on the Moon, who brought back rock samples that provided a glimpse of another planetary body much different from Earth. This new perspective ushered in the modern era where Earth is viewed as a planet and its constitution, history, and character are compared to those of other planets.

In 1980 another breakthrough came from evidence that Earth was struck by a large meteoroid 65 million years ago and that the impact probably caused the extinction of dinosaurs and many of the other living things on the planet at the time (Alvarez et al., 1980). Within a few years it became evident that some meteorites found on Earth came from Mars (Bogard and Johnson, 1983). These two developments underscored the idea, which had begun with studies of impact craters on Earth and the Moon, that Earth must be viewed in its astronomical context; for example, life could be terminated by uninvited extraterrestrial objects or imported from other Solar System planets!

Over the past 20 years the transformation of Earth science has continued. Major advances in technology that allow Earth to be observed much better at both large and small scales, continuing planetary exploration, and advanced computing have all contributed. We can now see into minerals and discern individual atoms, measure the properties of rocks at the immense pressures and temperatures inside Earth, watch continents drift and mountains grow in real time, and understand how organisms evolve and interact with Earth based on their DNA. We have also been able to extract new information from meteorites that tells us about how planets form and even about how the interiors of stars work. Armed with new tools, Earth science is turning to the deeper fundamental questions—the origin of Earth; the origin of life; the structure and dynamics of planets; the connections between life, climate, and Earth’s interior; and what the Earth may hold for humankind in the future.

SCOPE AND PURPOSE OF THIS REPORT

At the request of the U.S. Department of Energy, the National Science Foundation, the U.S. Geological Survey, and the National Aeronautics and Space Administration, the National Academies established a committee to propose and explore grand Earth science questions being pursued today. The charge to the committee, given below, provided unusual freedom in the selection of topics, without regard to agency-specific issues, such as mission relevance and implementation.

The committee will formulate a short list of grand research questions driving progress in the solid-Earth sciences. The research questions will cover a variety of spatial scales and temporal scales, from subatomic to planetary and from the past (billions of years) to the present and beyond. The questions will be written in a clear, compelling way and will be supported by text and figures that summarize progress to date and outline future challenges. This report will not discuss implementation issues (e.g., facilities, recommendations aimed at specific agencies) or disciplinary interests.

Our response to this charge has been to attempt to capture the scope and aspirations of what might best be referred to as geological and planetary science, which is another way of saying solid-Earth science. Research in this area draws on nearly every scientific discipline. However, research questions that are mainly the domain of other subdisciplines of Earth science—such as ocean, atmospheric, or space science—are discussed to the extent they are linked to solid-Earth science.

The committee began by developing criteria for what constitutes a “grand” question. Our definition of grand questions was partly determined by the small number requested in the charge, which led us to aim for 7 to 10 questions, and partly by a desire for the questions to meet at least two of the following criteria:

  • it transcends the boundaries of a narrow subfield of geological and planetary science;

  • it deals with eternal issues, such as the origins of Earth and life;

  • it is connected with phenomena that have significant impact on human well-being.

Our ultimate objective was to capture in this series of questions the essential scientific issues that constitute the frontier of Earth science at the start of the 21st century. It is our hope that these questions and our descriptions of them are as compelling as we believe the science to be and that this short report is useful to those who would like to understand more about where Earth science stands, how it got there, and where it might be headed. We have attempted to make the text accessible to managers of scientific programs, graduate students, and colleagues in sister disciplines who have



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