Getting Real About the Oxygen Biosignature

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Oxygen, which makes up about 21 percent of the Earth atmosphere, has been embraced as the best biosignature for life on faraway exoplanets. New research shows that detecting distant life via the oxygen biosignature is not so straight-forward, though it probably remains the best show we have. (NASA)

I remember the first time I heard about the atmospheres of distant exoplanets and how could and would let us know whether life was present below.

The key was oxygen or its light-modified form, ozone.  Because both oxygen and ozone molecules bond so quickly with other molecules — think rust or iron oxide on Mars, silicon dioxide in the Earth’s crust — it was said that oxygen could only be present in large and detectable quantities if there was a steady and massive source of free oxygen on the planet.

On Earth, this of course is the work of photosynthesizers such as planets, algae and cyanobacteria, which produce oxygen as a byproduct.  No other abiotic, or non-biological, ways were known at the time to produce substantial amounts of atmospheric oxygen, so it seemed that an oxygen signal from afar would be a pretty sure sign of life.

But with the fast growth of the field of exoplanet atmospheres and the very real possibility of having technology available in the years ahead that could measure the components of those atmospheres, scientists have been busy modelling exoplanet formations, chemistry and their atmospheres.

One important goal has been to search for non-biological ways to produce large enough amounts of atmospheric oxygen that might fool us into thinking that life has been found below.

And in recent years, scientists have succeeded in poking holes in the atmospheric oxygen-means-life scenario.

Oxygen bonds quickly with many other molecules. That means has to be resupplied regularly to be present as O2 in an atmosphere . On Earth, O is mostly a product of biology, but elsewhere it might be result of non-biological processes. Here is an image of oxygen bubbles in water.

Especially researchers at the University of Washington’s Virtual Planetary Laboratory (VPL) have come up with numerous ways that exoplanets atmospheres can be filled (and constantly refilled) with oxygen that was never part of plant or algal or bacteria photo-chemistry.

In other words, they found potential false positives for atmospheric oxygen as a biosignature, to the dismay of many exoplanet scientists.

In part because she and her own team were involved in some of these oxygen false-positive papers, VPL director Victoria Meadows set out to review, analyze and come to some conclusions about what had become the oxygen-biosignature problem.

The lengthy paper (originally planned for 6 pages but ultimately 34 pages because research from so many disciplines was coming in) was published last month in the journal Astrobiology.  It seeks to both warn researchers about the possibilities of biosignature false-positives based on oxygen detection, and then it assures them that there are ways around the obstacles.

“There was this view in the community that oxygen could only be formed by photosynthesis, and that no other process could make O2,”  Meadows told me.  “It was a little simplistic.  We now see the rich complexity of what we are looking at, and are thinking about the evolutionary paths of these planets.

 

Artist’s impression of the exoplanet GJ 1132 b, which orbits the red dwarf star GJ 1132.  Earlier this year, astronomers managed to detect the atmosphere of this Earth-sized planet and have determined that water and methane are likely prevalent in the atmosphere.  (Max Planck Institute for Astronomy)

“What I see is a maturing of the field.  We have models that show plausible ways for oxygen to be produced without biology, but that doesn’t mean that oxygen is no longer an important biosignature.

“It is very important.  But it has to be seen and understood in the larger context of what else is happening on the planet and its host star.”

Before moving forward, perhaps we should look back a bit at the history of oxygen on Earth.

For substantial parts of our planet’s history there was only minimal oxygen in the atmosphere, and life survived in an anaerobic environment.  When exactly oxygen went from a small percentage of the atmosphere to 21 percent of the atmosphere is contested, but there is broader agreement about the source of the O2 in the atmosphere.  The source was photosynthesis, most importantly coming from cyanobacteria in the oceans.

As far back as four billion years ago, photosynthesis occurred on Earth based on the capturing of the energy of near infrared light by sulfur-rich organisms, but it did not involve the release of oxygen as a byproduct.

A chart showing the percentage rise in oxygen in Earth’s atmosphere over the past 3.8 billion years. The great oxidation event occurred some 2.3 billion years ago, but it took more than a billion additional years for the build-up to have much effect on the composition of the planet’s atmosphere.

Then came the the rise of cyanobacteria in the ocean and their production of oxygen.  With their significantly expanded ability to use photosynthesis, this bacterium was able to generate up to 16 times more energy than its counterparts, which allowed it to out-compete and explode in reproduction.

It took hundreds of millions of years more, but that steady increase in the cyanobacteria population led to what is called the “Great Oxidation Event” of some 2.3 billion years ago, when oxygen levels began to really climb in Earth’s atmosphere.  They did level off and remained well below current levels for another billion years, but then shot up in the past billion years.

As Meadows (and others) point out, this means that life existed on Earth for at least two billion years years without producing a detectable oxygen biosignature.  It’s perhaps the ultimate false negative.

But as biosignatures go, oxygen offers a lot.  Because it bonds so readily with other elements and compounds, it remains unbonded or “free” O2 only if it is being constantly produced.  On Earth, the mode of production is overwhelmingly photosynthesis and biology.  What’s more, phototrophs — organism that manufacture their own food from inorganic substances using light for energy — often produce reflections and seasonally dependent biosignatures that can serve as secondary confirmations of biology as the source for abundant Oin an atmosphere.

So in a general way, it makes perfect sense to think that O in the atmosphere of an exoplanet would signify the presence of photosynthesis and life.

Victoria “Vikki” Meadows is the director of the Virtual Planetary Institute at the University of Washington, which has been an important engine for NASA’s Astrobiology Institute (NAI) since 2001.  Among its many lines of research, her group focuses on the Earth as a template for understanding exoplanets, and so Meadows is holding up a rock here as a whimsical nod to that approach.  (University of Washington.)

The problem arises because other worlds out there orbiting stars very different than our own can have quite different chemical and physical dynamics and evolutionary histories, with results at odds with our world.

For instance, when it comes to the non-biological production of substantial amounts of oxygen that could collect in the atmosphere, the dynamics involved could include the following:

Perhaps the trickiest false positive involves the possible non-biological release of O2 via the photolysis of water — the breaking apart of H2O molecules by light.  On Earth, the water vapor in the atmosphere condenses into liquids after reaching a certain height and related temperature, and ultimately falls back down to the surface.  How and why that happens is related to the presence of large amounts of nitrogen in our atmosphere.

But what if an exoplanet atmosphere doesn’t have a lot of an element like nitrogen that allows the water to condense?  Then the water would rise into the stratosphere, where it would be subject to intense UV light,. The molecule would be split, and an H atom would fly off into space — leaving behind large amounts of oxygen that had nothing to do with life.  This conclusion was reached by Robin Wordsworth and Raymond Pierrehumbert of the University of Chicago and was published by the The Astrophysical Journal.

Another recently proposed mechanism to generate high levels of abiotic oxygen, first described by Rodrigo Luger and Rory Barnes of Meadow’s VPL team, focuses on the effects of the super-luminous phase of young stars on any rocky planets that might be orbiting them.

Small-mass M dwarfs in particular can burn much brighter when they are young, exposing potential planets around those stars to very high levels of radiation for as long as one billion years.

Modeling suggests that during this super-luminous phase a terrestrial planet that forms within what will become the main sequence habitable zone around an M dwarf star may lose up to several Earth ocean equivalents of water due to evaporation and hydrodynamic escape, and this can lead to generation of large amounts of abiotic O via the same H2O photolysis process.

Red dwarf, or M stars, are the most common in the cosmos.  They start off with a long period of extremely high luminosity and radiation before evolving into low-energy cool (and red) stars.  While a mature red dwarf star might have habitable zone planets that appear today to have characteristics conducive to life,  exoplanet modelers have determined that many of those red dwarf stars may well have lost their oceans during their early  long exposure to intense radiation. This is an artist rendering of three exoplanets around a red dwarf star. (ESO/M. Kornmesser)

Non-biological oxygen can also build up on an exoplanet, according to a number of researchers, if the host star sends out a higher proportion of far ultraviolet light than near ultraviolet.  The dynamics of photo-chemistry are such, they argue, that the excess far ultraviolet radiation would split CO2 to an extent that O2 would build up in the atmosphere.

There are other potential scenarios that would produce an oxygen false positive, and almost all of them involve radiation from the host star driving chemistry in the planet’s atmosphere, with the planetary environment then allowing O2 to build up.  While some of these false positive mechanisms can produce enough oxygen to make a big impact on their planets, some may not produce enough to even be seen by telescopes currently being planned.

As Meadows tells it, it was Shawn Domagal-Goldman of NASA Goddard and VPL who first brought the issue of oxygen false-positives to her attention. It was back in 2010 after he found an anomaly in his photo-chemical code results regarding atmospheric oxygen and exoplanets, and followed it. Since that initial finding, several other VPL researchers discovered new ways to produce O2 without life, and often while undertaking research focused on a different scientific goal.

Six years later, when she was writing up a VPL annual report, it jumped out that the group (and others) had found quite a few potential oxygen false positives — a significant development in the field of biosignature detection and interpretation.  That’s when she decided that an analysis and summary of the findings would be useful and important for the exoplanet community.  “Never let it be said that administrative tasks can’t lead to inspiration!” she wrote to me.

While Meadows does not downplay the new challenges to defining oxygen and ozone as credible biosignatures, she does say that these new understandings can be worked around.

Some of that involves targeting planets and stars for observation that don’t have the characteristics known to produce abiotic oxygen.  Some involves finding signatures of this abiotic oxygen that can be identified and then used to discard potential false positives.  And perhaps most telling, the detection of methane alongside free oxygen in an exoplanet atmosphere would be considered a powerful signature of life.

The Virtual Planetary Laboratory investigates the potential habitability of extrasolar planets. The research will help in predicting the habitability of discovered bodies like the Earth-size planets orbiting TRAPPIST-1 and the planet orbiting our closest neighbor, Proxima Centauri. (NASA)

The official goal of Meadows’ VPL is to wrestle with this question: “How would we determine if an extrasolar planet were able to support life or had life on it already?”

This has led her to a highly interdisciplinary approach, bringing together fifty researchers from twenty institutions.  In addition to its leading role in the NASA Astrobiology Institute, the VPL is also part of a broad NASA initiative to bring together scientists from different locales and disciplines to work on issues and problems of exoplanet research — the Nexus for Exoplanet System Science, or NExSS.

Given this background and these approaches, it is hardly surprising that Meadows would be among the first to see the oxygen-false positive issue in both scientific and collective terms.

“I wanted the community to have some place to go to when thinking about Ofalse positives,” she said. “We’re learning now about the complexity and richness of exoplanets, and this is essential for preparing to do the best job possible {in terms of looking for signs of life on exoplanets} when we get better and better observations to work with.”

“This story needed to be told now. Forewarned is forearmed.”

 

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The Search for Exoplanet Life Goes Broad and Deep

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The scientific lessons learned over the centuries about the geological, chemical and later biological dynamics of Earth are beginning to enter the discussion of exoplanets, and especially which might be conducive to life. This is an artist's view of the young Earth under bombardment by asteroids, one of many periods with conditions likely to have parallels in other solar systems. (NASA's Goddard Space Flight Center Conceptual Image Lab)
The scientific lessons learned over the centuries about the geological, chemical and later biological dynamics of Earth are beginning to enter the discussion of exoplanets, and especially which might be conducive to life. This is an artist’s view of the young Earth under bombardment by asteroids, one of many periods with conditions likely to have parallels in other solar systems. (NASA’s Goddard Space Flight Center Conceptual Image Lab)

I had the good fortune several years ago to spend many hours in meetings of the science teams for the Curiosity rover, listening in on discussions about what new results beamed back from Mars might mean about the planet’s formation, it’s early history, how it gained and lost an atmosphere, whether it was a place where live could begin and survive.  (A resounding ‘yes” to that last one.)

At the time, the lead of the science team was a geologist, Caltech’s John Grotzinger, and many people in the room had backgrounds in related fields like geochemistry and mineralogy, as well as climate modelers and specialists in atmospheres.  There were also planetary scientists, astrobiologists and space engineers, of course, but the geosciences loomed large, as they have for all Mars landing missions.

Until very recently, exoplanet research did not have much of that kind interdisciplinary reach, and certainly has not included many scientists who focus on the likes of vulcanism, plate tectonics and the effects of stars on planets.  Exoplanets has been largely the realm of astronomers and astrophysicists, with a sprinkling again of astrobiologists.

But as the field matures, as detecting exoplanets and inferring their orbits and size becomes an essential but by no means the sole focus of researchers, the range of scientific players in the room is starting to broaden.  It’s a process still in its early stages, but exoplanet breakthroughs already achieved, and the many more predicted for the future, are making it essential to bring in some new kinds of expertise.

A meeting reflecting and encouraging this reality was held last week at Arizona State University and brought together several dozen specialists in the geo-sciences with a similar number specializing in astronomy and exoplanet detection.  Sponsored by NASA’s Nexus for Exoplanet Systems Science (NExSS), NASA Astrobiology Institute (NAI) and the National Science Foundation,  it was a conscious effort to bring more scientists expert in the dynamics and evolution of our planet into the field of exoplanet study, while also introducing astronomers to the chemical and geological imperatives of the distant planets they are studying.

Twenty years after the detection of the first extra-solar planet around a star, the time seemed ripe for this coming together — especially if the organizing goal of the whole exoplanet endeavor is to search for signs of life beyond Earth.

 

Our vast body of knowledge about the formation, processes and evolution of Earth will become increasingly important in the exoplanet field as new generations of instruments make different and more precise kinds of measurements possible. Using Earth dynamics as a guide, those measurements will be made into models of what might be occurring on the exoplanets. The artist rendering of exoplanet Upsilon Andromedea g by Ron Howard.
Our vast body of knowledge about the formation, processes and evolution of Earth will become increasingly important in the exoplanet field as new generations of instruments make different and more precise kinds of measurements possible. Using Earth dynamics as a guide, those measurements will be made into models of what might be occurring on the exoplanets. The artist rendering of exoplanet Upsilon Andromedae g is by Ron Howard, Black Cat Studios.

Ariel Anbar, a biogeochemist at ASU, was one of the leaders of the meeting and the call for a broader exoplanet effort.

“The astronomical community has been pushing hard to make very difficult measurement, but they really haven’t been thinking much about the planetary context of what they’re finding.  And for geoscience, our people haven’t thought much about astronomical observations because they are so focused on Earth.”

“But this makes little sense because exoplanets open up a huge new field for geoscientists, and the astronomers absolutely need them to make the calls on what many of the measurements of the future actually mean.”

What’s more, the knowledge of researchers familiar with the dynamics of Earth will be essential when planet hunters and planet characterizers put together their wish lists for what kind of instruments are included in future telescopes and spectrographs.  For instance, a deep knowledge would be useful of the Earth’s carbon cycle, or what makes for a stable planetary climate, or what minerals and chemistry a habitable planet probably needs.

And then there are all the false positives and false negatives that could come with detections (or non-detections) of possible signatures of life.  The search for life beyond Earth has already had two highly-public and controversial seeming detections of extraterrestrial life — first by the Viking landers in the 1970s and the Mars meteorite ALH84001 in the mid 1990s.  The two are now considered inconclusive at best, and discredited at worst.

The risk of a similar, and even more complex, confusing and ultimately controversial, discovery of signs of life on an exoplanet are great.  The Arizona State workshop debated this issue at length.

President’s Professor at ASU’s School of Earth and Space Exploration and Department of Chemistry and Biochemistry.
Ariel Anbar, President’s Professor at ASU’s School of Earth and Space Exploration and School of Molecular Sciences. He hopes that the drive to understand exoplanets will push his field to develop a missing general theory for the evolution of Earth and Earth-like planets.

What they came away with was the understanding that while one or two measured biosignatures from a distant planet would be enormously exciting, a deeper understanding of the planet’s atmosphere, interior, chemical makeup and relationship to its host star are pretty much required to make a firm conclusion about biological vs non-biological origins.  (Here is a link to an introductory and cautionary tale to the workshop by another of its organizers, astrophysicist Steven Desch.)

And so the issues under debate were:  Does a planet need plate tectonics to be able to support life?  (Yes on Earth, perhaps elsewhere.) Would the detection of oxygen in an exoplanet atmosphere signify the presence of life? (Possibly, but not definitively.)  Does the chemical and mineral composition of a planet determine its ability to support life? (As far as we can tell, yes.)  Does photosynthesis inevitably lead to an oxygen atmosphere?  (It’s complicated.)

All these issues and many more serve to make the case that exoplanet science and Earth or planetary science need each other.

This is by no means an entirely new message — the Virtual Planetary Laboratory at the University of Washington has taken the approach for a decade from the standpoint of astronomy and the New Earths team of the NAI from a geological standing point.   But still, its urgency and proposed reach was  quite unusual.

It is also a reflection of both the success and direction of exoplanet science, because scientists have — or will have in the years ahead — the instruments and knowledge to learn more about an exoplanet than its location.  The James Webb Space Telescope is expected to provide much advanced ability to read the chemical compositions exoplanet atmospheres, as will a new generation of mammoth ground-based telescopes under construction and (scientists in the field fervently hope) a NASA flagship mission for the 2030s that would be able to directly image exoplanets with great precision.

But really, it’s when more and better measurements come in that the hard work begins.

Transmission spectrum of exoplanet MIT
Information about the make-up of exoplanets comes largely by studying the transmission spectra produced as the planet crosses in front of its star.  The spectra can identify some of the elements and compounds present around the exoplanet. Christine Naniloff/MIT, Julien De Wit.

 

Astrophysicist Steve Desch, for instance,  believes it is highly important to know what Earth-sized planets are like without life.  Starting with a biologically dead exoplanet in the Earth-sized ballpark, it would be possible to get a far better idea of the signatures of a similar planet with life.  But that’s a line of thinking that Earth scientists and geochemists are not, he said, used to addressing.  He felt the ASU workshop provided some consciousness-raising about the kinds of issues that are important to the exoplanet community, and to the Earth scientist, too.

Scientists from the geoscience side see similar limitations in the thinking of exoplanet astronomers.  Christy Till, a geologist and volcano specialist at ASU, said that at the close of the three-day workshop, she wasn’t at all sure that exoplanet scientists have been aware of just how complex the issue of “habitability” will be.

“Our field has learned over the decades that the solid interior of a planet is a big control on whether that planet can be habitable — along with the presence of volcanoes, the cycling elements like carbon and iron, and a relatively stable climate.  These issues were not widely discussed in terms of exoplanets, so I think we can help move the research further.”

Till is relatively new to thinking about exoplanets, brought into the field by the indisciplinary ASU (and NExSS/NAI) approach. But she said it has been most exciting to have the potential usefulness of her kind of knowledge expand on such a galactic scale.

Although the amount of detailed information about exoplanets is very limited, Till (and others) said what is and will be available can be used to create predictive models.  Absent the models that researchers can start building now, future information coming in could easily be misunderstood or simply missed.

ASU geologist and assistant professor Christy Till, a relatively new and enthusiastic member of the exoplanet community. (Abigail Wiebel)
ASU geologist and assistant professor Christy Till, a relatively new and enthusiastic member of the exoplanet community. (Abigail Wiebel)

While the usefulness of geosciences is being largely embraced in the exoplanet field, there are clear caveats.  If Earth becomes the model for what is needed for life in the cosmos, then is the field falling into a new version of the misguided Earth-centric view that long dominated astronomy and cosmology?

With that concern in mind, astronomer Drake Deming of the Harvard-Smithsonian Center for Astrophysics made the case for collecting potential biosignatures of all kinds.  Since we don’t know how potential life on another planet might have formed, we also may well be unaware of what kind of signatures it would put out.  ASU geochemist Everett Shock was similarly wary of relying too heavily on the Earth model when trying to understand planets that may seem similar but are inevitably different.

And Ariel Anbar felt challenged by his more complete realization post-workshop that the exoplanets available to study for the foreseeable feature will not be Earth-sized, but will be “Super-Earths” with radii up to 1.5 times as great as that of our planet.  A proponent of much greater exoplanet-geoscience collaboration, he said the Earth science community has a big job ahead figuring out how the processes and dynamics understood on Earth would actually apply on these significantly larger relatives.

One participant at the workshop pretty much personifies the interdisciplinary bridge under construction , and he was encouraged by the extensive back-and-forth between the space scientists and the Earth scientists.

Shawn Domogal-Goldman, a research space scientist at the Goddard Space Flight Center and a leader of the NExSS group, is an expert in ancient earth as well the astrophysics of exoplanet detection and characterizing.  His view is that the Earth provides 4.5 billion years of physical, chemical, climatic and biological dynamics  that need to be mined for useful insights about exoplanets.

“For me, and I think for others, we’ll look back at this meeting years from now and say to ourselves, ‘We were there at the beginning of something big.'”

 

 

 

 

 

 

 

 

 

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