013; Garrod 2019), has shown that a competition between attempted aggregation frequency and successful launching a clearing solar system midplane determines the success of Oort Cloud emplacements, with the majority occurring 100–500 Myr after the start of the solar system. Only a handful of Oort Cloud comets are known to contain abundant hypervolatile majority ices (e.g., N2, CO, CH4 ices), representing a negligible (~10-3) fraction of all the known Oort Cloud comets. Yet we know that hypervolatile species existed in the protosolar nebula, since we detect them on the surfaces of the large KBOs, in the atmospheres of the giant planets, and as minority (few % vs water) species in Oort Cloud and inner-system Jupiter Family comets. This can be explained as a matter of timing. As shown in Lisse+ 2021 and Steckloff+ 2021, any small icy solar system body found in regions from the Kuiper Belt inward, including the giant planet region, will lose, via insolation heating, its hypervolatile majority ices to vacuum within 10 Myrs of the optical thinning, or “clearing”, of the solar system’s primordial planetary disk (PPD). Thus we expect none but the very first icy planetesimal aggregational failures lucky enough to thread the busy early midplane to transport any hypervolatiles successfully to the Oort Cloud. These lucky few, if as large as the best studied hypervolatile rich Oort comet C/2016 R2 (Rnuc ~15 km), can then endure thousands of orbits' worth (i.e. Gyrs) of hypervolatiles loss upon perihelion passage. But the vast majority of the icy planetesimals launched into Oort Cloud orbits on 100–1000 Myr timescales will contain no icy volatiles, except as minority impurities in refractory ice matrices, reproducing the situation we observe today.