planet came to be the home of complex life, we must be able to fill in this enormous gap in the geological record. At present we can construct plausible, but still highly uncertain, models for the Hadean Earth, which are based on our present understanding of planet formation (Question 1), planetary interior processes and material properties (Questions 4 and 6), and climate (Question 7). These models are informed by observations of the Moon and other planets in the Solar System, by measurements made on meteorites and the oldest rocks and minerals on Earth and the Moon, and by our geological understanding of how the modern Earth works. A critical component of understanding Hadean climate is our knowledge of atmospheric processes, but despite the advanced state of models for the modern Earth atmosphere, our understanding of radically different types of planetary atmospheres is still rudimentary.

Recent studies have raised new hope of improving our understanding of the Hadean. New information continues to be gleaned from precise measurement of the isotopic and chemical compositions of ancient zircons and their mineral inclusions. Observations of the Moon, Mars, Venus, and the moons of Jupiter and Saturn have opened new windows for visualizing the early Earth and for documenting what may have been happening in the early Solar System. Comparison of meteorites with Earth rocks has led to better models of Earth’s early internal processes, including the formation of the metallic core, the implantation and loss of gaseous species from Earth’s interior, and the evolution of the crust and mantle.

The future is certain to provide additional breakthroughs. Capabilities for microanalysis of geological materials are improving, and hence the amount of information that can be extracted from even the tiniest samples of old rocks and minerals is increasing rapidly. With concerted effort, it is expected that many more ancient rocks and mineral samples will be found. More precise isotopic measurements are revealing clues to early planetary processes. Planned spacecraft missions to the Moon and Mars will provide critical information about the nature of planets in the Hadean. There is even a chance that pieces of Hadean Earth rocks will be found on the surface of the Moon, sent there by impacts on Earth in the same way that pieces of the Moon and Mars have been sent here.

QUESTION 3:
HOW DID LIFE BEGIN?

The origin of life stands as one of science’s deepest and most challenging questions. It is a historical problem that emerged during a time with little recorded history, so it must be approached mostly through theory and experiment—imaginative efforts to re-create our planet’s early conditions and establish plausible chemical routes to the emergence of life. The goal of understanding life’s beginnings has attracted scientists from geology and from many overlapping disciplines, especially subfields of organic chemistry and molecular biology. In an age of planetary exploration, the origin of life is also an astrobiological issue, currently investigated on Mars, where a sedimentary record of earliest planetary history is preserved, and potentially across the wider stretch of Universe where planets have been detected.

Some of the most fundamental mysteries about the origin of life are geological in nature: From what materials did life originate? When, where, and in what form did life first appear? At its most basic physical level, life is a chemical phenomenon, and because it arose billions of years ago, geologists are intensely interested in creating an accurate picture of the chemical building blocks available to early life.

Top-Down and Bottom-Up Approaches

In The Origin of Species, Charles Darwin (1859) hypothesized that new species arise by the modification of existing ones—that the raw material of life is life. Louis Pasteur, Darwin’s great Parisian contemporary, went a step further. Pasteur decisively refuted the doctrine of spontaneous generation, the long-held view that life can arise de novo from nonliving materials, declaring instead that life springs always from life (Pasteur, 1922-1939). These conclusions, among the most important of 19th-century science, require that forms of life developed in an unbroken pattern of descent through time, with modifications, to produce the biological diversity we see today. And indeed, students of fossils have painstakingly traced such a pattern backward for more than 3 billion years to the time of our planet’s infancy (Knoll, 2003).

Before then, however, somehow and somewhere, the tree of life had to take root from nonliving precursors. Scientists have tried to identify these precursors



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