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.