The Search for Organic Compounds On Mars Is Getting Results

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This photograph, taken by NASA's Mars Rover Curiosity in 2015, shows sedimentary rocks of the Kimberley Formation in Gale Crater. The crater contains thick deposits of finely-laminated mudstone that represent fine-grained sediments deposited in a standing body of water that persisted for a long period of time - long enough to allow sediments to accumulate to significant thickness. Image by NASA. Enlarge image. [8]
Sedimentary rocks of the Kimberley Formation in Gale Crater, as photographed in 2015. The crater contains thick deposits of finely-laminated mudstone from fine-grained sediments deposited in a standing body of water that persisted for a long period of time.  Scientists have now reported several detections of organic compounds — the building blocks of life in Gale Crater samples. (NASA/JPL-Caltech/MSSS)

One of the primary goals of the Curiosity mission to Mars has been to search for and hopefully identify organic compounds — the carbon-based molecules that on Earth are the building blocks of life.

No previous mission had quite the instruments and capacity needed to detect the precious organics, nor did they have the knowledge about Martian chemistry that the Curiosity team had at launch.

Nonetheless, finding organics with Curiosity was no sure things.  Not only is the Martian surface bombarded with ultraviolet radiation that breaks molecules apart and destroys organics, but also a particular compound now known to be common in the soil will interfere with the essential oven-heating process used by NASA to detect organics.

So when Jennifer Eigenbrode, a biogeochemist and geologist at the Goddard Space Flight Center and a member of the Curiosity organics-searching team,  asked her colleagues gathered for Curiosity’s 2012 touch-down whether they thought organics would be found, the answer was not pretty.

“I did a quick survey across the the team and I was convinced that a majority in the room were very doubtful that we would ever detect organics on Mars, and certainly not in the top five centimeters or the surface.”

Yet at a recent National Academies of Sciences workshop on “Searching for Life Across Space and Time,” Eigenbrode gave this quite striking update:

“At this point, I can clearly say that I am convinced, and I hope you will be too, that organics are all over Mars, all over the surface, and probably through the rock record.  What does that mean? We’ll have to talk about it.”

 The hole drilled into this rock target, called "Cumberland," was made by NASA's Mars rover Curiosity on May 19, 2013. Credit: NASA/JPL-Caltech/MSSS
The hole drilled into this rock target, called “Cumberland,” was made by NASA’s Mars rover Curiosity on May 19, 2013.  One of the samples found to have organics was from the Cumberland hole. (NASA/JPL-Caltech/MSSS)

This is not, it should be said, the first time that a member of the Curiosity “Sample Analysis on Mars”  (SAM) team has reported the discovery of organic material.   The simple, but very important organic gas methane was detected in Gale Crater,  as were chlorinated hydrocarbons. Papers by Sushil Atreya of the University of Michigan and  Daniel Glavin and Caroline Freissinet from Goddard, along with other team members from the SAM team, have been published on all these finds.

But Eigenbrode’s work and her comments at the workshop– which acknowledged the essential work of SAM colleagues — move the organics story substantially further.

That’s because her detections involve larger organic compounds, or rather pieces of what were once larger organics.  What’s more, these organics were found only when the Mars samples were cooked at over over 800 degrees centigrade in the SAM oven, while the earlier ones came off as detectable gases at significantly lower temperatures.

Goddard biogeochemist Jennifer Eigenbrode, who is an expert at detecting organic compounds in rocks, is using R&D funds to develop a simplified sample-processing method that could be applied to a robotic chemistry lab. Photo Credit: Chris Gunn Summer 2008
Goddard biogeochemist Jennifer Eigenbrode, an expert at detecting organic compounds in rocks, has found them in Martian samples collected by the Curiosity rover.
(Chris Gunn)

These latest carbon-based organics were most likely bound up inside minerals, Eigenbrode said. Their discovery now is a function of having an oven on Mars that, for the first time, can get hot enough to break them apart.

The larger molecules bring with them additional importance because, as Eigenbrode explained it, 75 to 90 percent of organic compounds are of this more complex variety.  What’s more, she said that the levels at which the compounds are present, as well as where they were found, suggests a pretty radical conclusion:  that they are a global phenomenon, most likely found around the planet.

Her logic is that the overall geochemistry of soil at Gale Crater as read by Curiosity instruments is quite similar to the chemistry of samples tested by earlier rovers at two other sites on Mars, Gusev Crater and Meridiani Planum.

Many Mars scientists are comfortable with taking these parallel bulk chemistry readouts — the sum total of all the chemicals found in the samples — and inferring that much of the planet has a similar chemical makeup.

Taking the logic a step further, Eigenbrode proposed to the assembled scientists that the signatures of carbon-based organics are also a global phenomenon.

“I think it just might be,” she told the NAS workshop, which was organized by the Space Studies Board. “We’ll have to find out more, but I think there’s a good possibility.”

That’s quite a jump — from a situation not long ago when no organics had been knowingly  detected on Mars, to one where there’s a possibility they are everywhere.

The Sample Analysis on Mars instrument has the job of searching for, among other xxx, organics on Mars. And it seems to have succeeded, despite some major obstacles. (NASA/Goddard Space Flight Center)
The Sample Analysis on Mars (SAM) instrument has the job of searching for, among other targets, organics on Mars.  It heats the scooped or drilled samples to as much as 860 degrees C, cooking them until compounds come off in a gas form.  Then it sniffs the gases and identifies them.  It is the most complex instrument on Curiosity and has come up with important results, despite some major obstacles. (NASA/Goddard Space Flight Center)

And actually, they should be found everywhere.  Not only do organic molecules rain down from the sky embedded in asteroids and interstellar dust, but they can also be formed abiotically out of chemicals on Mars and, just possibly, can be the products of biological activity.

The fact that Mars surely has had organics on its surface and elsewhere has made the non-detection of organics a puzzle.  In fact, that conclusion of “no organics present” following the Viking landings in the mid 1970s set the Mars program back several decades.  If there weren’t even organic compounds to be found, the thinking went, then a search for actual living creatures was pointless.

As is now apparent, the Viking instrument used to detect organics didn’t have the necessary diagnostic power that SAM has. What’s more, the scientists working with it did not know about a particular chemical on the Martian surface that was skewing the results.  Plus the scientists may well have misunderstood their own findings.

First with the question of technological muscle.  The oven associated with the search for organics is part of a Gas Chromatograph Mass Spectrometer (GCMS), and it heats and breaks apart dirt and rock samples for analysis of their chemical makeup. The oven on the Viking landers only went up to 500 degrees C.  But the SAM oven on Curiosity goes hotter. It detected signs of organics between 500 and 850 degrees C.

In addition, NASA’s Phoenix lander discovered in 2008 that the Martian soil contained the salt perchlorate, which when burned in a GCMS oven can mask the presence of organics.  And finally, the Viking landers actually did detect organics in the form of simple chlorinated hydrocarbons.  They were determined at the time to be contamination from Earth, but the same compounds have been detected by Curiosity, suggesting that Viking might actually have found Martian, rather than Earthly, organics.

Image taken by Viking 2 on Mars in 1976. Results from both Viking landers reported no organic material in their samples, strongly suggesting there was no chance of current or past life. Recent readings by the SAM instrument on the Curiosity rover suggest the Viking conclusions were not correct, and that the instruments then did not have the capacity to detect Martian organics. NASA
Image taken by Viking 2 on Mars in 1976. Results from both Viking landers reported no organic material in their samples, strongly suggesting there was no chance of current or past life. Recent readings by the SAM instrument on the Curiosity rover suggest the Viking conclusions were not correct, and that the instruments then did not have the capacity to detect Martian organics. (NASA)

What makes carbon-based organic compounds especially interesting to scientists is that life is made of them and produces them.  So one source of the organics in Martian samples could be biology, Eigenbrode said.  But she said there were other potential sources that might be more plausible.

Organics, for instance, can be formed through non-biological geothermal and hydrothermal processes on Earth, and presumably on Mars too.  In addition, both meteorites and interstellar dust are known to contain organic compounds, and they rain down on Mars as they do on Earth.

Eigenbrode said the organics being detected could be coming from any one source, or from all of them.

Asked at the workshop what concentrations of organics were found, she replied with a grin that more light will be shed on the question at next week’s American Geophysical Union meeting.

The detection of a growing variety of organics on Mars adds to the conclusion already reached by the Curiosity team — that Mars was once much wetter, warmer and by traditional definitions “habitable.”  That doesn’t mean that life ever existed there, but rather that what are considered basic basic conditions for life were present for many millions of years.

Eigenbrode said that the detection of these carbon-based compounds is important in terms of both the distant past and the perhaps mid-term future.

For the past, it means that organics in a substantial reservoir of water like the one at Gale Crater some 3.6 billion years ago could have been a ready source of energy for microbial life.  The microbes would then have been heterotrophs, which get their nutrition from organic material.    Autotrophs, simpler organisms, are  capable of synthesizing their own food from inorganic substances using light or chemical energy.

But Eigenbrode also sees the organics as potentially good news for the future — for possibly still living microbes on Mars and also for humans who might be trying to survive there one day.

“Thinking forward, the organic matter could be really important for farming — a ready energy source provided by the carbon,”  she said.

Just what a human colony on Mars some day might need.

 

 

 

 

 

 

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The Ancient Mars Water Story, Updated

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Rendering of Gale Lake some 3.5 billion years ago, when Mars was warmer and much wetter. The Curiosity mission is finding that Gale Crater water-changed rock is everywhere.
Rendering of Gale Lake some 3.5 billion years ago, when Mars was warmer and much wetter. The Curiosity mission is finding that rocke in Gale Crater changed by water everywhere. (Evan Williams, with data from the Mars Reconnaissance Orbiter HIRISE project)

Before the Curiosity rover landed on Mars, NASA’s “follow the water”maxim had already delivered results that suggested a watery past and just maybe some water not far below the surface today that would periodically break through on sun-facing slopes.

While tantalizing — after all, the potential presence of liquid water on a exoplanet’s surface is central to concluding that it is, or once was, habitable — it was far from complete and never confirmed via essential ground-truthing.

Curiosity famously provided that confirmation early on with the discovery of pebbles that had clearly been shaped in the presence of flowing surface water, followed by the months in Yellowknife Bay which proved geologically, geochemically and morphologically the long-ago presence of substantial amounts of early Martian water.

Some of the earliest drilling was into mudstone that looked very much like a dried up basin or marsh, and that was exactly what Curiosity scientists determined it was, at a minimum.  It took many months for Curiosity leaders to ever use the word “lake” to describe what had once existed on the site, but now it is a consensus description.

Since the presence of a fossil lake was confirmed and announced, the water story has taken something of a backseat as the rover made its challenging and revelatory way across the lowlands of Gale Crater, through some dune fields and onto the Murray formation — a large geological unit that is connected to the base of Mount Sharp itself.  And all along the path of the rover’s traverse mudstone and sandstone were present, a clear indication of ever larger amounts of water.

I spoke recently with geologist and biogeologist John Grotzinger, the former NASA chief scientist for Curiosity and now a member of the science team, to get a sense of how things had progressed for the Gale water story.  He said there was no longer any doubt that the crater was once quite filled with water.

“We have  not seen a single rock at Gale that doesn’t say that the planet was wet.  In the areas where the rover has driven, I’d be very comfortable now in saying that the surface and ground water was often present for millions to tens of millions of years.”

Gale crater mudstone
Gale Crater mudstone at the Kimberly site. (NASA/JPL-Caltech)

Grotzinger said that the depth of the lakes, basins and playas clearly varied and are not well defined, but the rover’s newest extended mission will shed some light on the issue.  That’s because it is now (four years-plus after landing)  going to be actually climbing Mount Sharp, it’s original mission goal.

This is of great interest because Mars scientists already know that ahead lies fields of hematite, sulfates and phyllosilicates (clay), all minerals identified from orbit that can only form in water.  These deposits higher up the mountain can make the case for a deeper Gale Lake, or they could tell of up-welling ground water.  But in either case, they make the case for a watery ancient Mars.

There are innumerable ways in which this Gale water story is important.  Since it has been pretty well established that Gale Crater was formed by an asteroid impact 3.8 to 3.7 billion years ago, Grotzinger said that there is some consensus around the view that the water was present at least in the 3.5 to 3.6 billion years ago range.

While those are indeed ancient times — the planet was formed about 4.5 billion years ago —  it is quite a bit more recent than what was earlier considered to be the end of the period that early Mars might have supported surface water.  In those more conventional models, by 3.5 billion years ago Mars was parched, very cold, and had only the remnants of a protective atmosphere and magnetic field.  Yet now it appears that water was common, maybe plentiful.

“Clearly,” Grotzinger said, “there has to be some rethinking about ancient Mars and water.  It used to be that watery Mars was thought of as being in the 4 billion years time frame.  That has to be revised.”

 

Curiosity arm at Murray buttes, in the Murray formation. The endless acres of mudstone are visible. (NASA/JPL-Malin & Edgett)
Curiosity arm at Murray buttes, in the Murray formation. The endless acres of mudstone are visible. (NASA/JPL-Caltech/Malin and Edgett)

This presence of substantial amounts of water as late (or later) than 3.5 billion years ago has presented a major problem for Mars climate scientists.  By their calculations, there was essentially no way that abundant surface water could be present at that time — especially because of the “faint young sun” paradox.

As first put forward by Carl Sagan and colleagues, the paradox is this:  astrophysicists know from the study of stars like our sun that they begin with some 70 percent of the luminosity they will ultimately and gradually reach, and that as a result Mars (and Earth) would have been much colder in early days than it is now.   And it’s very cold indeed now.

There has been much discussion in recent years of various ways that a greenhouse effect could have warmed Mars (and Earth) during that early period, but noting conclusive or consensus-building has been identified geochemically on the surface or in the remaining atmosphere of Mars.

What’s more, in order for Gale Lake — and no doubt many others like it – to survive for as long as it apparently did requires a water cycle to replenish the water that is lost.  This is where one of the most intriguing and controversial questions about the Mars water story comes in.

It has long been known that much of the northern section of Mars is significantly lower than the southern highlands, and that the lowlands have far fewer geological features.  These observations led to the hypothesis some time ago that there was once a large northern ocean on Mars that could replenish the lakes and rivers of the south.

Artist rendering of a possible northern ocean on Mars. (NASA/ JPL-Caltech.)
Artist rendering of a possible northern ocean on Mars. (NASA Goddard Space Flight Center.)

The possible presence of such an ocean has been studied and debated for some time, but Grotzinger said only now is he “getting more sympathetic to the notion.”  Many others are also becoming more open to being persuaded  because it is extremely difficult to explain the proven existence of large amounts of surface water elsewhere on Mars without such a big liquid source.

Other recent findings and insights are pointing to the existence of a northern ocean as well.  For instance, a paper by Michael Mumma and Geronimo Villanueva of the NASA Goddard Astrobiology Center published last year in the journal Science estimated that a Martian ocean once covered 19 percent of the planet.  They used ratio measurements of the presence of two variants of water — regular H2O and deuterium or “heavy water” — to conclude that vast amounts of regular water had escaped from Mars over the eons.

And just this summer a team led by Alexis Palmero Rodriguez from the Planetary Science Institute in Tuscon, Arizona found evidence of what they described as ancient tsunami waves on Mars.  If confirmed, they could help explain one of the puzzling issues surrounding a potential northern ocean — that features of a shoreline have not been detected so far.

“So, we think this is going to remove a lot of the uncertainty that surrounds the ocean hypothesis,” Rodriguez told BBC News as the tsunami finding. “Features that have in the past been interpreted as relating to an ocean have been controversial; they can be explained by several, alternative processes. But the features we are describing – such as up-slope flows including large boulders – can only be explained in terms of tsunami waves.”

Co-author Alberto Fairen of the Centre for Astrobiology in Madrid said that the team concluded that a big meteorite impact triggered the first tsunami wave about 3.4 billion years ago.

He said the wave was composed of liquid water and formed widespread backwash channels to carry the water back to the ocean.

Their work, which was published in Scientific Reports, centers on two connected regions of Mars, known as Chryse Planitia and Arabia Terra — quite far from Gale Crater.

Tsunami-born sediments (arrow) inundate the land in an upslope direction (towards bottom-right)
Possible tsunami-deposited sediments (arrow) inundate the land in an upslope direction, towards bottom-right. (Alexis Rodruigez, Lunar and Planetary Institute)

While the potential existence of an ancient Martian ocean remains the subject of hot debate, the overall Mars water story is now considered pretty firm and with major implications for the potential presence of life on the planet.  Early Mars has already been deemed “habitable” by Curiosity scientists in terms of its geochemistry and more, and the presence of lakes or ocean water on the surface for tens of millions of years (or more) could certainly provide conducive places for life to form.

So a next step for Mars science is to determine what kind of Martian minerals best preserve organic material and potential signatures of long-ago life.  This field of study is called taphonomy, and Grotzinger was at a taphonomy conference at Williams College when I spoke with him.

ohn P. Grotzinger is the Fletcher Jones Professor of Geology at California Institute of Technology and chair of the Division of Geological and Planetary Sciences.
John P. Grotzinger is the Fletcher Jones Professor of Geology at California Institute of Technology and chair of the Division of Geological and Planetary Sciences. He spent four years as chief scientist for the Curiosity mission. (NASA)

“We’re definitely turning the corner from habitability to taphonomy,” Grotzinger said.  “This is to prepare for the 2020 mission”  to Mars, during which intriguing rocks will be identified for future sample returns to Earth.

The way that exoplanets are studied now and will be in the future is, of course, quite different from what is possible on Mars.

But there are strong parallels in terms of the importance of water and understanding the atmospheric make-up and geochemistry, and there’s this widely-accepted maxim from the world of astrobiology:  if a second form of life is ever found on Mars or anywhere else in our solar system, the likelihood that life is common in the cosmos grows exponentially.

Clearly, to have life start twice in our one solar system would make the search for life in other solar systems that much more compelling and pressing.

 

 

 

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The Habitable Zone Gets Poked, Tweaked and Stretched to the Limits

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To find another planet like Earth, astronomers are focusing on the "Goldilocks" or habitable zone around stars--where it's not too hot and not too cold for liquid water to exist on the surface. (NASA)
To find another planet like Earth, astronomers are focusing on the “Goldilocks” or habitable zone around stars–where it’s not too hot and not too cold for liquid water to exist on the surface. (NASA)

For more than 20 years now — even before the first detection of an extra-solar planet — scientists have posited, defined and then debated the existence and nature of a habitable zone.  It’s without a doubt a central scientific concept, and  the idea has caught on with the public (and the media) too.  The discovery of “habitable zone planets” has become something of a staple of astronomy and astrophysics.

But beneath the surface of this success is a seemingly growing discomfort about how the term is used. Not only do scientists and the general public have dissimilar understandings of what a habitable zone entails, but scientists have increasingly divergent views among themselves as well.

And all this is coming to the fore at a time when a working definition of the habitable zone is absolutely essential to planning for what scientists and enthusiasts hope will be a long-awaited major space telescope focused first and foremost on exoplanets.  If selected by NASA as a flagship mission for the 2030s, how such a telescope is designed and built will be guided by where scientists determine they have the best chance of finding signs of extraterrestrial life — a task that has ironically grown increasingly difficult as more is learned about those distant solar systems and planets.

Most broadly, the habitable zone is the area around a star where orbiting planets could have conditions conducive to life.  Traditionally, that has mean most importantly orbiting far enough from a star that it doesn’t become a desiccated wasteland and close enough that it is not forever frozen.  In this broad definition, the sometimes presence of liquid water on the surface of a planet is the paramount issue in terms of possible extraterrestrial life.

 The estimated habitable zones of A stars, G stars and M stars are compared in this diagram. More refinement is needed to better understand the size of these zones. Image credit: NASA/JPL-Caltech/MSSS.

The estimated habitable zones of A stars, G stars and M stars are compared in this diagram. More refinement is needed to better understand the size of these zones. Image credit: NASA/JPL-Caltech/MSSS.

It was James Kasting of Penn State University, Daniel Whitmire, then of Louisiana State University, and Ray Reynolds of NASA’s Ames Research Center who defined the modern outlines of a habitable zone, though others had weighed in earlier.  But Kasting and the others wrote with greater detail and proposed a model that took into account not only distance from the host star, but also the presence of planetary systems that could maintain relatively stable climates by cycling essential compounds.

Their concept became something of a consensus model, and remains an often-used working definition.

But with the detection now of thousands of exoplanets, as well as a better understanding of potential habitability in our solar system and the workings of atmospheric gases around planets, some scientists argue the model is getting outdated.  Not wrong, per se, but perhaps not broad enough to account for the flood of planetary and exoplanetary research and discovery since the early 1990s.

Consider, first our own habitable zone:  Two bodies often discussed as potentially habitable are the moons Europa and Enceladus. Both are far from the solar system’s traditional habitable zone, and are heated by gravitational forces from Jupiter and Saturn.

And then there’s the Mars conundrum.  The planet, now viewed as unable to support life on the surface, is currently within the range of our sun’s habitable zone.  Yet when Mars was likely quite wet and warmer and “habitable” some 3.5 billion years ago — as determined by the Curiosity rover team — it was outside the traditional habitable zone because the sun was less luminous and so Mars would ostensibly be frozen.

Remnants of an ancient alluvial fan have been found at Gale Crater, Mars, indicating that water flowed there for long periods of time billions of years ago.
Remnants of an ancient alluvial fan have been found at Gale Crater, Mars, indicating that water flowed there for long periods of time billions of years ago. Traditional habitable zone models cannot account for this wet and warm period on ancient Mars.  (NASA/JPL-Caltech)

Just as the source of heat keeping water on the moons liquid is not the sun, scientists have also proposed that even giant and distant planets with thick atmospheres of molecular hydrogen, a powerful greenhouse gas, could maintain liquid water on their surfaces.  Some have suggested that a hydrogen-rich atmosphere could keep a planet ten times further from the sun than Earth warm enough for possible life.

It was Raymond Pierrehumbert  at University of Chicago and Eric Gaidos of the University of Hawaii who first proposed this possibility in 2011, but others have taken it further.  Perhaps most forcefully has been Sara Seager at MIT, who has argued that the exoplanet community’s definition of a habitable zone needs to be broadened to keep up with new thinking and discoveries.  This is what she wrote in an influential 2013 Science paper:

“Planet habitability is planet specific, even with the main imposed criterion that surface liquid water must be present. This is because the huge range of planet diversity in terms of masses, orbits, and star types should extend to planet atmospheres and interiors, based on the stochastic nature of planet formation and subsequent evolution. The diversity of planetary systems extends far beyond planets in our solar system. The habitable zone could exist from about 0.5 AU out to 10 AU (astronomical units, the distance from the sun to the Earth) for a solar-type star, or even beyond, depending on the planet’s interior and atmosphere characteristics. As such, there is no universal habitable zone applicable to all exoplanets.”

Seager even makes room for the many rogue planet floating unconnected to a solar system as possible candidates, with the same kind of warming deep hydrogen covering that Pierrehumbert proposed. Clearly, her goal is to add exoplanets that are far less like Earth to the possible habitable mix.

 

In this artist's concept shows "The Behemoth," an enormous comet-like cloud of hydrogen bleeding off of a warm, Neptune-sized planet just 30 light-years from Earth. The hydrogen is evaporating from the planet due to extreme radiation from the star, but on many exoplanets it remains a thick covering. (NASA, ESA, and G. Bacon, STScI)
In this artist’s concept shows “The Behemoth,” an enormous comet-like cloud of hydrogen bleeding off of a warm, Neptune-sized planet just 30 light-years from Earth. The hydrogen is evaporating from the planet due to extreme radiation from the star, but on many exoplanets it remains a thick covering. (NASA, ESA, and G. Bacon, STScI)

Meanwhile, scientists have been adding numerous conditions beyond liquid surface water to enable a planet to turn from a dead to a potentially habitable one.  Kasting and Whitmore did include some of these conditions in their initial 1993 paper, but the list is growing.  A long-term stable climate is considered key, for instance, and that in turn calls for the presence of features akin to plate tectonics, volcanoes, magnetic fields and cycling into the planet interior of carbon, silicates and more.  Needless to say, these are not planetary features scientists will be able to identify for a long time to come.

So the disconnect grows between how exoplanet hunters and researchers use the term “habitable zone” and how the public understands its meaning.  Scientists describe a myriad of conditions and add that they are “necessary but not sufficient.”  Meanwhile, many exoplanet enthusiasts in the public are understandably awaiting a seemingly imminent discovery of extraterrestrial life on one of the many habitable zone planets announced.  (In fairness, no Earth-sized planet orbiting a sun-like star has been identified so far.)

Kasting, for one, does not see all this questioning of the necessary qualities of a habitable zone as a problem.

“Push back is what scientists do; we’re brought up to question authority.  My initial work is over 20 years old and a lot has been learned since then.  Not all things that are written down are correct.”

James Kasting of Penn State University, a pioneer in defining a habitable zone.
James Kasting of Penn State University, a pioneer in defining a habitable zone.

But in this case, he says, a lot of the conventional habitable zone concept is pretty defensible.

What’s more, it’s practical and useful.  While not discounting the possibility of life on exo-moons, on giant planets surrounded by warming molecular hydrogen or other possibilities, he says that the technical challenges to making a telescope that could capture the light necessary to analyze these moons or far-from-their-star planets would be so faint as to be undetectable given today’s (or even tomorrow’s) technology.  With those two exoplanet-focused telescopes (LUVOIR and Hab-Ex) now under formal study for a possible mission in the 2030s, Kasting thinks it’s essential to think inside, rather than outside, the box.

“I think that when the teams sit down and think about the science and technology of those projects, our habitable zone is the only one that make sense.  If you design a telescope to capture possible evidence of life as far out as 10 AU, you give up capability to study with the greatest precision planets close in the traditional habitable zone.  That doesn’t mean the telescope can’t look for habitable worlds outside the traditional habitable zone, but but don’t design the telescope with that as a high priority.  Better to focus on what we know does exist.”

Coming soon:  The Habitability Inde

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The Search for Exoplanet Life Goes Broad and Deep

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

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

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

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

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

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

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

 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 

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

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

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

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

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

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

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

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

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

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

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

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

 

 

 

 

 

 

 

 

 

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