Giant Impact: An Efficient Mechanism for the Devolatilization of Super-Earths

Abstract

Mini-Neptunes and volatile-poor super-Earths coexist on adjacent orbits in proximity to host stars such as Kepler-36 and Kepler-11. Several post-formation processes have been proposed for explaining the origin of the compositional diversity between neighboring planets: the mass loss via stellar XUV irradiation, degassing of accreted material, and in-situ accumulation of the disk gas. Close-in planets are also likely to experience giant impacts during the advanced stage of planet formation. This study examines the possibility of transforming volatile-rich super-Earths / mini-Neptunes into volatile-depleted super-Earths through giant impacts. We present the results of three-dimensional hydrodynamic simulations of giant impacts in the accretionary and disruptive regimes. Target planets are modeled with a three-layered structure composed of an iron core, silicate mantle and hydrogen/helium envelope. In the disruptive case, the giant impact can remove most of the H/He atmosphere immediately and homogenize the refractory material in the planetary interior. In the accretionary case, the planet is able to retain more than half of the original gaseous envelope, while a compositional gradient suppresses efficient heat transfer as the planetary interior undergoes double-diffusive convection. After the giant impact, a hot and inflated planet cools and contracts slowly. The extended atmosphere enhances the mass loss via both a Parker wind induced by thermal pressure and hydrodynamic escape driven by the stellar XUV irradiation. As a result, the entire gaseous envelope is expected to be lost due to the combination of those processes in both cases. Based on our results, we propose that Kepler-36b may have been significantly devolatilized by giant impacts, while a substantial fraction of Kepler-36c's atmosphere may remain intact. Furthermore, the stochastic nature of giant impacts may account for the observed large dispersion in the mass-radius relationship of close-in super-Earths and mini-Neptunes (at least to some extent).

Atmosphere Impact Losses

“…The aim of these models is to distill the somewhat complex and complicated nature of impacts to their essential physics, which allows us to gain an intuitive understanding of atmospheric mass loss results and to apply them over the whole range of possible impactor sizes. These models are not suitable to determine the precise outcome of a single specific impact, which for example have been investigated by Shuvalov (2009) and Liu et al (2015), but capture the overall mass loss results in an averaged sense, e.g. once averaged over various impact geometries and angles.…”

“…These works integrate the hydrodynamic equations of motion of the planetary atmosphere and determine the amount of atmospheric mass loss for a given ground velocity that launches a strong shock into the atmosphere. Recently Liu et al (2015) presented the first results of three dimensional hydrodynamic simulations of atmospheric mass loss that model the giant impact as well as the atmospheric mass loss together. Here we calculate the atmospheric loss due to a given ground motion.…”

Planetary magnetic field control of ion escape from weakly magnetized planets

“…This thermal energy can be gained from photo-heating (e.g. Yelle 2004), giant impacts (Liu et al 2015), or core-accretion (Ginzburg et al 2016). Thermally-driven atmospheric escape is thought to lead to the gap between super-earths and mini-neptunes (Owen and Wu 2017;Ginzburg et al 2018;Biersteker and Schlichting 2018).…”

Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. III. A High Mass and Low Envelope Fraction for the Warm Neptune K2-55b*

“…Unless the smaller planets collided farther out in the disk where collisional velocities were lower and the newly formed K2-55b subsequently migrated inward to 0.0347 au via planetesimal scattering, this scenario is unlikely to explain the formation of K2-55b. Alternatively, the presence of a gaseous envelope before the collision might have made the collision less destructive (e.g., Liu et al 2015). The logical observational test for this scenario is to measure the spin-orbit alignment of the system via the Rossiter-McLaughlin effect (McLaughlin 1924;Rossiter 1924), but the host star is too faint to permit such a precise measurement with current facilities.…”

“…A third possibility is that K2-55b formed as a "regular" subSaturn with a typical envelope fraction but then lost most of its envelope to a single late giant impact (e.g., Inamdar & Schlichting 2015Liu et al 2015;Schlichting et al 2015). More massive planets are less vulnerable to envelope loss via either photoevaporation or impacts (Lopez & Fortney 2013;Inamdar & Schlichting 2015), suggesting that a late giant impact could have had a more catastrophic effect for K2-55b than for a Saturn-mass planet.…”