Artist conception of early Earth. (NASA/JPL-Caltech)
We all know that life has not been found so far on any planet beyond Earth — at least not yet. This lack of discovery of extraterrestrial life has long been used as a knock on the field of astrobiology and has sometimes been put forward as a measure of Earth’s uniqueness.
But the more recent explosion in exoplanet discoveries and the next-stage efforts to characterize their atmospheres and determine their habitability has led to rethinking about how to understand the lessons of life of Earth.
Because when seen from the perspective of scientists working to understand what might constitute an exoplanet that can sustain life, Earth is a frequent model but hardly a stationary or singular one. Rather, our 4.5 billion year history — and especially the almost four billion years when life is believed to have been present — tells many different stories.
For example, our atmosphere is now oxygen-rich, but for billions of years had very little of that compound most associated with complex life. And yet life existed.
The same with temperature. Earth went through snowball or slushball periods when most of the planet’s surface was frozen over. Hardly a good candidate for life, and yet the planet remained habitable and inhabited.
And in its early days, Earth had a very weak magnetic field and was receiving only 70 to 80 percent as much energy from the sun as it does today. Yet it supported life.
“It’s often said that there’s an N of one in terms of life detected in the universe,” that there is but one example, said Timothy Lyons, a biogeochemist and distinguished professor at University of California, Riverside.
“But when you look at the conditions on Earth over billion of years, it’s pretty clear that the planet had very different kinds of atmospheres and oceans, very different climate regimes, very different luminosity coming from the sun. Yet we know there was life under all those very different conditions.
“It’s one planet, but it’s silly to think of it as one planetary regime. Each of our past chapters is a potential exoplanet.”
A particularly extreme phase of our planet’s history is called the “Snowball Earth” period. During these episodes, the Earth’s surface was entirely or largely covered by ice for millions of years, stretching from the poles to the tropics. One such freezing happened over 700 to 800 million years ago in the Pre-Cambrian, around the time that animals appeared. Others are now thought to have occurred much further back in time. They varied in duration and extent but during a full-on snowball event, life could only survive in ice-free refuges, or where sunlight managed to penetrate through the ice to allow photosynthesis. No image credit.
Lyons is the principal investigator for one of the newer science teams selected to join the NASA Astrobiology Institute (NAI), an interdisciplinary group hat calls itself “Alternative Earths.”
Consisting of 23 scientists from 14 institutions, its self-described mission is to address and answer these questions: How has Earth remained persistently inhabited through most of its highly changeable history? How has the presence of very different kinds of lifeforms been manifested in the atmosphere, and simultaneously been captured in what would become the rock record? And how might this approach to early Earth help in the search for life beyond Earth?
“The idea that early Earth can help us understand other planets and moons, especially in our solar system, is certainly not new,” Lyons said. “Scientists have studied possible Mars analogues and extreme life for years. But we’re taking it to the next level with exoplanets, and pushing hard on the many ways that conditions on early Earth can help us study exoplanet atmospheres and habitability.”
The importance of this work was apparent at a recent workshop on biosignatures held by NASA’s initiative, the Nexus for Exoplanet System Science (NExSS.) As Earth scientists, Lyons and his group are expert at finding proxy records in ancient rocks that hold information important to exoplanet scientists (among others) want to know.
Those proxy fingerprints occur as elemental, molecular, and isotopic properties preserved in rocks that correspond to ancient characteristics in the ocean or atmosphere that can no longer be observed directly.
“We can’t measure the pH in ancient oceans, and we can’t measure the composition of ancient atmospheres,” Lyons said. “So what we have to do is go to the chemistry of ocean and land deposits formed at the same time and look for the chemical fingerprints locked away and preserved.”
At the exoplanet biosignatures workshop, Lyons was struck by how eager exoplanet modelers were to learn about the proxy chemicals they could profitably put in their models for clues about how distant planet atmospheres might form and behave. It’s clear that no single element or compound will be a silver bullet for understanding whether there’s life on an exoplanet, but a variety of proxy results together can begin to tell an important story.
The element chromium and its isotopes have become important proxies for the measurement of oxygen levels in the atmosphere of early Earth and have led to some revised theories about when those concentrations jumped. Understanding the potential makeup of early Earth’s atmosphere and oceans is a pathway to understanding exoplanets. No image credit.
“We told them about the range of things they should be modeling and, wow, they were interested. I was thinking at the time that ‘you guys really need us — and vice versa.’”
Some of the researchers most intrigued by potentially new geochemical proxies from the University of Washington’s Virtual Planetary Laboratory, They’ve been a pioneer in modeling how different atmospheric, geological, stellar and other factors characterize particular kinds of planets and solar systems and their possibilities for life.
In keeping with the growing connection between exoplanet and Earth science, Lyons just brought one of the VPL top modellers, Edward Schwieterman, to UC Riverside for a postdoc as part of the Alternative Earths project.
Among his initial projects will take the new data being generated by the Alternative Earths team about the atmosphere and oceans of early Earth, and model what would happen on a planet with that kind of atmosphere if it was orbiting a very different type of star from our own.
“It’s a direct use of early Earth research on exoplanet studies, and is exactly the kind of work we plan to do be doing,” Lyons said. “Eddie is the perfect bridge between the lessons learned from early Earth and their implications for exoplanets.”
Banded iron formations at Karijini National Park, Western Australia. The layers of reddish iron point to an early ocean poor in oxygen and rich in dissolved iron. These formations date most commonly from the periods just before and right after the Great Oxidation Event, which spanned from about 2.4 to 2.0 billion years ago. Their distributions over times and their chemical properties are key proxies for the tempo and fabric of the earliest permanent oxygenation of Earth’s atmosphere. No image credit.
Lyons, along with colleagues Christopher Reinhard of Georgia Tech and Noah Planavsky of Yale and other members of their Alternative Earths team, are especially focused on an effort to understand Earth’s atmosphere—as tracked in the rock record—over the eons and especially the levels of oxygen present.
The concentration of oxygen in the atmosphere is now about about 21 percent and, by some estimates, reached as high as 35 percent within the past 500 million years.
In comparison, early Earth had but trace amounts of oxygen for two billion years before what is called the Great Oxidation Event—when marine O2-producing photosynthesis outpaced reactions that consumed O2 and allowed for the beginnings of its permanent accumulation in the atmosphere. Estimated to have occurred 2.4 billion years ago, it began (or was part of) an oxidizing process that led to ever more complex life forms over the following one to two billion years.
There is a spirited scientific debate underway now about whether that “Great Oxidation Event” triggered permanently high levels of oxygen in the atmosphere and the oceans, or whether it began an up and down process through which the presence of oxygen was quite unstable and still well below current levels until relatively recent times.
Lyons and Reinhard are of the “boring billion” school, arguing that oxygen levels did not head continuously upwards after the Oxidation Event, but rather stayed relatively stable and still very low for most of a billion and half years after the Great Oxidation Event and continued to challenge O2-requiring life—for almost a third of Earth history.
This would be primarily an Earth science issue if not for the fact that oxygen — on its own and in conjunction with other compounds — is among the most prominent and promising biosignatures that exoplanet scientists are looking for.
In fact, not that long ago, it was widely accepted that a discovery of oxygen and/or ozone in the atmosphere of a planet pretty much proved, or at least strongly suggested, the presence of some sort of biology on the planet below. That view has been modified of late by the identification of ways that free oxygen can be formed abiotically (without the presence of photosynthesis and life), potentially producing false positives for potential life.
While the field is a long way from an active search for direct, in situ fingerprints of life on exoplanets light years away, oxygen and its relationship with other atmospheric gases remains a lodestar in thinking about what biosignatures to search for. The technology is already in place for characterizing the compositions of very distant atmospheres.
And this is where, for Lyons, Reinhard and others, things get both interesting and complicated.
For more than a billion years before the Great Oxidation Event Earth demonstrably supported life. It consisted mostly of anaerobic microbes that did just fine without oxygen, but in many cases needed and produced of methane, an organic compound with one carbon atom and four hydrogen.
So if an exoplanet scientist from a distant world were to search for life on Earth during that period via the detection of oxygen only, they would entirely miss the presence of an already long history of life. Searching for the large-scale presence of methane might have been more productive, though that is a source of rigorous debate as well.
An image of a rock with fossilized stromatolites, tiny layered structures from 3.7 billion years ago that are remnants from a community of microbes. Found in a part of Greenland new exposed by melting glaciers, Australian scientists reported in the journal Nature that the stromatolites lived on an ancient seafloor at a time when Earth’s skies were orange and its oceans green. They describe the stromatolites as perhaps the oldest fossil found so far on Earth, although chemical suggestions of life may extend further back in time . (Allen Nutman/University of Wollongong)
Because both oxygen and methane can be formed without life, a current gold standard for detecting future biosignatures on exoplanets is the presence of the two together. Because of the way the two interact, they would remain in an atmosphere together only if both were being replenished on a substantial, on-going scale. And as far as is now understood, the only way to do that is through biology.
Yet as described by Reinhard, the most current research suggest that oxygen and methane were probably were in the Earth’s atmosphere together at levels that would be detectable from afar. There was a lot of methane in the very early Earth atmosphere, and there has been a lot of oxygen for the past 600 million years or so, but as one grew in concentration the other declined — and during the “boring billion” both were likely low.
“So we have a complicated situation here where using the best exoplanet biosignatures we have now, intelligent beings looking at Earth over the past 4.5 billion years would not find a convincing signature of life for most, or maybe all, of that time if they relied only on co-occurrence of oxygen and methane,” Reinhard said. Yet there has been life for at least 3.7 billion years, and those beings studying Earth would have come up with a very false negative.
Lyons insists this is should not be a source of pessimism in the search for life on exoplanets, instead it is a “call to arms for new and more creative possibilities rather than the lowest hanging fruit.” It’s a challenge “to help us sharpen our thinking in a search that was never going to be easy.”
And the best test bed available for coming up with different answers, he said, may very well be the many different Earths that have come and gone on our planet.
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About Many Worlds
There are many worlds out there waiting to fire your imagination.
Marc Kaufman is an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer, and is the author of two books on searching for life and planetary habitability. While the “Many Worlds” column is supported by the Lunar Planetary Institute/USRA and informed by NASA’s NExSS initiative, any opinions expressed are the author’s alone.
This site is for everyone interested in the burgeoning field of exoplanet detection and research, from the general public to scientists in the field. It will present columns, news stories and in-depth features, as well as the work of guest writers.
The Nexus for Exoplanet System Science (NExSS) is a NASA research coordination network dedicated to the study of planetary habitability. The goals of NExSS are to investigate the diversity of exoplanets and to learn how their history, geology, and climate interact to create the conditions for life. NExSS investigators also strive to put planets into an architectural context — as solar systems built over the eons through dynamical processes and sculpted by stars. Based on our understanding of our own solar system and habitable planet Earth, researchers in the network aim to identify where habitable niches are most likely to occur, which planets are most likely to be habitable. Leveraging current NASA investments in research and missions, NExSS will accelerate the discovery and characterization of other potentially life-bearing worlds in the galaxy, using a systems science approach.
The National Aeronautics and Space Administration (NASA) is the agency of the United States government that is responsible for the nation’s civilian space program and for aeronautics and aerospace research.
President Dwight D. Eisenhower established the National Aeronautics and Space Administration (NASA) in 1958 with a distinctly civilian (rather than military) orientation encouraging peaceful applications in space science. The National Aeronautics and Space Act was passed on July 29, 1958, disestablishing NASA’s predecessor, the National Advisory Committee for Aeronautics (NACA). The new agency became operational on October 1, 1958.
Since that time, most U.S. space exploration efforts have been led by NASA, including the Apollo moon-landing missions, the Skylab space station, and later the Space Shuttle. Currently, NASA is supporting the International Space Station and is overseeing the development of the Orion Multi-Purpose Crew Vehicle and Commercial Crew vehicles. The agency is also responsible for the Launch Services Program (LSP) which provides oversight of launch operations and countdown management for unmanned NASA launches. Most recently, NASA announced a new Space Launch System that it said would take the agency’s astronauts farther into space than ever before and lay the cornerstone for future human space exploration efforts by the U.S.
NASA science is focused on better understanding Earth through the Earth Observing System, advancing heliophysics through the efforts of the Science Mission Directorate’s Heliophysics Research Program, exploring bodies throughout the Solar System with advanced robotic missions such as New Horizons, and researching astrophysics topics, such as the Big Bang, through the Great Observatories [Hubble, Chandra, Spitzer, and associated programs. NASA shares data with various national and international organizations such as from the [JAXA]Greenhouse Gases Observing Satellite.