Frank Sohl

The interior structure of Mercury: what we know, what we expect from BepiColombo

Abstract The BepiColombo mission is planned to very accurately measure the gravity field, the topography, and the tidal Love numbers of Mercury. In this paper, we review our present knowledge of the interior structure and show how the data from BepiColombo can be used to improve on our knowledge. We show that our present estimates of the core mass and volume depend mostly on our confidence in cosmochemically constrained values of the average silicate shell and core densities. The moment of inertia (MOI) C about the rotation axis will be determined very accurately from the degree 2 components of the gravity field and from measurements of the obliquity and the libration frequency of the rotation axis. The ratio Cm/C between the MOI of the solid planet to the MOI of the planet, both about the rotation axis, will additionally be obtained. If the core is liquid or if there is a liquid outer core, Cm/C will be around 0.5. In this case, Cm can be identified with the MOI of the silicate shell. If the core is solid, Cm/C will be about 1. The MOI C can be used to test and refine present models but will most likely not per se help to increase the confidence in the two-layer model beyond the present level, at least if there is a substantial inner core. C and Cm/C can be used to calculate the inner core radius and the outer core density, assuming the silicate shell density and the inner core density are given by cosmochemistry. The accuracy of the outer core density estimate depends largely on the confidence in the cosmochemical data. The inner core radius can be determined to the accuracy of the densities if the inner core radius is greater than 0.5 core radii. These values can be checked against the Love number of the planet. The higher order components of the gravity field can be used to estimate core–mantle boundary undulations and crust thickness variations. The former will dominate the gravity field at long wavelength, while the latter will dominate at short wavelengths.

Rocky super-Earth interiors - Structure and internal dynamics of CoRoT-7b and Kepler-10b

Aims. We present interior structure models of the recently discovered exoplanets CoRoT-7b and Kepler-10b addressing their bulk compositions, present thermal states, and internal dynamics. We investigate how mantle convection patterns are influenced by the depth-dependence of thermodynamic parameters (e.g., thermal expansivity and conductivity) caused by the extended pressure and temperature ranges within rocky super-Earths. Methods. To model the interior of rocky exoplanets, we construct a four-layer structural model solving the mass and energy balance equations in conjunction with a generalized Rydberg equation of state providing the radial density distribution within each layer. The present thermal state is calculated according to a modified mixing-length approach for highly viscous fluids. Furthermore, the obtained internal structure is used to carry out two-dimensional convection simulations to visualize the mantle convection pattern within massive exoplanets such as CoRoT-7b and Kepler-10b. Results. Both CoRoT-7b and Kepler-10b most likely have large iron cores and a bulk composition similar to that of Mercury. For a planetary radius of Rp = (1.58 ± 0.10) R⊕, a revised total mass of Mp = (7.42 ± 1.21) M⊕, and the existence of a third planet in the CoRoT-7 planetary system, calculations suggest that an iron core of 64 wt-% and a silicate mantle of 36 wt-% is produced owing to the relatively high average compressed density of ρavg = (10.4±1.8) g cm −3 . Kepler-10b’s planetary radius and total mass yield an iron core of 59.5 wt-%, which complements the silicate mantle of 40.5 wt-%. An enhanced radiogenic heating rate owing to CoRoT-7b’s young age (1.2−2.3 Gyr) raises the radial distribution of temperature by only a few hundred Kelvin, but reduces the viscosity by an order of magnitude. The planform of mantle convection is found to be strongly modified for depth-dependent material properties, with hot plumes rising across the whole mantle and cold slabs, which stagnate in the mid-mantle because of the loss of buoyancy. Conclusions. We use a new model approach to determine the detailed interior structures and present thermal states of CoRoT-7b and Kepler-10b. Both planets are found to be enriched in iron. The results imply that modest radiogenic heating does not play a significant role in determining the internal structure of rocky exoplanets. The depth-dependence of thermodynamic properties, however, strongly influences the mantle convection patterns within exoplanets such as CoRoT-7b and Kepler-10b. This may have a significant effect on the thermal evolution and magnetic field generation of close-in super-Earths.

Structural and tidal models of Titan and inferences on cryovolcanism

Titan, Saturns largest satellite, is subject to solid body tides exerted by Saturn on the timescale of its orbital period. The tide-induced internal redistribution of mass results in tidal stress variations, which could play a major role for Titans geologic surface record. We construct models of Titans interior that are consistent with the satellites mean density, polar moment-of-inertia factor, obliquity, and tidal potential Love number k2 as derived from Cassini observations of Titans low-degree gravity field and rotational state. In the presence of a global liquid reservoir, the tidal gravity field is found to be consistent with a subsurface water-ammonia ocean more than 180 km thick and overlain by an outer ice shell of less than 110 km thickness. The model calculations suggest comparatively low ocean ammonia contents of less than 5 wt % and ocean temperatures in excess of 255 K, i.e., higher than previously thought, thereby substantially increasing Titans potential for habitable locations. The calculated diurnal tidal stresses at Titans surface amount to 20 kPa, almost comparable to those expected at Enceladus and Europa. Tidal shear stresses are concentrated in the polar areas, while tensile stresses predominate in the near-equatorial, midlatitude areas of the sub- and anti-Saturnian hemispheres. The characteristic pattern of maximum diurnal tidal stresses is largely compliant with the distribution of active regions such as cryovolcanic candidate areas. The latter could be important for Titans habitability since those may provide possible pathways for liquid water-ammonia outbursts on the surface and the release of methane in the satellites atmosphere.

Nitrate and nitrite variability at the seafloor of an oxygen minimum zone revealed by a novel microfluidic in-situ chemical sensor

Microfluidics, or lab-on-a-chip (LOC) is a promising technology that allows the development of miniaturized chemical sensors. In contrast to the surging interest in biomedical sciences, the utilization of LOC sensors in aquatic sciences is still in infancy but a wider use of such sensors could mitigate the undersampling problem of ocean biogeochemical processes. Here we describe the first underwater test of a novel LOC sensor to obtain in situ calibrated time-series (up to 40 h) of nitrate+nitrite (ΣNOx) and nitrite on the seafloor of the Mauritanian oxygen minimum zone, offshore Western Africa. Initial tests showed that the sensor successfully reproduced water column (160 m) nutrient profiles. Lander deployments at 50, 100 and 170 m depth indicated that the biogeochemical variability was high over the Mauritanian shelf: The 50 m site had the lowest ΣNOx concentration, with 15.2 to 23.4 μM (median=18.3 μM); while at the 100 site ΣNOx varied between 21.0 and 30.1 μM over 40 hours (median = 25.1μM). The 170 m site had the highest median ΣNOx level (25.8 μM) with less variability (22.8 to 27.7 μM). At the 50 m site, nitrite concentration decreased fivefold from 1 to 0.2 μM in just 30 hours accompanied by decreasing oxygen and increasing nitrate concentrations. Taken together with the time series of oxygen, temperature, pressure and current velocities, we propose that the episodic intrusion of deeper waters via cross-shelf transport leads to intrusion of nitrate-rich, but oxygen-poor waters to shallower locations, with consequences for benthic nitrogen cycling. This first validation of an LOC sensor at elevated water depths revealed that when deployed for longer periods and as a part of a sensor network, LOC technology has the potential to contribute to the understanding of the benthic biogeochemical dynamics.

Revealing Titan's interior

Gravity field measurements by the Cassini spacecraft suggest that Titans interior was too cold for the primordial mixture of ice and rock to melt and fully separate. The interior structure and composition of solar system bodies are key to understanding their origin and evolution. Saturns largest icy moon, Titan, and the jovian moons, Ganymede and Callisto, are of similar size, mean density, and primordial ice-rock fraction from which the satellites formed. Titan is distinct due to its dense nitrogen atmosphere, with methane as the next most abundant constituent, which precludes direct observations of the surface. Before the arrival of the Cassini-Huygens spacecraft to study the Saturn system in 2004, little was known about the nature of Titans interior—information as to its origin, evolution, and the rate at which it degasses was limited. On page 1367 of this issue, Iess et al. (1) report evidence based on the analysis of its gravitational field that the interior was much colder than previously thought, and thereby impeded substantial melting and subsequent separation of the primordial ice-rock mixture.

Interior structure models of terrestrial exoplanets and application to CoRoT-7 b

In this study, we model the internal structure of CoRoT-7b as a type example for a terrestrial extrasolar planet using mass and energy balance constraints. Our results suggest that the deep interior is predominantly composed of dry silicate rock, similar to the Earth’s Moon. A central iron core, if present, would be relatively small and less massive (< 15 wt.% of the planet’s total mass) as compared to the Earth’s (core mass fraction 32.6 wt.%). Furthermore, a partly molten near-surface magma ocean could be maintained, provided surface temperatures were sufficiently high and the rock component was mainly composed of Earth-like mineral phase assemblages.

Physical state of the deep interior of the CoRoT-7b exoplanet

The present study takes the CoRoT-7b exoplanet as an analogue for massive terrestrial planets to investigate conditions, under which intrinsic magnetic fields could be sustained in liquid cores. We examine the effect of depth-dependent transport parameters (e.g., activation volume of mantle rock) on a planets thermal structure and the related heat flux across the core mantle boundary. For terrestrial planets more massive than the Earth, our calculations suggest that a substantial part of the lowermost mantle is in a sluggish convective regime, primarily due to pressure effects on viscosity. Hence, we find substantially higher core temperatures than previously reported from parameterized convection models. We also discuss the effect of melting point depression in the presence of impurities (e.g., sulfur) in iron-rich cores and compare corresponding melting relations to the calculated thermal structure. Since impurity effects become less important at the elevated pressure and temperature conditions prevalent in the deep interior of CoRoT-7b, iron-rich cores are likely solid, implying that a self-sustained magnetic field would be absent.

Erde und Mond

Wenn man die Erde vom All aus betrachtet und ihre unverwechselbare blaue Farbung, die dynamischen Wolkenformationen und diese zarte Lufthulle sieht, bekommt man eine vage Ahnung davon, wie einzigartig, verletzlich und wertvoll unser Heimatplanet doch ist. Kein anderer der inneren terrestrischen Planeten ist so reich an Wasser, dass auf der Erde das schier kostbarste im Universum ermoglicht hat – das Leben. Was genau die Erde so einzigartig macht, werden wir in diesem Kapitel beleuchten.

Jenseits der Neptunbahn

Noch viele Jahrzehnte nach seiner Entdeckung im Jahr 1930 galt Pluto mit einer mittleren Entfernung von 39,4 AE zur Sonne als der auserste Planet des Sonnensystems. Heute gilt Pluto nur als das zuerst entdeckte Mitglied einer neuen Klasse von eisigen Objekten, die sich bevorzugt in den auseren Regionen des Sonnensystems aufhalten. Spatestens seit dem Jahr 1992 wissen wir, dass es dort auser Pluto noch viele weitere Himmelskorper mit Durchmessern von 100 bis uber 1000 Kilometern und Umlaufbahnen jenseits der Neptunbahn gibt. Dort sind auch die Kometen angesiedelt. In ihnen ist die alteste und ursprunglichste Materie des Sonnensystems gespeichert. Bis in eine Entfernung von zwei Lichtjahren von der Sonne entfernt werden Billionen von Kometen vermutet. Immer wieder dringen einzelne von ihnen ins innere Sonnensystem vor und bescheren uns mit ihrem Schweif dann spektakulare Naturereignisse.

Uranus und Neptun

Die spektakularen Vorbeifluge der Raumsonde Voyager 2 an Uranus im Januar 1986 und Neptun im August 1989 revolutionierten damalige Vorstellungen von der Beschaffenheit der Planeten des auseren Sonnensystems, denn es zeigte sich, dass deren innere Struktur und chemische Zusammensetzung sich deutlich von der der beiden „Gasriesen“ Jupiter und Saturn unterschied. Jupiter, der ahnlich unserer Sonne vor allem aus Wasserstoff und Helium besteht, hat beispielsweise nur einen verhaltnismasig kleinen Eis- und Gesteinsanteil von etwa vier bis zwolf Prozent seiner Gesamtmasse. Demgegenuber entfallen bei Uranus und Neptun bis zu 85 Prozent der Gesamtmasse allein auf den Kern, der vorwiegend aus Wasser-, Methan- und Ammoniakeis besteht und der aufgrund des hohen Drucks ein flussiges Materialverhalten zeigt. Wasserstoff und Helium sammeln sich eher in den auseren dunnen Atmospharenhullen von Uranus und Neptun an. Nicht zuletzt vor dem Hintergrund der Entdeckung zahlreicher ahnlich groser und massereicher Planeten um andere Sterne in unserer kosmischen Umgebung ist das Interesse an Uranus und Neptun erneut geweckt worden.