False Positives, False Negatives; The World of Distant Biosignatures Attracts and Confounds

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This artist’s illustration shows two Earth-sized planets, TRAPPIST-1b and TRAPPIST-1c, passing in front of their parent red dwarf star, which is much smaller and cooler than our sun. NASA’s Hubble Space Telescope looked for signs of atmospheres around these planets. (NASA/ESA/STScI/J. de Wit, MIT)

What observations, or groups of observations, would tell exoplanet scientists that life might be present on a particular distant planet?

The most often discussed biosignature is oxygen, the product of life on Earth.  But while oxygen remains central to the search for biosignatures afar, there are some serious problems with relying on that molecule.

It can, for one, be produced without biology, although on Earth biology is the major source.  Conditions on other planets, however, might be different, producing lots of oxygen without life.

And then there’s the troubling reality that for most of the time there has been life on Earth, there would not have been enough oxygen produced to register as a biosignature.  So oxygen brings with it the danger of both a false positive and a false negative.

Wading through the long list of potential other biosignatures is rather like walking along a very wet path and having your boots regularly pulled off as they get captured by the mud.  Many possibilities can be put forward, but all seem to contain absolutely confounding problems.

With this reality in mind, a group of several dozen very interdisciplinary scientists came together more than a year ago in an effort to catalogue the many possible biosignatures that have been put forward and then to describe the pros and the cons of each.

“We believe this kind of effort is essential and needs to be done now,” said Edward Schwieterman, an astronomy and astrobiology researcher at the University of California, Riverside (UCR).

“Not because we have the technology now to identify these possible biosignatures light years away, but because the space and ground-based telescopes of the future need to be designed so they can identify them. ”

“It’s part of what may turn out to be a very long road to learning whether or not we are alone in the universe”.

 

Artistic representations of some of the exoplanets detected so far with the greatest potential to support liquid surface water, based on their size and orbit.  All of them are larger than Earth and their composition and habitability remains unclear. They are ranked here from closest to farthest from Earth.  Mars, Jupiter, Neptune an Earth are shown for scale on the right. (Planetary Habitability Laboratory, managed by the University of Puerto Rico at Arecibo.)

The known and inferred population of exoplanets — even small rocky exoplanets — is now so vast that it’s tempting to assume that some support life and that some day we’ll find it.  After all,  those billions of planets are composed of same basic chemical elements as Earth and are subject to the same laws of physics.

That assumption of life widespread in the galaxies may well turn out to be on target.  But assuming this result, and proving or calculating a high probability of finding extraterrestrial life, are light years apart.

The timing of this major community effort is hardly accidental.  There is a National Academy of Sciences effort underway to review progress in the science of reading possible biosignatures from distant worlds, something that I wrote about recently.

Edward Schwieterman, spent six years at the University of Washington’s Virtual Planetary Laboratory.  He now works with the NASA Astrobiology Institute Alternative Earths team UCR.

The results from the NAS effort will in term flow into the official NAS decadal study that will follow and will recommend to Congress priorities for the next ten or twenty years.  In addition, two NASA-ordered science and technology definition teams are currently working on architectures for two potential major NASA missions for the 2030s — HabEx (the Habitable Exoplanet Imaging Mission) and Luvoir (the Large Ultraviolet/Optical/Infrared Surveyor.)

The two mission proposals, which are competing with several others, would provide the best opportunity by far to determine whether life exists on other distant planets.

With these formal planning and prioritizing efforts as a backdrop, NASA’s Nexus for Exoplanet System Science (NExSS) called for a biosignatures workshop in the fall of 2016 and brought together scientists from many disciplines to wrestle with the subject.  The effort led to the white paper submitted to NAS and will result in and will result in the publication of series of five detailed papers in the journal Astrobiology this spring.” The overview paper with Schwieterman as first author, which has already been made available to the community for peer review, is expected to lead off the package.

So what did they find?  First off, that Earth has to be their guide.

“Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet,” the paper reads. “Aided by the universality of the laws of physics and chemistry, we turn to Earth’s biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere.

Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a state-of-the-art overview of our current understanding of potential exoplanet biosignatures including gaseous, surface, and temporal biosignatures.”

In other words, potential biosignatures in the atmosphere, on the ground, and that become apparent over time.  We’ll start with the temporal:

These vegetation maps were generated from MODIS/Terra measurements of the Normalized Difference Vegetation Index (NDVI). Significant seasonal variations in the NDVI are apparent between northern hemisphere summer  and winter. (Reto Stockli, NASA Earth Observatory Group, using data from the MODIS Land Science Team.)

Vegetation is probably clearest example of how change-over-time can be a biosignature.  As these maps show and we all know, different parts of the Earth have different seasonal colorations.  Detecting exoplanetary change of this sort would be a potentially strong signal, though it could also have some non-biological explanations.

If there is any kind of atmospheric chemical corroboration, then the time signal would be a strong one.  That corroboration could come in seasonal modulations of biologically important gases such as CO2 or O2.  Changes in cloud cover and the periodic presence of volcanic gases can also be useful markers over time.

Plant pigments themselves which have been proposed as a surface biosignature.  Observed in the near infrared portion of the electromagnetic spectrum, the pigment chlorophyll — the central player in the process of photosynthesis — shows a sharp increase in reflectance at a particular wavelength.  This abrupt change is called the “red edge,” and is a measurement known to exist only which chlorophyll engaged in photosynthesis.

So the “red edge,” or parallel dropoffs in reflectance of other pigments on other planets, is another possible biosignature in the mix.

And then there is “glint,” reflections from exoplanets that come from light hitting water.

True-color image from a model (left) compared to a view of Earth from the Earth and Moon Viewer (http://www.fourmilab.ch/cgi-bin/Earth/). A glint spot in the Indian Ocean can be clearly seen in the model image.

Since biosignature science essentially requires the presence of H2O on a planet, the clear detection of an ocean is part of the process of assembling signatures of potential life.  Just as detecting oxygen in the atmosphere is important, so too is detecting unmistakable surface water.

But for reasons of both science and detectability, the chemical make-up exoplanet atmospheres is where much biosignature work is being done.  The compounds of interest include (but are not limited to) ozone, methane, nitrous oxide, sulfur gases, methyl chloride and less specific atmospheric hazes.  All are, or have been, associated with life on Earth, and potentially on other planets and moons as well.

The Schwieterman et al review looks at all these compounds and reports on the findings of researchers who have studied them as possible biosignatures.  As a sign of how broadly they cast their net, the citations alone of published biosignature papers number more than 300.

(Sara Seager and William Bains of MIT, both specialists in exoplanet atmospheres, have been compiling a separate and much broader list of potential biosignatures, even many produced in very small quantities on Earth.  Bains is a co-author on one of the five biosignature papers for the journal Astrobiology.)

All this work, Schwieterman said, will pay off significantly over time.

“If our goal is to constrain the search for life in our solar neighborhood, we need to know as much as we possibly can so the observatories have the necessary capabilities.  We could possibly save hundreds of millions or billions of dollars by constraining the possibilities.”

“The strength of this compilation is the full body of knowledge, putting together what we know in a broad and fast-developing field,” Schwieterman said. ”

He said that there’s such a broad range of possible biosignatures, and so many conditions where some might be more or less probable, that’s it’s essential to categorize and prioritize the information that has been collected (and will be collected in the future.)

“We have a lot of observations recorded here, but they will all have their ambiguities,” he said.  “Our goal as scientists will be to take what we know and work to reduce those ambiguities. It’s an enormous task.”

 

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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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