“The Archean Eon stands out for being so incredibly distant, and incredibly distinct, from modern Earth,” University of Washington astrobiologist, Tyler Robinson,Telled The Daily GalaxyAbout the eon that life on Earth first appeared. “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.
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 is 2018 StudyThis increases the time window in which life could have appeared on these 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 on planetary evolution. Professor at Boston University Earth and Environment 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. NASA image credit.
Although the conditions of the early Earth were long a mystery to scientists, NASA and University of Washington researchers have created a way for them to account the unknown variables of that time. They have also discovered that the conditions of the early Earth may have been more moderately than previously believed.
These findings were applied to other rocky planets by the researchers. Their results are published in the Proceedings of the National Academy of Sciences.
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 imagine early Earth as being wrought with volcanic eruptions and bubbling hot lava. Others see a world in darkness and encased by 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. Volcanoes release carbon into the atmosphere via outgassing carbon dioxide. Then carbonic acid falls to the surface, dissolving the rocks, and releasing the ions. They eventually reach the oceans via the rivers and form calcium carbonate. This process results in carbon being locked up in rocks. Seawater flowing through the ocean crust dissolves surrounding rocks and releases ions. This creates new carbonate rock which then locks in atmospheric carbon. 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.”
Temperature rises with an increase in carbon dioxide emissions. Seafloor weathering decreases as temperature rises. 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.
Researchers compared their model to the past 100 million years Earth history. We now know more about this period. This study is the first to use a realistic, consistent representation of the process to determine the Earth’s early history.
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.”
Comparing the Archean Earth (left), with present day Earth. Because of the high levels of iron ions, Archean oceans are green. The proto-continents that are orange represent magnesium-rich protocontinents before the age of plate tectonics. Image credit to 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. SchwietermanSend 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). It would be thrilling to detect high levels of methane and carbon dioxide in a temperate terrestrial exoplanet. This would provide 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 are on Deck
Future telescopes such as the James Webb Space Telescope, (right), will monitor the atmospheres of distant planets for signs of life. The Earth’s atmosphere (top left) contains several gases that indicate the existence of life, primarily oxygen or ozone. New research shows that the combination of carbon dioxide and abundant methane would have been a sign of life for the Earth’s early days (bottom left). NASA/Wikimedia Commons/Joshua Krissansen-Totton
“NASA concept missions such as the Habitable Exoplanet Observatory – HabEx 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 University of Washington Astrobiology Program wrote 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. The JWST will offer a new way to search for an atmosphere and study its compositions for a select few 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 KaltennegerAssociate professor of Astronomy at the College of Arts and Sciences and Director of the Carl Sagan Institute, said 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 toFive distinct Earth epoch models 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 ShporerResearch Scientist with the MIT Kavli Institute for Astrophysics and Space Research via Cornell UniversityNASA Astrobiology University of Washington, Lisa Kaltenegger, Victoria Meadows, Edward W. Schwieterman, Tyler Robinson
Avi Shporer Research Scientist, MIT Kavli Institute for Astrophysics and Space Research. A Google ScholarAvi was once a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL). His motto, not surprisingly, is a quote from Carl Sagan: “Somewhere, something incredible is waiting to be known.”
