Early Earth –A Model for Emerging Life on Alien Planets?

 Early Earth

 

“The Archean Eon stands out for being so incredibly distant, and incredibly distinct, from modern Earth,” University of Washington astrobiologist, Tyler Robinson, told The Daily Galaxy about the eon when life on Earth likely emerged. “The conditions on this near-alien version of Earth are so unique that the James Webb Space Telescope (JWST) should be able to distinguish Archean-like features from signatures more synonymous with modern Earth, Mars, or Venus. Of course, the entire astronomical community, including exoplanet scientists, are excitedly awaiting the launch of JWST and some first hints of its true capabilities.”

Early Earth’s regulatory carbon and seafloor weathering process would occur on any rocky planet with water. “There’s nothing special about these processes,” says Joshua Krissansen-Totton from the University of Washington’s astrobiology program and Virtual Planetary Laboratory. “We know pre-solar nebulae contained the ingredients for life; we also know countless exoplanets with those ingredients exist in habitable zones.”

This 2018  study widens the window of time on which life could have emerged on those planets. 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,” said Krissansen-Totton, “but that would restrict origins to locally warm environs like hydrothermal vents.”

Mars once had most of what Earth has going for it, or so we think”

The study also has implications for planetary evolution. Boston University Earth and Environment professor Andrew Kurtz, who was not part of the study, 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 time-frame and likelihood of life persisting elsewhere is greater than first thought”

An artist’s concept of the early Earth is shown above. 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, concluded that the time-frame and likelihood of life persisting elsewhere is greater than first thought.

Earth’s 4.5-billion-year History Leaves Room for Many Geological Phases

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 Krissansen-Totton.

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 difficult part is 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).

Disequilibrium –The Unmistakable Signal of Alien Biological Life

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 then 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.

A “Natural Thermostat”

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

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,” said Krissansen-Totton.

“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],” explained Kurtz, who was not part of the study.

The researchers tested their model against the last 100 million years of Earth history, about which we know far more details. 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 reasonable climate and pH history that is physically sensible and mathematically internally consistent.”

 

Early and Modern Earth

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.

Eon Models

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 (7.0 is neutral). Kurtz notes that this result is consistent with an influential 2002 paper arguing the likelihood of a “cool early Earth”.

The James Webb Space Telescope is best suited to characterize the atmospheres of planets transiting nearby M dwarf stars”

Habitable Exoplanet Observatory –Detecting an Earth Analog

“The detectability of atmospheric constituents on an exoplanet depends on the type and sensitivity of the instrumentation, the wavelengths observed (e.g., visible or infrared), and the observing mode (such as transit spectroscopy or direct-imaging spectroscopy) in addition to the composition of the target planet’s atmosphere,” UC Riverside astrobiologist  Edward W. Schwieterman wrote in an email to The Daily Galaxy.

“JWST is best suited to characterize the atmospheres of planets transiting nearby M dwarf stars,” Schwieterman continued in his email. “The spectral features of carbon dioxide and methane, which we may expect to be abundant in the atmospheres of planets like the Archean Earth, are more readily observable by JWST than more difficult to detect gases like molecular oxygen (O2). Detection of high levels of both methane and carbon dioxide on a temperate terrestrial exoplanet would be exciting because it would indicate a chemical disequilibrium between those carbon gases, which is potential evidence for life. Even the detection of evidence of simple life in the atmosphere of another world would be a revolutionary scientific discovery.” 

Future Telescopes on Deck

Future telescopes like the James Webb Space Telescope (right) will observe the atmospheres of distant planets to seek evidence of life. Earth (top left) has several gases in its atmosphere that reveal the presence of life, primarily oxygen and ozone. The new study finds that for the early Earth (bottom left), the combination of abundant methane and carbon dioxide would provide an alternative sign of life. NASA/Wikimedia Commons/Joshua Krissansen-Totton

 

Planetary Atmosphere Disequilibrium

“NASA concept missions such as the Habitable Exoplanet Observatory (HabEx) and LUVOIR (The Large UV/Optical/IR Surveyor), would be better able to detect molecular oxygen on planets like the Earth today with reflected visible light direct-imaging observations.” Schwieterman added.

Victoria Meadows, Director of the University of Washington Astrobiology Program, wrote in an email to The Daily Galaxy: “I’m not sure that JWST will be able to identify a planet as being in an Archean phase, because the molecules we are most likely sensitive to have been present throughout Earth’s history.  But, the JWST will provide an exciting new capability to search for the presence of an atmosphere, and even study their compositions, for a few select Earth-sized targets.  The observations will be challenging, but JWST may be able to detect the presence of carbon dioxide and methane in the atmosphere of a habitable zone terrestrial exoplanet, two molecules that have been present in our atmosphere for much of Earth’s history.”

The Last Word — Identifying Life on the Universe’s ‘Pale Blue Dots’

“I am very excited about upcoming JWST observations. JWST will for the first time allow us to probe the atmosphere of small rocky exoplanets,” Lisa Kaltenneger, associate professor of astronomy in the College of Arts and Sciences and director of the Carl Sagan Institute, told The Daily Galaxy. “The atmospheric composition on Earth changed considerably over time which left markers in Earth’s air that JWST can now search for.”

Our Earth and the air we breathe have changed drastically since Earth formed 4.5 billions years ago”

“These new generation of space- and ground-based telescopes coupled with our models will allow us to identify planets like our Earth out to about 50 to 100 light-years away,” said Kaltenegger, about forthcoming telescopes like NASA’s James Webb Space Telescope, scheduled to launch in March 2021, or the Extremely Large Telescope in Antofagasta, Chile, scheduled for first light in 2025, astronomers could watch as an exoplanet transits in front of its host star, revealing the planet’s atmosphere.

“Our Earth and the air we breathe have changed drastically since Earth formed 4.5 billions years ago,” Kaltenegger said in a Cornell University article–referring to five distinct models of Earth epochs her team created to provide a template for how we can characterize a potential exo-Earth – from a young, prebiotic Earth to our modern world– “and for the first time, this paper addresses how astronomers  trying to find worlds like ours, could spot young to modern Earth-like planets in transit, using our own Earth’s history as a template.”

“The models,” Kaltenegger explained, “allow us to explore at what point in Earth’s evolution a distant observer could identify life on the universe’s ‘pale blue dots’ and other worlds like them.”

According to Kaltenegger’s models, if astronomers can find exoplanets with nearly 1% of Earth’s current oxygen levels, those scientists will begin to find emerging biology, ozone and methane – and can match it to ages of the Earth templates. “Our transmission spectra show atmospheric features, which would show a remote observer that Earth had a biosphere as early as about 2 billion years ago,” she said.

Avi Shporer, Research Scientist, with the MIT Kavli Institute for Astrophysics and Space Research via Cornell University, NASA Astrobiology, University of Washington, Lisa Kaltenegger, Victoria Meadows, Edward W. Schwieterman, Tyler Robinson

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