The Hadean Eon

This is the end of the discussion of astronomy, and the rest of this book focuses on Earth. The Hadean Eon occupied the first 700 million years (4.6 - 3.9 Ga) of Earth’s history. Hadean means hell, but much of this eon was probably not as hellish as the name implies. Although it was not habitable for humans, the climate of the Hadean was the essential first step in the evolution of life on Earth. With periodic methane atmospheres, UV light, liquid water, and chemicals such as hydrogen cyanide and hydrogen sulfide, it might be called the Garden of Eden for the origin of life. Life possibly evolved twice after periods of intense asteroid bombardment, in the middle Hadean Eon and after the Late Heavy Bombardment at the end of the Hadean Eon.

The supernova that caused the collapse of the molecular cloud that formed the sun emitted highly radioactive Aluminum 26 (²⁶Al), which impregnated the molecular cloud that formed the solar system. The half-life of ²⁶Al is 700,000 years, which means that half of whatever level of ²⁶Al that originally existed in a planetesimal would have decayed within 700,000 years. Then, half of the remaining ²⁶Al would decay within the next 700,000 years. This is a very fast decay compared to uranium, which has a half-life of 4.5 billion years. Because of its rapid decay, ²⁶Al releases a tremendous amount of heat and melts planetesimals and planets. This causes differentiation, in which the surface of the planet melts and iron and other heavy substances sink to the core, and silicates, organic elements, and water, essential to life, rise to the surface.

The earth cooled to the point that water was on the surface after a few hundred million years; however, much, or all, of the earth's surface melted again during the Late Heavy Bombardment (LHB), 3.9 Ga, and the only remaining geologic evidence of the Hadean Eon is tiny zircon crystals that survived the high surface temperatures of the LHB. These crystals contain high fractions of uranium. The decay of uranium to lead has a half-life of 4.5 billion years so they are ideal for radiometric dating of the Hadean Eon. Because zircon crystals only form in liquid water, and a zircon crystal is dated 4.37 billion years ago, we known that liquid water and a solid crust existed only 200 million years after the initial formation of the earth.

Scientists estimate the temperature of the Earth’s crust, and thus the intensity of asteroid bombardment vs. time, by looking at the zircon crystals from the Hadean Eon. After the crust solidified, the hottest temperatures and thus the most intense bombardment period took place between 3.95 and 3.85 billion years ago,[1] which is strong support for the fact that there was a period of intense asteroid bombardment during the Late Heavy Bombardment.

The Late Heavy Bombardment and other periods of asteroid bombardment added a “Late Veneer” to the earth after the original heavy elements sank to the center of the earth in the process of differentiation. The quantity of precious metals is 100 times greater than expected since most should have fallen to the core of the earth along with iron and other heavy elements.

Earth's initial (primary) atmosphere was primarily hydrogen and helium; however, Earth's gravitational field is not strong enough to hold on to these light elements. In contrast, water, carbon dioxide, nitrogen, and oxygen gases are too heavy to escape Earth's gravitational field. The reason for this is that all molecules in gases have the same kinetic energy, which means that lighter gases such as hydrogen and helium travel at much higher velocities than heavier molecules such as oxygen. Thus, the gases that were beneficial to life remained in Earth's atmosphere, but those that were detrimental escaped into space. Volcanic emissions supplied the secondary atmosphere of water, carbon dioxide, and methane. Brief periods of asteroid impact on Earth generated highly reducing methane atmospheres that lasted approximately one million years. These intervals were critical for the origin of life, which required a highly reducing atmosphere. Algae converted carbon dioxide over the billions of years of the Precambrian.

Elizabeth Bell and collaborators discovered possible signs of life in the mid Hadean Eon.[2] They analyzed 10,000 zircon crystals and found 656 with dark specks. Of these 656 zircon crystals, 79 looked like they might contain graphite, which is a sign of life. Of the 79 crystals, one contained graphite. The delta carbon 13 test (δ¹³C _(PPD)) measures the ratio of ¹³C in comparison to ¹²C. Living organisms prefer ¹²C isotope and preferentially exclude ¹³C. The graphite had a delta carbon 13 (δ¹³C _(PPD)) value of -25, which is a strong indication of low ¹³C and the fact that the carbon came from a living organism. Coauthor Mark Harrison stated, "It was nerve-wracking to manipulate the sole tiny zircon fragment — about half the width of a hair on your head — containing the graphite inclusions." This possible evidence of life in 1 in 10,000 zircon crystals is not proof of life. There could be many possible causes of this result, such as research error, but it justifies future investigations of life in the Hadean Eon.

References

[1] Abbott, Sunshine S. et al. A search for thermal excursions from ancient extraterrestrial impacts using Hadean zircon Ti-U-Th-Pb depth profiles. PNAS 2012;109:34:13486-13492

[2] Bell, Elizabeth A., Patrick Boehnke, T. Mark Harrison, and Wendy L. Mao. "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon." Proceedings of the National Academy of Sciences 112, no. 47 (2015): 14518-14521

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Artist's representation of Hadean Eon during the period prior to the cooling of the crust, or just after a asteroid bombardment. Credit: Tim Bertelink. Used here per CC BY-SA 4.0