Geo- and Cosmochemistry

DFG SPP 1385

The First 10 Million Years of the Solar System - a Planetary Materials Approach

Summary

Our solar system formed 4.6 Ga ago from a collapsing cloud of interstellar gas and dust. Questions about the very origin of our Earth and planets - and the prospect of detecting Earth-like, potentially life-harbouring planets - are of tremendous public and scientific interest. The most important and critical step in building the Earth and other planets was probably the coalescence of planetesimals out of dust in the first few million years of our solar system’s existence. The mechanisms and rates involved in this step can be revealed by analysing material left over from this growth process and accessible as meteorites or cometary dust samples returned by the STARDUST mission. The SPP aims to initiate and provide the means for comprehensive and interdisciplinary studies on key aspects of planetesimal formation in the early solar system. The SPP is particularly timely due to the new research opportunities provided by the availability of extraterrestrial materials and new analytical technology, combined with a dramatic increase in planned space missions by nations around the world. The program will crosslink both established and young research groups and will provide training and education opportunities for promising young scientists. The proposed research is of national strategic importance, as it will maintain our national capability of analysing extraterrestrial material from future sample return missions.

Scientific Motivation

The discovery of numerous extra solar system planets and the increased activity of national and international space exploration missions have dramatically increased interest in planetary formation and evolution. This renewed interest has brought to light a dramatic deficit in the understanding of the physical and chemical conditions during the first few million years of our solar system and their variation in space and time. The goal of this planned SPP is to study primarily extraterrestrial material in combination with experiments and theoretical models to unravel the history and development of our solar system from the first formation of dust to the differentiation of planetesimals. Important questions will include:

• When, where and how did the first solids form and what was their composition and structure?
• How did the first solids combine to form large bodies and what were the chemical and physical conditions of these coagulation processes?
• When and how did the first planetesimals form and differentiate?

In addition to new ideas, new technology is definitely one of the major driving forces of progress in research on early solar system processes. Previous and ongoing research on highly heterogeneous matter from small solar system bodies, asteroids and comets demonstrate the need for higher precision structural, chemical and isotope analyses (e.g., short-lived radiogenic isotopes; non-traditional isotope systems) and the need to be able to study increasingly smaller samples in order to be able to elucidate the history of individual grains. In the wake of these research efforts new instrumental capabilities have been developed, with constantly improving higher precision and better spatial resolution. Primitive extraterrestrial material preserves tiny grains (most of them in the sub-micrometer range) that date back to the very origin of our solar system; some grains even predate the formation of the solar system. These presolar grains have enormously stimulated research on nucleosynthesis in stars. With the most advanced instruments like NanoSIMS, it is now possible to determine the isotopic composition of some elements in an area less than ca. 0.3 µm in diameter, thus allowing to constrain the physical and chemical conditions of their formation. Major advances in mass spectrometry in recent years have led to new levels of precision and allow unprecedented resolution of early solar system processes and chronology. In addition to these analytical advancements, sophisticated numerical and laboratory experiments meanwhile allow to simulate particle growth, annealing, evaporation and condensation processes of solids under conditions similar to those prevailing in protoplanetary disks.

Another important driving force of early solar system research is the increasing quantity and diversity of extraterrestrial material now available for study in our laboratories. In the last 15 years the total mass of available extraterrestrial material has increased significantly by the discovery of more than 30 000 meteorites from cold and hot deserts, which include previously unknown meteorite classes and several unique and highly primitive chondritic meteorites. Very recently, the STARDUST mission brought back dust samples of unequivocal cometary origin, allowing insight into the presumably most primitive type of solar system material. It is very likely that, based on the findings of the STARDUST investigations, we will be able to discover and identify more material of cometary origin that exists already within the collection of extraterrestrial dust sampled in the stratosphere or recovered from ice cores, but has not been recognized as such.

The interest in the physical and chemical conditions of planetary formation processes has also increased drastically due to the discovery of extrasolar planets during the last 10 years. To date >250 extrasolar planets have been discovered and upcoming observational programs (e.g. COROT satellite) will likely detect thousands of new planetary systems, some of them can be expected to harbor Earth-like planets. The birthplaces of planets - protoplanetary disks around young stars - can be observed with unprecedented detail, and Mg-rich silicates like forsterite and enstatite, which are fundamental components of any silicate dominated planet, can be detected by infrared astronomy. Furthermore, computational capabilities have reached a stage, where much more realistic models become possible that simulate structure, temperature and other physico-chemical parameters in protoplanetary disks, including solid state and gas phase chemistry.

All these new developments are perfect preconditions for this timely structured research effort on early solar system materials and processes. Through such a planned concerted research program that combines research groups with very different expertise, a significant improvement for the understanding of the origin and early evolution of our solar system is to be expected.

A major goal of the planned SPP will be the development of a combined quantitative physical and chemical model with unprecedented high temporal resolution for the formation of solid matter and its modification during the first few million years of the solar system. Establishing a time-line for physical and chemical processes will be a major research focus of the SPP since the sequence of events from dust to planetesimal formation holds the key for the understanding of the genesis of our planetary system as it exists today. In this respect dating key events and processes with isotopes is an indispensable tool. Of particular use will be chronometers with high resolution that is offered by short lived isotope systems such as 26Al-26Mg, 60Fe-60Ni, or 182Hf-182W. The latter isotope system has provided new high precision ages for silicate-metal segregation, e.g. occurring during condensation, metamorphism and core formation. One of the major recent scientific breakthroughs was the recognition that some iron meteorites are older than chondrites and have a similar age as CAIs. This new chronology requires high precision chronology for other key processes because the conventional model that chondrites are among the oldest objects and the building blocks of all evolved planetesimals needs further testing.

The planned SPP will focus on the early stage of planetesimal formation using a wide spectrum of mostly geoscientific techniques and approaches. This focus of the program is defined in a way to complement other major research projects without duplication and to ensure maximum gain for the planned projects. The predominantly geoscientific approach avoids duplication of studies based on astrophysical approaches, and it will allow collaboration although with a complementary focus with the existing DFG research group FOR 759. Also, the focus on the early stages of planetesimal formation avoids overlap with the just terminated SPP “Mars and the terrestrial planets”, which was highly successful, but had a strong emphasis on processes active in the interior and on surfaces of large planets, and their evolution through all stages of Solar system history. The focus of the planned SPP is defined to ensure an optimum number of participating groups, without getting too large. The planned size is appropriate to allow effective communication among collaborating groups, but also large enough to guarantee synergetic “added value” effects. Finally, the question of how to make planets from dust is probably the most interesting problem when seeing our solar system and the planet we live on in context, particularly concerning the question: Are “terrestrial planets” like the Earth common in the universe or are they exceptional? This topic is currently in the focus of public interest.

On a long term perspective, we expect enormous benefits for the Earth and Planetary Sciences in general and in Germany specifically. The special challenge of analyzing highly heterogeneous (and rare) extraterrestrial matter is (and has always been) a trigger for major improvement in instrumental and technical capabilities. It can be expected that improving and applying analytical techniques with significantly increased precision and spatial resolution will find numerous applications in non-extraterrestrial geoscientific investigations.

Large astronomical programs e.g. COROT, GAIA (astrometry), or DARWIN are underway to discover thousands of planets within the next few years, including Earth-like bodies. Protoplanetary disks are being observed with increasing sensitivity and spatial resolution, e.g. by the Hubble space and (planned) James Webb space telescopes, the Spitzer IR satellite and by ground based telescopes. ESA small body missions are underway (DAWN, ROSETTA) and sample return missions are planned. The Japanese mission Hayabusa is on the way back to Earth with a yet unknown amount of material from asteroid Itokawa, possibly arriving in 2010. Sample return missions from Moon and Mars are also part of the upcoming ESA programs (NEXT/Aurora, Cosmic Vision). In anticipation of these materials to become available for research in the foreseeable future, it is highly desirable to have well-prepared ground-based laboratories that are capable to obtain high precision and spatially well resolved analyses of tiny amounts of precious extraterrestrial matter. Training young scientists will ensure that the future research environment will be suitable for the innovative and creative study of these materials.

Major Scientific Focus and Goals

Elucidating all aspects and details of the origin and evolution of our solar system is a major long term endeavor. Focusing on certain key aspects and timely questions, however, will allow major progress if the efforts can be pooled and focused as it is planned for this proposed SPP. The focus of the SPP will be on achieving an improved understanding of the processes active during the first few (<10) million years of our solar system through the quantitative study of extraterrestrial material, laboratory experiments and modeling. The main research themes focus on basic processes that ultimately led to formation of chemically, mineralogically and isotopically different planetesimals that eventually became building blocks of larger planets.

Solar system raw material – presolar grains, short-lived and stable isotopes

About 4.6 billion years ago, an interstellar cloud of gas and dust collapsed by a gravitational instability. A smaller fragment separated and formed a flattened disk with a central protostar, the protosun. The surrounding gas and dust formed a protoplanetary disk, the so-called solar nebula .. A certain fraction of solar nebula dust formed by previous condensation in stellar outflows of evolved stars. These dust species are isotopically anomalous “presolar grains”, e.g. SiC or some silicates and oxides that can be found in primitive meteorites or interplanetary and cometary dust. Another important solar nebula ingredient were short lived radioactive nuclides (e.g. 26Al, 60Fe) that were synthesized by coexisting neighboring, massive evolved stars or by a nearby supernova explosion. Major questions concerning the violent birthplace of the solar system are:

• How heterogeneous was the solar nebula in space and time? How, when and from which stellar sources were short-lived nuclides injected into the solar nebula? Was there a supernova explosion nearby, and were its products incorporated in all solar system solids?
• How much presolar material was thoroughly processed in the solar nebula or even before in the interstellar medium? What are the sources of the various presolar grain populations and can we understand the origin of their isotope anomalies?
• How efficient was radial mixing in the solar nebula? What were the processes that transported material from hot inner regions to the outer solar nebula into asteroid and comet forming regions?
• What can primitive extraterrestrial matter from comets - particularly STARDUST mission samples from comet Wild-2 - tell us about this early stage? Can STARDUST studies enable us to identify cometary material in interplanetary dust populations?

Chondrites and chemical differentiation of the protoplanetary disk

Coagulation of sub-micrometer sized dust, partly accompanied by evaporation and condensation processes led to formation of the first solids: cm sized Ca, Al rich inclusions (CAIs), mm sized chondrules, coexisting with micrometer sized dust. These are the basic constituents of primitive meteorites, called chondrites. Mixing of these constituents or their precursor material determines the bulk chemical and isotopic composition of planetesimals and subsequently larger planets. Chemical fractionation processes modified the whole solar nebula, e.g. causing a depletion of volatile elements in the inner solar system. Key scientific questions addressing this important highly dynamic stage in the formation of the most primitive solid material include:

• How and when did the different components observed in chondrites (e.g., refractory inclusions, chondrules, matrix) form? Were they separated physically in the solar nebula? What was their chemical contribution to the bulk composition of planetesimals?
• What processes led to the heterogeneous distribution of elements in the nebular disk?
• When and where, and how long did short scale heating processes, evaporation and condensation, mineral formation, and large scale chemical differentiation occur?
• What is the origin of material with different nucleosynthetic pathways in primitive and evolved planetesimals? How can we recognize such material in extraterrestrial samples?

Growth, metamorphism and differentiation of planetesimals

Kilometer sized planetesimals formed by coagulation processes from dust and small particles. Further growth from 1 km to asteroid size was fast, as it was dominated by gravitational forces. Some asteroids heated up strongly and differentiated into a metallic core and a silicate mantle, others underwent milder thermal and/or aqueous metamorphism, comets stayed rather cold. The weaker the heating, the better the small bodies could preserve pre-accretional textures and constituents. Material from all these different evolutions stages is available in meteorite collections and accessible to direct study. The major questions that will be addressed using the physical, chemical and mineralogical information obtained on these materials include:

• How and when did coagulation of dust lead to the formation of the first planetesimals?
• What were the physical conditions that led to the differentiation of early planetesimals? What was/were the heat source(s)?
• When did core formation occur and how were the chemical elements distributed between geochemical reservoirs in these planetary bodies?
• Is it possible to develop a model that reconciles the thermal evolution with the chemical differentiation of small bodies?

Meteorites as terrestrial building blocks

The vast majority of the meteorites in our collections are derived from asteroids and they show different degrees of chemical fractionation and alteration. It is probably a fact that planets, including the Earth, are made up of material possibly represented by meteorites. However, it is not known which of the different materials that still exist in the solar system (e.g., CAIs, chondrites, stardust, differentiated meteorites) contributed to the formation of the Earth and what their relative contributions are. Due to Jupiter’s enormous size it likely prevented planet formation in the asteroid belt, the main source of today´s meteorites. However, some asteroids may have been derived from the inner solar system and resemble precursor material of the terrestrial planets. A fundamental question is to identify building blocks of the planets that formed at various distance from the sun and evaluate their relative contributions to planetesimal and planet formation.

• What processes led to the heterogeneous distribution of elements in the solar system
• What processes led to the formation and modification of different meteorite classes
• Which meteorite classes or groups originate from individual bodies and how did these early planetesimals differentiate internally?
• Can we model the composition of the Earth as derived from meteoritic building blocks?

Placing our solar system in context – links to astrophysics

The recent decade has brought significant instrumental advances in both geoscientific laboratory analyses of extraterrestrial materials, observational astronomy and computational astrophysics. Increasing capability of observing fine grained protosolar matter or dust in protoplanetary systems has led to an increasing convergence of these two communities. For example, basic fractionation processes such as the forsterite-enstatite fractionation, which is known from chondrites, is just starting to be incorporated into protoplanetary disk models, and can be observed by infrared spectroscopy in protoplanetary disks. Similarly, other mineral components that occur in traditional cosmochemical condensation sequences or are real components of primitive chondrites, e.g. highly refractory minerals as occurring in CAIs (hibonite, spinel, melilite, diopside, anorthite) or low temperature minerals (carbonates, hydrous minerals) were observed or should be observable by IR astronomy in protoplanetary disks.

Radial outward mixing in protoplanetary disks and in the early solar system is a concept that is hotly debated in both cosmochemistry and astrophysics. This has been stimulated by fundamental results of the STARDUST mission and by IR observations of solar system comets and extrasolar protoplanetary disks. Radial transport mechanisms seem indispensable to explain the survival of free floating CAIs over 2-3 Ma in the solar nebula, before incorporation into chondritic parent bodies, or the separation of refractory from volatile components by radial removal of condensed refractory components.