1. Introduction and Scientific Motivation

 

Scientific motivation.

The only key to understand how the Earth developed towards a habitable planet is to understand its earliest geological evolution. This task is best accomplished by combining the record from the Earth’s oldest rocks and from extraterrestrial samples. So far, research efforts in this field were hampered by the availability of suitable samples and by methodological limitations. The increasing access to ancient terrestrial rock suites and the increasing amount of available extraterrestrial samples combined with fundamental technical breakthroughs in the past years now make a concerted research approach particularly timely. This is also reflected in numerous new university appointments in Germany with a research focus on the early evolution of the Earth and the solar system. Many of these groups, also including a substantial number of promising young scientists, have just been established. Consequently, there is the urgent need for a structured research program in German Earth Sciences that will permit these groups to coordinate their internationally highly visible research programs to a much better extent. An SPP program will enable these groups to combine research approaches and techniques from different Earth Science disciplines such as geology, geochemistry, cosmochemistry, planetology, geobiology and geophysical modelling.

 

Origin of the Earth.

The origin and earliest history of the Earth and the emergence of life have been eminent issues in science for hundreds of years. Following the discovery of radioactive decay more than a century ago, it took nearly another 100 years to understand that the apparently simple questions “How old is the Earth?” and “What is it made of?” are not well posed. Modern theories of Earth formation now argue that the Earth formed over several million years involving several important steps. First, small planetesimals formed from dispersed interstellar material and/or condensed solar system matter. Formation of planetesimals took about 5 million years. Subsequently, the Earth formed by gravitational interaction and collisions of such planetesimals, involving much longer timescales (ca. 100 million years). Importantly, the planetesimals making up the Earth likely originated from various heliocentric distances, thus implying considerable radial mixing within the protoplanetary disk around the young sun. The asteroid belt between Mars and Jupiter contains the only remnants of such old planetesimals, and studying these materials provides essential clues on the earliest history of the Earth. More than 45,000 fragments of these asteroids have been delivered to the Earth as meteorites, allowing to study the history and composition of the Earth’s potential building materials at great detail in the laboratory. Over the past years, a wealth of new extraterrestrial samples became available to the research community, making concerted studies on such samples a particularly timely endeavour.

 

The Earth’s “dark period”.

Our understanding of processes active on the Early Earth relies on the geological rock record. However, the oldest rocks are only ca. 4.0 billion years and the oldest minerals nearly 4.4 billion years old, and therefore little has been known about the first 500 million years of Earth’s history (Hadean Eon, >4.0 billion years). During this important episode major steps of Earth’s evolution occurred, such as formation of the Earth’s metal core with a magnetic field and of the silicate mantle via a deep magma ocean. Formation of this magma ocean was triggered by giant collisions during the Earth’s growth, one of which caused the formation of the Moon. However, the origin of Earth’s inventory in volatile elements, including water, one of the key requirements for life, is still controversial. According to so-called heterogeneous accretion models, Earth’s volatile inventory was likely acquired during late addition of more oxidised material to an initially volatile-depleted and reducing Earth. Alternatively, so-called homogenous accretion models argue for late volatile loss during giant impact collisions and cosmic erosion. Virtually all information on Earth’s “dark period” has to be retrieved from indirect approaches such as trace element or isotope geochemistry, experimental approaches or numerical modeling. During the past years the potential of these techniques has drastically improved, largely driven by methodological breakthroughs.

 

The first continents and the evolution of the ocean-atmosphere system.

During the Archean eon (4.0 to 2.5 billion years), important geodynamic processes began to operate on Earth, including formation of the first continental crust, the evolution of a depleted upper mantle, the onset of plate tectonics, and the rise of free atmospheric oxygen. The oldest continental rocks and minerals on Earth are found in remote areas such as northern Canada, Greenland, Western Australia and southern Africa. Evidence for the first liquid water and oceans on the early Earth stems from the isotope compositions of the oldest minerals and from the presence of ancient ocean floor volcanic rocks and sediments. Water has also played a pivotal role for the onset of subduction zone processes, erosion and ore formation. Not least, the presence of water in its liquid form also set the stage for the evolution of life. There is also evidence that Earth had already lost its earliest atmosphere during its dark period, and possibly some of its earliest crust to space. Phases of intense impact events may have significantly shaped the surface of the early Earth and may have replenished the Earth’s inventory of volatile elements such as water. The rise of atmospheric oxygen in the Late Archean has set the stage for more complex life on Earth. In the past years, a markedly increasing amount of almost pristine Archean rock samples has become available, that for the first time permit concerted studies of Archean processes in unprecedented detail.