Exoplanet interiors and habitability

More than 1000 exoplanets with a radius smaller than twice that of the Earth are currently known, mainly thanks to space missions dedicated to the search of exoplanets. Mass and radius estimates, which are only available for a fraction (∼ 10%) of the exoplanets, provide an indication of the bulk composition and interior structure and show that the diversity in exoplanets is far greater than in the Solar System. Geophysical studies of the interior of exoplanets are key to understanding their formation and evolution, and are also crucial for assessing their potential habitability since interior processes play an essential role in creating and maintaining conditions for water to exist at the surface or in subsurface layers. For lack of detailed observations, investigations of the interior of exoplanets are guided by the more refined knowledge already acquired about the Solar System planets and moons, and are heavily based on theoretical modelling and on studies of the behaviour of materials under the high pressure and temperature conditions in planets. Here we review the physical principles and methods used in modelling the interior and evolution of exoplanets with a rock or water/ice surface layer and identify possible habitats in or on exoplanets

Testing Lorentz symmetry with planetary orbital dynamics

Planetary ephemerides are a very powerful tool to constrain deviations from the theory of General Relativity using orbital dynamics. The effective field theory framework called the Standard-Model Extension (SME) has been developed in order to systematically parametrize hypothetical violations of Lorentz symmetry (in the Standard Model and in the gravitational sector). In this communication, we use the latest determinations of the supplementary advances of the perihelia and of the nodes obtained by planetary ephemerides analysis to constrain SME coefficients from the pure gravity sector and also from gravity-matter couplings. Our results do not show any deviation from GR and they improve current constraints. Moreover, combinations with existing constraints from Lunar Laser Ranging and from atom interferometry gravimetry allow us to disentangle contributions from the pure gravity sector from the gravity-matter couplings.Comment: 12 pages, 2 figures, version accepted for publication in Phys. Rev.

Lunar Seismology: An Update on Interior Structure Models

An international team of researchers gathered, with the support of the Interna- tional Space Science Institute (ISSI), (1) to review seismological investigations of the lunar interior from the Apollo-era and up until the present and (2) to re-assess our level of knowl- edge and uncertainty on the interior structure of the Moon. A companion paper (Nunn et al. in Space Sci. Rev., submitted) reviews and discusses the Apollo lunar seismic data with the aim of creating a new reference seismic data set for future use by the community. In this study, we first review information pertinent to the interior of the Moon that has become available since the Apollo lunar landings, particularly in the past ten years, from orbiting spacecraft, continuing measurements, modeling studies, and laboratory experiments. Fol- lowing this, we discuss and compare a set of recent published models of the lunar interior, including a detailed review of attenuation and scattering properties of the Moon. Common features and discrepancies between models and moonquake locations provide a first esti- mate of the error bars on the various seismic parameters. Eventually, to assess the influence of model parameterisation and error propagation on inverted seismic velocity models, an inversion test is presented where three different parameterisations are considered. For this purpose, we employ the travel time data set gathered in our companion paper (Nunn et al. in Space Sci. Rev., submitted). The error bars of the inverted seismic velocity models demon- strate that the Apollo lunar seismic data mainly constrain the upper- and mid-mantle struc- ture to a depth of ∼1200 km. While variable, there is some indication for an upper mantle low-velocity zone (depth range 100–250 km), which is compatible with a temperature gradi- ◦ent around 1.7 C/km. This upper mantle thermal gradient could be related to the presence of the thermally anomalous region known as the Procellarum Kreep Terrane, which contains a large amount of heat producing elements

Pre-mission InSights on the Interior of Mars

Abstract The Interior exploration using Seismic Investigations, Geodesy, and Heat Trans- port (InSight) Mission will focus on Mars’ interior structure and evolution. The basic structure of crust, mantle, and core form soon after accretion. Understanding the early differentiation process on Mars and how it relates to bulk composition is key to improving our understanding of this process on rocky bodies in our solar system, as well as in other solar systems. Current knowledge of differentiation derives largely from the layers observed via seismology on the Moon. However, the Moon’s much smaller diameter make it a poor analog with respect to interior pressure and phase changes. In this paper we review the current knowledge of the thickness of the crust, the diameter and state of the core, seismic attenuation, heat flow, and interior composition. InSight will conduct the first seismic and heat flow measurements of Mars, as well as more precise geodesy. These data reduce uncertainty in crustal thickness, core size and state, heat flow, seismic activity and meteorite impact rates by a factor of 3–10× relative to previous estimates. Based on modeling of seismic wave propagation, we can further constrain interior temperature, composition, and the location of phase changes. By combining heat flow and a well constrained value of crustal thickness, we can estimate the distribution of heat producing elements between the crust and mantle. All of these quantities are key inputs to models of interior convection and thermal evolution that predict the processes that control subsurface temperature, rates of volcanism, plume distribution and stability, and convective state. Collectively these factors offer strong controls on the overall evolution of the geology and habitability of Mars

The interior structure of terrestrial planets : an application to Mars

The Earth's interior structure is well known from seismic experiments. From them we precisely know the size and state of the core and the density and rheological properties within the planet. This information, complemented with rock sample analysis, has been used to constrain the thermal state of the Earth and deduce the composition of the mantle and the core. By contrast, we know very little about the interior structure of other terrestrial planets. Our knowledge is limited to the bulk properties - such as the average densities and moment of inertia (except Venus) - and the fact that they all have at least a partially liquid core. Their composition and thermal state are only weakly constrained. With the aim of elucidating the interior structure, we construct detailed interior structure models applicable to terrestrial planets. We then show how knowledge of the model parameters can be inferred from the data provided by space missions, earth-bound measurements, and laboratory experiences from high pressure physics. As a case study, we infer the model parameters for two different data sets related to the planet Mars. The main aim of the first study is to constrain the state, size, and composition of the core by using existing geodesy data. In this study we do not infer the thermal state and mineralogy of the mantle but use results that have been obtained from independent studies relating to the thermal evolution and composition of Mars. The objective of the second study is to ascertain whether the mineralogy and the thermal state of the Martian mantle can be inferred by combining geodesy, seismic, and magnetic induction data. The first study shows that Mars has no solid inner core and that the liquid core contains a large fraction of sulfur. The absence of a solid inner is in agreement with the absence of a global magnetic field. We estimate the radius of the core to be 1616 +/-137 km and its sulfur concentration to be 12+/-6wt%. We also show that it is possible for Mars to have a thin layer of perovskite at the bottom of the mantle. The second study shows that the temperature of the mantle and its mineralogical composition can be determined with a high confidence. The robustness of our method is also tested by using incomplete and missing data sets.(PHYS 3) -- UCL, 201

Exoplanet interiors and habitability

status: publishe

Volcanism and outgassing of stagnant-lid planets: Implications for the habitable zone

© 2017 Elsevier B.V. Rocky exoplanets are typically classified as potentially habitable planets, if liquid water exists at the surface. The latter depends on several factors like the abundance of water but also on the amount of available solar energy and greenhouse gases in the atmosphere for a sufficiently long time for life to evolve. The range of distances to the star, where surface water might exist, is called the habitable zone. Here we study the effect of the planet interior of stagnant-lid planets on the formation of a secondary atmosphere through outgassing that would be needed to preserve surface water. We find that volcanic activity and associated outgassing in one-plate planets is strongly reduced after the magma ocean outgassing phase for Earth-like mantle compositions, if their mass and/or core-mass fraction exceeds a critical value. As a consequence, the effective outer boundary of the habitable zone is then closer to the host star than suggested by the classical habitable zone definition, setting an important restriction to the possible surface habitability of massive rocky exoplanets, assuming that they did not keep a substantial amount of their primary atmosphere and that they are not in the plate tectonics regime.status: publishe

Libration and tides of synchronously rotating icy satellites

International audienceAs is well known for the Moon, the gravitational forcing from a central planet not only induces tides in synchronously rotating satellites but also small periodic variations in the rotation rate, or forced longitudinal librations. We calculate the effect of the tidal deformations on the forced longitudinal librations of icy satellites. For entirely solid satellites, we show that the forced librations are well approximated by the rotation variations for a rigid body. When the satellite has a global internal subsurface ocean, different solid layers, separated by the liquid layer, can rotate differently, leading to additional torques between layers. Moreover, the layers also respond differently to tidal forcing. As a result, the forced longitudinal librations of the surface will be affected by the liquid layer and the global internal structure of the satellite. Results are presented for the Galilean satellites and Titan, satellites for which accurate rotation data is already available thanks to the Cassini mission or will be measured in the future by missions to the Jupiter system, under study by ESA and NASA. Observations of rotation data have the potential of yielding information on the interior structure of icy satellites and may reveal the existence of putative subsurface oceans