321.01 — Modeling the evolution of ice-rich planets
Dina Prialnik1; Attay Kovetz2
1 Department of Geosciences, Tel Aviv University (Tel Aviv, Israel) 2 School of Physics, Tel Aviv University (Tel Aviv, Israel)
Ice-rich bodies in the Solar System and beyond are of great interest to the pursuit of extraterrestrial life and present a challenge to our understanding of the formation and evolution of planetary systems. If
formed at the outskirts of the planetary disk, they must have grown slowly, by accretion of planetes- imals, without extended atmospheres. Among the hundred and more rocky exoplanets recently discov- ered in planetary systems, a fraction must be of sim- ilar ice-rich composition and structure. By means of a numerical code, we follow continuously the long- term growth of a planet by accretion of ice-rich mate- rial, as well as its subsequent evolution up to present time. The assumed composition includes silicate rock and ice in a compressed or porous structure, de- pending on local pressure. We use a second-order Birch-Murnaghan equation of state, with an adap- tive coefficient, depending on the total mass of the planet, and include radiogenic heating, latent heat and gravitational potential energy in the energy bal- ance equation. We allow for flow of water through the pores.
Two cases are discussed, for two different accre- tion rates. The typical structure that develops dur- ing the long-term evolution is an ice-depleted core, a liquid-filled middle layer (where saturation is al- most unity and temperatures are in the liquid water range), and a top ice-rich cold layer. The outer layer is porous, with porosity increasing from close to zero at the lower boundary, to almost 0.6 near the surface. During the accretion phase, the surface temperature rises well above the local equilibrium temperature, allowing the planet to cool, but also allowing subli- mation of some of the accreted ice. Hence, in the final model, the ice/rock ratio varies widely throughout the planet, and is different from the ratio assumed for the accreted material. In conclusion, we show that the emerging structure of such a planet, grown from a small embryo, should be differentiated at the end of a 1-2 Gyr accretion phase, with an ice-depleted rocky core, an intermediate region filled with liquid water and an outer ice-rich layer.
321.02 — Hot and Steamy, Cold and Icy, or Temper- ate and Habitable: Modeling the Early Evolution of Water World Exoplanets
Nadejda Marounina1; Leslie A. Rogers1
1 Department of Astronomy and Astrophysics, University of Chicago (Chicago, Illinois, United States)
Waterworlds are water-rich (>1% water by mass) ex- oplanets that never attained masses sufficient to ac- crete or retain large amounts of H/He nebular gas. Waterworlds are especially timely given the discov- ery and characterization of the TRAPPIST-1 plane- tary system, which hosts several planets that may contain several percents to several tens of percent of water by mass.
To date, studies of the interior structure of water- world exoplanets have either assumed that the plan- ets are cold and icy (with interiors structures similar to scaled-up versions of Jupiter’s moon Ganymede) or that the planets are hot and steamy (with most of their water in extended envelopes of supercritical steam). Models have not yet demonstrated the evo- lution of waterworlds from an initial post-accretion hot and steamy state to habitable and temperate con- ditions (with surface or subsurface water oceans).
We have performed the most detailed calculations to date of the post-accretional thermal evolution of waterworlds with pure water envelopes. We ac- count for the condensation of liquid water and high- pressure ices as the planets cool, along with the tem- perature dependence of water opacities in the in- frared and visible. In this presentation, we will de- lineate the regions of the parameter space (orbital separation, planet mass, radius, and water envelope mass) wherein waterworlds are likely to be hot and steamy, cold and icy, and temperate and (potentially) habitable.
Our results have important implications for both the habitability of waterworlds and its observable characteristics — i.e. the apparent transit radius, mean planet density, and atmospheric spectra.
321.03 — How magmatic degassing of C, O, and H affects Earth’s early atmosphere
Frank Sohl1; Gianluigi Ortenzi1,2; Lena Noack2; Claire
Guimond2; Julia Schmidt2; Sara Vulpius2
1 Institute of Planetary Research, German Aerospace Center (DLR) (Berlin, Germany)
2 Department of Earth Sciences, Freie Universität (Berlin, Ger- many)
The build-up of the Earth’s early atmosphere is a key point to investigate the planet’s evolution and poten- tial habitability. In this research we investigate the volcanic outgassing of C, O, and H and the related development of the atmosphere using the equilib- rium and mass balance method for volatile specia- tion. We estimate the outgassing process soon af- ter the magma ocean crystallization, simulating both magma production and lithostatic pressure effect on volatile solubility. Considering the volume of melt produced, we calculate the composition and pres- sure variation in the accumulated outgassed atmo- sphere during the early Earth’s evolution. Our re- sults indicate that the outgassed chemical compo- sition is mainly affected by the oxidation state of the mantle and by the pressure dependence on the volatile solubility. The early Earth history was char- acterized by core-mantle segregation. During this
process, the oxidation state of the mantle evolved from an initially reduced state to the more oxidiz- ing present-day value. The different melt redox states would have affected the evolution and chemi- cal composition of the early Earth atmosphere. The volatile species vary from H2 and CO in reducing
state to H2O and CO2in oxidizing conditions similar
to present days. We find that both chemical and pres- sure variations of the atmosphere are directly linked to the evolution of the mantle. The abundances of the outgassed species are also affected by the different solubility of the volatiles. Considering the pressure increase of magmatic reservoirs due to formation of the overlying crust, the outgassed volatile species change according to their different solubility, influ- encing the degassing process and thus atmosphere composition. Furthermore, planetary mass and ra- dius may affect melt production and atmospheric thickness. The coupling of the volatile speciation to the melt production suggests that this technique is useful to describe the early Earth evolution but it has also the potential to investigate the habitability of planets other than the Earth including rocky exo- planets.
321.04 — The Limits on Interior Pressures and Tem- peratures of Likely Super Earths
Wendy Panero1; Cayman Unterborn2
1 School of Earth Sciences, Ohio State University (Columbus, Ohio, United States)
2 Arizona State University (Tempe, Arizona, United States) The interior composition of structure, composition, and dynamics of exoplanetary Super Earths are not observable. Recently described observational trends suggest that rocky exoplanets, that is, planets with- out significant volatile envelopes, are likely limited to ≤1.5 Earth radii. This likely upper limit in the radii of purely-rocky Super-Earth exoplanets, the maximum expected core-mantle boundary pressure and adiabatic temperature is relatively moderate, 630 GPa and 5000 K, while the maximum central core pressure varies between 1.5 and 2.5 TPa. We fur- ther find that for planets with radii less than 1.5 Earth radii, core-mantle boundary pressure and adi- abatic temperature are mostly a function of planet radius and insensitive to planet structure. The pres- sures and temperatures of rocky exoplanet interiors, then, are less than those explored in recent shock- compression experiments, ab-initio calculations, and planetary dynamical studies. We further show that the extrapolation of relevant equations of state does not introduce significant uncertainties in the struc- tural models of these planets. Mass-radius models
are more sensitive to bulk composition than any un- certainty in the equation of state, even when extrap- olated to TPa pressures.
321.05 — On the Concept of Multi-Stable Tectonic States: The Un/Inevitability of Plate Tectonics
Matthew Weller1; June Wicks2
1 Earth, Environmental and Planetary Sciences, Brown University (Providence, Rhode Island, United States)
2 Earth & Planetary Sciences, Johns Hopkins University (Balti- more, Maryland, United States)
With the plethora of recently discovered exoplanets, it is a natural question to consider how many of these bodies could operate within a plate tectonic regime, and host life, as we observe for the Earth. However, our understanding of Earth’s evolution is far from complete. Geologic evidence suggests that plate tec- tonics may not have operated on the early Earth, with both the timing of onset and the length of activity far from certain. Uncertainty about the initiation of plate tectonics, and the initial tectonic state for Earth has been extended to extra-solar planets. It is an open question of whether terrestrial planets larger and more massive than Earth are more or less likely to have plate tectonics, with groups arguing that a single plate regime should be favoured, while oth- ers argue that plate tectonics should dominate. Re- cently, tectonic bi-stability (multiple stable, energet- ically allowed solutions) has been shown to be dy- namically viable, both from analytical analysis and through numeric experiments in two and three di- mensions. From scaling analysis, high-temperature planets with a large contribution from internal heat- ing (radiogenics or tidal sources) will operate in dif- ferent velocity-stress scaling regimes compared to cooler-temperature planets that may have a larger relative contribution from core heating. Thus, differ- ences in predictions for plate tectonics on exoplan- ets may in part result from different model assump- tions being more appropriate to different times in the evolution of a terrestrial-type planet. This indicates that multiple tectonic modes may operate on a single planetary body at different times within its temporal evolution. It can then be shown that identical planets at similar stages of their evolution may exhibit differ- ent tectonic regimes due to random fluctuations. We will discuss a new framework of planetary evolution that is based on general physical principals, as op- posed to particular rheologies, that further incorpo- rates the potential of tectonic regime transitions and multiple tectonic-state viability at equivalent physi- cal and chemical conditions.
321.06 — New Giant Planet Physics from Statistical Population Models
Daniel Thorngren1,4; Peter Gao3; Jonathan Fortney2 1 Physics, University of California, Santa Cruz (Santa Cruz, Cali- fornia, United States)
2 Astronomy and Astrophysics, University of California, Santa Cruz (Santa Cruz, California, United States)
3 Astronomy, University of California, Berkeley (Berkeley, Califor- nia, United States)
4 Physics, Université de Montréal (Montréal, Quebec, Canada) The dramatic rise in the number of well- characterized giant exoplanets has enabled a powerful approach to understanding their physics: combining detailed physical models with statistical population models to infer their physical proper- ties. Here I will present two new results from this approach. The first is that hot Jupiters quickly re- inflate in response to their parent stars brightening on the main sequence. This reinflation can only occur this rapidly if the inflationary mechanism is depositing heat below the radiative-convective boundary (RCB) of the planet’s atmosphere. Thus delayed cooling mechanisms are ruled out, as is our present understanding of Ohmic dissipation, which relies heavily on the insulating effects of shallowly deposited heat. The second key result is that in order to explain their radii, the interior entropies of hot Jupiters must be so high that the RCB will be pushed up to much lower pressures than previously appreciated. A typical hot Jupiter will have its RCB at around ten bars, rather than the hundreds of bars to 1 kbar usually considered. This has important implications for atmosphere modelling, including 3D general circulation models, 1D models used for analyzing spectroscopic data, and cloud models that rely heavily on an understanding of cold traps and vertical mixing.