Periodic Reporting for period 1 - MERCURYREFINEMENT (Refining our view on planet Mercury, in anticipation for the BepiColombo mission.)
Période du rapport: 2020-07-01 au 2022-06-30
This fellowship is devoted to refining knowledge of planet Mercury, and prepare scientific model development and laboratory data for interpretation of upcoming measurements by ESA’s BepiColombo space mission (launched in 2018, entering Mercury-orbit in 2025). The objective is to improve constraints on Mercury’s evolution and interior structure from available, new, and upcoming data, and contribute to the understanding of planet formation and magnetic field generation in terrestrial planetary cores.
An interdisciplinary approach is taken that involves improving methods for studying the generation of Mercury’s magnetic field, improving constraints on Mercury’s interior from geophysical and new experimental data, and improving Mercury’s surface chronology which defines constraints on Mercury’s evolution.
A detailed thermal evolution model is developed, currently the best available tool for studying Mercury’s thermal evolution and long-term dynamo forcing. New experimental high-pressure data on metallic alloys have been obtained and integrated in interior structure models of planet Mercury, which examine the potential of geodetic measurements by BepiColombo to constrain the composition of Mercury’s core and mantle. An improved crater production function for estimating the formation age of Mercury’s surface is developed, but requires additional development.
An interdisciplinary approach is taken that involves improving methods for studying the generation of Mercury’s magnetic field, improving constraints on Mercury’s interior from geophysical and new experimental data, and improving Mercury’s surface chronology which defines constraints on Mercury’s evolution.
A detailed thermal evolution model is developed, currently the best available tool for studying Mercury’s thermal evolution and long-term dynamo forcing. New experimental high-pressure data on metallic alloys have been obtained and integrated in interior structure models of planet Mercury, which examine the potential of geodetic measurements by BepiColombo to constrain the composition of Mercury’s core and mantle. An improved crater production function for estimating the formation age of Mercury’s surface is developed, but requires additional development.
The new developed thermal evolution model solves inconsistencies in former thermal evolution models that are specifically problematic for application to Mercury and the study of dynamo action in its core. Most thermal evolution models for terrestrial planets assume an adiabatic profile for the entire planetary core either because the core of the object of interest is thought to be convective or the core is small, such that simplifying the temperature profile as an adiabat is justified. Implementation of an adiabatic profile in Mercury’s core is not justified, because both a stratified upper part of the liquid core and the inner core are thought to be conductive and the core constitutes ~70wt% of the planet. Additionally, available parameterized planetary thermal evolution models are applicable to a convective mantle. Convection in Mercury’s thin mantle, however, is difficult to persist over the planet’s entire evolution.
We developed a new thermal evolution model that incorporates conductive transport of heat though the inner core and though the stratified liquid layer in the outer core, and is coupled to a new parametrized evolution of the mantle that converges smoothly from convective to conductive state if convection halts (figure 1). This thermal evolution model is equipped with a formulation of the available entropy for dynamo generation in the convective part of Mercury’s core.
Preliminary results have been published in a peer reviewed scientific article (Knibbe and Van Hoolst, 2021), and are presented at the Europlanet Science Congress in 2021. A scientific article that presents the global thermal evolution model is in preparation.
Experimental data on liquid iron-rich metallic alloys are important for interior structure modeling of Mercury, consisting for ~70% of (largely liquid) metal. In this project, newly acquired experimental data of liquid Fe-Si-C metal has been integrated in interior structure models of Mercury. These models are constrained by the planet’s mass, the moment of inertia (MOI) of Mercury’s mantle, and the MOI of the total planet, derived from Mercury’s gravity field, the physical librations of Mercury’s surface, and the obliquity of its spin-axis. These models provided clarity on the potential of geodetic measurements on Mercury by BepiColombo to constrain the composition of Mercury’s core and mantle.
Additional synchrotron experiments are performed to constrain equation of state parameters of Fe-S-Si liquid metals at pressures up to about 20 GPa, in beamtime approved by the Spring-8 synchrotron facility in Sayo, Japan. The first experimental session is performed in May 2022. Follow up beamtime has been allocated at Spring-8 in January 2023.
Melting experiments have been conducted on Fe-S-Si-C metals at 4.2 and 7.5 GPa at the Vrije Universiteit in Amsterdam.
Newly acquired experimental data on the equation of state of the liquid Fe-Si-C system with new interior structure models have been published in a peer-reviewed scientific article (Knibbe et al., 2021), and are presented at the AGU fall meeting (2021). Publication of recently finished melting experiments and the ongoing experimental project at the Spring-8 synchrotron facility is foreseen in the coming years.
Mercury’s surface records geological events that constrain Mercury’s interior evolution. For example, volcanic plains record the end of large-scale volcanism, an important constraint for Mercury’s mantle evolution, and discovered magnetized crust record at time when Mercury hosted an active dynamo, an important constraint for Mercury’s core evolution.
Surface emplacement ages are for Mercury estimated by relating statistics of surface craters to the influx of asteroid impacts. The production rate of craters is governed by a crater production function (CPF). CPFs for Mercury are calibrated to cratered and lunar surface terrains that are radiometrically dated using rock samples collected by NASA’s Apollo missions.
A population model of asteroids in the inner solar system, which partitions asteroids in bins of asteroid size and orbital parameters, is an essential ingredient for calibrating a CPF for Mercury. CPFs for Mercury are based on asteroid population models from more than a decade ago, while the knowledge of asteroids has strongly improved over the last two decades.
In this fellowship, a contemporary CPF is constructed using a contemporary asteroid population model, and is compared with a reference CPF developed using an older asteroid population model. A classical mathematical framework (Wetherill, 1965) is used to compute Asteroid-Moon and Asteroid-Mercury collision probabilities, which neglects orbital precession of eccentricity and inclination for the asteroids and target object. This classical method seemed a natural approach to take, since it has been standard practice in the development for all other CPFs for Mercury.
Results show that the impact rate on Mercury of the contemporary CPF is lower than that of the reference CPF, such that older surface emplacement ages are obtained with the new CPF. However, neglecting precession of eccentricity and inclination of asteroids and target object appears to be unreasonable, because many asteroids that collied with Mercury significantly preces in eccentricity and inclination. Therefore, this work needs to be revisited with a method for computing asteroid-target collision rates that accounts for the precession eccentricity and inclination of the asteroid and target object, for example the method described in Pokorny and Vokrouhlicky (2013).
Dissemination of this work needs to await recalculations with an improved mathematical framework.
We developed a new thermal evolution model that incorporates conductive transport of heat though the inner core and though the stratified liquid layer in the outer core, and is coupled to a new parametrized evolution of the mantle that converges smoothly from convective to conductive state if convection halts (figure 1). This thermal evolution model is equipped with a formulation of the available entropy for dynamo generation in the convective part of Mercury’s core.
Preliminary results have been published in a peer reviewed scientific article (Knibbe and Van Hoolst, 2021), and are presented at the Europlanet Science Congress in 2021. A scientific article that presents the global thermal evolution model is in preparation.
Experimental data on liquid iron-rich metallic alloys are important for interior structure modeling of Mercury, consisting for ~70% of (largely liquid) metal. In this project, newly acquired experimental data of liquid Fe-Si-C metal has been integrated in interior structure models of Mercury. These models are constrained by the planet’s mass, the moment of inertia (MOI) of Mercury’s mantle, and the MOI of the total planet, derived from Mercury’s gravity field, the physical librations of Mercury’s surface, and the obliquity of its spin-axis. These models provided clarity on the potential of geodetic measurements on Mercury by BepiColombo to constrain the composition of Mercury’s core and mantle.
Additional synchrotron experiments are performed to constrain equation of state parameters of Fe-S-Si liquid metals at pressures up to about 20 GPa, in beamtime approved by the Spring-8 synchrotron facility in Sayo, Japan. The first experimental session is performed in May 2022. Follow up beamtime has been allocated at Spring-8 in January 2023.
Melting experiments have been conducted on Fe-S-Si-C metals at 4.2 and 7.5 GPa at the Vrije Universiteit in Amsterdam.
Newly acquired experimental data on the equation of state of the liquid Fe-Si-C system with new interior structure models have been published in a peer-reviewed scientific article (Knibbe et al., 2021), and are presented at the AGU fall meeting (2021). Publication of recently finished melting experiments and the ongoing experimental project at the Spring-8 synchrotron facility is foreseen in the coming years.
Mercury’s surface records geological events that constrain Mercury’s interior evolution. For example, volcanic plains record the end of large-scale volcanism, an important constraint for Mercury’s mantle evolution, and discovered magnetized crust record at time when Mercury hosted an active dynamo, an important constraint for Mercury’s core evolution.
Surface emplacement ages are for Mercury estimated by relating statistics of surface craters to the influx of asteroid impacts. The production rate of craters is governed by a crater production function (CPF). CPFs for Mercury are calibrated to cratered and lunar surface terrains that are radiometrically dated using rock samples collected by NASA’s Apollo missions.
A population model of asteroids in the inner solar system, which partitions asteroids in bins of asteroid size and orbital parameters, is an essential ingredient for calibrating a CPF for Mercury. CPFs for Mercury are based on asteroid population models from more than a decade ago, while the knowledge of asteroids has strongly improved over the last two decades.
In this fellowship, a contemporary CPF is constructed using a contemporary asteroid population model, and is compared with a reference CPF developed using an older asteroid population model. A classical mathematical framework (Wetherill, 1965) is used to compute Asteroid-Moon and Asteroid-Mercury collision probabilities, which neglects orbital precession of eccentricity and inclination for the asteroids and target object. This classical method seemed a natural approach to take, since it has been standard practice in the development for all other CPFs for Mercury.
Results show that the impact rate on Mercury of the contemporary CPF is lower than that of the reference CPF, such that older surface emplacement ages are obtained with the new CPF. However, neglecting precession of eccentricity and inclination of asteroids and target object appears to be unreasonable, because many asteroids that collied with Mercury significantly preces in eccentricity and inclination. Therefore, this work needs to be revisited with a method for computing asteroid-target collision rates that accounts for the precession eccentricity and inclination of the asteroid and target object, for example the method described in Pokorny and Vokrouhlicky (2013).
Dissemination of this work needs to await recalculations with an improved mathematical framework.
The new thermal evolution model will become a leading tool for studying the thermal evolution and the long-term dynamo forcing of Mercury.
The new experimental data on the equation of state and melting behavior of iron metals at high pressure provide new constraints on behavior of important candidate core alloys for terrestrial planets. This is essential for constraining the bulk composition of terrestrial planets.
The new experimental data on the equation of state and melting behavior of iron metals at high pressure provide new constraints on behavior of important candidate core alloys for terrestrial planets. This is essential for constraining the bulk composition of terrestrial planets.