Periodic Reporting for period 3 - VapLoss (The chemical consequences of vapour loss during planetary accretion)
Berichtszeitraum: 2023-06-01 bis 2024-11-30
Planets are thought to form in a disk surrounding a central star like our Sun by a process called accretion, in which initially dust-sized particles in a disk surrounding our (proto-)Sun aggregate to meter-sized clumps, which continue to grow to km-sized objects and then collect surrounding material by gravitational attraction to grow further to planet-sized objects like our Earth. It Is thought that due this accretion process, planets are a mixture of the smaller precursor material from which they grew. Chemical analysis of terrestrial as well as extra-terrestrial material enable us to investigate what kind of material ended up in the Earth and from what part of the so-called proto-Solar disk this material originated.
One type of such chemical analysis comprises nucleosynthetic anomalies in isotopic ratios of certain elements. These anomalies are produced during nucleosynthesis in stars and not modified afterwards by any chemical processes that take place during planetary growth. The nucleosynthetic anomalies in Earth imply that it formed from material very similar to a type of primitive meteorite called enstatite chondrites that formed quite close to the Sun. In contrast, the relative abundances of the elements in Earth are very different from those in enstatite chondrites. They are instead similar to the element abundances of a different type of primitive meteorite called carbonaceous chondrites, which formed quite far away from the Sun beyond the snowline (approximately where Jupiter is at present).
These contrasting implications suggest that current models of planet formation are flawed. A potential explanation for the contrasting views obtained from these two types of chemical observations, could be that planetary accretion is chemically an open system. In that case, the composition of a planet is not simply a mixture of its precursor material, but something different, for instance due to loss of vaporised material during extremely hot episodes that occur due to energy release in the early Solar System (e.g. conversion of accretional impact energy to heat). Such partial vaporisation might have led to loss of several tens of percent of the original mass of the Earth, for instance. Such large mass loss by vaporisation could have caused major changes in the element abundances of the Earth, making it appear similar to carbonaceous chondrites after such vapour loss while it was actually similar to enstatite chondrites before this vapour loss.
However, testing this hypothesis of changing planetary compositions due to vapour loss during accretion is hampered by missing information on how much vapour would be lost during accretion as well as on how elements distribute between silicate liquid and vapour, the latter being required to model how vapour loss changes the chemistry of a growing planet. This project aims to provide this missing information through experiments containing silicate liquid and vapour, through chemical analyses on these experiments as well as on meteorites, and through numerical modelling. The final aim is to determine to what extent planetary accretion affects the characteristics of the planets in our Solar System, and evaluate whether the same process of chemical fractionation due to vapour loss also affects growth of planets surrounding other stars (i.e. exo-planets).
One type of such chemical analysis comprises nucleosynthetic anomalies in isotopic ratios of certain elements. These anomalies are produced during nucleosynthesis in stars and not modified afterwards by any chemical processes that take place during planetary growth. The nucleosynthetic anomalies in Earth imply that it formed from material very similar to a type of primitive meteorite called enstatite chondrites that formed quite close to the Sun. In contrast, the relative abundances of the elements in Earth are very different from those in enstatite chondrites. They are instead similar to the element abundances of a different type of primitive meteorite called carbonaceous chondrites, which formed quite far away from the Sun beyond the snowline (approximately where Jupiter is at present).
These contrasting implications suggest that current models of planet formation are flawed. A potential explanation for the contrasting views obtained from these two types of chemical observations, could be that planetary accretion is chemically an open system. In that case, the composition of a planet is not simply a mixture of its precursor material, but something different, for instance due to loss of vaporised material during extremely hot episodes that occur due to energy release in the early Solar System (e.g. conversion of accretional impact energy to heat). Such partial vaporisation might have led to loss of several tens of percent of the original mass of the Earth, for instance. Such large mass loss by vaporisation could have caused major changes in the element abundances of the Earth, making it appear similar to carbonaceous chondrites after such vapour loss while it was actually similar to enstatite chondrites before this vapour loss.
However, testing this hypothesis of changing planetary compositions due to vapour loss during accretion is hampered by missing information on how much vapour would be lost during accretion as well as on how elements distribute between silicate liquid and vapour, the latter being required to model how vapour loss changes the chemistry of a growing planet. This project aims to provide this missing information through experiments containing silicate liquid and vapour, through chemical analyses on these experiments as well as on meteorites, and through numerical modelling. The final aim is to determine to what extent planetary accretion affects the characteristics of the planets in our Solar System, and evaluate whether the same process of chemical fractionation due to vapour loss also affects growth of planets surrounding other stars (i.e. exo-planets).
The experiments with silicate liquid and vapour have to be performed at conditions with equipment and methodologies not traditionally available in Earth and planetary science institutes. Thus far, three people are working on the project (the principal investigator and two young researchers) together with three external collaborators and one more person to be employed on the project soon. A custom-designed furnace has been installed as a major first step in the project. This furnace enables us to perform the experiments with silicate liquid and vapour that form an important part of the project. Currently, work is being performed to fine-tune the full methodology for the experiments with this furnace so that we can extract the chemical distribution of elements between silicate liquid and vapour. Additionally, the team has gathered the necessary meteorites for chemical analyses and is developing a high-precision method to perform the most important isotopic analyses on them. Finally, we developed a numerical model that describes, in a realistic manner, the physical processes that control the amount of vapour loss from a planetary body, as a function of its size and heat budget (i.e. difference between heat input and output).
We expect to soon finalise the development of the methodology for the experiments to study the distribution of elements between silicate liquid and vapour, which together with the new furnace, will enable us to mimic the temperature and pressure conditions that occur during vapour loss from growing planetary objects. This would form significant progress beyond the current state of the art in our field of research. From these experiments we expect to obtain the distribution of a suite of elements between silicate liquid and vapour as a function of temperature, which will form the basis for a model describing the chemical fractionation between these two phases. Together with the numerical model calculating how much vapour is lost from growing planetary bodies, we thus expect that at the end of our project we can predict how planets change their chemical composition during their growth. The methodology in development for high-precision isotopic analysis on meteorites will augment the observational constraints to which we will compare our modelled predictions to determine to what extent planetary accretion is an open-system process that modifies the composition of a planet relative to what is expected from simple mixing of its precursor material.