Planetary Protection is a “Wicked” Problem

The Viking landers were baked for 30 hours after assembly, a dry heat sterilization that is considered the gold standard for planetary protection.  Before the baking, the landers were given a preliminary cleaning to reduce the number of potential microbial spores.  The levels achieved with that preliminary cleaning are similar to what is now required for a mission to Mars unless the destination is an area known to be suitable for Martian life.  In that case, a sterilizing equivalent to the Viking baking is required.  (NASA)

The only time that a formally designated NASA “life detection” mission was flown to another planet or moon was when the two Viking landers headed to Mars forty years ago.

The odds of finding some kind of Martian life seemed so promising at the time that there was little dispute about how much energy, money and care should be allocated to making sure the capsule would not be carrying any Earth life to the planet.  And so after the two landers had been assembled, they were baked at more than 250 °F for three days to sterilize any parts that would come into contact with Mars.

Although the two landers successfully touched down on the Martian surface and did some impressive science, the life detection portion of the mission was something of a fiasco — with conflict, controversy and ultimately quite a bit of confusion.

Clearly, scientists did not yet know enough about how to search for life beyond Earth and the confounding results pretty much eliminated life-detection from NASA’s missions for decades.

But scientific and technological advances of the last ten years have put life detection squarely back on the agenda — in terms of future searches for fossil biosignatures on Mars and for potential life surviving in the oceans of Europa and Enceladus.  What’s more, both NASA and private space companies talk seriously of sending humans to Mars in the not-too-distant future.

With so many missions being planned, developed and proposed for solar system planets and moons, the issue of planetary protection has also gained a higher profile.  It seems to have become more contentious and to some seems far less straight-forward as it used to be.

A broad consensus appears to remain that bringing Earth life to another planet or moon, especially if it is potentially habitable, is a real possibility that is both scientifically and ethically fraught. But there are rumblings about just how much time, money and attention needs to be brought to satisfying the requirements of “planetary protection.”

In fact, it has become a sufficiently significant question that the first plenary session of the recent Astrobiology Science Conference in Mesa, Arizona was dedicated to it.  The issue, which was taken up in later technical sessions as well, was how to assess and weigh the risks of bringing Earth life to other bodies versus the benefits of potentially sending out more missions, more often and more cheaply.

It is not a simple problem, explained Andrew Maynard, director of the Risk Innovation Lab at Arizona State University.  Indeed, he told the audience of scientists that it was a “wicked problem,” a broadly used terms for issues that are especially complex and involve numerous issues and players.


A primary barrier to keeping microbes off spacecraft and instruments going to space is to build them in clean rooms, such as this one at JPL.  These large rooms with filtered air do help lower the count of microbes on surfaces, but the bacteria are everywhere and further steps are essential.  (NASA/JPL-Caltech)

As he later elaborated to me, other “wicked” risk-benefit problems include gene editing and autonomous driving — both filled with great potential and serious potential downsides.  Like travel to other planets and moons.

“This is subjective,” Maynard said, “but I’d put planetary protection on the more wicked end of the spectrum. It combines individual priorities and ethics  — what people and groups deeply believe is right — with huge uncertainties.  That makes it something never really experienced before and so escalates all factors of wickedness.”

Those groups include scientists (who very much don’t want Mars or another potentially habitable place to be contaminated with Earth life before they can get there), to advocates of greater space exploration (who worry that planetary protection will slow or eliminate some missions they very much want to proceed), to NASA mission managers (worried about delays and costs associated with planetary protections surprises.)

And then there’s the general public which might (or might not) have entirely different ethical concerns about the potential for contaminating other planets and moons with Earth life.

No wonder the problem is deemed wicked.

We’ll get into the pros and cons, but first some background:

I asked NASA’s Planetary Protection officer, Catharine Conley, whether Earth life has been transported to its most likely solar system destination, Mars.

Her reply:  “There are definitely Earth organisms that we’ve brought to Mars and are still alive on the spacecraft.”

Catharine “Cassie” Conley has been NASA’s Planetary Protection officer since 2006. There is only one other full-time official in the world with the same responsibilities, and he works for the European Space Agency. (NASA/W. Hrybyk)

She said it is quite possible that some of those organisms were brushed off the vehicles or otherwise were shed and fell to the surface. Because of the strong ultraviolet radiation and the Earth life-destroying chemical makeup of the soil, however, it’s unlikely the organisms could last for long, and equally unlikely that any would have made it below the surface.  Nonetheless, it is sobering to hear that Earth life has already made it to Mars.

Related to this reality is the understanding that Earth life, in the form of bacteria, algae and fungi and their spores, can be extraordinarily resilient.  Organisms have been discovered that can survive unimagined extremes of heat and cold, can withstand radiation that would kill us, and can survive as dormant spores for tens of thousands of years.

What’s more, Mars scientists now know that the planet was once much warmer and wetter, and that ice underlies substantial portions of the planet. There are even signs today of seasonal runs of what some scientists argue is very briny surface water.

So the risk of Earth life surviving a ride to another planet or moon is probably greater than imagined earlier, and the possibility of that Earth life potentially surviving and spreading on a distant surface (think the oceans of Europa and Enceladus, or maybe a briny, moist hideaway on Mars) is arguably greater too.  From a planetary protection perspective, all of this is worrisome.

The logic of planetary protection is, like almost everything involved with the subject, based on probabilities.  Discussed as far back as the 1950s and formalized in the 1967 Outer Space Treaty, the standard agreed on is to take steps that ensure there is less than a 1 in 10,000 chance of a spaceship or lander or instrument from Earth bringing life to another body.

This figure takes into account the number of microorganisms on the spacecraft, the probability of growth on the planet or moon where the mission is headed, and a series of potential sanitizing to sterilizing procedures that can be used.  A formula for assessing the risk of a mission for planetary protection purposes was worked out in 1965 by Carl Sagan, along with Harvard theoretical physicist Sidney Coleman.

Deinococcus radiodurans is an extremophilic bacterium, one of the most radiation-resistant organisms known. It can survive cold, dehydration, vacuum, and acid, and is therefore known as a polyextremophile and is considered perhaps the world’s toughest bacterium. It can withstand a radiation dose 1,000 times stronger than what would kill a person.

A lot has been learned since that time, and some in the field say it’s time to re-address the basics of planetary protection.  They argue, for instance, that since we now know that Earth life can (theoretically, at least) be carried inside a meteorite from our planet to Mars, then Earth life may have long been on Mars — if it is robust enough to survive when it lands.

In addition, a great deal more is known about how to sanitize a space vehicle without baking it entirely — a step that is both very costly and could prove deadly to the more sophisticated capsules and instruments.  And more is known about the punishing environment on the surface of Mars and elsewhere.

People ranging from Mars Society founder Robert Zubrin to Cornell University Visiting Scientist Alberto G. Fairén in Nature Geoscience have argued — and sometimes railed — against planetary protection requirements. NASA mission managers have often voiced their concerns as well.  The regulations, some say, slow the pace of exploration and science to avoid a vanishingly small risk.

Brent Sherwood, planetary mission formulation manager for JPL, is currently overseeing two proposed projects for New Frontiers missions.  One is to search for signs of life on Saturn’s moon Enceladus and the other for habitability on the moon Titan. (Brent Sherwood)

Brent Sherwood, program manager for solar system mission formulation at JPL, spoke at AbSciCon about what he sees as the need for a review and possibly reassessment of the planetary protection rules and regulations.  As someone who helps scientists put together proposals for NASA missions in the solar system, he has practical and long considered views about planetary protection.

He and his co-authors argue that the broad conversation that needs to take place should include scientists, ethicists, managers, and policy makers; and especially should include the generations that will actually implement and live with the consequences of these missions.

In the abstract for his talk, Sherwood wrote:

“The (1 chance in 10,000) requirement may not be as logically sound or deserving of perpetuation as generally assumed.  What status should this requirement have within an ethical decision-making process? Do we need a meta-ethical discussion about absolute values, rather than an arbitrary number that purports to govern the absolute necessity of preserving scientific discovery or protecting alien life?”

As he  later he told me: “I’m recommending that we be proactive and engage the broadest possible range of stakeholder communities…. With these big, hairy risk problems, everything is probabilistic and open to argument.  People are bad at thinking of very small and very big numbers, and the same for risks.  They tend to substitute opinion for fact.”

Sherwood is no foe of planetary protection.  But he said planetary protection is a “foundational” part of the space program, and he wants to make sure it is properly adapted for the new space era we are entering.

Elon Musk of SpaceX, Jeff Bezos of Blue Origins and NASA have all talked about potentially sending astronauts to Mars or establishing a colony on Mars in the decades ahead.  Many obstacles remain, but planning is underway. (Bryan Versteeg/

Planetary protection officer Conley contends that regular reviews are already built into the system.  She told me that every mission gets a thorough planetary protection assessment early in the process, and that there is no one-size-fits-all approach.  Rather, the risks and architecture of the missions are studied within the context of the prevailing rules.

In addition, she said, the group that oversees planetary protection internationally — the Committee on Space Research (COSPAR) — meets every two years and its Panel on Planetary Protection takes up general topics and welcomes input from whomever might want to raise issues large or small.

“You hear it said that there are protected areas on Mars or Europa where missions can’t go, but that’s not the case,” she said.  “These are sensitive areas where life just might be present now or was present in the past.  If that’s the case, then the capsule or lander or rover has to be sterilized to the level of the Viking missions.”

She said that she understood that today’s spacecraft are different from Viking, which was designed and built from scratch with planetary protection in mind.  Today, JPL and other mission builders get some of their parts “off the shelf” in an effort to make space exploration less expensive.

“We do have to balance the goals of exploration and space science with making sure that Earth life does not take hold.  We also have to be aware that taxpayer money is being spent.  But if a mission sent out returns a signal of life, what have we achieved if it turns out to be life we brought there?

“I see planetary protection as a great success story.  People identified a potential contamination problem back in the ’50s, put regulations into place, and have succeeded in avoiding the problem.  This kind of global cooperation that leads to the preventing of a potentially major problem just doesn’t happen that often.”

The global cooperation has been robust, Conley said, despite the fact that only NASA and the European Space Agency have a full-time planetary protection officer.  She cited the planning for the joint Russian-Chinese mission to the Martian moon Phobos as an example of other nations agreeing to very high standards.  She and her European Space Agency (ESA) counterpart traveled twice to Moscow to discuss planetary protection steps being taken.

Andrew Maynard is the director of Arizona State University’s Risk Innovation Lab and is a professor in School for the Future of Innovation in Society.  (ASU.)

So far, she said private space companies have been attentive to planetary protection as well.  Some of the commercial space activity in the future involves efforts to mine on asteroids, and Conley said there is no planetary protection issues involved.  The same with mining on our moon.

But should the day arrive that private companies such as SpaceX and Blue Origin seriously propose a human mission to Mars — as they have said they plan to — Conley said they would have the same obligations as any NASA mission.  The US has not yet determined how to ensure that compliance, she said, but companies already would need Federal Aviation Administration approval for a launch, and planetary protection is part of that.

Risk innovation expert Maynard, however, was not so sure about those protections.  He said he could imagine a situation where Elon Musk of SpaceX or Jeff Bezos of Blue Origin or any other space entrepreneur around the world would decide to move their launch to a nation that would be willing to provide the service without intensive planetary protection oversight.

“The risk of this may be small, but this is all about the potentially outsize consequences of small risks,” he said.

Maynard said that was hardly a likely scenario — and that commercial space pioneers so far have been supportive of planetary protection guidelines — but that he was well aware of the displeasure among some mission managers and participating scientists about planetary protection requirements.

Given all this, it’s easy to see how and why planetary protection advocates might feel that the floodgates are being tested, and why space explorers looking forward to a time when Mars and other bodies might be visited by astronauts and later potentially colonized are concerned about potential obstacles to their visions.

An artist’s rendering of a sample return from Mars.  Both the 2020 NASA Mars mission and the ESA-Russian mission are designed to identify and cache intriguing rocks for delivery to Earth in the years ahead. (Wickman Spacecraft & Propulsion)

This column has addressed the issue of “forward contamination” — how to prevent Earth life from being carried to another potentially habitable solar system body and surviving there.  But there is another planetary protection worry and that involves “backward contamination” — how to handle the return of potentially living extraterrestrial organisms to Earth.

That will be the subject of a later column, but suffice it to say it is very much on the global space agenda, too.

The Apollo astronauts famously brought back pounds of moon rocks, and grains of asteroid and comet dust have also been retrieved and delivered.  A sample return mission by the Russian and Chinese space agencies was designed to return rock or grain samples from the Martian moon Phobos earlier this decade, but the spacecraft did not make it beyond low Earth orbit.

However, the future will see many more sample return attempts.  The Japanese space agency JAXA launched a mission to the asteroid 162173 Ryugu in 2014 (Hayabusa 2) and it will arrive there next year.  The plan is to collect rock and dust samples and bring them back to Earth.  NASA’s OSIRIS-REx is also making its way to an asteroid, 101955 Bennu, with the goal of collecting a sample as well for return to Earth.

And in 2020 both NASA and ESA (with Russian collaboration) will launch spacecraft for Mars with the intention of preparing for future sample returns.  Sample return is a very high priority in the Mars and space science communities, and many consider it essential for determining whether there has ever been life on Mars.

So the “wicked” challenges of planetary protection are only going to mount in the years ahead.









Ocean Worlds: Enceladus Looks Increasingly Habitable, and Europa’s Ocean Under the Ice More Accessible to Sample

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

It wasn’t that long ago that Enceladus, one of 53 moons of Saturn, was viewed as a kind of ho-hum object of no great importance.  It was clearly frozen and situated in a magnetic field maelstrom caused by the giant planet nearby and those saturnine rings.

That view was significantly modified in 2005 when scientists first detected signs of the icy plumes coming out of the bottom of the planet.  What followed was the discovery of warm fractures (the tiger stripes) near the moon’s south pole, numerous flybys and fly-throughs with the spacecraft Cassini, and by 2015 the announcement that the moon had a global ocean under its ice.

Now the Enceladus story has taken another decisive turn with the announcement that measurements taken during Cassini’s final fly-through captured the presence of molecular hydrogen.

To planetary and Earth scientists, that particular hydrogen presence quite clearly means that the water shooting out from Enceladus is coming from an interaction between water and warmed rock minerals at the bottom of the moon’s ocean– and possibly from within hydrothermal vents.

These chimney-like hydrothermal vents at the bottom of our oceans — coupled with a chemical mixture of elements and compounds similar to what has been detected in the plumes — are known on Earth as prime breeding grounds for life.  One important reason why is that the hydrogen and hydrogen compounds produced in these settings are a source of energy, or food, for microbes.

A logical conclusion of these findings:  the odds that Enceladus harbors forms of simple life have increased significantly.

To be clear, this is no discovery of extraterrestrial life. But it is an important step in the astrobiological quest to find life beyond Earth.

“The key here is that Enceladus can produce fuel that could be used by biology,” said Mary Voytek, NASA’s senior scientist for astrobiology, referring to the detection of hydrogen.


This graphic illustrates how scientists on NASA’s Cassini mission think water interacts with rock at the bottom of the ocean of Saturn’s icy moon Enceladus, producing hydrogen gas (H2). It remains unclear whether the interactions are taking place in hydrothermal vents or more diffusely across the ocean. (NASA)

“So now on this moon we have many of the components associated with life — water, a source of energy and many of the important chemical building blocks.  Nothing coming from Cassini will tell is if there is biology there, but we definitely have found another important piece of evidence of possible habitability.”

The finding of molecular hydrogen (H2 rather a single hydrogen atom) in the Enceladus plumes was described in a Science paper lead by authors Hunter Waite and Christopher Glein of the Southwest Research Institute, headquartered in San Antonio.

They went through a number of possible sources of the hydrogen and then concluded that the clearly most likely one was that chemical interaction of cool water and hot rocks — both heated by tidal forces in the complex Saturn system — at the bottom of the global ocean.

“We previously thought that the water was heated but now we have evidence that the rocks are as well,” Waite told me.  “And the evidence suggests that the rock is quite porous, which means that water is seeping through on a large scale and producing these chemical interactions that have a byproduct of hydrogen.”

The moon Enceladus is the sixth largest in the Saturn system. This image was taken by Cassini in 2008. (NASA/JPL-Caltech, Space Science Institute.)

He said that the process could be taking place in and around those chimney-like hydrothermal vents,  or it could be more diffuse across the ocean floor.  The vent scenario, he said, was “easier to envision.”

What’s more, he said, the conditions during this water-rock interaction are favorable for the production of the gas methane, which has been detected in the Enceladus plume.

This is another tantalizing part of the Enceladus plume story because the earliest lifeforms on Earth are thought to have both consumed and expelled that gas.  At this point, however, Waite said there is no way to determine how the methane was formed, which would be a key finding if and when it is made.

“Our results leave us agnostic on the presence of life,” he said. “We don’t have enough information for that.”

“But we now can make a strong case that we have a very habitable environment on this moon.” It’s such a strong case, he said, that it would be almost as scientifically interesting to not find life there than to detect it.

One of the more interesting remaining puzzles is why the hydrogen is present in the plume in such unexpectedly substantial (though initially difficult to detect) amounts.  If there was a large microbial community under the ice, then it could plausibly be argued that there wouldn’t be so much hydrogen left if they were consuming it.

The possibilities:  Waite said that it could mean there is just a lot of “food” being produced for potential microbes to survive on in the ocean, or that other factors limit the microbe population size.  Or, of course, it could mean that there are no microbes at all to consume the hydrogen food.

Astronomers have twice found evidence of a plume of water vapor coming from the same location on Europa. Both plumes, photographed in UV light by Hubble, were seen in silhouette as the moon passed in front of Jupiter. (NASA/ESA/STScI/USGS)

News of the Enceladus discovery came on the same day that other researchers announced that strong evidence of detecting a similar plume on Jupiter’s moon Europa using the Hubble Space Telescope.

This was not the first plume seen on that larger moon of Jupiter, but is perhaps the most important because it appeared to be was spitting out water vapor in the same location as an earlier plume.  In other words, it may well be the site of a consistently or frequently appearing geyser.

“The plumes on Enceladus are associated with hotter regions,” said William Sparks of the Space Telescope Science Institute. “So after Hubble imaged this new plume-like feature on Europa, we looked at that location on the Galileo thermal map. We discovered that Europa’s plume candidate is sitting right on the thermal anomaly,”

Sparks led the Hubble plume studies in both 2014 and 2016, and their paper was published in The Astrophysical Journal.  He said he was quite confident, though not completely confident of the result because of the limits of the Hubble resolution.  A 100 percent confirmation, he said, will take more observations.

Since Europa has long been seen as a strong candidate for harboring extraterrestrial life, this is extraordinarily good news for those hoping to test that hypothesis.  Now, rather than devising a way to blast through miles of ice to get to Europa’s large, salty and billions-of-years-old ocean, scientists can potentially learn about the composition of water by studying the plume — as has happened at Enceladus.

As their paper concluded, “If borne out with future observations, these indications of an active Europan surface, with potential access to liquid water at depth, bolster the case for Europa’s potential habitability and for future sampling of erupted material by spacecraft.”

This is particularly exciting since NASA is actively developing a mission to Europa that would orbit the moon and could target the plume area for study.

NASA teams have also proposed a Europa lander — a mission that was rejected by the Trump administration in its budget proposals.  But discovery of  what might be a regularly-spurting plume just might change the equation.

The plumes of Enceladus originate in the long tiger stripe fractures of the south polar region pictured here. (Cassini Imaging Team, SSI, JPL, ESA, NASA)

The news about both Enceladus and Europa illustrates well the process by which the search for life beyond Earth — astrobiology — moves forward.

Like few other disciplines, astrobiology needs expertise coming from a broad range of fields, from astrophysicists, geochemists, biochemists, geologists, and more.

Hunter Waite, for instance, trained as an atmospheric  scientists and now builds mass spectrometers for spacecraft such as Cassini,  operates them in flight, and analyzes and reports the data.  He is something of a “plume” expert as well, and will follow up his team leading work on Enceladus as principal investigator of the Europa mass spectrometer that surely will investigate that other moon’s new-found plumes. (The Europa mission, called the Europa Clipper, is loosely scheduled to launch in 2022.)

His colleague, Christopher Glein, is a geochemist.  And the leader of the Europa plume-spotting team, William Sparks, is an astronomer.

Mary Voytek, NASA senior scientist for astrobiology.  (NASA)

Each discipline focuses on a part of the larger system that might, or might not, be habitable.  No single scientists or discipline of scientists is capable of detecting extraterrestrial life.

This has long been the view of NASA’s Voytek, who views astrobiology as a kind of very long-term scientific full-court press.

She is wary of overselling discoveries that involve the search for life beyond Earth and the origin of life here, saying that they sometimes are well-meaning “science fiction” more than science.

However, the Enceladus findings in particular have her excited.  A lot of questions remain, such as whether the water with molecular hydrogen is coming from a hydrothermal vent or across the ocean floor, and whether the amount of methane detected in the plume increases or decreases the likelihood of life on the ocean floor.

But her conclusion: “I think this puts Enceladus into a different category and definitely higher up on the index of habitability.”  Any potential life, she said, would almost surely be microbial, though it might be larger “if we get lucky.”


Waiting on Enceladus

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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















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

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


Enceladus and Water Worlds

Glittering geysers of water ice erupt from Saturn's enigmatic moon Enceladus as seen during a previous flyby. The plumes are backlit by the sun, which is almost directly behind the moon. The moon's dark side that we see here is illuminated by reflected Saturn-shine. Today, the Cassini spacecraft flew right through the plumes in order to let its instruments 'taste' them. Credit: NASA/JPL/SSI/Ugarkovich
Glittering geysers of water ice erupt from Saturn’s enigmatic moon Enceladus as seen during a previous flyby. The plumes are backlit by the sun, which is almost directly behind the moon. The moon’s dark side that we see here is illuminated by reflected Saturn-shine.  Credit: NASA/JPL/SSI/Ugarkovich

As if the prospect of billions of potentially habitable exoplanets wasn’t enough to get people excited, what about all those watery exo-moons too?

The question arises as the Cassini mission makes its final pass near the now famous geysers at the south pole of the moon Enceladus ,scheduled for Saturday.  The plumes are currently in darkness and so it’s a perfect time to tease out a particularly compelling aspect of the Enceladus story:  how hot is the inside of the mini-moon.  Earlier measurements of the water ice spray took place when the sun was on that southern pole, so this will be the first time Cassini can measure precisely how much of the already detected heat comes from the moon’s interior.

The expectation is that much of the heat does indeed come from inside, warmed substantially by tidal forces and perhaps hydrothermal vents that together serve to keep liquid a subsurface ocean all around the moon.  As a result, the evolving scientific view is that tiny Enceladus, one of 63 moons of Saturn, just may have the ingredients and characteristics that put it into an improbable habitable zone.

“Step by step, we’re learning about an environment that seemed impossible not long ago,” said Cassini Mission Scientist Linda Spilker.  “We know that Enceladus has some rocky core, and that it touches the liquid water.  We also know that some of the compounds identified in the geysers can only be formed when rock is in contact with hot water, and that must be happening at the bottom of the moon’s ocean.  All the pieces are coming together to tell us that the moon has an ocean that might be able to support life.”

NASA's Cassini spacecraft captured this view as it neared icy Enceladus for its closest-ever dive past the moon's active south polar region. The view shows heavily cratered northern latitudes at top, transitioning to fractured, wrinkled terrain in the middle and southern latitudes. The wavy boundary of the moon's active south polar region -- Cassini's destination for this flyby -- is visible at bottom, where it disappears into wintry darkness. This view looks towards the Saturn-facing side of Enceladus. North on Enceladus is up and rotated 23 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Oct. 28, 2015. The view was acquired at a distance of approximately 60,000 miles (96,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 45 degrees. Image scale is 1,896 feet (578 meters) per pixel.
The Cassini spacecraft, sponsored by NASA, the European Space Agency and the Italian space Agency,  captured this view on Oct. 28 as it neared Enceladus. The wavy boundary of the moon’s active south polar region — Cassini’s destination for this flyby — is visible at bottom. The image was taken in visible light with the Cassini spacecraft narrow-angle camera from approximately 60,000 miles away. (Cassini Imaging Team, SSI, JPL, ESA, NASA)

That a moon might have habitable conditions is not a new idea:  science fiction great Arthur C. Clarke (as well as many scientists and now members of Congress) have pressed for a mission to Jupiter’s moon Europa because its internal ocean has been identified as similarly promising.

But what is so compelling about Enceladus is that its potential habitability pretty much came out of nowhere.  While Europa is the sixth largest moon in the solar system, Enceladus is but 370 miles in diameter.  It is covered in ice, but in 2004 its four parallel “tiger stripe” fractures were discovered, leading to the conclusion that some kind of volcanic action was taken place beneath them.  Spilker was a scientist with the Voyager mission that passed by Saturn in 1980-81 and said that Enceladus was then in relative darkness and made little impression on the team.  “We definitely missed the tiger stripes,” she said.

The plumes emerge from the south pole region, not far from the tiger stripe fractures, and appear to come from  near-surface pockets of liquid water. (The oceans of Europa are not nearly as accessible, lying 6 to 20 miles below the icy surface.)  Making Enceladus even more interesting, the 70 geysers spit out organic chemicals known as the building blocks of life along with its water ice.

The chemical composition of the plumes of Enceladus's includes hydrocarbons such as ammonia, methane and formaldehyde in trace amounts similar to the makeup of many comets. (NASA)
The chemical composition of the plumes of Enceladus’s includes hydrocarbons such as ammonia, methane and formaldehyde in trace amounts similar to the makeup of many comets.  The presence of the organic compounds suggests that very interesting chemistry is taking place where the moon’s oceans touch its core. (NASA, ESA)

So Enceladus (and perhaps Europa, too) provide a kind of emerging “proof of concept” that ice-covered water worlds can and do exist elsewhere.  (Jupiter’s giant moons Ganymede and Callisto also have massive ocean, but far below their surfaces and sandwiched between layers of ice.)  In fact, subterranean oceans may be common because, in recent years, scientists have come to understand that water — especially in its vapor and ice stages — is ubiquitous in the solar system, the galaxy and the universe.  Comets, which are generally half water ice and half rock, are one of numerous delivery systems.

And we already have, of course, one good example of what would generally be considered a waterworld without the ice covering — Earth.  But that’s not all, even without leaving our solar system.   We look at planets such as Mars and Venus and now see desiccated landscapes.  Yet it is broadly accepted that Mars once was quite wet on the surface, based on findings from the Curiosity mission and years of satellites imaging, and some have speculated that Venus might once have been wet as well.

So are there many waterworlds or aquaworlds with surface liquid water out there?

“It would be fair to say there is a consensus view that exoplanets and moons with lots of water are all over the place,” said Joel Green, an exoplanet scientist with the Space Telescope Science Institute. “The known presence of so much H2O makes that non-controversial.”

Kepler-62e has been described as being a possible waterworld, with large oceans. UPR Arecibo
Kepler-62e has been described as being a possible waterworld, with large oceans. UPR Arecibo


What is indeed controversial is whether any waterworlds, or even potential waterworlds, have been detected. There actually has been wide coverage of waterworld discoveries far, far away,  and even some declared confirmations.  But so far at least, those confirmations have not lasted.  As early as 2004, the Hubble Space Telescope identified the signature of water vapor in the atmosphere of an exoplanet, and similar detections have followed.  But that information and more that scientists have collected and modeled about H2O on exoplanets has never been sufficient to make a confirmation stick.

“It’s a very challenging detection to make, and many don’t think we haven’t gotten there yet,” Green said.  Especially challenging is the detection of liquid water, since it does not show up using optical, infrared, ultraviolet or any other kind of light, and so can’t be identified with a spectroscope. It’s presence can only be inferred based on other conditions.  Water vapor and water ice are, however, detectable via spectroscope.

But waterworld theories abound.  For instance, scientists know that Neptune and Uranus in the outer part of our solar system consist of vast amounts of water ice, and they also know that planets tend to change orbits and sometimes migrate closer to their suns.  Marc Kurchner of NASA’s Goddard Space Flight Center has proposed that if similar planets were to migrate inward in different solar systems, the result could be a very wet planet, with oceans hundreds of miles deep.

There is a general tendency to associate the presence of water with the presence of life, and the ubiquity of water in the galaxies with the likelihood of finding life.   But while life is found everywhere that water exists on Earth, that does not at all mean that the discovery of water elsewhere means life will be present, too.

All it means is that one of many prerequisites for life (as we know it) will have been met. But as Enceladus shows, when water is present, all kinds of interesting things start of happen.

A view of Enceladus’ southern hemisphere in enhanced color (IR-green-UV). The “tiger stripe” fractures, the source of plumes venting gas and dust into space, are prominently visible in the center. {NASA/JPL-Caltech/SSI/Lunar and Planetary Institute, Paul Schenk (LPI, Houston)
A view of Enceladus’ southern hemisphere in enhanced color (IR-green-UV). The “tiger stripe” fractures, the source of plumes venting gas and dust into space, are prominently visible in the center. {NASA/JPL-Caltech/SSI/Lunar and Planetary Institute, Paul Schenk (LPI, Houston)