Early habitability of Earth. The NAI has supported Stephen Mojzsis (University of Colorado team) and others (e.g., Mark Harrison, University of California, Los Angeles, team) to investigate the oldest rocks and to use ancient zircons to characterize the environment of the young Earth. One result is evidence for an ocean and hydrological cycle in the Hadean Eon, the first 500 million years of Earth’s history.2,3 For additional details see Box 2.1.
The rise of oxygen and Earth’s “middle age.” NAI support was critical in fostering a new interest in the Archean and Proterozoic Eons, the geological periods from approximately 3.9 billion to 2.5 billion and from 2.5 billion to 542 million years ago, respectively. The NAI sponsored collaborative deep-drilling projects and isotopic studies to document the co-evolution of Earth’s biota with the rise of atmospheric oxygen. Findings include new evidence of oxygen before the so-called Great Oxidation Event, improved understanding of the timing of this event, and evidence that this event led to a Proterozoic world unlike what came before or after.4-8 For additional details see Box 2.2.
Snowball Earth. The NAI supported fieldwork by Paul Hoffman (Harvard University) and his students to provide high-resolution stratigraphic and geochemical data needed to refine the hypothesis that Earth was, at times, completely covered with ice during the period from 850 million to 630 million years ago. Snowball Earth and other extreme events are now considered a natural aspect of Earth’s evolution on long timescales. Other NAI investigators at the California Institute of Technology and Arizona State University have investigated the implications of this period for the evolution of life.9-12 For additional details see Box 2.3.
Microbial mat ecology. In situ studies, led by the NAI team at NASA’s Ames Research Center, of the Guerrero Negro hypersaline microbial mats (modern representatives of one of Earth’s earliest and most pervasive
EARLY HABITABILITY OF EARTH
Direct information concerning the first 500 million years of Earth history—the Hadean Eon, approximately 4.0 billion to 4.5 billion years ago—is very limited, since practically no crustal rocks from that time have survived. Researchers do know that asteroids and comets collided with Earth much more frequently than they do today, and astronomers also tell us that the Sun was about 30 percent fainter then, so that Earth may have been cold, unless there was a large greenhouse effect to trap the Sun’s heat and raise surface temperatures above the freezing point. Also of special interest is the apparent fact that life arose on Earth either during or shortly after the Hadean Eon.
Understanding the chemical state of the earliest atmosphere and ocean is critical to any theory of the origins of life on Earth. Stephen Mojzsis (University of Colorado team) and colleagues have been investigating the geological record, including the use of ancient zircons to determine the environment on the earliest Earth. The oldest rocks, found in Australia, Canada, and Greenland, are less than 4.0 billion years old. Some of the zircons they contain are much older; oxygen isotope dating places some of these zircons at ages up to 4.3 billion years. Mojzsis and colleagues conclude that these zircons were formed from magmas containing a significant component of reworked continental crust that formed in the presence of water at Earth’s surface. This result is consistent with the presence of a hydrosphere interacting with the crust within only 200 million years of Earth’s Moon-forming event.
C.E. Manning, S.J. Mojzsis, and T.M. Harrison, “Geology, Age and Origin of Supracrustal Rocks at Akilia, West Greenland,” American Journal of Science 306: 303-366, 2003.
S.J. Mojzsis, T.M. Harrison, and R.T. Pidgeon, “Oxygen-isotope Evidence from Ancient Zircons for Liquid Water at the Earth’s Surface 4,300 Myr Ago,” Nature 409: 178-181, 2001.