The Astrobiology Institute
Moffett Field - June 4, 2001
Initially, the surface of our planet was a fiery, molten stew. These early hellish conditions inspired scientists to call the time period from 4.5 to 3.8 billion years ago the "Hadean" era.
But the Earth was not molten all throughout the Hadean. Within a few million years, the crust cooled and water vapor rained down to form the oceans, although the inside of the planet still remained very hot. It is thought that life soon after may have made its first appearance on Earth, either in the newly formed oceans, or in clay or rocks within the Earth's crust.
The Sun at that time was much dimmer than it is today - it has steadily brightened since the Solar System formed. Yet geological evidence says that even after the Earth cooled from its initial molten conditions, it was so warm 3.8 billion years ago (Ga) that there weren't any glaciers. In other words, it must have been much warmer at that time than it is today.
How could a dimmer Sun have kept the planet warmer? Some have suggested that massive amounts of greenhouse gases like carbon dioxide (CO2) could have trapped in heat. But a study published in the Journal of Geophysical Research - Planets by Norman Sleep and Kevin Zahnle says the early Earth had very little CO2 in the atmosphere during both the Hadean and the Archean (3.8 - 2.5 Ga) eras.
Sleep and Zahnle came to this conclusion by analyzing the implications of differences between the modern and early Earth. For one thing, the early Earth had a faster plate tectonic cycle in the late Hadean and throughout the Archean. This more active cycle tended to bury more CO2 in the crust and mantle, as shifting tectonic plates plowed carbonates and other minerals under the planet's surface.
They also point out that the early Earth was bombarded by many comets and asteroids, especially during the Hadean. The resulting impact ejecta, they say, would have reacted with CO2, further reducing the amount of free CO2 in the atmosphere.
"Asteroid and comets eject great masses of material from their craters when they hit the Earth," says Sleep. "The ejected material is fine-grained, often glassy, and very reactable. It reacted with the available CO2 in the air and the ocean. This kept CO2 at low levels on the early Earth. It works a little bit like having a CO2 scrubber around."
With little CO2 in the atmosphere, unless other greenhouse gases were abundant during the Hadean and Archean, there would not have been anything to prevent heat from escaping into space.
How then, could the late Hadean/early Archean have been warm? The scientists suggest that methane could have slowly built up during the Hadean, and by 3.8 Ga. it could have been abundant enough to warm the Earth.
Although Sleep admits that methane would have been rare before life arrived, he suggests that pre-biotic methane could have been produced by chemical reactions between water and the metallic iron in asteroids.
This prebiotic methane may have helped spur along the development of life on the planet. Then, once life emerged, methane-producing organisms could have driven atmospheric methane levels high enough to keep the planet warm.
Life on Ice
To support the theory that pre-biotic methane may have played an important role in the development of life during the Hadean, Sleep cites the Miller-Urey experiments conducted in the 1950s. These experiments tried to replicate the environmental conditions of the primitive Earth to see if organic compounds would form.
When methane was included, the experiments showed that complex reduced carbon compounds, such as amino acids, were formed by non-biological processes. This seemed to prove that complex organic molecules can build up from very simple organic molecules, such as methane.
The precise gas mixture used in the Miller-Urey experiments has since fallen out of favor with scientists as the most likely pre-biotic atmosphere. Still, methane could have been a component of the Earth's early atmosphere.
But even if pre-biotic methane and CO2 were a part of the Hadean atmosphere, their levels were so low that the Earth was probably very cold back then. In fact, Sleep and Zahnle suggest a variation on the "Snowball Earth" hypothesis: that at various times, the Earth during the Hadean era was so cold it was almost entirely covered with a crust of ice.
According to the Snowball Earth hypothesis put forward by some scientists, the Earth went through at least two and possibly up to six "snowball" phases between 750 million and 580 million years ago, during the Neoproterozoic era.
The average temperature during these deep-freeze periods would have been around minus 50 degrees Celsius (minus 58 F), and a kilometer-thick layer of ice may have covered most of the planet's surface.
Volcanoes probably dotted the icy landscape, expelling gases and creating isolated islands of heat. All but a tiny fraction of the planet's primitive organisms on the surface would have died.
Although the Neoproterozoic era occurred long after the Hadean, Sleep and Zahnle see a correlation between their argument about the lack of atmospheric CO2 on early Earth and the Snowball Earth hypothesis.
"Evidence for a few extremely cold episodes - Snowball Earths around 2.3 and 0.6 to 0.8 billion years ago - indicates that pCO2 (partial pressure, or concentration of CO2) was not always large enough to keep things warm and thereby casts a little more doubt on the hypothesis that CO2 was the only important greenhouse gas on ancient Earth," their report reads.
One of the problems facing the Snowball Earth hypothesis is figuring out how the Earth transitioned from a Snowball phase to a warming phase, not just once, but several times. Some have suggested the Earth warmed up over time due to huge amounts of CO2 trapped under the ice.
But Sleep says that much CO2 could not have built up because of large open areas in the ice. These open areas would allow the sea to absorb CO2 from the atmosphere.
This air-sea exchange occurs today wherever CO2 levels between the ocean and air are out of balance, even in ice-covered areas like Antarctica.
"The usual story has a CO2 build-up in the air ending the snowball episode suddenly," says Sleep. But "even local areas of open water would've prevented a massive trapping of CO2 in the atmosphere out of equilibrium with the ocean."
Sleep suggests instead that atmospheric methane could have built up in great enough abundance to push the Earth out of its Snowball phases.
The last Snowball Earth episode is thought to have occurred just before the Cambrian explosion 575 to 525 million years ago, when life experienced a huge leap in evolution. Multi-cellular life diversified at a fast rate, forming most of the major groups of animals still around today.
Some scientists believe the environment may have contributed to this burst of evolution, so the last Snowball Earth phase - if it happened - could have played a role. Long periods of isolation, along with the extreme environments on a Snowball Earth, would likely have contributed to genetic change.
But could life have originated during a Hadean Snowball Earth? The popular theory for life's origins has it taking place at the searing hot hydrothermal vents at the bottom of the oceans, but Sleep thinks it is more likely that life originated under cold conditions.
"A freeze-thaw cycle creates chemical disequilibrium, and hence energy for life," says Sleep. "The hydrothermal vents also supply disequilibrium, but it is much easier to start life around the freezing point of water than the boiling point because an inept high-T organism [an organism that cannot handle high temperatures] does poorly.
"Still, whether this occurs on the Earth or Mars or even elsewhere in the inner solar system is unclear. But there is extensive photosynthesis and life within modern sea ice. Freezing concentrates fluid by removing water to make ice, and it also isolates numerous microenvironments in the remaining fluid - or brine - within the ice."
For the future, Sleep says he hopes to further understand the role carbon dioxide played in the early Earth's environment. Another goal of his work is to figure out the timeline of environmental conditions soon after the Moon was formed. The Moon formed during the Hadean, when two planet-sized bodies collided. The resulting larger body became the Earth while the smaller one became the Moon.
"We are trying to further quantify subduction of CO2 and how long hot conditions lasted on the Earth after the moon-forming impact," Sleep says. But this work will be difficult, because the rock record only goes back to 3.8 Ga. "There are no intact rocks on the Earth from before that time," but "there may be some information from very old zircon crystals in younger sediments."
NASA Astrobiology Institute
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