Rocky Interior Research Articles

Interior Characterization in Multiplanetary Systems: TRAPPIST-1

Abstract Interior characterization traditionally relies on individual planetary properties, ignoring correlations between different planets of the same system. For multiplanetary systems, planetary data are generally correlated. This is because the differential masses and radii are better constrained than absolute planetary masses and radii. We explore such correlations and data specific to the multiplanetary system of TRAPPIST-1 and study their value for our understanding of planet interiors. Furthermore, we demonstrate that the rocky interior of planets in a multiplanetary system can be preferentially probed by studying the densest planet representing a rocky interior analog. Our methodology includes a Bayesian inference analysis that uses a Markov chain Monte Carlo scheme. Our interior estimates account for the anticipated variability in the compositions and layer thicknesses of core, mantle, water oceans, and ice layers, as well as a gas envelope. Our results show that (1) interior estimates significantly depend on available abundance proxies and (2) the importance of interdependent planetary data for interior characterization is comparable to changes in data precision by 30%. For the interiors of TRAPPIST-1 planets, we find that possible water mass fractions generally range from 0% to 25%. The lack of a clear trend of water budgets with orbital period or planet mass challenges possible formation scenarios. While our estimates change relatively little with data precision, they critically depend on data accuracy. If planetary masses varied within ±24%, interiors would be consistent with uniform (∼7%) or an increasing water mass fractions with orbital period (∼2%–12%).

The Contribution of Dwarf Planets to the Origin of Low-inclination Comets by the Replenishment of Mean Motion Resonances in Debris Disks

Abstract In this work we explore a new dynamical path for the delivery of low-inclination comets. In a configuration formed by an interior giant planet and an exterior massive debris disk, where the mass is accounted for by the 50 largest objects in the disk, the strongest mean motion resonances of the giant, located along the belt, are replenished with new material (test particles) due to the influence of the 50 massive objects. Once in resonance, slow chaotic diffusion stirs the orbital elements of the cometary nuclei enough to encounter the giant and to be scattered by it. When the disk is massive enough, both resonant and non-resonant particles are stirred quickly to encounter the giant and form a scattered disk component, greatly increasing the rate for the delivery of cometary material to the inner part of the system. This mechanism is applicable both to the solar system and extrasolar systems in general. Preliminary results, using a disk as massive as the classical Kuiper Belt, indicate that the mechanism proposed here can account for about a tenth of the required injection rate to maintain the population of ecliptic comets in steady state. In a more massive belt of 0.25 M ⊕, an estimated rate of around 0.6 new comets per year is found. Such a high rate would pose a serious risk for the habitability of rocky interior planets, yet would resemble the late heavy bombardment that was present in the early solar system.

A New Desalination Pump Helps Define the pH of Ocean Worlds

Abstract We study ocean exoplanets, for which the global surface ocean is separated from the rocky interior by a high-pressure ice mantle. We describe a mechanism that can pump salts out of the ocean, resulting in oceans of very low salinity. Here we focus on the H2O–NaCl system, though we discuss the application of this pump to other salts as well. We find our ocean worlds to be acidic, with a pH in the range of 2–4. We discuss and compare between the conditions found within our studied oceans and the conditions in which polyextremophiles were discovered. This work focuses on exoplanets in the super-Earth mass range (∼2 M ⊕), with water composing at least a few percent of their mass. However, the principle of the desalination pump might extend beyond this mass range.

A Universal Break in the Planet-to-star Mass-ratio Function of Kepler MKG Stars

Abstract We follow the microlensing approach and quantify the occurrence of Kepler exoplanets as a function of planet-to-star mass ratio, q, rather than planet radius or mass. For planets with radii ∼1–6 R ⊕ and periods <100 days, we find that, except for a normalization factor, the occurrence rate versus q can be described by the same broken power law with a break at ∼3 × 10−5 independent of host type for hosts below 1 M ⊙. These findings indicate that the planet-to-star mass ratio is a more fundamental quantity in planet formation than planet mass. We then compare our results to those from microlensing for which the overwhelming majority satisfies the M host < 1 M ⊙ criterion. The break in q for the microlensing planet population, which mostly probes the region outside the snowline, is ∼3–10 times higher than that inferred from Kepler. Thus, the most common planet inside the snowline is ∼3–10 times less massive than the one outside. With rocky planets interior to gaseous planets, the solar system broadly follows the combined mass-ratio function inferred from Kepler and microlensing. However, the exoplanet population has a less extreme radial distribution of planetary masses than the solar system. Establishing whether the mass-ratio function beyond the snowline is also host type independent will be crucial to build a comprehensive theory of planet formation.

Detection of the Atmosphere of the 1.6 M⊕ Exoplanet GJ 1132 b

Abstract Detecting the atmospheres of low-mass, low-temperature exoplanets is a high-priority goal on the path to ultimately detecting biosignatures in the atmospheres of habitable exoplanets. High-precision HST observations of several super-Earths with equilibrium temperatures below 1000 K have to date all resulted in featureless transmission spectra, which have been suggested to be due to high-altitude clouds. We report the detection of an atmospheric feature in the atmosphere of a 1.6 transiting exoplanet, GJ 1132 b, with an equilibrium temperature of ∼600 K and orbiting a nearby M dwarf. We present observations of nine transits of the planet obtained simultaneously in the griz and JHK passbands. We find an average radius of 1.43 ± 0.16 for the planet, averaged over all the passbands, and a radius of 0.255 ± 0.023 for the star, both of which are significantly greater than previously found. The planet radius can be decomposed into a “surface radius” at ∼1.375 overlaid by atmospheric features that increase the observed radius in the z and K bands. The z-band radius is 4σ higher than the continuum, suggesting a strong detection of an atmosphere. We deploy a suite of tests to verify the reliability of the transmission spectrum, which are greatly helped by the existence of repeat observations. The large z-band transit depth indicates strong opacity from H2O and/or CH4 or a hitherto-unconsidered opacity. A surface radius of 1.375 ± 0.16 allows for a wide range of interior compositions ranging from a nearly Earth-like rocky interior, with ∼70% silicate and ∼30% Fe, to a substantially H2O-rich water world.

The GAPS programme with HARPS-N at TNG

Binary stars hosting exoplanets are a unique laboratory where chemical tagging can be performed to measure with high accuracy the elemental abundances of both stellar components, with the aim to investigate the formation of planets and their subsequent evolution. Here, we present a high-precision differential abundance analysis of the XO-2 wide stellar binary based on high resolution HARPS-N@TNG spectra. Both components are very similar K-dwarfs and host planets. Since they formed presumably within the same molecular cloud, we expect they should possess the same initial elemental abundances. We investigate if the presence of planets can cause some chemical imprints in the stellar atmospheric abundances. We measure abundances of 25 elements for both stars with a range of condensation temperature $T_{\rm C}=40-1741$ K, achieving typical precisions of $\sim 0.07$ dex. The North component shows abundances in all elements higher by $+0.067 \pm 0.032$ dex on average, with a mean difference of +0.078 dex for elements with $T_{\rm C} > 800$ K. The significance of the XO-2N abundance difference relative to XO-2S is at the $2\sigma$ level for almost all elements. We discuss the possibility that this result could be interpreted as the signature of the ingestion of material by XO-2N or depletion in XO-2S due to locking of heavy elements by the planetary companions. We estimate a mass of several tens of $M_{\oplus}$ in heavy elements. The difference in abundances between XO-2N and XO-2S shows a positive correlation with the condensation temperatures of the elements, with a slope of $(4.7 \pm 0.9) \times 10^{-5}$ dex K$^{-1}$, which could mean that both components have not formed terrestrial planets, but that first experienced the accretion of rocky core interior to the subsequent giant planets.

THORIUM ABUNDANCES IN SOLAR TWINS AND ANALOGS: IMPLICATIONS FOR THE HABITABILITY OF EXTRASOLAR PLANETARY SYSTEMS

We present the first investigation of Th abundances in solar twins and analogues to understand the possible range of this radioactive element and its effect on rocky planet interior dynamics and potential habitability. The abundances of the radioactive elements Th and U are key components of a planet?s energy budget, making up 30%?50% of the Earth?s. Radiogenic heat drives interior mantle convection and surface plate tectonics, which sustains a deep carbon and water cycle and thereby aides in creating Earth?s habitable surface. Unlike other heat sources that are dependent on the planet?s specific formation history, the radiogenic heat budget is directly related to the mantle concentration of these nuclides. As a refractory element, the stellar abundance of Th is faithfully reflected in the terrestrial planet?s concentration. We find that log varies from 59% to 251% that of solar, suggesting extrasolar planetary systems may possess a greater energy budget with which to support surface to interior dynamics and thus increase their likelihood to be habitable compared to our solar system.

TERRESTRIAL PLANET FORMATION IN THE PRESENCE OF MIGRATING SUPER-EARTHS

Super-Earths with orbital periods less than 100 days are extremely abundant around Sun-like stars. It is unlikely that these planets formed at their current locations. Rather, they likely formed at large distances from the star and subsequently migrated inward. Here we use N-body simulations to study the effect of super-Earths on the accretion of rocky planets. In our simulations, one or more super-Earths migrates inward through a disk of planetary embryos and planetesimals embedded in a gaseous disk. We tested a wide range of migration speeds and configurations. Fast-migrating super-Earths ($\tau_{mig} \sim$0.01-0.1 Myr) only have a modest effect on the protoplanetary embryos and planetesimals. Sufficient material survives to form rocky, Earth-like planets on orbits exterior to the super-Earths'. In contrast, slowly migrating super-Earths shepherd rocky material interior to their orbits and strongly deplete the terrestrial planet-forming zone. In this situation any Earth-sized planets in the habitable zone are extremely volatile-rich and are therefore probably not Earth-like.

Formation of ridges on Europa above crystallizing water bodies inside the ice shell

Jupiter’s second Galilean satellite, Europa, is a Moon-sized body with an icy shell and global ocean approximately 100km thick surrounding a rocky interior. Its surface displays extensive tectonic activity in a geologically recent past. Europa’s most ubiquitous surface features, double ridges, have a central trough flanked by two raised edifices. Double ridges can extend hundreds of kilometers and appear genetically related to cracks formed in the Europan ice shell. The origin of the raised flanks has been the center of much debate and many models have been proposed. There are also ridges without a central trough, single ridges. These ridges are far less common than their double ridge counterparts. However, there are locations where along-strike changes in ridge type appear to occur. We explore an elastic model in which the ridges form in response to crystallization of a liquid water intrusion. In our model, liquid water fills tension cracks that open in the Europan crust in response to tidal stress or perhaps overpressure of a subsurface ocean. The crack would be long and essentially continuous, similar to dikes on Earth, explaining the remarkable continuity and lack of segmentation of Europan ridges. The freezing of the water would cause a volume expansion, compressing and buckling the adjacent crust. We find that the geometry of the intruding water body controls the shape of the resulting ridges, with single ridges forming above sill-like intrusions and double ridges above dike-like intrusions. In order to match the ridge heights observed for double ridges we would need approximately 1.5km2 of water intruded at a shallow depth in the ice shell, potentially over the course of multiple events. Deeper intrusions result in a broader, lower amplitude ridge than shallow intrusions.

MASSES, RADII, AND ORBITS OF SMALL KEPLER PLANETS: THE TRANSITION FROM GASEOUS TO ROCKY PLANETS

We report on the masses, sizes, and orbits of the planets orbiting 22 Kepler stars. There are 49 planet candidates around these stars, including 42 detected through transits and 7 revealed by precise Doppler measurements of the host stars. Based on an analysis of the Kepler brightness measurements, along with high-resolution imaging and spectroscopy, Doppler spectroscopy, and (for 11 stars) asteroseismology, we establish low false-positive probabilities for all of the transiting planets (41 of 42 have a false-positive probability under 1%), and we constrain their sizes and masses. Most of the transiting planets are smaller than 3X the size of Earth. For 16 planets, the Doppler signal was securely detected, providing a direct measurement of the planet's mass. For the other 26 planets we provide either marginal mass measurements or upper limits to their masses and densities; in many cases we can rule out a rocky composition. We identify 6 planets with densities above 5 g/cc, suggesting a mostly rocky interior for them. Indeed, the only planets that are compatible with a purely rocky composition are smaller than ~2 R_earth. Larger planets evidently contain a larger fraction of low-density material (H, He, and H2O).

Water on the Moon

The Moon is not wet like Earth, and it doesn’t have frozen icecaps like those on Mars. But it might be just a tiny bit damp, both inside and on its surface.

The Dark Red Spot on KBO Haumea

AbstractKuiper belt object 136108 Haumea is one of the most fascinating bodies in our solar system. Approximately 2000 × 1600 × 1000 km in size, it is one of the largest Kuiper belt objects (KBOs) and an unusually elongated one for its size. The shape of Haumea is the result of rotational deformation due to its extremely short 3.9-hour rotation period. Unlike other 1000 km-scale KBOs which are coated in methane ice the surface of Haumea is covered in almost pure H2O-ice. The bulk density of Haumea, estimated around 2.6 g cm−3, suggests a more rocky interior composition, different from the H2O-ice surface. Recently, Haumea has become the second KBO after Pluto to show observable signs of surface features. A region darker and redder than the average surface of Haumea has been identified, the composition and origin of which remain unknown. I discuss this recent finding and what it may tell us about Haumea.

True polar wander on Europa from global-scale small-circle depressions

The tectonic patterns and stress history of Europa are exceedingly complex and many large-scale features remain unexplained. True polar wander, involving reorientation of Europa's floating outer ice shell about the tidal axis with Jupiter, has been proposed as a possible explanation for some of the features. This mechanism is possible if the icy shell is latitudinally variable in thickness and decoupled from the rocky interior. It would impose high stress levels on the shell, leading to predictable fracture patterns. No satisfactory match to global-scale features has hitherto been found for polar wander stress patterns. Here we describe broad arcuate troughs and depressions on Europa that do not fit other proposed stress mechanisms in their current position. Using imaging from three spacecraft, we have mapped two global-scale organized concentric antipodal sets of arcuate troughs up to hundreds of kilometres long and 300 m to approximately 1.5 km deep. An excellent match to these features is found with stresses caused by an episode of approximately 80 degrees true polar wander. These depressions also appear to be geographically related to other large-scale bright and dark lineaments, suggesting that many of Europa's tectonic patterns may also be related to true polar wander.

Hydrothermal plume dynamics on Europa: Implications for chaos formation

Hydrothermal plumes may be responsible for transmitting radiogenic or tidally generated heat from Europa's rocky interior through a liquid ocean to the base of its ice shell. This process has been implicated in the formation of chaos regions and lenticulae by melting or exciting convection in the ice layer. In contrast to earlier work, we argue that Europa's ocean should be treated as an unstratified fluid. We have adapted and expanded upon existing work describing buoyant plumes in a rotating, unstratified environment. We discuss the scaling laws governing the flow and geometry of plumes on Europa and perform a laboratory experiment to obtain scaling constants and to visualize plume behavior in a Europa‐like parameter regime. We predict that hydrothermal plumes on Europa are of a lateral scale (at least 25–50 km) comparable to large chaos regions; they are too broad to be responsible for the formation of individual lenticulae. Plume heat fluxes (0.1–10 W/m2) are too weak to allow complete melt‐through of the ice layer. Current speeds in the plume (3–8 mm/s) are much slower than indicated by previous studies. The observed movement of ice blocks in the Conamara Chaos region is unlikely to be driven by such weak flow.

Evidence for a weakly stratified Europan ocean sustained by seafloor heat flux

We interpret chaos‐type features on the surface of Europa as melt‐through structures formed by rotationally confined, steady and/or episodic oceanic plumes that rise to the base of the ice shell from magmatically heated regions of the seafloor. Smaller lenticular features in the vicinity of chaos‐type regions might be formed by baroclinically unstable vortices that spin off the main convective plume or by persistent heating from localized hydrothermal venting sites. The ocean is assumed to be weakly stratified because of turbulent convection generated by heating from below and cooling from above. Seafloor heating, maintained by tidal dissipation in the rocky interior, generates an estimated global heat flux of 8.7×1012 W and limits the mean ice thickness to 2–5 km. For seafloor heat sources with radii r that are less than the ocean's deformation radius, rD = ND/|ƒ| (N is the Brunt‐Väisälä frequency, D is the water depth, and ƒ is the Coriolis parameter), the diameters of chaos‐type regions are expected to diminish from O (100 km) within equatorial regions to O (10 km) at high latitudes (assuming spatially uniform water depth and density structure). Where r > rD, the scale of the source region determines the scale of the melt‐through features. Provided there is sufficient time before refreezing, ice rafts in large melt‐through regions are imbedded in episodes of preferentially anticyclonic circulation, corresponding to clockwise (counterclockwise) motions in the northern (southern) hemisphere. We calculate that 1021 J were required to melt the ice in the ∼100 km diameter Conamara Chaos region and that for a steady, localized heat flux F ≈ 1011 W (∼1% of the global heat flux) it took ∼1000 years for the initial melt‐through to occur. Assuming that ice raft displacements in Conamara Chaos occurred during a major melt‐through event, maximum current speeds in the region were O (10 cm s−1), and refreezing occurred within ∼20 hours. A lack of well‐defined ice drift in other major melt‐through regions suggests that these regions formed through episodes of melting and refreezing that modified the existing structure but left little time for the establishment of organized advective motion.

Evidence for a subsurface ocean on Europa.

Ground-based spectroscopy of Jupiter's moon Europa, combined with gravity data, suggests that the satellite has an icy crust roughly 150 km thick and a rocky interior. In addition, images obtained by the Voyager spacecraft revealed that Europa's surface is crossed by numerous intersecting ridges and dark bands (called lineae) and is sparsely cratered, indicating that the terrain is probably significantly younger than that of Ganymede and Callisto. It has been suggested that Europa's thin outer ice shell might be separated from the moon's silicate interior by a liquid water layer, delayed or prevented from freezing by tidal heating; in this model, the lineae could be explained by repetitive tidal deformation of the outer ice shell. However, observational confirmation of a subsurface ocean was largely frustrated by the low resolution (>2 km per pixel) of the Voyager images. Here we present high-resolution (54 m per pixel) Galileo spacecraft images of Europa, in which we find evidence for mobile 'icebergs'. The detailed morphology of the terrain strongly supports the presence of liquid water at shallow depths below the surface, either today or at some time in the past. Moreover, lower-resolution observations of much larger regions suggest that the phenomena reported here are widespread.

Proceedings

Ordinary General Meeting 9 December 1970 at 5.00 p.m. This meeting was devoted to a presentation by staff of The British Petroleum Company Ltd, on Alaskan exploration and the development of the Prudhoe Bay Oilfield. 1. The general geology of northern Alaska By R. Stoneley. Alaska is a country of over half-a-million square miles. It can be divided into three major geological-physiographical provinces: southern Alaska formed of the arcuate Pacific Mountain System (the Aleutian-Alaska and Coastal Ranges, separated by depressions), central or interior Alaska which is the counterpart of the inter-montane plateaux of the western United States, and northern Alaska, the extension of the Rocky Mountain System and Interior Plains. Northern Alaska, on which attention was focussed in the lecture, includes the Brooks Range, the Arctic Foothills and the treeless tundra of the Arctic Coastal Plain or Slope, in all an area of about 125000 square miles. The Brooks Range is a continuous chain of mountains composed of Palaeozoic and early Mesozoic sedimentary rocks which in the western and central part are thrust northwards over the Mesozoic rocks of the foothills. The Arctic foothills are underlain by a very thick succession of Cretaceous flysch and molasse which in the southern part show complex fold and thrust structures comparable with the foothills structures of the Canadian Rockies. In the northern foothills the Cretaceous beds exposed at the surface are folded into a series of long, narrow anticlines and broader synclines which for the most part die out at depth in a