Magma oceans and enhanced volcanism on TRAPPIST-1 planets due to induction heating

Low-mass M stars are plentiful in the Universe and often host small, rocky planets detectable with the current instrumentation. Recently, seven small planets have been discovered orbiting the ultracool dwarf TRAPPIST-1\cite{Gillon16,Gillon17}. We examine the role of electromagnetic induction heating of these planets, caused by the star's rotation and the planet's orbital motion. If the stellar rotation and magnetic dipole axes are inclined with respect to each other, induction heating can melt the upper mantle and enormously increase volcanic activity, sometimes producing a magma ocean below the planetary surface. We show that induction heating leads the three innermost planets, one of which is in the habitable zone, to either evolve towards a molten mantle planet, or to experience increased outgassing and volcanic activity, while the four outermost planets remain mostly unaffected.Comment: Published in Nature Astronomy; https://www.nature.com/articles/s41550-017-0284-

Escape and fractionation of volatiles and noble gases from Mars-sized planetary embryos and growing protoplanets

Planetary embryos form protoplanets via mutual collisions, which can lead to the development of magma oceans. During their solidification, large amounts of the mantles' volatile contents may be outgassed. The resulting H$_2$O/CO$_2$ dominated steam atmospheres may be lost efficiently via hydrodynamic escape due to the low gravity and the high stellar EUV luminosities. Protoplanets forming later from such degassed building blocks could therefore be drier than previously expected. We model the outgassing and subsequent hydrodynamic escape of steam atmospheres from such embryos. The efficient outflow of H drags along heavier species (O, CO$_2$, noble gases). The full range of possible EUV evolution tracks of a solar-mass star is taken into account to investigate the escape from Mars-sized embryos at different orbital distances. The envelopes are typically lost within a few to a few tens of Myr. Furthermore, we study the influence on protoplanetary evolution, exemplified by Venus. We investigate different early evolution scenarios and constrain realistic cases by comparing modeled noble gas isotope ratios with observations. Starting from solar values, consistent isotope ratios (Ne, Ar) can be found for different solar EUV histories, as well as assumptions about the initial atmosphere (either pure steam or a mixture with accreted H). Our results generally favor an early accretion scenario with a small amount of accreted H and a low-activity Sun, because in other cases too much CO$_2$ is lost during evolution, which is inconsistent with Venus' present atmosphere. Important issues are likely the time at which the initial steam atmosphere is outgassed and/or the amount of CO$_2$ which may still be delivered at later evolutionary stages. A late accretion scenario can only reproduce present isotope ratios for a highly active young Sun, but then very massive steam atmospheres would be required.Comment: 61 pages, 7 figures, 3 tables, accepted to Icaru

A grid of upper atmosphere models for 1--40 MEARTH planets: application to CoRoT-7 b and HD219134 b,c

There is growing observational and theoretical evidence suggesting that atmospheric escape is a key driver of planetary evolution. Commonly, planetary evolution models employ simple analytic formulae (e.g., energy limited escape) that are often inaccurate, and more detailed physical models of atmospheric loss usually only give snapshots of an atmosphere's structure and are difficult to use for evolutionary studies. To overcome this problem, we upgrade and employ an already existing upper atmosphere hydrodynamic code to produce a large grid of about 7000 models covering planets with masses 1 - 39 Earth mass with hydrogen-dominated atmospheres and orbiting late-type stars. The modeled planets have equilibrium temperatures ranging between 300 and 2000 K. For each considered stellar mass, we account for three different values of the high-energy stellar flux (i.e., low, moderate, and high activity). For each computed model, we derive the atmospheric temperature, number density, bulk velocity, X-ray and EUV (XUV) volume heating rates, and abundance of the considered species as a function of distance from the planetary center. From these quantities, we estimate the positions of the maximum dissociation and ionisation, the mass-loss rate, and the effective radius of the XUV absorption. We show that our results are in good agreement with previously published studies employing similar codes. We further present an interpolation routine capable to extract the modelling output parameters for any planet lying within the grid boundaries. We use the grid to identify the connection between the system parameters and the resulting atmospheric properties. We finally apply the grid and the interpolation routine to estimate atmospheric evolutionary tracks for the close-in, high-density planets CoRoT-7 b and HD219134 b,c...Comment: 21 pages, 4 Tables, 15 Figure