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4. Major Scientific Focus and Goals


Important scientific issues.

The planned SPP will focus on the central question how the Earth became a habitable planet and how conditions favourable for life could be sustained over geological history. This will be achieved by investigating the early planetary and geological processes that controlled its evolution to the only known habitable planet. A wide spectrum of Earth Science disciplines will be involved to solve this question, including geology, geochemistry, cosmochemistry, geobiology and geophysical modelling. This Earth Science focus is unique in its concept and complements other research projects without duplication. The three most critical issues to study with respect to the Earth’s habitability are:

  • The composition and sources of the Earths building materials.
  • The early internal processing of the Earth leading to its primordial differentiation into a metal core, a silicate mantle, and the continental crust, including mineral ressources.
  • The evolution of the Earth’s early ocean-atmosphere system and the rise of free oxygen in the atmosphere.


4.1 The composition and sources of Earth’s building materials

The emergence and sustainability of life on the early Earth have been possible, because volatiles such as water and an adequate supply of nutrients were available. To understand this unique chemical inventory of the Earth, it has to be placed in a solar system context. In cosmochemistry, the chemical elements are classified according to their 50% condensation temperatures in the early solar nebula. Refractory elements and their compounds commenced to condense at ca. 1800 K, while volatile elements and their compounds (such as water) did not condense until ca. 1000 K and lower temperatures were reached. On Earth, there is a characteristic decrease of element abundances with increasing volatility. Nevertheless, enough volatiles are still present on Earth for a sufficient supply of water. To fully understand element delivery to the Earth, it is essential to study the remnants of the first asteroids in the solar system that are only preserved in the asteroid belt between Mars and Jupiter. Such remnants are delivered as meteorites to the Earth, and their compositional and structural variety allows studying the potential building material of the Earth in an astonishingly detailed fashion. The most eminent scientific questions with respect to the Earth’s building phase to be addressed in the proposed SPP are:

  • Does the refractory and volatile element inventory on Earth sample different regions of the solar system?
  • What is the source of the Earth’s volatiles and when were they delivered?
  • Which groups of meteorites provided suitable building blocks of the Earth?
  • Are chondritic meteorites true references for the bulk Earth composition?
  • How was the Earth’s composition affected by the Moon-forming giant impact?


4.2 The early internal processing of Earth leading to its primordial differentiation into metal core, silicate mantle, and continental crust.

The Earth’s stable continental surface was an important parameter for the emergence and evolution of life. This required a rapid primordial differentiation of the young Earth into metal core, silicate mantle, and a crust. Processes such as crust-mantle differentiation, the initiation of subduction processes, erosion and crustal recycling into the mantle might have changed the Earth’s surface dramatically and also account for the Earth’s important mineral ressources. There is now clear evidence that the Moon-forming giant impact removed the Earth`s primordial atmosphere and triggered formation of a deep terrestrial magma ocean in the Hadean Eon (>4.0 Ga). This event was followed by degassing that led to formation of a new atmosphere and to formation of the first protocrust. Valuable insights into the earliest history of the Earth are provided by early Archean rocks. During the Archean Eon, (between 4.0 Ga and 2.5 Ga) important geodynamic processes like plate tectonics began to operate. Over the past years, an ever increasing amount of geological samples from Archean terrains became available for study. In addition, the Earth’s oldest minerals were discovered, being represented by up to 4.4 Ga old detrital zircons from W-Australia that represent our only direct vestiges of the Hadean Earth. Therefore, little is known about this “dark age” of Earth’s history and virtually any information about Earth’s first 500 million years has to be retrieved from emerging approaches such as trace element or isotope geochemistry, experiments or modeling. The key questions to be adresssed in the proposed SPP with respect to the Earth’s primordial differentiation are:

  • What were the physical and chemical consequenses of the terrestrial Magma Ocean?
  • What was the composition of the Earth’s first crust?
  • How and when did the first continental crust and mineral ressources form on Earth?
  • When did plate tectonics begin to operate on Earth?
  • What was the role of impacts during formation of the first crust and emergence of life?


4.3 The early evolution of Earth’s early ocean-atmosphere system and the rise of free oxygen in the atmosphere

The emergence and evolution of life on Earth is directly linked to the evolution of the ocean-atmosphere system. The first evidence for life is at least 3.5 Ga old, but the ambient conditions then and over most of the geological history thereafter were much different from today. Importantly, a much more reduced atmosphere suggests a completely different ocean chemistry at the time, and the first life thrived under substantially different environmental conditions. One of the key parameters relevant to the co-evolution of life and environment is the atmospheric oxygen level. The first significant rise of free oxygen occurred ca. 2.4 billion years ago and is known as the Great Oxidation Event (GOE). However, newly developed isotope proxies suggest that minute amounts of free oxygen were at least episodically and locally available even much earlier in Earth history. Reconstructing the co-evolution of early life and early environmental conditions is based on the investigation of well-preserved ancient sedimentary successions, referred to as deep-time research. Over the past years, a substantial amount of well preserved ancient sediment samples and new techniques have become available to Earth Sciences, permitting to adress the following SPP-relevant issues:

  • Was the first atmosphere formed above a magma ocean and how did it evolve?
  • How did the atmospheric oxygen level evolve over time?
  • How did the composition of the oceans evolve over time?
  • How did ocean temperature and acidity evolve?
  • What were the boundary conditions for the emergence of life in the Archean?
  • How can biosignatures in ancient sediments be discriminated from abiotic signatures?



Stand alone features.

An Earth Science based SPP “Building a habitable Earth” combines for the first time the research efforts of many different disciplines and research groups within the German Earth Sciences, namely geology, geochemistry, cosmochemistry, geobiology and geophysical modelling approaches. The scope of our proposed SPP is focused within Earth Sciences in order to optimize the number of participating groups, without getting too broad for effective communication. At the same time, the scope will be sufficiently large to guarantee synergetic added value effects through integrated studies on shared sample sets, new method applications and interdisciplinary training of young scientists. The initiative is to ca. 90% university-based and mirrors a number of new university appointments in this exciting research area. On a long term perspective, we therefore expect large benefits for the German research community in part.