We used a thermal model of an iron core to calculate magnetodynamo evolution in Earth-mass rocky planets to determine the sensitivity of dynamo lifetime and intensity to planets with different mantle tectonic regimes, surface temperatures, and core properties. The heat flow at the core-mantle boundary (CMB) is derived from numerical models of mantle convection with a viscous/pseudo-plastic rheology that captures the phenomenology of plate-like tectonics. Our thermal evolution models predict a long-lived (~8 Gyr) field for Earth and similar dynamo evolution for Earth-mass exoplanets with plate tectonics. Both elevated surface temperature and pressure-dependent mantle viscosity reduce the CMB heat flow but produce only slightly longer-lived dynamos (~8-9.5 Gyr). Single-plate ("stagnant lid") planets with relatively low CMB heat flow produce long-lived (~10.5 Gyr) dynamos. These weaker dynamos can cease for several billions of years and subsequently reactivate due to the additional entropy production associated with inner core growth, a possible explanation for the absence of a magnetic field on present-day Venus. We also show that dynamo operation is sensitive to the initial temperature, size, and solidus of a planet's core. These dependencies would severely challenge any attempt to distinguish exoplanets with plate tectonics and stagnant lids based on the presence or absence of a magnetic field.
Nominally anhydrous minerals in rocky planet mantles can sequester oceans of water as a whole, giving a constraint on bulk water inventories. Here we predict mantle water capacities from the thermodynamically-limited solubility of water in their constituent minerals. We report the variability of mantle water capacity due to (i) host star refractory element abundances that set mineralogy, (ii) realistic mantle temperature scenarios, and (iii) planet mass. We find that planets large enough to stabilise perovskite almost unfailingly have a dry lower mantle, topped by a high-water-capacity transit…
A sufficient amount of water is required at the surface to develop water oceans. A significant fraction of water, however, remains in the mantle during magma ocean solidification, and thus the existence of water oceans is not guaranteed even for exoplanets located in the habitable zone. To discuss the likelihood of ocean formation, we built two models to predict the rate of mantle degassing during the magma ocean stage and the subsequent solid-state convection stage. We find that planets with low H$_2$O/CO$_2$ ratios would not have a sufficient amount of surface water to develop water oceans i…
Volatile molecules containing hydrogen, carbon, and nitrogen are key components of planetary atmospheres. In the pebble accretion model for rocky planet formation, these volatile species are accreted during the main planetary formation phase. For this study, we modelled the partitioning of volatiles within a growing planet and the outgassing to the surface. The core stores more than 90\% of the hydrogen and carbon budgets of Earth for realistic values of the partition coefficients of H and C between metal and silicate melts. The magma oceans of Earth and Venus are sufficiently deep to undergo …
Stellar Abundances as Probes of Rocky Exoplanet Interiors: The Mantle Composition and Mineralogy of GJ 486b
National Institute of Science Education and Research（国家科学教育研究中心）；Homi Bhabha National Institute（霍米·巴巴国立研究所）；Indian Institute of Science Education and Research (IISER) Mohali（印度科学教育与研究学院（旁遮普邦））；Indian Institute of Science Education and Research, Kolkata（印度科学教育与研究学院（加尔各答））；Indian Institute of Technology, Kharagpur（印度理工学院卡拉格普尔分校）
Water cycling between Earth's mantle and surface has previously been modelled and extrapolated to rocky exoplanets, but these studies neglected the host star. M-dwarf stars are more common than Sun-like stars and at least as likely to host temperate rocky planets (M-Earths). However, M dwarfs are active throughout their lifetimes; specifically, X-ray and extreme ultraviolet (XUV) radiation during their early evolution can cause rapid atmospheric loss on orbiting planets. The increasing bolometric flux reaching M-Earths leads to warmer, moister upper atmospheres, while XUV radiation can photodi…
We present a thermal evolution model coupled with a Henyey solver to study the circumstances under which a rocky planet could potentially host a dynamo in its liquid iron core and/or magma ocean. We calculate the evolution of planet thermal profiles by solving the energy balance equations for both the mantle and the core. We use a modified mixing length theory to model the convective heat flow in both the magma ocean and solid mantle. In addition, by including the Henyey solver, we self-consistently account for adjustments in the interior structure and heating (cooling) due to planet contracti…
The ideal exoplanets to search for life are those within a star's habitable zone. However, even within the habitable zone planets can still develop uninhabitable climate states. Sustaining a temperate climate over geologic ($\sim$Gyr) timescales requires a planet contain sufficient internal energy to power a planetary-scale carbon cycle. A major component of a rocky planet's energy budget is the heat produced by the decay of radioactive elements, especially $^{40}$K, $^{232}$Th, $^{235}$U and $^{238}$U. As the planet ages and these elements decay, this radiogenic energy source dwindles. Here w…
We present a series of papers dedicated to modelling the accretion and differentiation of rocky planets that form by pebble accretion within the lifetime of the protoplanetary disc. In this first paper, we focus on how the accreted ice determines the distribution of iron between the mantle (oxidized FeO and FeO$_{1.5}$) and the core (metallic Fe and FeS). We find that an initial primitive composition of ice-rich material leads, upon heating by the decay of $^{26}$Al, to extensive water flow and the formation of clay minerals inside planetesimals. Metallic iron dissolves in liquid water and pre…
We explore the heating and differentiation of rocky planets that grow by rapid pebble accretion. Our terrestrial planets grow outside of the ice line and initially accrete 28\% water ice by mass. The accretion of water stops after the protoplanet reaches a mass of $0.01\,M_{\rm E}$ where the gas envelope becomes hot enough to sublimate the ice and transport the vapour back to the protoplanetary disc by recycling flows. The energy released by the decay of $^{26}$Al melts the accreted ice to form clay (phyllosilicates), oxidized iron (FeO), and a water surface layer with ten times the mass of Ea…
The bulk density of a planet, as measured by mass and radius, is a result of planet structure and composition. Relative proportions of iron core, rocky mantle, and gaseous envelopes are degenerate for a given density. This degeneracy is reduced for rocky planets without significant gaseous envelopes when the structure is assumed to be a differentiated iron core and rocky mantle, in which the core mass fraction (CMF) is a first-order description of a planet's bulk composition. A rocky planet's CMF may be derived both from bulk density and by assuming the planet reflects the host star's major ro…
The geology of Earth and super-Earth sized planets will, in many cases, only be observable via their atmospheres. Here, we investigate secondary volcanic atmospheres as a key base case of how atmospheres may reflect planetary geochemistry. We couple volcanic outgassing with atmospheric chemistry models to simulate the growth of C-O-H-S-N atmospheres in thermochemical equilibrium, focusing on what information about a planet's mantle fO$_2$ and bulk silicate H/C ratio could be determined by atmospheric observation. 800K volcanic atmospheres develop distinct compositional groups as the mantle fO$…
The Dense, Rocky Core of Haumea Revealed by Satellite Dynamics
Florida Space Institute, University of Central Florida（佛罗里达空间研究所，中佛罗里达大学）；Brigham Young University Department of Physics & Astronomy（杨百翰大学物理与天文学系）；Northern Arizona University（北亚利桑那大学）；Space Telescope Science Institute（太空望远镜科学研究所）
We report the detection of an atmosphere on a rocky exoplanet, GJ 1132 b, which is similar to Earth in terms of size and density. The atmospheric transmission spectrum was detected using Hubble WFC3 measurements and shows spectral signatures of aerosol scattering, HCN, and CH$_{4}$ in a low mean molecular weight atmosphere. We model the atmospheric loss process and conclude that GJ 1132 b likely lost the original H/He envelope, suggesting that the atmosphere that we detect has been reestablished. We explore the possibility of H$_{2}$ mantle degassing, previously identified as a possibility for…
Geophysical and atmospheric implications of $f$O$_{2}$-dependent melting on rocky exoplanets
Kapteyn Astronomical Institute, University of Groningen（格罗宁根大学卡普坦天文研究所）；Netherlands eScience Center（荷兰e科学中心）；Interuniversity Micro Electronics Centre（跨大学微电子中心）；Institute of Geochemistry and Petrology, Department of Earth and Planetary Sciences, ETH Zürich（苏黎世联邦理工学院地球与行星科学系地球化学与岩石学研究所）；Institute of Astronomy, University of Cambridge（剑桥大学天文学研究所）
The thermal evolution of rocky planets on geological timescales (Gyr) depends on the heat input from the long-lived radiogenic elements potassium, thorium, and uranium. Concentrations of the latter two in rocky planet mantles are likely to vary by up to an order of magnitude between different planetary systems because Th and U, like other heavy r-process elements, are produced by rare stellar processes. Here we discuss the effects of these variations on the thermal evolution of an Earth-size planet, using a 1D parameterized convection model. Assuming Th and U abundances consistent with geochem…
Life on Earth emerged at the interface of the geosphere, hydrosphere and atmosphere. This setting serves as our basis for how biological systems originate on rocky planets. Often overlooked, however, is the fact that the chemical nature of a rocky planet is ultimately a product of galactic chemical evolution. Elemental abundances of the major rock-forming elements can be different for different stars and planets formed at different times in galactic history. These differences mean that we cannot expect small rocky exoplanets to be just like Earth. Furthermore, age of the system dictates starti…
Rocky planets both in and outside of our solar system are observed to have a range of core-mass fractions (CMFs). Imperfect collisions can preferentially strip mantle material from a planet, changing its CMF, and are therefore thought to be the most likely cause of this observed CMF variation. However, previous work that implements these collisions into N-body simulations of planet formation has struggled to reliably form high CMF super-Earths. In this work, we specify our initial conditions and simulation parameters to maximize the prevalence of high-energy, CMF-changing collisions in order t…
Star compositions are essential for examining densities and compositional ranges of rocky exoplanets, testing their similarity to Earth. Stellar elemental abundances and planetary orbital data show that of the ~5000 known minerals, exoplanetary silicate mantles will contain mostly olivine, orthopyroxene, and clinopyroxene, $\pm$ quartz, and magnesiuwustite at the extremes; wholly exotic mineralogies are likely absent. Understanding these exotic geological systems requires a better marriage of geological insights to astronomical data. The study of exoplanets is like a mirror, reflecting our inc…
The melt productivity of a differentiated planet's mantle is primarily controlled by its iron content, which is itself approximated by the planet's core mass fraction (CMF). Here we show that estimates of an exo-planet's CMF allows robust predictions of the thickness, composition and mineralogy of the derivative crust. These predicted crustal compositions allow constraints to be placed on volatile cycling between surface and the deep planetary interior, with implications for the evolution of habitable planetary surfaces. Planets with large, terrestrial-like, CMFs ($\geq$0.32) will exhibit thin…
Most Rocky Sub-Neptunes are Molten: Mapping the Solidification Shoreline for Gas Dwarf Exoplanets
Institute of Astronomy, University of Cambridge（剑桥大学天文研究所）；Department of Earth Sciences, University of Cambridge（剑桥大学地球科学系）；Atmospheric, Oceanic, and Planetary Physics, Department of Physics, University of Oxford（牛津大学物理学院大气、海洋与行星物理组）；Kapteyn Astronomical Institute, University of Groningen（格罗宁根大学卡普坦天文研究所）；Department of Earth and Planetary Sciences, ETH Zurich（苏黎世联邦理工学院地球与行星科学系）
