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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Putting Together a Community Strategy To Search for Extraterrestrial Life

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I regret that the formatting of this column was askew earlier; I hope it didn’t make reading too difficult.  But now those problems are fixed.

The scientific search underway for life beyond Earth requires input from many disciplines and fields. Strategies forward have to hear and take in what scientists in those many fields have to say. (NASA)

Behind the front page space science discoveries that tell us about the intricacies and wonders of our world are generally years of technical and intellectual development, years of planning and refining, years of problem-defining and problem-solving.  And before all this, there also years of brainstorming, analysis and strategizing about which science goals should have the highest priorities and which might be most attainable.

That latter process is underway now in regarding the search for life in the solar system and beyond, with numerous teams of scientists tackling specific areas of interest and concern and turning their group discussions into white papers.  In this case, the white papers will then go on to the National Academy of Sciences for a blue-ribbon panel review and ultimately recommendations on which subjects are exciting and mature enough for inclusion in a decadal survey and possible funding.

This is a generally little-known part of the process that results in discoveries, but scientists certainly understand how they are essential.  That’s why hundreds of scientists contribute their ideas and time — often unpaid — to help put together these foundational documents.

With its call for extraterrestrial habitability white papers, the NAS got more than 20 diverse and often deeply thought out offerings.  The papers will be studied now by an ad hoc, blue ribbon committee of scientists selected by the NAS, which will have the first of two public meetings in Irvine, Calif. on Jan. 16-18.

Shawn Domagal-Goldman, a leader of many NASA study projects and a astrobiologist at NASA’s Goddard Space Fight Center. (NASA)

Then their recommendations go up further to the decadal survey teams that will set formal NASA priorities for the field of astronomy and astrophysics and planetary science.  This community-based process that has worked well for many scientific disciplines since they began in the late 1950s.

I’m particularly familiar with two of these white paper processes — one produced at the Earth-Life Science Institute (ELSI) in Tokyo and the other with NASA’s Nexus for Exoplanet System Science (NExSS.)  What they have to say is most interesting.

This is what Shawn Domagal-Goldman, an astrobiologist at the Goddard Space Flight Center, had to say about their effort, which began 16 months ago with a workshop in Seattle:

Chaitanya Giri, a research scientist and the Earth-Life Science Institute in Tokyo. (Nerissa Escanlar)

“This is an ‘all-hands-on-deck’ problem, and we held a workshop to start drawing a wide variety of scientists to the problem. Once we did, the group gave itself an ambitious goal – to quantify an assessment of whether or not an exoplanet has life, based on remote observations of that world.

“Doing that will take years of collaboration of scientists like the ones at the meeting, from diverse backgrounds and diverse experiences.”

Chaitanya Giri, a research scientist at ELSI with a background in organic planetary chemistry and organic cosmochemistry, said that his work on the European Rosetta mission to a comet convinced him that it is essential to “develop technological capacities to explore habitable niches on various planetary bodies and find unambiguous signatures of life, if present.”  There is some debate about the organic molecules — the chemical building blocks of life — identified by Rosetta.

“Over the years there have been scattered attempts at building such instruments, but a coherent collaborative network was missing,” Giri said. “This necessity inspired me to put on this workshop,” which led to the white paper.

We’ll discuss the conclusions of the papers, but first at little about the decadal surveys:

NASA Decada:

Here are the instruction from the NAS to potential white paper teams working on life beyond Earth projects and issues:

  • Identify promising key research goals in the field of the search for signs of life in which progress is likely in the next 20 years.
  • Identify key technological challenges in astrobiology as they pertain to the search for life in the solar system and extrasolar planetary systems.
  • Identify key scientific questions in astrobiology as they pertain to the search for life in the solar system and extrasolar planetary systems
  • Discuss scientific advances that can be addressed by U.S. and international space missions and relevant ground-based activities in operation or funded and in development
  • Discuss how to expand partnerships (interagency, international and public/private) in furthering the study of life’s origin, evolution, distribution, and future in the universe

Quite a wide net, from specific issues to much broader ones.  But the teams submitting their papers are not expected to address all the issues, but only one or perhaps a related second.

The papers range from a SETI Institute call for a program to increase the use of artificial intelligence and machine learning to address a range of astrobiology issues; to tempting possibilities offered by teams already in the running for future missions to Europa or Enceladus or elsewhere; to recommendations from the Planetary Science Institute about studying and searching for microbialites, living carbonate rock structures once common on Earth and possibly on Mars as well.

 

Proposed White Paper Subjects

Saturn’s moon Enceladus and plume of water vapor flowing out from its South Pole. (NASA)

 

Microbialites are fresh water versions of the organic and carbonate structures called stromatolites — which are among the oldest signs of life detected on Earth.

 

The white paper from ELSI focuses how to improve and discover technology that can detect potential life on other planets and moons. It calls for an increasingly international approach to that costly and specialized effort.

The paper from Giri et al begins with a disquieting conclusion that only “lately, scattered efforts are being undertaken towards the R&D of the novel and as-yet space unproven ‘life-detection’ technologies capable of obtaining unambiguous evidence of extraterrestrial life, even if it is significantly different from {Earth} life. As the suite of space-proven payloads improves in breadth and sensitivity, this is an apt time to examine the progress and future of life-detection technologies.”

The paper points to one discovery in particular as indicative of what the team feels is necessary — an ability to search for life in regions theoretically devoid of life and therefore requiring novel detection
techniques or probes.

“For example,” they write, “air sampling in Earth’s stratosphere with a novel scientific cryogenic payload has led to the isolation and identification of several new species of bacteria; this was an innovative technique analyzing a region of the atmosphere that was initially believed to be devoid of life.”

Other technologies they see as promising and needing further development are high-sensitivity fluorescence microscopy techniques that may be able to detect extraterrestrial organic compounds with catalytic activity surrounded by membranes, i.e., extraterrestrial cells.  In addition, they support on-going and NASA-funded work on genetic samplers that could go to Mars and — if present — actually identify nucleic acid-based life.

“With back-to-back missions under development and proposed by various space agencies to the potentially habitable Mars, Enceladus, Titan, and Europa, this is a right time for a detailed envisioning of the technologies needed for detection of life,” Giri said in an e-mail.

 

Yellowknife Bay on Mars, where the rover Curiosity first found conditions that were habitable to life. The rover subsequently found many more habitable spots, but no existing or fossil microbial life so far. (NASA)

The NExSS white paper on potentially detectable biosignatures from distant exoplanets– one of four submitted by the group– is an especially ambitious one.  The NASA-sponsored effort brought in many top scientists working in the field of biosignatures, and in the past year has already resulted in the publication or submission of five major science papers in addition to the white paper.

In keeping with the interdisciplinary mission of NExSS, the paper brought in people from many fields and ultimately advocates for a Bayesian approach to exoplanet life detection (named after 18th century statistician and philosopher Thomas Bayes. )

In most basic terms, the Bayes approach describes the probability of an event based on prior knowledge of conditions that might be related to the event. A simple example:  Runners A and B have competed four times, and runner A won three times.  So the probability of A winner is high, right?  But what if the two competed twice on a rainy track and each won one race.  If the forecast for the day of the next race is rain, the probability of who will be the winner would change.

This  approach not only embraces probability as an essential way forward, but it is especially useful in terms of weighing probabilities involving many measurements and fields.   Because the factors involved in finding a biosignature are so complex and potentially confounding, they argue, the field has to think in terms of the probability that a number of biosignatures together suggest the presence of life, rather than a 100 percent certain detection (although that may some day be possible.)

Nancy Kiang of the Goddard Institute for Space Studies in New York, explores (among other subjects) the possibility of using photosynthetic pigments as biosignatures on exoplanets.

Nancy Kiang of the Goddard Institute for Space Studies in New York, explores (among other subjects) the possibility of using photosynthetic pigments as biosignatures on exoplanets.

 

Both Domagal-Goldman and collaborator Nancy Kiang of NASA’s Goddard Institute for Space Studies are eager to adopt climate modeling and it’s ability to use known characteristics of divergent sub-fields to put together a big picture. (Those two, along with Niki Parenteau of NASA Ames Research Center led the NExSS effort.)

“For instance,” Kiang said, “the general circulation model (GCM) at GISS simulates the global circulation patterns of a planet’s wind, heat, moisture, and gases, providing statistical behaviors of the simulated climate.”  She sees a similar possibility with exoplanets and biosignatures.

 

Such a computer model can take in data from different fields and come up with some probabilities.  The model “might tell us that a planet is habitable over a certain percent of its surface,” she said.

“A geochemist or planetary formation person might then tell us that if certain chemistry exists on that planet, it has good potential for prebiotic compounds to form. A biologist and geologist might tell us that certain surface signatures on the planet are plausible for either life or mineral background.” That’s not a robust biosignature, but the probability that it could be life is not zero, depending on origin of the signature.

“These different forms of information can be integrated into a Bayesian analysis to tell us the likelihood of life on the planet,” she wrote.

One arm of the NExSS team is already using the tools of climate modeling to predict how particular conditions on exoplanets would play out under different circumstances.

 

This is a plot of what the sea ice distribution could look like on a synchronously rotating ocean world. The star is off to the right, blue is where there is open ocean, and white is where there is sea ice.  (NASA/GISS/Anthony Del Genio)

I will return to the NExSS biosignatures white paper later, since it is so rich with cutting edge thinking about this upcoming stage in space science.  But I do want to include one specific recommendation made by the group, which calls itself the Exoplanet Biosignatures Workshop Without Walls (EBWWW).

What they say is necessary now is for more biologists to join the search for extraterrestrial life.

“The EBWWW revealed that the search for exoplanet life is still largely driven by astronomers and planetary scientists, and that this field requires more input from origins of life researchers and biologists to advance a process-based understanding for planetary biosignatures.

“This includes assessing the {already assessed probability} that a planet may have life, or a life process evolved for a given planet’s environment. These advances will require fundamental research into the origins and processes of life, in particular for environments that vary from modern Earth’s. Thus, collaboration between origins of life researchers, biologists, and planetary scientists is critical to defining research questions around environmental context.”

The recommendation, it seems to me, illustrates both the youth and a maturing of the field.

 

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