SETI Reconceived and Broadened; A Call for Community Proposals

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A screenshot from a time lapse video of radio telescopes by Harun Mehmedinovic and Gavin Heffernan of Sunchaser Pictures was shot at several different radio astronomy facilities—the Very Large Array (VLA) Observatory in New Mexico, Owens Valley Observatory in Owens Valley California, and Green Bank Observatory in West Virginia. All three of these facilities have been or are still being partly used by the SETI (Search for the Extraterrestrial Intelligence) program. You can watch the video at: https://www.youtube.com/watch?v=SrxpgUJoHRc
A screenshot from a time lapse video of radio telescopes by Harun Mehmedinovic and Gavin Heffernan of Sunchaser Pictures that was shot at several different radio astronomy facilities—the Very Large Array (VLA) Observatory in New Mexico, Owens Valley Observatory in Owens Valley California, and Green Bank Observatory in West Virginia. All three of these facilities have been or are still being partly used by the SETI (Search for the Extraterrestrial Intelligence) program.

Earlier this summer, Natalie Cabrol, the director of the Carl Sagan Center of the SETI Institute, described a new direction for her organization in Astrobiology Magazine, and I wrote a Many World column about the changes to come.

Cabrol’s Alien Mindscapes – Perspective on the Search for Extraterrestrial Intelligence” laid out a plan for the new approach to SETI that would take advantage of the goldmine of new exoplanet discoveries in the past decade, as well as the data from fast-advancing technologies.  These fresh angles and masses of information come, she wrote,  from the worlds of astronomy and astrophysics, as well as astrobiology and the biological, geological, environmental, cognitive, mathematical, social, and computational sciences.

In her article,  Cabrol said that a call would be coming for community input on how to develop of a Virtual Institute for SETI Research. Its primary goal, she said, would be to “understand how intelligent life interacts with its environment and communicates.”

That call for white papers has now gone out in a release from SETI, which laid out the questions the organization is looking to address:

Question 1: How abundant and diverse is intelligent life in the Universe?

The Virtual Institute will use data synergistically from astrobiology, biological sciences, space and planetary exploration, and geosciences to quantitatively characterize the potential abundance and diversity of intelligent life in the Universe. The spatiotemporal distribution of potential intelligent life will be considered using models of the physicochemical evolution of the Universe.

Question 2: How does intelligent life communicate?

By drawing from a combination of cognitive sciences, neuroscience, communication and information theory, mathematical sciences, bio-neural computing, data mining, and machine learning (among others), we will proactively explore and analyze communication in intelligent terrestrial species. Building upon these analyses, we will consider the physiochemical and biochemical models of newly discovered exoplanet environments to generate and map probabilistic neural and homolog systems, and infer the resulting range of viable alien sensing systems.

Question 3: How can we detect intelligent life?

Using the results (data and databases) of research conducted under Questions 1 and 2, we will consider the design and promising exploration strategies, instruments, exploration strategies, instruments, experimental protocols, technologies, and messaging (content and support) that may optimize the probabilities of detecting intelligent life beyond Earth.

And here is what SETI hopes interested scientists will do:

To support the goals and address the questions outlined above, we seek white papers that will serve as a foundation for the intellectual framework of the Virtual Institute’s roadmap – and that specifically describe: (a) scientific rationales (theories, hypotheses) as foundations for investigations; (b) concepts of experimental designs (methods, protocols, and metrics); (c) universal markers, signals, instruments, systems, technologies for communication; (d) target identification; and (e) ground- and space-based instrumentation, observing scenarios, instrument requirements, and exploration strategies.

To better understand the possible existence of intelligence and technology in the universe, and to learn how to detect it, we expect that proposals may draw from diverse scientific fields. These include astrobiology, astronomy/astrophysics, cognitive sciences, epistemology, geo- and environmental sciences, biosciences, mathematical sciences, social sciences, space sciences, communication theory, bioneural computing, machine learning, big data analytics, technology, instrument and software development, and other relevant fields.

White papers should be submitted in electronic form as PDF files to Dr. Nathalie Cabrol at ncabrol@seti.org. They should be no more than three pages in length, with a minimum 10-point font size. A figure can be included if of critical importance. It is anticipated that there will be an opportunity for interested respondents to present their contribution in person during a planned workshop in the summer of 2017.

Notification of opportunities to present will be made after the white paper deadline of February 17, 2017, and those most responsive to this call will be published in the Astrobiology Journal. Questions related to this call should be addressed to SETI Institute President and CEO Bill Diamond at bdiamond@seti.org

Here is the column I wrote when the Astrobiology Magazine paper came out in August:

Allen Telescope Array
SETI’s partially-built Allen Telescope Array in Northern California, the focus of the organization’s effort to collect signals from distant planets, and especially signals that just might have been created by intelligent beings.  (SETI)

For decades, the Search for Extraterrestrial Intelligence (SETI)  and its SETI Institute home base have been synonymous with the search for intelligent, technologically advanced life beyond Earth.  The pathway to some day finding that potentially sophisticated life has been radio astronomy and the parsing of any seemingly unnatural signals arriving from faraway star system — signals that just might be the product of intelligent extraterrestrial life.

It has been a lonely five decade search by now, with some tantalizing anomalies to decipher but no “eurekas.”  After Congress defunded SETI in the early 1990s — a Nevada senator led the charge against spending taxpayer money to look for “little green men” — the program has also been chronically in need of, and looking for, private supporters and benefactors.

But to those who know it better, the SETI Institute in Mountain View, California has long been more than that well-known listening program.  The Institute’s Carl Sagan Center for Research is home to scores of respected space, communication, and astrobiology scientists, and most have little or nothing to do with the specific message-analyzing arm of the organization.

And now, the new head of the Carl Sagan Center has proposed an ambitious effort to further re-define and re-position SETI and the Institute.  In a recent paper in the Astrobiology Journal, Nathalie Cabrol has proposed a much broader approach to the search for extraterrestrial intelligence, incorporating disciplines including psychology, social sciences, communication theory and even neuroscience to the traditional astronomical approach.

“To find ET, we must open our minds beyond a deeply-rooted, Earth-centric perspective, expand our research methods and deploy new tools,” she wrote. “Never before has so much data been available in so many scientific disciplines to help us grasp the role of probabilistic events in the development of extraterrestrial intelligence.

“These data tell us that each world is a unique planetary experiment. Advanced intelligent life is likely plentiful in the universe, but may be very different from us, based on what we now know of the coevolution of life and environment.”

The galaxay as viewed by the Hubble Space Telescope
With billions upon billions of galaxies, stars and exoplanets out there, some wonder if the absence of a SETI signal means none are populated by intelligent being.  Others say the search remains in its infancy, and needs new approaches.  The galaxy as viewed by the Hubble Space Telescope. (NASA/STScI)

She also wants to approach SETI with the highly interdisciplinary manner found in the burgeoning field of astrobiology — the search for signs of any kind of life beyond Earth. And in a nod to NASA’s Astrobiology Institute, which has funded most of her work, Cabrol went on to call for the establishment of a SETI Virtual Institute with participation from the global scientific community.

I had the opportunity recently to speak with Cabrol, who is a French-American astrobiologist with many years of research experience working with the NASA Mars rover program and with extremophile research as a senior SETI scientist.  She sees the SETI search for technologically advanced life as very much connected with the broader goals of the astrobiology field, which are focused generally on signs of potential microbial extraterrestrial life.  Yes, she said, SETI has thus far a distinctive and largely separate role in the overall astrobiology effort, but now she wants that role to be significantly updated and broadened.

“The time is right for a new chapter for us,” she said. “The origins of SETI were visionary — using the hot technology of the day {radio astronomy} to listen for signals.  But we don’t exactly know what to look and listen for.  We don’t know the ways that ET might interact with its own environment, and that’s a drawback when looking for potential communications we might detect.”

Cabrol foresees future SETI Institute research into neural systems and how they interact with the environment (“bioneural computing,”) much more on the theory and mechanisms of communication, as well as on big data analysis and machine learning.  And, of course, into how potential biosignatures might be detected on distant planets.

The ultimate goal, however, remains the same:  detecting intelligent life (if it’s out there.)

Nathalie Cabrol, director of SETI's Carl Sagan Institute, wants to expand and update SETI's approach to searching for intelligent life beyond our solar system. (NASA)
Nathalie Cabrol, director of SETI’s Carl Sagan Center, wants to expand and update SETI’s approach to searching for intelligent life beyond our solar system. (NASA)

But with so much progress in the sciences that could help improve the chances of finding evolved extraterrestrial life, she said, it’s time for SETI to focus on them as a way to expand the SETI vision and its strategies.

“The purpose is to expand the vision and strategies for SETI research and to break through the constraints imposed by imagining ET to be similar to ourselves,” she wrote. The new approach will “probe the alien landscapes and mindscapes, and generally further understanding of life in the universe.”

The Institute will soon put out a call for white papers on how to expand the SETI search beyond radio astronomy, with an emphasis on “life as we don’t know it.”  After getting those white papers — hopefully from scientists ranging from astronomers to evolutionary biologists — the Sagan Center  plans a workshop to create a roadmap.

Cabrol was emphatic in saying that the SETI search is not turning away from the original vision of its founders — especially astrophysicists Frank Drake, Jill Tarter and Carl Sagan — who were looking for a way to quantify the likelihood of intelligent and technologically-proficient life on distant planets.  Rather, it’s an effort to return to and update the initial SETI formulation, especially as expressed in the famed Drake Equation.

Drake Equation
The Drake Equatio,, as first presented in 1961 to a gathering of scientists at the National Radio Astronomy Observatory in Green Bank, W. Va.

“What Frank proposed was actually a roadmap itself,” Cabrol said.  “The equation takes into account how suitable stars are formed, how many planets they might have, how many might be Earth-like planets, and how many are habitable or inhabited.”

Drake’s equation was formulated for the pioneering Green Bank Conference more than 50 years ago, when basically none of the components of his formula had a number or range that could be associated with it.  That has changed for many of those components, but the answer to the original question — Are We Alone? — remains little closer to being answered.

“I’ve talked a great deal with my colleagues about what type of life can be out there,” she said.  “How different from Earth can it be?”

“Now we’re looking for habitable environments with life as we know it. But it’s time to add life as we don”t know it, too.  And that can help augment our targeting, help pinpoint better what we’re looking for.”

“We think one of the key issues is how ET communicates with its environment, and the great advances in neuroscience can help inform what we do.  The same with evolutionary biology.  Given an environment with life, we want to know, what kind of evolution might be anticipated.”

Connectivity network between disciplines showing the bridges and research avenues that link together space, planetary, and life sciences, geosciences, astrobiology, and cognitive and mathematical sciences. This representation is an expanded version of the Drake equation. It integrates all the historical factors now broken down in measurable terms and expanded to include the search for life we do not know using universal markers, and the disciplines, fields, and methods that will allow us to quantify them.
A diagram of the proposed SETI  “connectivity network” between disciplines showing the bridges and research avenues that link together space, planetary, and life sciences, geosciences, astrobiology, and cognitive and mathematical sciences. Cabrol describes it as  an expanded version of the Drake equation.  (Astrobiology Journal/SETI Institute.)

These are, of course, very long-term goals.  No extraterrestrial life has been detected, and researchers are just now beginning to debate and formulate what might constitute a biosignature on a faraway exoplanet or, what has more recently been coined, a “bio-hint.”

In her paper, Cabrol is also frank about the entirely practical, real-world reasons what SETI needs to change.

“Decades of perspective on both astrobiology and the Search for Extraterrestrial Intelligence (SETI) show how the former has blossomed into a dynamic and self-regenerating field that continues to create new research areas with time, whereas funding struggles  have left the latter starved of young researchers and in search of both a long-term vision and a development program.

“A more foundational reason may be that, from the outset, SETI is an all-or-nothing venture where finding a signal would be a world-changing discovery, while astrobiology is associated with related fields of inquiry in which incremental progress is always being made.”

Whatever changes arrive at the SETI Institute, it will continue with its trademark efforts — most importantly operating the Allen Telescope Array in Northern California and collaborations with numerous other SETI groups.  The array began its work in 2007 with 42 interconnected small radio telescopes, and  continues its constant search for incoming signals.  The SETI Institute had hoped to build the array up to 350 telescopes, but the funding has not been forthcoming.

Cabrol is clearly a scientific adventurer and risk taker.  During her extremophile research in Chile, she went scuba diving and free diving — that is, diving without scuba equipment — in the Licancabur Lake, some 20,000 feet above sea level.  It is believed to be an unofficial altitude record high-altitude for both kinds of diving.

With this kind of view of life, she is a logical candidate to bring substantial change to SETI.  The new primary questions for SETI and the institute to probe are: How abundant is intelligent life in the universe?  How does it communicate? How can we detect intelligent life?

As she concluded in her Astrobiology Journal article:

‘Ultimately, SETI’s vision should no longer be constrained by whether ET has technology, resembles us, or thinks like us. The approach presented here will make these attributes less relevant, which will vastly expand the potential sampling pool and search methods, ultimately increasing the odds of detection.

“Advanced, intelligent life beyond Earth is most likely plentiful, but we have not yet opened ourselves to the full potential of its diversity.”

 

 

 

 

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Waiting on Enceladus

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NASA's Cassini spacecraft completed its deepest-ever dive through the icy plume of Enceladus on Oct. 28, 2015. Credits: NASA/JPL-Caltech
NASA’s Cassini spacecraft completed its deepest-ever dive through the icy plume of Enceladus on Oct. 28, 2015. (NASA/JPL-Caltech)

Of all the possible life-beyond-Earth questions hanging fire, few are quite so intriguing as those surrounding the now famous plumes of the moon Enceladus:  what telltale molecules are in the constantly escaping jets of water vapor, and what dynamics inside the moon are pushing them out?

Seldom, if ever before, have scientists been given such an opportunity to investigate the insides of a potentially habitable celestial body from the outside.

The Cassini mission to Saturn made its closest to the surface (and last) plume fly-through a year ago, taking measurements that the team initially said they would report on within a few weeks.

That was later updated by NASA to include this guidance:  Given the important astrobiology implications of these observations, the scientists caution that it will be several months before they are ready to present their detailed findings.

The reference to “important astrobiology implications” certainly could cover some incremental advance, but it does seem to at least hint of something more.

I recently contacted the Jet Propulsion Lab for an update on the fly-through results and learned that a paper has been submitted to the journal Nature and that it will hopefully be accepted and made public in the not-too-distant future.

All this sounds most interesting but not because of any secret finding of life — as some might infer from that official language.  Cassini does not have the capacity to make such a detection, and there is no indication at this point that identifiable byproducts of life are present in the plumes.

What is intriguing is that the fly-through was only 30 miles above the moon’s surface — the closest pass through a plume ever by Cassini — and so presumably its instruments produced some new and significant findings.

The scientists writing the paper could not, of course, discuss their findings before publication.  But Jonathan Lunine, a Cornell University planetary scientist and physicist on the Cassini mission with a longtime and deep interest in Enceladus, was comfortable discussing what is known about the moon and what Cassini (and future missions) still have to explain.

And thanks to that briefing, it became apparent that whatever new findings are coming, they will not make or break the case for the moon as a habitable place. Rather, they will essentially add to a strong case that has already been made.

“I think the evidence shows that Enceladus is the most promising target (for finding life beyond Earth) in the solar system,” Lunine told me.

enceladus geyser
Icy cyrogeysers erupt at the southern pole of Enceladus. (NASA/JPL-Caltech/Space Science Institute)

Any new findings from the October 28, 2015 fly-through would certainly be useful in terms of understanding the habitability of the moon, but he said that the logical question to ask now takes the story much further.  “Is the moon inhabited? That’s what I want to know now.”

That Lunine is such an enthusiastic supporter of a habitable Enceladus is not surprising:  He is concept principal investigator for the Enceladus Life Finder, probably the most advanced of several proposed missions looking for NASA support.

What is surprising, at least to those who have not followed Enceladus developments with the intensity they seem to deserve, is how much is already known about the moon and its potential for supporting life.

I’ll lay out Lunine’s case, but first a little background:

Enceladus is one of the 53 (or more) moons of Saturn, and is roughly the width of Colorado– about 310 miles in diameter.  It is one of Saturn’s major inner moons, is covered in ice and as a result reflects a lot of light and is one of the brightest objects in the sky.   But it didn’t attract much scientific attention until 2005 when those water vapor and dust plumes were detected shooting out from its south pole by Cassini.

Further study strongly suggests that Enceladus has a global liquid ocean between its rocky core and its icy surface, and the plumes, or geysers, consist of water pushed out through cracks in that surface.  The ocean is small in comparison to that of Jupiter’s watery moon Europa — the first is roughly the volume of Lake Superior while the latter has more water than all the oceans of the Earth put together — but as Lunine put it, “bacterium could do just fine in a Lake Superior-sized ocean.”

The history of Enceladus and its ocean are little understood in comparison with Europa, which Lunine said has probably had a stable ocean under its ice cover for billions of years.  But unlike Europa, Enceladus has that singular advantage of constantly spitting out its insides for us to study and gradually understand.  (Yes, researchers using the Hubble Space Telescope have detected what they concluded could be some water vapor plumes on Europa, too, but that finding is not confirmed.)

The equatorial surface of Enceladus is a beyond frigid  -340 degrees Fahrenheit,  but the temperatures around the southern polar fractures are a still cold but much warmer -100 to -130 degrees Fahrenheit. What’s important is the huge difference in temperatures — in the range of 200 degrees Fahrenheit.

The presumed sources of the heat are friction caused by gravitational forces from Saturn, and scalding heat from the core that enters the water through hydrothermal vents.

enceladus has a large -- 60/40 or 70/30
Scientists estimate that the ratio of rock to water and ice on Enceladus is in the range of 65 percent rock to 35 percent H2O..  NASA

So, what is known about the geysers being pushed out of Encedadus, and about the dynamics causing the phenomenon?

Already published papers report that the water vapor, which can extend out three times the diameter of the moon, is salty, filled with fine dust particles, and contains molecules including carbon dioxide, methane, molecular nitrogen, propane, acetylene, formaldehyde and traces of ammonia.  While none of these compounds are a biosignature per se, many are associated with life.

Recent analysis of some of the dust particles concluded that they were from the floor of the Enceladus ocean, and based on their characteristics appear to have been formed by the interaction of water and rock.  The most logical site for this kind of interaction is at hydrothermal vents, where heat from the core makes its way up into the water.  Some have argued that life on Earth may well have started at potentially similar hydrothermal vents on early Earth.

Jonathan Lunine is the David C. Duncan Professor of xxx at Cornell University, and Director, Center for Radiophysics and Space Research. He's also a member of the Cassini team.
Jonathan Lunine is the David C. Duncan Professor in the Physical Sciences at Cornell University, and Director of the Cornell Center for Astrophysics and Planetary Science. He’s also a longtime member of the Cassini team. (Cornell)

One of the primary goals of that final close fly-through was to collect data that would allow the Cassini scientists to measure how much hydrothermal activity is occurring within Enceladus.

If substantial amounts can be detected, that increases the chances for the existence around of vents of simple forms of life.  Measurements for hydrothermal activity depend on the detection of methane (which has already occurred) and of molecular hydrogen (which scientists were looking for in that final fly-through.)  Measurements for molecular hydrogen can be difficult to make, which might explain some of the time lag.

At the low altitude, the team also expected to be more sensitive to the possible presence of heavier and more massive molecules — including organics — that would not be observed during previous, higher-altitude passes through the plume.

One potentially complicating issue is that measurements of the pH of the water has come back with quite high alkaline levels.  If that is limited to areas around hydrothermal vents then it isn’t a problem for life, Lunine said.  But if it was far more widespread, it could be.

So these are some of the results we are now awaiting.  But to Lunine (and others on and off the Cassini team) the case for habitability and a possible home for life on Enceladus has already been made.

“What we already know is that the ocean has the general characteristics of habitability.  Obviously, it has liquid water and so there’s an energy source keeping it from all freezing.  It appears to have varied thickness but is still global. It’s salty and has organic molecules, as well as those small grains of silica.  The simplest model for why they exist is that water is cycling through quite warm rock at the base of the ocean, dissolving silica and delivering it to the ocean.

“Put this and more together and you have a signal, a big red arrow pointing to this moon saying it may well support life, and needs to be explored more and soon.”

The plumes of Enceladus originate in the long tiger stripe fractures of the south polar region pictured here. Detailed models support conclusions that the plumes arise from near-surface pockets of liquid water at temperatures of 273 kelvins (0 degrees Celsius). (Cassini Imaging Team, SSI, JPL, ESA, NASA)
The plumes of Enceladus originate in the long tiger stripe fractures of the south polar region pictured here. Detailed models support conclusions that the plumes arise from near-surface pockets of liquid water at temperatures of 273 kelvins (0 degrees Celsius). (Cassini Imaging Team, SSI, JPL, ESA, NASA)

But even if the upcoming Enceladus paper adds significantly to the habitable moon story, another mission to study the plumes may be long in coming.  Limited resources are the major reason why but so too is the congressionally-mandated mission to Europa, a target not dissimilar to Enceladus.

Texas congressman John Culberson has pushed long and hard for the Europa mission (or missions), arguing that the Jovian moon offers our best chance of finding extraterrestrial life in the solar system.  That huge and stable ocean is such a tempting target that the miles of ice encasing it are not seen as an deal-breaking obstacle. (“Thick-icers” and “thin-icers” are in constant debate about how deep that ice might go.)

That Europa is promising in terms of astrobiology is a conclusion that many scientists agree with, and NASA seems eager to cooperate. But it is nonetheless quite unusual to have Congress require NASA to mount a specific and costly mission and to set a timetable for doing it — as Congress did for Europa in 2015.

The congressional requirement follows years of waiting for a Europa mission.  The Galileo mission to Jupiter produced convincing information starting in 1998 that the moon had a large ocean under its ice surface, but almost two decades have gone by without an Europa-specific mission.

Lunine, and others, are pressing to make sure that doesn’t happen with Enceladus.  Last year he proposed a NASA Discovery mission to the moon that wasn’t selected, and has ideas for other sorts of NASA efforts.

“It was a sixteen or seventeen year odyssey to get a mission planned for Europa, and we just hope that doesn’t happen with Enceladus,” he told me.  “We could be testing for bio-activity there and really, where else would that make so much sense?”

 

 

 

 

 

 

 

 

 

 

 

 

 

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Coming to Terms With Biosignatures

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Exoplanets are much too far away for missions to visit and explore, so scientists are learning about them remotely.  That includes the question of whether they might support life — an aspect of exoplanet science that is getting  new attention. This is artist Ron Miller’s impression of an exoplanet.
Exoplanets are much too far away for missions to visit and explore, so scientists are learning about them remotely. That includes the question of whether they might support life — an aspect of exoplanet science that is getting new attention. This is artist Ron Miller’s impression of an exoplanet.

The search for life beyond our solar system has focused largely on the detection of an ever-increasing number of exoplanets, determinations of whether the planets are in a habitable zone, and what the atmospheres of those planets might look like.  It is a sign of how far the field has progressed that scientists are now turning with renewed energy to the question of what might, and what might not, constitute a sign that a planet actually harbors life.

The field of “remote biosignatures” is still in its early stages, but a NASA-sponsored workshop underway in Seattle has brought together dozens of researchers from diverse fields to dig aggressively into the science and ultimately convey its conclusions back to the exoplanet community and then to the agency.

While a similar NASA-sponsored biosignatures workshop put together a report in 2002, much has changed since then in terms of understanding the substantial complexities and possibilities of the endeavor.  There is also a new sense of urgency based on the observing capabilities of some of the space and ground telescopes scheduled to begin operations in the next decade, and the related need to know with greater specificity what to look for.

“The astrobiology community has been thinking a lot more about what it means to be a biosignature,” said Shawn Domogal-Goldman of the Goddard Space Flight Center, one of the conveners of the meeting.  Some of the reason why is to give advice to those scientists and engineers putting together space telescope missions, but some is the pressing need to maintain scientific rigor for the good of one of humankind’s greatest challenges.

“We don’t want to spend 20 years of our lives and billions in taxpayer money working for a mission to find evidence of life, and learn too late that our colleagues don’t accept our conclusions,” he told me.  “So we’re bringing them all together now so we can all learn from each other about what would be, and what would not be, a real biosignature.”

 

How to measure the chemical signatures in the atmosphere of a transiting exoplanet. The total light measured off-transit (B in the lower left figure) decreases during the transit, when only the light from the star is measured (A). By subtracting A from B, we get the planet counterpart, and from this the “chemical fingerprints” of the planet atmosphere can be revealed. Credits: NASA/JPL-Caltech.
How to measure the chemical signatures in the atmosphere of a transiting exoplanet. The total light measured off-transit (B in the lower left figure) decreases during the transit, when only the light from the star is measured (A). By subtracting A from B, we get the planet counterpart, and from this the “chemical fingerprints” of the planet atmosphere can be revealed. ( NASA/JPL-Caltech)

The three-day workshop is bringing together some 50 scientists ranging from astronomers, astrobiologists and planetary scientists to microbiologists and specialists in photosynthesis.  Organized by NASA’s Nexus for Exoplanet System Science (NExSS) — an initiative created to encourage interdisciplinary collaboration — it has been tasked with putting together a report for the larger exoplanet community and ultimately for NASA.

The first day of the workshop featured a review of previous work on biosignatures, which initially put forward the presence of oxygen in an exoplanet atmosphere as a strong and almost certain sign that biology was at work below. This is because oxygen, which is a byproduct of much life, bonds quickly with other molecules and so would be undetectable unless it was continuously replenished.

But as outlined by Victoria Meadows, director of the Virtual Planet Laboratory at the University of Washington, more recent research has shown large amounts of oxygen can be produced without biology under a number of (usually extreme)  conditions.  There has been a resulting focus on potential false positive signals regarding oxygen and other molecules.

From another perspective, Tim Lyons, a biogeochemist from the University of California, Riverside, used the early and middle Earth as an example how easy it is to arrive at a false negative result.

He said that current thinking is that for as long as two billion years, Earth was inhabited but the lifeforms produced little oxygen.  If analyzed from afar for all those years, the result would be a complete misreading of life on Earth.

With these kinds of false positives and negatives in mind, Meadows said that the current approach to understanding biosignatures is to look beyond a single molecule to the broader planetary and solar environment.

“We have to look not just at single biosignatures, but at their their context on the planet. How might life have modified an environment in a potentially detectable way?  And having stepped back a bit, does the biosignature make sense?”

As one example, while oxygen alone is no longer considered a sure biosignature, oxygen in an atmosphere in the presence of methane would be convincing because of the known results of the chemical interactions of the two.

 

Schematic for the concept of considering all small molecules in the search for biosignature gases. The goal is to start with chemistry and generate a list of all small molecules and filter them for the set that is stable and volatile in temperature and pressure conditions relevant for exoEarth planetary atmospheres. Further investigation relates to the detectability: the sources and sinks that ultimately control the molecules’ accumulation in a planetary atmosphere of specific conditions as well as its spectral line characteristics. Geophysically or otherwise generated false positives must also be considered. In the ideal situation, this overall conceptual process would lead to a finite but comprehensive list of molecules that could be considered in the search for exoplanet biosignature gases. Figure credit: S. Seager and D. Beckner.
Schematic for the concept of considering all small molecules in the search for biosignature gases.
The goal is to start with chemistry and generate a list of all small molecules and filter them for the set that is stable and volatile in temperature and pressure conditions relevant for exoEarth planetary atmospheres. In the ideal situation, this overall conceptual process would lead to a finite but comprehensive list of molecules that could be considered in the search for exoplanet biosignature gases. (S. Seager and D. Beckner)

 

In part because of the false positive/false negative issues involving oxygen, some have begun a concerted effort to produce a list of additional possible biosignatures.  William Bains, a member of Sara Seager’s team at the Massachusetts Institute of Technology, described the blunderbuss approach they have adopted:  examining some 14,000 compounds simple (fewer than six non-hydrogen atoms) and stable enough to exist in the atmosphere of an exoplanet.

In their Astrobiology Journal article, Seager, Bains and colleagues wrote that “To maximize our chances of recognizing biosignature gases, we promote the concept that all stable and potentially volatile molecules should initially be considered as viable biosignature gases.”

Elaborating during the workshop, Bains asked:  “Why does life produce the gases that it does? We really don’t know, so we’re bringing in everything as a possibility.”   Not surprisingly, he said, “The more you search, the more you find.”

And as for the possibility of life existing in extreme environments, Bains referred to the microbes known to live in radioactive environments, in plastic, and virtually everywhere else on Earth.

Because the science of remote biosignatures is still in its early stages, the unknowns can seem to overwhelm the knowns, making the whole endeavor seem near impossible.  After all, it’s proven extremely difficult to determine whether there was ever life on “nearby” Mars, and scientists have Martian meteorites to study and rovers sending back information about the geology, the geochemistry, the weather, the atmospheric conditions and the composition of the planet.

By comparison, learning how to probe the atmospheres of faraway exoplanets and assess what might or might not be a biosignature will have to be done entirely with next generation space telescopes and the massive ground telescopes in development.  The information in the photons they collect will tell scientists what compounds are present, whether liquid water is present on the surface, and potentially whether the surface is changing with seasons.  And then the interpretation begins.

That’s why Mary Voytek, the originator of NExSS and the head of the NASA astrobiology program, said at the workshop that the goal was to test and ultimately provide as many biosignatures as possible.  She wants many molecules potentially associated with life to be identified and then studied and restudied in the same critical way that oxygen has been — embraced for the biosignature possibilities it offers, and understood for the false positives and false negatives that might mislead.

“What we need is an arsenal,” she said, as many ways to sniff out the byproducts of exoplanet life as that daunting task demands.

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