Putting Together a Community Strategy To Search for Extraterrestrial Life

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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
Several white papers discussed the desirability of sending a proble to Saturn’s moon Enceladus.  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 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.
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 example of how Earth planet modeling can be used for exoplanets 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 minority and the maturing of the field.
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Marc Kaufman
Marc Kaufman is the author of two books about space: "Mars Up Close: Inside the Curiosity Mission” and “First Contact: Scientific Breakthroughs in the Search for Life Beyond Earth.” He is also an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer. While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.

To contact Marc, send an email to marc.kaufman@manyworlds.space.

The Mars Water Story Gets Ever More Interesting

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Enchanced-color traverse section of Martian icy scarps in late spring to early summer. Arrows indicate locations where relatively blue material is particularly close to the surface. Image taken by HiRISE camera on Mars Reconnaissance Orbiter. (NASA/JPL/UNIVERSITY OF ARIZONA/USGS )

Huge escarpments of quite pure water ice have been found in the Southern Highlands of Mars — accessible enough that astronauts might some day be able to turn the ice into water, hydrogen and oxygen.

Some of these deposits are more than 100 meters thick and begin only a meter or two below the surface.

These are among the conclusion from a new paper in the journal Science that describes these previously unknown water ice reserves.  While Mars scientists have long theorized the presence of subsurface ice under one-third of the planet, and even exposed bits of it with the Phoenix lander, the consensus view was that Martian ice was generally cemented with soil to form a kind of permafrost.

But the “scarp” ice described by Colin Dundas of the U.S. Geological Survey and colleagues is largely water ice without much other material.  This relative purity, along with its accessibility, would make the ice potentially far more useful to future astronauts.

“The ice exposed by the scarps likely originated as snow that transformed into massive ice sheets, now preserved beneath less than 1 to 2 meters of dry and ice-cemented dust or regolith,” the authors write. The shallow depths, the write “make the ice sheets potentially accessible to future exploration.”

The bright red regions contain water ice, as determined by measurements by the High-Resolution Imaging Science Experiment (HiRISE) on NASA’s Mars Reconnaissance Oribter. (NASA)

The importance is clear:  These sites are “very exciting” for potential human bases as well, says Angel Abbud-Madrid, director of the Center for Space Resources at the Colorado School of Mines in Golden, who led a recent NASA study exploring potential landing sites for astronauts.

Water is a crucial resource for astronauts, because it could be combined with carbon dioxide, the main ingredient in Mars’s atmosphere, to create oxygen to breathe and methane, a rocket propellant. And although researchers suspected the subsurface glaciers existed, they would only be a useful resource if they were no more than a few meters below the surface. The ice cliffs promise abundant, accessible ice, Abbud-Madrid told Science Magazine.

While the discovery adds to the view that Mars is neither bone-dry now nor was earlier in its history, it does not necessarily add to the question of where all the Martian water has gone or how much was originally there.

That’s because the paper describes the huge ice deposits as the result of snowfall over more recent eons that was packed into its current form, rather than water that might have been present during the warmer wetter periods of Mars history. With this in mind, Dundas said in an email that his team’s work does not add to what is known about the early Mars water budget.

As for the age of the water ice, he said “we can’t put an accurate number on it at this time, but the icy units are lightly cratered. Others in the community have proposed snowfall during periods of high axial tilt within the last few million years.”

So the ice is relatively young. But that doesn’t mean it has no story to tell.  Exposed ice, like exposed rock, always has a story to tell.

“We expect the vertical structure of Martian ice-rich deposits to preserve a record of ice deposition and past climate,” the authors write.

The eight scarps studied were steep and faced the poles. All were in the mid-latitudes, and therefore far from the polar ice sheets.

The lander Phoenix dug into the soil of the northern polar region and found cemented ice as well as pure ice several inches down. (NASA)

NASA has long had a motto for exploring Mars and other sites beyond Earth of “follow the water.”  That has been expanded to “follow the carbon” and “follow the organics,” but the water is still a guidepost of sorts of where life, or its remnants, might be found.  Now with these large and seemingly accessible deposits of water ice, “follow the water” takes on a new meaning for potential future astronauts in search of essential chemical components.

Still, the issue of just how much water there is and has been on Mars is a central to piecing together the planet’s history and how much of the planet might have one day been eminently habitable.

The last decade of Mars exploration and observation has led most Mars scientists to conclude that the planet once had rivers, lakes and possibly a northern ocean. That water is almost entirely (or perhaps entirely) gone from the surface now, and understanding where it went is certainly key to understanding the history of the planet.

While much no doubt escaped to space after the early protective Mars magnetic field and atmosphere largely disappeared, researchers say there remains a lot of Mars water to be accounted for.

An article in the journal Nature last month reported the possibility of large amounts of water mixing with Martian basalts long ago and forming a broadly water-rich crust.  The authors of that paper, led by Jon Wade of Oxford’s Department of Earth Sciences, described modeling that found water on early Mars could be absorbed into spongy rock at a far greater rate than on Earth.

In an accompanying review, geochemist and cosmochemist Tomohiro Usui of the Earth-Life Science Institute in Tokyo, supported the notion, and added another possibility that he has published on as well.

“Ground ice might also account for the missing water reservoir on Mars,” he wrote. “Subsurface radar-sounder measurements have detected an anomaly in an electrical property of rocks in the planet’s northern hemisphere, which implies that massive ice deposits are embedded among or between layers of sediment and volcanic materials at a depth of 60–80 m.”

Usui wrote that the ground-ice model has also been proposed based of analyses of hydrogen isotopes in Martian meteorites and of the shapes and characteristics of craters. Indeed, the crater study indicated that the subsurface water ice has a volume comparable to the size of the ancient oceans.

Where did the large amounts of water once present on Mars go. Some clearly was lost to the atmosphere, but some researchers are convinced that much is underground as ice or incorporated into minerals. (Nature)

 

Dundas agreed that the new paper was a continuation of earlier work, rather than something entirely new. “We’ve known for some time that there is shallow ground ice within a meter of the surface, and there have been recent radar detections of ice sheets tens of meters thick,” he said in his email. “What our work does is provide some three-dimensional information at high resolution that helps tie things together.”

Dundas et al reported that the fractures and steep angles found indicate that the ice is cohesive and strong. What’s more, bands and variations in color suggest that the ice contains distinct layers, which could be used to understand changes in Mars’ climate over time.

Because the ice is only visible where surface soil has been removed, the paper says it is likely that ice near the surface is more extensive than detected in this study.

And that could be very important to astronauts on future missions to Mars.

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Marc Kaufman
Marc Kaufman is the author of two books about space: "Mars Up Close: Inside the Curiosity Mission” and “First Contact: Scientific Breakthroughs in the Search for Life Beyond Earth.” He is also an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer. While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.

To contact Marc, send an email to marc.kaufman@manyworlds.space.

Two Tempting Reprise Missions: Explore Titan or Bring Back a Piece of A Comet

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Dragonfly is a quadcopter lander that would take advantage of the environment on Titan to fly to multiple locations, some hundreds of miles apart, to sample materials and determine the composition of the surface.  A central goal would be to analyze Titan’s organic chemistry and assess its habitability. (NASA)

Unmanned missions to planets and moons and asteroids in our solar system have been some of NASA’s most successful efforts in recent years, with completed or on-going ventures to Mars, Saturn, Jupiter, the asteroid Bennu, our moon, Pluto, Mercury and bodies around them all.   On deck are a funded mission to Europa, another to Mars and one to the unique metal asteroid 16 Psyche orbiting the sun between Mars and Jupiter.

We are now closer to adding another New Frontiers class destination to that list, and NASA announced this week that it will be to either Saturn’s moon Titan or to the comet 67P/Churyumov-Gerasimenko.

After assessing 12 possible New Frontiers proposals, these two made the cut and will receive $4 million each to further advance their proposed science and technology. One of them will be selected in spring of 2019 for launch in the mid 2020s.

With the announcement, associate administrator for NASA’s Science Mission Directorate Thomas Zurbuchen described the upcoming choice as between two “tantalizing investigations that seek to answer some of the biggest questions in our solar system today.”

Those questions would be:  How did water and other compounds essential for life arrive on Earth?  Comets carry ancient samples of both, and so can potentially provide answers.

And with its large inventories of nitrogen, methane and other organic compounds, is Titan potentially habitable?  Then there’s the added and very intriguing prospect of visiting the methane lakes of that frigid moon.

The CAESAR mission would return to the nucleus of  comet explored by the European Space Agency’s Rosetta mission, and its lander Philae.  (NASA)

Both destinations selected have actually been visited before.

The European Space Agency’s Rosetta mission orbited the comet 67P/Churyumov-Gerasimenko comet for two years and deployed a lander, which did touch down but sent back data for only intermittently for several days.

And the NASA’s Cassini-Huygens mission to Saturn passed by Titan regularly during its decade exploring that system, and the ESA’s Huygens probe did land on Titan and sent back information for a short time.

So both Rosetta and Cassini-Huygens began the process of understanding these distant and potentially revelatory destinations, and now NASA is looking to take it further.

The Titan “Dragonfly” mission, for instance, would feature a “quadcopter” or “rotorcraft” — a vehicle that is part helicopter and part drone.  It’s designed to hop hundreds of miles around the moon to sample what has already been determined to be a surface with many organic compounds and liquid methane lakes.

Those already identified hydrocarbon seas may contain amino acids and other interesting molecules, making Titan a test bed of sorts on how life arose on Earth. With spectrometers, drills, and cameras, Dragonfly would split its mission between science in the air and on the ground.

Although frigid, Titan is otherwise now known to be a relatively benign place, and the rotorcraft could potentially survive for several years. That could give the team time to “evaluate how far prebiotic chemistry has progressed in an environment where we know we have the ingredients for life,” said lead investigator Elizabeth Turtle from the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.

“Titan is a unique ocean world, with lakes and rivers of liquid methane flowing across its surface,” she said.  But it is also very cold. Average surface temperatures are -290 degrees Fahrenheit,  so any potential lifeforms would face some major challenges.

A methane lake near the northern pole of Saturn’s moon, Titan. The image was taken using radar on the Cassini spacecraft. NASA/JPL-Caltech

The comet mission CAESAR (Comet Astrobiology Exploration SAmple Return) will take a very different route than Rosetta did and will use different propulsion that will get the spacecraft to the comet in four years.  The plan is to then perform a “touch and go” maneuver with an extended arm to pick up 100 grams of precious Rosetta material from the nucleus.

And then unlike with Rosetta, the plan is to cache the sample and bring it back to Earth for intensive study.

By returning to 67P/Churyumov-Gerasimenko, a body mapped in detail by Rosetta, the mission is “able to design our spacecraft specifically for the conditions we know,” said Steven Squyres of Cornell University, principal investigator for the mission.

During a news conference, Squyres said that “comets are among the most scientifically important objects in the solar system, but they’re also among the most poorly understood.

“They’re the most primitive building blocks of planets; they contain materials that date from the very earliest moments of solar system formation and even before. Comets were a source of water for the Earth’s oceans, and critically they were a source of organic molecules that contributed to the origin of life.”

Squyres told me after the announcement that the CAESAR instruments will allow for more precise measurements than from Rosetta, and that a successful sample return could potentially change our understanding of the Earth’s history substantially.  He said the sample collection would also importantly include gases.

NASA has already succeeded with comet sample return — the Stardust mission to fly through the plume of comet Wild-2.  It did gather some dust, but it didn’t land on the nucleus like Rosetta did and CAESAR would.

Sample return is clearly a high priority for NASA and other space agencies.  The NASA 2020 mission to Mars is designed to collect and cache rock samples for later return, and two other sample return missions are underway.  Both are to asteroids, with one launched last year by NASA and the other by the Japanese space agency JAXA in 2014.

The nucleus of the 67P/Churyumov-Gerasimenko comet, where the CAESAR mission hopes to be headed in the 2020s. (ESA)

What “Dragonfly” would return is data about a moon shrouded in haze and unlike any other body in our solar system.

When the Huygen’s probe descended to the moon in early 2005, planetary scientists weren’t sure if the moon’s surface was covered in oceans methane and ethane.  So Huygens was designed to float if solid ground wasn’t to be found.

As the lander sailed through the haze it took hundreds of aerial images that showed an alien but strikingly Earth-like landscape of mountains, dry floodplains and what appeared to be river deltas. Huygens did land on something solid, though liquid methane flowed nearby and the Huygens cameras could see intricate channels cutting into the surface.

When Huygens touched down, it did so with a soft thud and a short slide across the frozen surface. Later analysis of the lander’s telemetry showed that Huygens sank around 4.7 inches into the surface on first contact, bounced and slid before coming to a stop.

This knowledge made possible a Titan project with a lander that can jump from one spot to another quite far away.  Those methane lakes, it is now understood, are largely found near the northern pole, and so floatation equipment is no longer necessary.

The Huygens descent to the surface of Titan, as recorded in 2005.  (ESA/NASA)

Squyres, a professor of physical science at Cornell University, is also principal investigator for the long-lasting Mars exploration rovers, one of which operated successfully for 8 years and the other –Opportunity — is still exploring Mars 13 years after landing.

He told me that his long years heading the Mars rover mission had convinced him that his greatest satisfaction in science comes from identifying a plausible space project, putting together a team of scientists, engineers and managers who can plausibly pull it off, and then work nonstop for years putting together a proposal to get it selected and funded. His team now numbers about 150 people and will soon grow much larger.

It was that sense of “shared struggle,” he said, that gave him enormous satisfaction. That came, of course, with a happy ending in several competitions, but he said he also knows the disappointment of not being selected for others.

Two other missions were selected for additional, though limited, funding under the New Frontiers program.  One is headed by NASA Ames Research Center astrobiologist Chris McKay and will be funded to develop “cost-effective techniques that limit spacecraft contamination and thereby enable life detection measurements on cost-capped missions.”

The larger mission his team proposed would go to the water vapor plume spurting out of Saturn’s moon Enceladus, with the goal of searching for signs of life.

The two projects selected were no doubt something of a disappointment to researchers who have longed for NASA to return to Venus.  As the study of exoplanets progresses, one of the key questions facing scientists is why Earth and Venus evolved so differently.  Both are within our sun’s habitable zone, but Venus experienced a runaway greenhouse effect that parched the surface and pushed surface temperatures to a led-melting 870 degrees F.

Three of the proposed New Frontiers missions were to Venus but, as in other recent competitions, none were selected by NASA.  One of the Venus proposals, led by Goddard’s Lori Glaze, was selected for further technology development.

Venus hasn’t been visited by a NASA spacecraft in decades, leading to hopes that one of the New Frontiers finalists would be a mission there. But it was not to be. ESA operated the Venus Express mission orbiting the planet from 2006 to 2014 and the Japanese Space Agency has a probe (Akatsuki) here now. (NASA)

Also noticeably absent among the finalists was any pr0ject going to the moon, especially since the Trump administration has made a lunar colony a priority.  Apparently a New Frontiers mission is not what the administration has in mind.

New Frontiers initiative is the largest NASA planetary exploration program open to outside competition and leadership.  But NASA does set the priorities, as put forward by the planetary science and astrobiology communities.

Previous spacecraft launched under New Frontiers include New Horizons, which surveyed Pluto and is now due to visit MU69, an icy object in the farthest reaches of the solar system; Juno, now in orbit around Jupiter; and OSIRIS-REx, launched last year, which will collect samples from an asteroid and return them to Earth.

Since the New Frontiers program began at the beginning of the century, NASA has selected two missions for each decade, making them the most frequent of the major agency missions.  Costs are capped at $1 billion — with $850 million for the mission and $150 million for the launch.  So far, the money has been demonstrably well spent.

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Marc Kaufman
Marc Kaufman is the author of two books about space: "Mars Up Close: Inside the Curiosity Mission” and “First Contact: Scientific Breakthroughs in the Search for Life Beyond Earth.” He is also an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer. While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.

To contact Marc, send an email to marc.kaufman@manyworlds.space.

Artificial Intelligence Has Just Found Two Exoplanets: What Does This Mean For Planet Hunting?

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There are now two known eight-planet solar systems in the galaxy. Artificial intelligence was used to comb through the data collected three years ago by the Kepler Space Telescope and its algorithms helped find Kepler 90-1, the eight planet in that solar system.  (NASA)

By Elizabeth Tasker

The media was abuzz last week with the latest NASA news conference. A neural network — a form of artificial intelligence or machine learning — developed at Google had found two planets in data previously collected by NASA’s prolific Kepler Space Telescope. It’s a technique that could ultimately track-down our most Earth-like planets.

The new exoplanets orbit stars already known to host planetary systems, Kepler-90 and Kepler-80. While both are only slightly larger than the Earth, their two-week orbits makes these worlds too hot to be considered likely candidates for hosting life. Moreover, the systems are thousands of light years away, putting the planets out of range of atmospheric studies that could test their habitability.

With over 3,500 exoplanets already discovered, you might be forgiven for finding these additions underwhelming. However, while other planets in the same system have been known about for several years, these two Earth-sized worlds were previously overlooked. The difference is not a new telescope, but an exploration of the data with a different kind of brain.

The Kepler Space Telescope searches for planets using the transit technique; detecting small dips in amount of starlight as the planet passes in front of the star. As planets are much smaller than stars, picking out this tiny light drop is a tricky task. For a Jupiter-sized planet orbiting a star like our Sun, the decrease in brightness is only about 1%. For an Earth-sized planet, the signal becomes so small it is right on the edge of what Kepler is able to detect. This makes their dim wink extremely difficult to spot in the data.

Kepler Space Telescope collected data on planet transits around distant stars for four years, and the information has provided  — and will continue providing —  a goldmine for planet hunters.  A severe malfunction in 2013 had robbed Kepler of its ability to stay pointed at a target without drifting off course, but the spacecraft was stabilized and readjusted to observe a different set of stars.  (NASA)

The discovery paper published in the Astronomical Journal combined the expertise of Christopher Shallue from Google’s artificial intelligence project, Google Brain, and Andrew Vanderburg, a NASA Sagan Postdoctoral Fellow and astronomer at the University of Texas at Austin. The researchers explored using a neural network to shake ever harder to find worlds out of the Kepler data.

It is a technique that is being used across a wide range of disciplines, but what exactly does a neural network do?

Neural networks are computer algorithms inspired by the way the brain recognizes patterns. For example, as a child you learned to recognize buses. It is unlikely anyone sat you down and presented a set of rules for identifying a bus. Rather, buses were repeatedly pointed out to you on the street and your brain found its own set of similarities within these examples. The idea behind a neural network is similar. Rather than telling a computer how to identify a feature such as the dip in light from a planet, the network is fed many examples and allowed to determine the features to get a consistently correct result.

This is a very successful way of developing pattern recognition software, making neural networks one of the newest tools in town used from image recognition to stock market trends. A key strength is dealing with large quantities of data to produce a consistent result.

Kepler has observed about 200,000 stars and another 200,000 will be the target for the Transiting Exoplanet Survey Satellite (TESS) to be launched next year. And if that analysis still looks doable with a bit of elbow grease, the NASA exoplanet archive has just added 18 million light curves from the UKIRT Microlensing survey.

In addition to being slow, humans can also be inconsistent (I once tried to flag down a lorry instead of a bus before I’d had my morning tea). This is especially true when trying to tease out the faint signature of Earth-sized worlds at the limit of the telescope’s capabilities. While Kepler has an automated pipeline to identify likely planets, simulated data suggests it recovers just 26% of Earth-sized planets on orbits similar to our own. Exploring new ways to handle these huge data sets is therefore a top priority.

While neural networks all learn to identify patterns from a series of examples, there are different choices for their structure. In their discovery paper, Shallue and Vanderburg try three different network architectures. The one they find the most successful is known as a “Convolution Neural Network”, which is commonly used in image classification.

Neural networks are loosely inspired by the structure of the human brain: “Neurons” do a simple computation and then pass information to the next layer of neurons. In this way, a computer can “learn” to identify a dog in an image, or an exoplanet in a Kepler light curve.  (Google)

This utilizes the fact that neighboring data points may form related structures, examining attributes such as the maximum and minimum of small local groups of points to hunt for features. This makes sense when your input data is the light from a star being consecutively dimmed by the passage of a planet.

In this first exploration, the neural network searched for undiscovered planets in known systems. The network found a total of 30 possible new planets, four of which it assigned a probability greater than 0.9 of this being a true detection. Based on the network’s performance when tested on known planets, this level of probability corresponded to a correctly identified planet 96% of the time.

These four candidates were then examined by Shallue and Vanderburg for alternative reasons for the dip in the light curve. Such false positives can be caused by the star being part of a binary system, where the stellar siblings periodically eclipse one another to produce small drops in their combined light. One candidate fell foul of having a close stellar neighbor which may have been causing this effect, while a second candidate showed a light dip that increased over time; an effect not expected by a planet. For the remaining two possibilities, there were no obvious reservations. These were really two new planets; Kepler-90i and Kepler-80g.

While neither new exoplanet is likely to be Earth-like, both belong to intriguing planetary systems. Kepler-80g is the outermost world of a compact system of six planets, all with orbits between 1 – 10 days. The outer five planets form a “resonant chain”; a musical-sounding term that means that the duration of the orbits of neighboring planets are neat integer ratios (in this case, either 2:3 or 3:4).

This orderly line-up is seen in the orbits of the Jovian moons, Io, Europa and Ganymede, and more recently, in the TRAPPIST-1 exoplanet system that hit the headlines last February. Computer models suggest that resonant orbits are formed when planets migrate inwards from a location further out from their star. This is likely how such a close stack of planets exists so close to the star, where we do not expect a lot of planet-building dust and gas.

The second planet hit the media headlines because its addition made Kepler-90 the first known star other than our own Sun to host eight planets. Also like our Solar System, the Kepler-90 planets have the giant gaseous worlds further from the star and the smaller rocky planets closer in. However, these planets all sit within the orbit of the Earth around the Sun, suggested that they too migrated inwards from colder reaches where ice could solidify and help build-up the mass of the giant planets.

Kepler-90i is 2,545 light-years away from Earth and orbits its host star in 14.45 days. (NASA)

Notably, Kepler-90i is right at the limit of what Kepler is sensitive enough to detect. This means the system may well have more planets that are too small and distant from their star for Kepler to spot.

In addition to finding these small planets, the size of their planetary systems underscores the potential of the neural network. The evolution of a planet depends heavily on its neighbors. The Earth may have been a dry world if our gas giants had not swept in icy meteorites to deliver oceans to our surface. Mars’s build-up of ice changes substantially over time as the planet’s axis wobbles due to the looming presence of Jupiter.

Such conditions can be modeled, but only if the full planetary system is known. Uncovering the planets around known host stars helps constrain models of how planets form and evolve, and even hint at which worlds may have remained temperate enough to develop life. Picking out the smaller worlds in a starlight signature crowded by other planets is as tricky as spotting a bus in the morning rush hour before tea; it could need this computer algorithm on the job.

Last week’s announcement may show the beginning of a new regime of planet hunting; one where we shake-out the smaller worlds hidden in noisy data. This could provide us both with more small planets and many more multi-planet systems, helping us pin down the most likely places we may find another planet like our own or even one most likely to be completely alien.

 

Elizabeth Tasker is a planetary scientist at the Japanese space agency JAXA and the Earth-Life Science Institute in Tokyo.  Her newly-released book is titled “The Planet Factory.”

 

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Marc Kaufman
Marc Kaufman is the author of two books about space: "Mars Up Close: Inside the Curiosity Mission” and “First Contact: Scientific Breakthroughs in the Search for Life Beyond Earth.” He is also an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer. While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.

To contact Marc, send an email to marc.kaufman@manyworlds.space.

A New Way to Find Signals of Habitable Exoplanets?

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Scientists propose a new and more indirect way of determining whether an exoplanet has a good, bad or unknowable chance of being habitable.  (NASA’s Goddard Space Flight Center/Mary Pat Hrybyk)

The search for biosignatures in the atmospheres of distant exoplanets is extremely difficult and time-consuming work.  The telescopes that can potentially take the measurements required are few and more will come only slowly.  And for the current and next generation of observatories, staring at a single exoplanet long enough to get a measurement of the compounds in its atmosphere will be a time-consuming and expensive process — and thus a relatively infrequent one.

As a way to potentially improve the chances of finding habitable conditions on those exoplanets that are observed, a new approach has been proposed by a group of NASA scientists.

The novel technique takes advantage of the frequent stellar storms emanating from cool, young dwarf stars. These storms throw huge clouds of stellar material and radiation into space – traveling near the speed of light — and the high energy particles then interact with exoplanet atmospheres and produce chemical biosignatures that can be detected.

The study, titled “Atmospheric Beacons of Life from Exoplanets Around G and K Stars“, recently appeared in Nature Scientific Reports. 

“We’re in search of molecules formed from fundamental prerequisites to life — specifically molecular nitrogen, which is 78 percent of our atmosphere,” said Airapetian, who is a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and at American University in Washington, D.C. “These are basic molecules that are biologically friendly and have strong infrared emitting power, increasing our chance of detecting them.”

The thin gauzy rim of the planet in foreground is an illustration of its atmosphere. (NASA’s Goddard Space Flight Center)

So this technique, called a search for  “Beacons of Life,” would not detect signs of life per se, but would detect secondary or tertiary signals that would, in effect, tell observers to “look here.”

The scientific logic is as follows:

When high-energy particles from a stellar storm reach an exoplanet, they break the nitrogen, oxygen and water molecules that may be in the atmosphere into their individual components.

Water molecules become hydroxyl — one atom each of oxygen and hydrogen, bound together. This sparks a cascade of chemical reactions that ultimately produce what the scientists call the atmospheric beacons of hydroxyl, more molecular oxygen, and nitric oxide.

For researchers, these chemical reactions are very useful guides. When starlight strikes the atmosphere, spring-like bonds within the beacon molecules absorb the energy and vibrate, sending that energy back into space as heat, or infrared radiation. Scientists know which gases emit radiation at particular wavelengths of light.  So by looking at all the radiation coming from the that planet’s atmosphere, it’s possible to get a sense of what chemicals are present and roughly in what amounts..

Forming a detectable amount of these beacons requires a large quantity of molecular oxygen and nitrogen.  As a result, if detected these compounds would suggest the planet has an atmosphere filled with biologically friendly chemistry as well as Earth-like atmospheric pressure.  The odds of the planet being a habitable world remain small, but those odds do grow.

“These conditions are not life, but are fundamental prerequisites for life and are comparable to our Earth’s atmosphere,” Airapetian wrote in an email.

Stellar storms and related coronal mass ejections are thought to burst into space when magnetic reconnections in various regions of the star.  For stars like our sun,  the storms become less frequent within a relatively short period, astronomically speaking.  Smaller and less luminous red dwarf stars, which are the most common in the universe, continue to send out intense stellar flares for a much longer time.

Vladimir Airapetian is a senior researcher
at NASA Goddard and a member of NASA’s  Nexus for Exoplanet System Science (NExSS) initiative.

The effect of stellar weather on planets orbiting young stars, including our own four billion years ago, has been a focus of Airapetian’s work for some time.

For instance, Airapetian and Goddard colleague William Danchi published a paper in the journal Nature last year proposing that solar flares warmed the early Earth to make it habitable.  They concluded that the high-energy particles also provided the vast amounts of energy needed to combine evenly scattered simple molecules into the kind of complex molecules that could keep the planet warm and form some of the chemical building blocks of life.

In other words, they argue, the solar flares were an essential part of the process that led to us.

What Airapetian is proposing now is to look at the chemical results of stellar flares hitting exoplanet atmospheres to see if they might be an essential part of a life-producing process as well, or of a process that creates a potentially habitable planet.

Airapetian said that he is again working with Danchi, a Goddard astrophysicist, and the team from heliophysics to propose a NASA mission that would use some of their solar and stellar flare findings.  The mission being conceived, the Exo Life Beacon Space Telescope (ELBST),  would measure infrared emissions of an exoplanet atmosphere using direct imaging observations, along with technology to block the infrared emissions of the host star.

For this latest paper, Airapetian and colleagues used a computer simulation to study the interaction between the atmosphere and high-energy space weather around a cool, active star. They found that ozone drops to a minimum and that the decline reflects the production of atmospheric beacons.

They then used a model to calculate just how much nitric oxide and hydroxyl would form and how much ozone would be destroyed in an Earth-like atmosphere around an active star. Earth scientists have used this model for decades to study how ozone — which forms naturally when sunlight strikes oxygenin t he upper atmosphere — responds to solar storms.  But the ozone reactions found a new application in this study; Earth is, after all, the best case study in the search for habitable planets and life.

Will this new approach to searching for habitable planets out?

“This is an exciting new proposed way to look for life,” said Shawn Domagal-Goldman, a Goddard astrobiologist not connected with the study. “But as with all signs of life, the exoplanet community needs to think hard about context. What are the ways non-biological processes could mimic this signature?”

 

A 2012 coronal mass ejection from the sun. Earth is placed into the image to give a sense of the size of the solar flare, but our planet is of course nowhere near the sun. (NASA, Goddard Media Studios)

Today, Earth enjoys a layer of protection from the high-energy particles of solar storms due to its strong magnetic field.  However, some particularly strong solar events can still interact with the magnetosphere and potentially wreak havoc on certain technology on Earth.

The National Oceanic and Atmospheric Administration classifies solar storms on a scale of one to five (one being the weakest; five being the most severe). For instance, a storm forecast to be a G3 event means it could have the strength to cause fluctuations in some power grids, intermittent radio blackouts in higher latitudes and possible GPS issues.

This is what can happen to a planet with a strong magnetic field and a sun that is no longer prone to sending out frequent solar flares.  Imagine what stellar storms can do when the star is younger and more prone to powerful flaring, and the planet less protected.

Exoplanet scientists often talk of the possibility that a particular planet was “sterilized” by the high-energy storms, and so could never be habitable.  But this new research suggests that some stellar storms could have just the opposite effect — making the planet more habitable.

 

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Marc Kaufman
Marc Kaufman is the author of two books about space: "Mars Up Close: Inside the Curiosity Mission” and “First Contact: Scientific Breakthroughs in the Search for Life Beyond Earth.” He is also an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer. While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.

To contact Marc, send an email to marc.kaufman@manyworlds.space.