EPS Colloquium: David Yuen

Senior Research Scientist, Columbia University

Impact of Equation of State on Dynamics of Terrestrial Exo-Solar Planets

I will present  some new  results for the internal structure and convective dynamics of large  terrestrial (rocky) exo-solar planets, which may exist several light years away in other solar systems in our galaxy. Previous works have focussed only on individual eco-solar planets, without any regard a systematic understanding of the influence from the amount of mass or size of the planet. By going out to 20 Earth masses we show for the first time  that pressure and temperature can reach several Terapascal (TPa) and 10000 Kelvin respectively in the silicate mantle of these planets. Recent ab initio calculations have predicted that the main constituent magnesium-silicate mineral of the Earth’s mantle can fully dissociate, in a stepwise fashion, int the oxides SiOand MgO under these P,T conditions (Umemoto et al., 2017). Based on ab initio  results based using quantum mechanics   for the properties of relevant phases and the new phase boundaries by Umemoto et al.  (2017) we have modelled the internal structure of large terrestrial planets with an Earth-like core   mass fraction of 0.3 ( earth-like assumption )and one to twenty times the Earth’s mass. We found that full dissociation into oxides occurs inside planets that are more massive than thirteen Earth masses when pressure at the core mantle boundary exceeds ∼ 2.4 TPa.  Our modeling results of compressible mantle convection for exo-planets with mass valueswithin the range 1-20 Earth masses show strong differences in the internal dynamical structure and the con vection dynamics between the different cases of planets with varying  masses. First, considering the post-perovskite phase as the lowest pressure form of MgSiO3, the number of phase transitions

  •  increases fom zero in the smallest case to four transitions, with five layers of different mineral asociations, for the larger planet cases 13M. The bottom layer of the latter cases corresponds to the layer of oxides. Furthermore,  we also observe three regimes of convective dynamics, with:

     1) smaller planets (less  than∼ 4M), showing vigorous convection, 

    2) intermediate cases (less∼ 12M), with sluggish penetrative convection, concentred in a single shallow mantle zone of higher flow velocity, and 

    3) large planets, ( bigger than ∼ 12M), showing vigorous convection in two zones near the top and bottom, separated by a high viscosity mid-mantle region mantle with sluggish convection.