Ice lines, planetesimal composition and solid surface density in the solar nebula

Abstract

To date, there is no core accretion simulation that can successfully account for the formation of Uranus or Neptune within the observed 2-3 Myr lifetimes of protoplanetary disks. Since solid accretion rate is directly proportional to the available planetesimal surface density, one way to speed up planet formation is to take a full accounting of all the planetesimal-forming solids present in the solar arXiv:0806.3788v2 [astro-ph] 4 Dec 2008 -2nebula. By combining a viscously evolving protostellar disk with a kinetic model of ice formation, which includes not just water but methane, ammonia, CO and 54 minor ices, we calculate the solid surface density of a possible giant planetforming solar nebula as a function of heliocentric distance and time. Our results can be used to provide the starting planetesimal surface density and evolving solar nebula conditions for core accretion simulations, or to predict the composition of planetesimals as a function of radius.We find three effects that favor giant planet formation by the core accretion mechanism: (1) a decretion flow that brings mass from the inner solar nebula to the giant planet-forming region, (2) the fact that the ammonia and water ice lines should coincide, according to recent lab results from Collings et al. (2004), and (3) the presence of a substantial amount of methane ice in the trans-Saturnian region. Our results show higher solid surface densities than assumed in the core accretion models of Pollack et al. (1996) by a factor of 3-4 throughout the trans-Saturnian region. We also discuss the location of ice lines and their movement through the solar nebula, and provide new constraints on the possible initial disk configurations from gravitational stability arguments.

Aqueous alteration on main belt primitive asteroids: Results from visible spectroscopy

“…The snow line has been located within or nearby the asteroid belt (Lunine, 2006;Cyr et al, 1998), and, following the recent nebular kinetics models results, it may have migrated as the nebula evolved, sweeping across the entire asteroid belt (Dodson-Robinson et al, 2009). According to the latest dynamical models, it has also been suggested that part of the main belt primitive asteroids were formed either between the giant planets or in the Trans-neptunian region (Walsh et al, 2011), and then injected into the inner Solar System.…”

Astrobiology: An astronomer's perspective

“…This result is simplistic as it assumes a bare grain sitting alone at 1 AU with approximate grain properties. However, more detailed radiation transfer calculations confirm the general picture [70,71,72,73]. This well known result suggests that the solar nebular disk, and extra-solar protoplanetary disks, should have what is called a "snow-line".…”

DISCOVERING THE GROWTH HISTORIES OF EXOPLANETS: THE SATURN ANALOG HD 149026b

“…We set up an experiment to test our hypothesis that the same model protostellar disk that produced Saturn, containing 8.6 g cm −2 of planetesimals at 9.5 AU (Dodson- Robinson et al 2008Robinson et al , 2009, with surface density doubled to match the heavy element content of HD 149026, could account for HD 149026b's large core. In a successful outcome, the planet would reach its current mass within a typical disk lifetime of 2-3 Myr (Haisch et al 2001) and contain the observed proportion of solid core and atmospheric gas.…”

“…At 1.3 solar masses, HD 149026b is essentially a Sun-like star, so we begin by assuming its protostellar disk had the same total mass (gas+solid) as the solar nebula. We reconstruct the solid surface density profile Σ(R) of the HD 149026b disk by scaling the Σ(R) curves predicted for the solar nebula (Dodson- Robinson et al 2009) upward by a factor of 2. Figure 1 shows time shapshots of Σ(R) during the planetesimal-building epoch of the HD 149026 disk.…”