the technical or scientific background needed to comprehend what is discussed.

Our most difficult problem in selecting the grand questions was to distill from a large number of topics and questions the “most worthy” candidates. To do so the committee canvassed the broad geological community and deliberated in meetings and telephone conferences. After arriving at 10 grand questions, the committee set about writing, as well as soliciting written contributions from other scientists. Some of our questions present truly awesome challenges and may not be fully understood for decades, if ever. Others seem more tractable, and significant progress may be made in a matter of years. Overall, we have included most of what the committee regards as the important issues and also most of what was suggested by the respondents to our canvassing effort. There was, in fact, a fair degree of consensus about what constitutes a grand question and which ones should be included here.

GRAND RESEARCH QUESTIONS FOR THE 21ST CENTURY

Although we started by simply identifying the overarching questions we believe to be driving modern Earth science, we found that these questions can be grouped into four broad themes. These themes constitute the four chapters of the report, and within each chapter are descriptions of the grand questions. Chapter 1 deals with origins—the origin of Earth and other Solar System planets, Earth’s earliest history, and the origin of life. Chapter 2 treats the workings of Earth’s interior and its surface manifestations and includes a question on material properties and their fundamental role in Earth processes. Chapter 3 addresses the habitability of the surface environment—climate and climate change and Earth–life interactions. Chapter 4 focuses on geologica10 hazards and Earth resources—earthquakes and volcanoes and modern environmental issues associated with water and other fluids in and on Earth.

The following is a summary of the 10 grand research questions identified by the committee:

  1. How did Earth and other planets form? The Solar System, with its tantalizing geometric patterns and its wide variety of planets and moons, presents intriguing questions that become more nuanced as we make new observations from spacecraft and more exacting measurements on meteorites. While it is generally agreed that the Sun and planets all coalesced out of the same nebular cloud, it is still not known how Earth obtained its particular chemical composition, at least not in enough detail to understand its subsequent evolution or why the other planets ended up so different from ours and from each other. Earth, for example, has retained a life-giving inventory of volatile substances, including water, but Earth is far different from every other planet in this regard. Advanced computing capabilities are enabling development of more credible models of the early Solar System, but further measurements of other Solar System bodies and extrasolar planets and objects appear to be the primary pathway to furthering our understanding of the origin of Earth and the Solar System.

  2. What happened during Earth’s “dark age” (the first 500 million years)? It is now believed that in the later stages of Earth’s formation, a Mars-sized planet collided with it, displacing a huge cloud of debris that became our Moon. This collision added so much heat to Earth that the entire planet melted. Little is known about how this magma soup differentiated into the core, mantle, and lithosphere of today or how Earth developed its atmosphere and oceans. The so-called Hadean Eon is a critical link in our understanding of planetary evolution, but we have little information about it because there are almost no rocks of this age preserved on Earth. Clues about this time period are accumulating, however, as we learn more about meteorites and other planets and extract new information from ancient crystals of zircon on Earth.

  3. How did life begin? The origin of life is one of the most intriguing, difficult, and enduring questions in science. Because life in the Solar System arose billions of years ago, some of the most fundamental questions about its origin are geological. Our knowledge of the materials from which life originated, and where, when, and in what form it first appeared, stems from geological investigations of rocks and minerals that represent the only remaining evidence. When life first arose, the conditions at Earth’s surface may have been much different than today’s, and one critical challenge is to de-



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