Habitability of the young Earth could boost the chances of life elsewhere

An artist’s concept of the early Earth. While still fairly inhospitable compared to today’s standards, the early Earth may have had a more moderate climate and ocean temperature and pH than had been thought. Image credit: NASA.

The conditions on the early Earth have long been a mystery, but researchers from NASA and the University of Washington have now devised a way to account for the uncertain variables of the time, in turn discovering that the conditions of early Earth may have been more moderate than previously thought.

By applying these findings to other rocky planets, the researchers, whose results are published in the Proceedings of the National Academy of Sciences, conclude that the time-frame and likelihood of life persisting elsewhere is greater than first thought.

Given that we have no rocks or other material from Earth’s first 500 million years, approximations of conditions on our planet during that time have varied widely. Some picture early Earth as wrought by volcanic eruptions and bubbling with lava, while others envision a world asleep and encased in ice. Earth’s 4.5-billion-year history leaves room for many geological phases and “people have used all kinds of different geochemical datasets to get some measure of surface conditions,” says the study’s lead author Joshua Krissansen-Totton from the University of Washington.

The researchers focused on the Archean Eon, 4 billion to 2.5 billion years ago, shortly after the formation of Earth’s crust, atmosphere, and oceans. It’s also when life likely emerged.

The difficultpartis in deducing ocean pH and global temperature, about which estimates fluctuate drastically, from alkaline to corrosively acidic and from –25 to 85 degrees Celsius (–13 to 185 degrees Fahrenheit).

A natural thermostat

Earth’s carbon cycle holds the key to constraining these variables. Volcanos push carbon into the atmosphere by outgassing carbon dioxide, then carbonic acid rains down to the surface, dissolving rocks and releasing the ions inside, which eventually reach the oceans via rivers and form calcium carbonate. The net result of this process is that carbon in the air is locked up in rocks. Similarly, seawater circulating through the ocean crust dissolves the surrounding rock, releasing ions that them form new carbonate rocks, which also locks up atmospheric carbon in the crust. Some of this carbon is subducted back into the planet’s mantle and starts the cycle anew as it’s outgassed again by volcanoes.

These weathering processes are temperature dependent; Krissansen-Totton likens it to a “natural thermostat.”

A schematic of the carbon cycle on the early Earth, in which carbon enters the ocean from the atmosphere and eventually becomes part of carbon-bearing rocks on the sea floor that undergo weathering, dissolving the carbon. Image credit: Creative Commons BY-NC-ND.

If carbon dioxide emissions increase, the temperature increases; if the temperature increases, seafloor weathering increases. Because it took billions of years to create Earth’s continents, less land existed on the early Earth, so seafloor weathering had a particularly significant regulatory impact on Earth’s temperature and vice versa.

Researchers applied their understanding of the carbon cycle based on data from the last 100 years and, instead of choosing any single theory regarding ocean composition and climate, they “picked the broadest range for the unknown and then calculated the range of possibilities for climate and ocean pH,” Krissansen-Totton tells Astrobiology Magazine.

“The researchers came up with new ways to describe how carbon in sediment and rock pore water is consumed by chemical reactions [in seafloor weathering],” explains Boston University Earth and Environment professor Andrew Kurtz, who was not part of the study.

A robust picture of early Earth

The researchers tested their model against the last 100 million years of Earth history, about which we know far more details, for a paper they published last year. This new study is the first to deploy a realistic and self-consistent representation of the process and to apply that to the early Earth. 

The simulations aren’t exact and don’t resolve all uncertainties, but according to Krissansen-Totton, they provide “robust” information about early Earth. Kurtz affirms that the results “produce a seemingly reasonably climate and pH history that is physically sensible and mathematically internally consistent.”

A comparison between the Archean Earth (left) and present day Earth. The Archean oceans appear green as a result of a high amount of iron ions present. The orange shapes represent magnesium-rich proto-continents, before the era of plate tectonics. Image credit: Ming Tang/University of Maryland.

The first half-billion years of the Earth’s life is a period called the Hadean Eon, so-named because of its hellish heat. However, the study’s results challenge the notion that Earth remained scorching hot well into the Archean Eon. After the heat from Earth’s formation dissipated, the researchers’ models suggest that the climate and ocean pH were surprisingly moderate: between 0 and 50 degrees Celsius (32–122 degrees Fahrenheit) with a pH of between 6.2 and 7.7 (7 is neutral). Kurtz notes that this result is consistent with an influential 2002 paper arguing the likelihood of a “cool early Earth.”

Krissansen-Totton believes the regulatory carbon/seafloor weathering process would occur on any rocky planet with water. “There’s nothing special about these processes,” he says. We know pre-solar nebulae contained the ingredients for life; we also know countless exoplanets with those ingredients exist in habitable zones. The study widens the window of time on which life could have emerged on those planets.

More opportunities for life

The model doesn’t resolve debates about exactly when or where life emerged, but it steers scientists in productive directions for further research. For example, “if you believe life on Earth started at high temperatures, that could still be true,” Krissansen-Totton tells Astrobiology Magazine, “but that would restrict origins to locally warm environs like hydrothermal vents.”

The study also has implications for planetary evolution. Kurtz points out that “Mars once had most of what Earth has going for it, or so we think: water on the surface, carbon dioxide in the atmosphere, and silicate rocks,” which seem to support the possibility of life having once existed there. Scientists believe Mars’ atmosphere was vented into space via solar winds, but questions remain as to what upset the Red Planet’s cyclical balance, as well as whether other planets could experience such drastic conditional changes.

The work was supported by NASA Astrobiology through the Exobiology & Evolutionary Biology Program and the Virtual Planetary Laboratory, as well as through the Earth and Space Science Fellowship program.