Cool Early Earth Theory

               Although geoscientists have proposed numerous theories about the Earth’s early history, the problem is that no known rocks have survived from the first 500 million years. Consequently, large-scale inferences have been drawn from scant evidence. That situation may be changing. In 2002, several geoscientists led by John W. Valley[1] of the University of Wisconsin — Madison published their findings that a constant range of values of oxygen isotope ratios (¹⁸O/¹⁶O)[2] in single zircon crystals collected from the Jack Hills metamorphosed conglomerate in Western Australia suggested that certain continental crusts formed as early as 4.4 to 4.0 Ga (giga anni or billion years ago) and implied somewhat temperate conditions on Earth throughout most of the Archaean eon, meaning liquid water and relatively low temperatures characterized the environment. The surface temperatures inferred through the research were low enough for liquid water being cooled to, near, or below the boiling point, 212° F. The range of δ¹⁸O (oxygen isotope) values, as well as quartz and other trace inclusions within the zircon crystals analyzed by Valley and his colleagues, is constant throughout the Archaean eon (4.4-2.6 Ga), suggesting a long interval of uniform conditions and processes that were conducive to liquid water oceans, stable continental-granitic crust, and possibly life.

According to Valley, liquid water present in the early Earth would have formed oceans rather than a thick, steam-rich atmosphere expected in a Hadean-type early Earth. Meteorite impacts during that period may have been less frequent than previously thought. Consequently, the Cool Early Earth Hypothesis contrasts with earlier ideas that surface lava and subsurface magma covered the Earth, which led to the first 500 million years of Earth history being named Hadean or hell-like. Their research has called into question whether the well-accepted Giant-Impact model for the origin of the Earth-Moon system is compatible with that information and suggested that maybe a planetoid capture model should be considered instead. In addition, Valley’s studies of oxygen isotope ratios (which are commonly used as temperature proxies) in the Jack Hills zircons suggest that the Earth’s surface temperatures did not change appreciably between 4.4 and 2.6 billion years ago,[3] again implying a cool early Earth.

In the last decade, debate on the origins of life has been focused on a deceptively simple question: did life on Earth start in a hot or cold environment? Many earlier researchers — especially the world-famous chemist Stanley Miller, who by firing an electrical current through a chamber containing methane, ammonia, hydrogen, and water obtained amino acids, which are considered by many to be the building blocks of life — argued that the first cells arose in the near boiling waters of hot springs or geothermal vents. However, a small but increasingly prominent band of scientific dissenters insists on life arising from cool oceans. Of special interest is recent research conducted at University of Colorado-Boulder’s Laboratory for Atmospheric and Space Physics by then doctoral student Feng Tian, who published the results (“A Hydrogen-Rich Early Earth Atmosphere”) in the April 7, 2005, issue of Science Express, the online edition of Science Magazine. Tian’s work indicated that from to 30 to 40 percent of the early atmosphere was hydrogen, implying the existence of a more favorable environment for the creation of pre-biotic organic compounds, specifically amino acids, and therefore the possibility of life. Many scientists believe that the early Earth was practically devoid of an atmosphere, or if it had one it was formed by volcanic outgassing of materials trapped in the Earth’s interior, which would have been rich in carbon dioxide, sulfur dioxide, methane, ammonia, and hydrogen sulfide.

However, one possible conclusion from Tian’s research is that the hydrogen-poor, carbon dioxide-rich Mars/Venus-like model of Earth’s early atmosphere that many chemists have relied on for the last four or more decades may lack critical elements, particularly hydrogen. In such atmospheres, organic molecules would not be produced by photochemical reactions or electrical discharges. The premise that early Earth had a hot, carbon dioxide-dominated atmosphere long after its formation caused many scientists to search for evidence of the origin of life in hydrothermal vents in the ocean, fresh-water hot springs, or those brought to Earth from space via meteorites or stellar dust. But Tian and the research team concluded that even if the atmospheric carbon dioxide concentrations were high, hydrogen concentrations would have been larger, with a hydrogen mixing ratio of more than 30 percent. Which would mean that production of pre-biotic organic compounds through the agency of electrical discharge or photochemical reactions may have been sufficient to generate life and more efficient than either exogenous delivery or synthesis in hydrothermal systems. Consequently, the organic soup in the oceans and ponds on early Earth may have created more favorable conditions for the origin of life than has been believed previously.

In an article published in Nature in 2008, Hopkins et al,[4] geoscientists from UCLA, examined over 400 Hadean zircons from Jack Hills. Their results imply a near-surface heat flow about three to five times lower than estimates of Hadean global heat flow, indicating that the magmas from which the Jack Hills Hadean zircons crystallized were formed largely in an underthrust environment that may have been similar to modern convergent margins. Meaning that plate movements may have already begun more than four billion years ago, a condition supporting the Cool Early Earth Theory.

In November 2009 two geoscientists from Stanford University published a study in Nature that analyzed hydrogen and oxygen isotope ratios in 3.4 billion-year-old ocean floor chert in South Africa. Their findings suggest that the early ocean was much more temperate and that life likely diversified and spread across the Earth sooner than has been generally theorized. The approach chosen by Michael Hren and Mike Tice, both Stanford University graduate students at the time, used isotope ratio data to calculate upper and lower bounds for the range of water temperature and composition that could have given rise to the observed ratios. They determined that the ocean temperature could not have been more than 104° F and may have been lower in some parts, indicating that the chemical composition of the ancient world ocean was significantly different from today’s world ocean, and not the 150° to 185° F as had been previously assumed by many researchers. The research implications are many but include that if the composition of the Archean ocean was significantly different from that of today, then the ancient atmosphere must have been different as well because gases move across the air-water boundary with considerable ease since the ocean and lower atmosphere are in a rough equilibrium. The hydrogen-oxygen rations found by Hren and Tice mean that over several billion years the ocean lost large amounts of hydrogen to the atmosphere to bring the hydrogen isotope ratio in seawater to where it is today. And since oxygen, not hydrogen, has built up in Earth’s atmosphere over that same period of time, the atmosphere must have discharged that hydrogen to space.

Author’s Note: Several atmospheric scientists reject Tian’s concept, arguing among other things that the assumptions behind the cold exosphere featured in the Tian model are too unrealistic for the exosphere temperature to be relevant and that that temperature needs more careful analysis as it affects the rate of hydrogen escape, especially considering that today’s escape of hydrogen is predominantly non-thermal. According to that criticism, Tian et al. incorrectly dismiss the importance of non-thermal hydrogen escape from early Earth by making comparisons with the low non-thermal escape characteristics on Venus. However, Venus possesses no magnetic field and thus features different escape physics. Interested readers should be sure to follow this critical and very exciting discussion in the professional journals since this discussion is far from over.

---

[1] See: Valley, John W. 2005. “A Cool Early Earth?” Scientific American, vol. 293, no. 4, pp. 58-65.

[2] Geoscientists calculate the proportion of oxygen-18, which is a rare isotope having eight protons and ten neutrons representing about 0.2 percent of all oxygen on Earth, to oxygen-16, the most common oxygen isotope with eight protons and eight neutrons comprising about 99.8 percent of all oxygen. These isotopes do not undergo radioactive decay or spontaneously change with time and therefore are stable. But, the proportions of ¹⁸O and ¹⁶O incorporated into a crystal as it forms differ depending on the ambient temperature at the time of formation. The ¹⁸O/¹⁶O ratio is well known for magmas formed in the Earth’s mantle, which always have about the same oxygen isotope ratio. Those ratios are calculated relative to that of seawater and expressed in what is called delta (δ) notation.

[3] Valley, J. W.; Peck; W. H.; King, E. M. and Wilde, S. A., 2002. “A cool early Earth,” Geology, vol. 30; no. 4; pp. 351-354.

[4] Hopkins, M.; Harrison, T. M.; and & Manning, C. E., 2008. “Low heat flow inferred from >4 Gyr zircons suggests Hadean plate boundary interactions,” Nature, vol. 456, no. 7221, pp. 493-496.