Origin and Evolution of the Cometary Reservoirs

Abstract:

Comets have three known reservoirs: the roughly spherical Oort Cloud (for long-period comets), the flattened Kuiper Belt (for ecliptic comets), and, surprisingly, the asteroid belt (for main-belt comets). Comets in the Oort Cloud are thought to have formed in the region of the giant planets and then placed in quasi-stable orbits at distances of thousands or tens of thousands of AU through the gravitational effects of the planets and the Galaxy. Originally the planets were thought to have formed in place. However, Fernández and Ip (1984) proposed that Jupiter would have migrated slightly inward, while Saturn and (especially) Uranus and Neptune would have migrated outward as they interacted with a massive disk of planetesimals. Malhotra (1993) showed that Pluto's orbit in the 3:2 resonance with Neptune was a natural outcome if Neptune captured Pluto into resonance while it migrated outward. Building on this work, Tsiganis et al. (2005) proposed the "Nice" model, in which the giant planets formed closer together than they are now, and, perhaps many hundreds of Myr later, underwent a dynamical instability that led to a flood of comets and asteroids throughout the Solar System (Gomes et al. 2005). In this scenario, it is somewhat a matter of luck whether an icy planetesimal ends up in the Kuiper Belt or Oort Cloud (Brasser and Morbidelli 2013), as a Trojan asteroid (Morbidelli et al. 2005, Nesvorný and Vokrouhlický 2009, Nesvorný et al. 2013), or as a distant "irregular" satellite of a giant planet (Nesvorný et al. 2007). Comets could even have been captured into the asteroid belt (Levison et al. 2009). The remarkable finding of two "inner Oort cloud" bodies, Sedna and 2012 VP₁₁₃ (Brown et al. 2004, Sheppard and Trujillo 2014), suggests that the Sun formed in a denser environment, i.e., in a star cluster (Brasser et al. 2006, Kaib and Quinn 2008, Brasser et al. 2012).

My talk will focus on how, in spite of this unexpected complexity, we can attempt to surmise the histories of comets from clues such as the size distributions of their nuclei (Lamy et al. 2004, Fraser et al. 2014) and current ideas about rapid formation of planetesimals through "pebble accretion" (Johansen et al. 2012). I will also discuss new insights from Rosetta's recent arrival at ecliptic comet 67P and the close approach of long-period comet C/2013 A1 to Mars in October.