Cornell University astronomers have created five supercomputer models representing key points from our planet's evolution, like chemical snapshots through Earth's own geologic epochs.

The models will be spectral templates for astronomers to use in the approaching new era of powerful telescopes, and in the hunt for Earth-like planets in distant solar systems.

"These new generations 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 Lisa Kaltenegger, associate professor of astronomy and director of the Carl Sagan Institute.

For the research and model development, Kaltenegger, doctoral student Jack Madden and Zifan Lin authored "High-Resolution Transmission Spectra of Earth through Geological Time," published in Astrophysical Journal Letters. {module INSIDE STORY}

"Using our own Earth as the key, we modeled five distinct Earth epochs to provide a template for how we can characterize a potential exo-Earth - from a young, prebiotic Earth to our modern world," she said. "The models also 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."

Kaltenegger and her team created atmospheric models that match the Earth of 3.9 billion years ago, a prebiotic Earth when carbon dioxide densely cloaked the young planet. A second throwback model chemically depicts a planet free of oxygen, an anoxic Earth, going back 3.5 billion years. Three other models reveal the rise of oxygen in the atmosphere from a 0.2% concentration to modern-day levels of 21%.

"Our Earth and the air we breathe have changed drastically since Earth formed 4.5 billion years ago," Kaltenegger said, "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."

In Earth's history, the timeline of the rise of oxygen and its abundance is not clear, Kaltenegger said. But, 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," Kaltenegger said.

Using 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 transit in front of its host star, revealing the planet's atmosphere.

"Once the exoplanet transits and blocks out part of its host star, we can decipher its atmospheric spectral signatures," Kaltenegger said. "Using Earth's geologic history as a key, we can more easily spot the chemical signs of life on the distant exoplanets."

Scripps Oceanography researcher's assertion bolstered by series of new studies

New research lends credence to an unorthodox retelling of the story of early Earth first proposed by a geophysicist at Scripps Institution of Oceanography at UC San Diego.

In a study appearing March 15 in the journal Earth and Planetary Science Letters, Scripps Oceanography researchers Dave Stegman, Leah Ziegler, and Nicolas Blanc provide new estimates for the thermodynamics of magnetic field generation within the liquid portion of the early Earth's mantle and show how long that field was available.

The paper provides a "door-opening opportunity" to resolve inconsistencies in the narrative of the planet's early days. Significantly, it coincides with two new studies from UCLA and Arizona State University geophysicists that expand on Stegman's concept and apply it in new ways. {module INSIDE STORY}

"Currently we have no grand unifying theory for how Earth has evolved thermally," Stegman said. "We don't have this conceptual framework for understanding the planet's evolution. This is one viable hypothesis."

The trio of studies are the latest developments in a paradigm shift that could change how Earth history is understood.

It has been a bedrock tenet of geophysics that Earth's liquid outer core has always been the source of the dynamo that generates its magnetic field. Magnetic fields form on Earth and other planets that have liquid, metallic cores, rotate rapidly, and experience conditions that make the convection of heat possible.

In 2007, researchers in France proposed a radical departure from the long-held assumption that the Earth's mantle has remained entirely solid since the very beginnings of the planet. They argued that during the first half of the planet's 4.5-billion-year history, the bottom third of Earth's mantle would have had to have been molten, which they call "the basal magma ocean." Six years later, Stegman and Ziegler expanded upon that idea, publishing the first work showing how this once-liquid portion of the lower mantle, rather than the core, could have exceeded the thresholds needed to create Earth's magnetic field during that time.

The Earth's mantle is made of silicate material that is normally a very poor electrical conductor. Therefore, even if the lowermost mantle were liquid for billions of years, rapid fluid motions inside it wouldn't produce large electrical currents needed for magnetic field generation, similar to how Earth's dynamo currently works in the core. Stegman's team asserted the liquid silicate might actually be more electrically conductive than what was generally believed.

"Ziegler and Stegman first proposed the idea of a silicate dynamo for the early Earth," said UCLA geophysicist Lars Stixrude. The idea was met with skepticism because their early results "showed that a silicate dynamo was only possible if the electrical conductivity of silicate liquid was remarkably high, much higher than had been measured in silicate liquids at low pressure and temperature."

A team led by Stixrude used quantum-mechanical supercomputations to predict the conductivity of silicate liquid at basal magma ocean conditions for the first time.

According to Stixrude, "we found very large values of the electrical conductivity, large enough to sustain a silicate dynamo." The UCLA study appeared in an academic journal. 

In another paper, Arizona State geophysicist Joseph O'Rourke applied Stegman's concept to consider whether it's possible that Venus might have at one point generated a magnetic field within a molten mantle.

These new studies are signs that the premise is starting to take hold, but is still far from being widely accepted.

"No one is going to believe it until they do it themselves and now two other highly esteemed scientists have done it themselves," said Stegman.

"The pioneering studies of Dave Stegman and his collaborators directly inspired my work on Venus," said O'Rourke. "Their recent paper helps answer a question that vexed scientists for many years: How has Earth's magnetic field survived for billions of years?"

If Stegman's premise is correct, it would mean the mantle could have provided the young planet's first magnetic shield against cosmic radiation. It could also underpin studies of how tectonics evolved on the planet later in history.

"If the magnetic field was generated in the molten lower mantle above the core, then Earth had protection from the very beginning and that might have made life on Earth possible sooner," Stegman said.

"Ultimately, our papers are complementary because they demonstrate that basal magma oceans are important to the evolution of terrestrial planets," said O'Rourke. "Earth's basal magma ocean has solidified but was key to the longevity of our magnetic field."