Testing the 'wet versus dry' late veneer hypothesis

The Earth's formative years remain an enigmatic era of our planet. Only recently - and with the help of novel technological advances - we are beginning to gain an unprecedented view into this geological Dark Age. During the accretionary phase of the solar system about 4.6 billion years ago, an infant Earth quickly grew by cataclysmic collisions with fellow proto-planets that crossed its path around the young Sun. Emerging as the victor of these catastrophic events, the cannibalistic growth of our planet terminated in a final collision with a Mars-sized planetary embryo about 4.5 billion years ago. What followed was a mere sprinkling of the Earth by meteoritic material. Yet, this 'late accretion' could have been vital for converting it into a habitable 'blue planet'.

While several lines of evidence support such a general growth model of the Earth, the chemical composition of the material added to the Earth during the 'late accretion' remains a topic of debate. It has been suggested that this material was made of meteorites enriched in volatile elements, like hydrogen, nitrogen, and carbon. Such a 'wet' late accretion may thus represent an important source of gaseous and liquid compounds on Earth today, like water, carbon dioxide and noble gases. However, this contradicts our observation that the most common meteorites found today are depleted in volatile elements. Therefore, a 'dry' late accretion has been postulated.

Testing the hypothesis of a 'wet versus dry' late accretion has important implications for a series of geological and biological questions. It is assumed that, besides adding environmental key compounds such as water to the Earth, a 'wet' late accretion may have added primitive organic compounds, which could have been vital for the origin of Life on Earth. Also, small amounts of water in the Earth's mantle may be a vital parameter for triggering and sustaining mantle convection. This global-scale convection operated over geological times and prevented the Earth from becoming a life-barren and 'dead' desert planet like Mars.

Unfortunately, the same global-scale mantle convection processes that are keeping our planet geologically 'alive' have resulted in the pervasive dissolution and homogenisation of the late accretion component within the Earth. This lack of physical evidence of a late accretion component represents the prime reason for not having been able to test the 'wet versus dry' hypothesis in the past. However, a pioneer study related to this proposal has revealed a single set of terrestrial samples that still contains remnants of this late accretion component. This will offer a unique and exciting opportunity to directly study its chemical composition.

The samples of the study originated as silicate melts in the Earth's mantle and were formed by fusion of the late accretion component and concomitant 'normal' mantle material. An important aspect of our approach will be the ability to isolate the key chemical signature of the late accretion component from the mantle matrix. We will apply a sophisticated combination of geochemical and novel isotopic tracers that will extract a unique fingerprint of the meteorite material of which the late accretion component was made of. We will then compare this fingerprint with the known and unique composition of 'wet' and 'dry' meteorites. This will enable us to directly test the 'wet versus dry' late accretion hypothesis for the first time.

These results will contribute to our understanding of how the Earth evolved into a habitable 'blue planet' and will complement current studies carried out by the PI as part of his NERC Advanced Research Fellowship.

The results of the proposed project and the methods involved will have immediate and intermediate benefits to a wide group outside the academic circle. These include:

1) Commercial private sector
Mining industry/precious metals trading: A result of the study will be a better understanding of the planetary processes that delivered precious metal to the Earth during the formational years of the Solar system. It will also give detailed insights into the question of why these metals are so scarce in the Earth's crust. As such, the outcome of this study will be of wider interest to commercial companies involved in the mining and trading of precious metal resources.

Industry: The isotope geochemical methods developed and applied by the PI and other members of the MAGIC group are used far beyond the fields of Earth Sciences. Members of the MAGIC group apply stable isotope techniques to research in medical and life sciences and, most recently, the rapidly growing field of nanoparticle toxicology. The MAGIC group has an excellent track record of publishing method development work in the peer-reviewed literature to ensure that researchers within the science and commercial private sector can readily implement these novel procedures.


2) Policy makers
This proposal will contribute to our understanding of the origin of water on Earth. Water is a vital constituent of our daily diet. It is also a driving force in controlling our climate either as part of the oceans or as vapour in the atmosphere. There is increased concern about limited access to clean water supply and manmade climate change. Despite the essential role water plays in all kinds of human existence, we all so often treat it as a granted commodity.

Understanding the unique processes that delivered this essential life-sustaining ingredient to Earth will increase public awareness of the impending problems mankind is facing with regards to dwindling water supplies on one side and an increased number of flooding events on the other. As such the outcomes of this study, will be of general interest to policy makers as well as to NGO concerned with climate research and sustainability.

3) Press, media, museums, science education, lay scientific organisations and wider public
In the public awareness, the origin of water on Earth is intimately linked to the question of how and when life formed on our planet. This public interest is further enhanced by current space missions looking for evidence of water on other extra-terrestrial objects, such as Mars and the Moon. Accordingly, there is also an increased interest in the topic of how and when water has been added to the Earth.

There is a general public interest in natural sciences. This is expressed by the large number of popular scientific journals and scientific TV programs on natural sciences in the UK but also worldwide. The origin of our planet is a regular theme of many popular scientific publications and shows. Yet, we still know only very little about this crucial time of Earth's history and, accordingly, new scientific developments are often embraced by the press and broadcasting media. It is anticipated that new scientific results concerning this topic, to which the proposed research will directly contribute, will lead to a more complete picture and an improved public awareness of the huge time- and length-scales involved in planetary formation.

Over the past years, the PI has been involved in several contributions to the media and public lectures (see 'previous track record'). The media as well as lay science organisations are keen on disseminating this information. Therefore, important UK-based and worldwide immediate beneficiaries of the project proposed will be news channels, radio programmes, newspapers, popular scientific journals, scientific TV programs, natural history museums, geology education programs in schools, and lay geologists and meteoriticists associations.