Enceladus and Water Worlds

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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 back lit 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.  (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.  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 water worlds or aquaplanets 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)
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On Super-Earths, Sub-Neptunes and Some Lessons They Teach

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Part 1 of 2

The discovery of a menagerie of exoplanets sized greater than Earth and smaller than Neptune has changed thinking about planets and solar systems. The radius of Neptune is almost 4 times greater than Earth’s, and the planet’s mass is 17 times greater than our planet. (NASA)
The discovery of a menagerie of exoplanets sized greater than Earth and smaller than Neptune has changed thinking about planets and solar systems. The radius of Neptune is almost 4 times greater than Earth’s, and the planet’s mass is 17 times greater than our planet. (NASA)

When the first exoplanet was identified and confirmed 20 years ago, there was enormous excitement, a sense of historic breakthrough and, with almost parallel intensity, sheer bewilderment. The planet, 51 Pegasi B, was larger than Jupiter yet orbited its parent star in 4 days. In other words, it was much closer to its star than Mercury is to ours and so was extremely hot.

According to theories of the time about planetary formation and solar system organization, a hot Jupiter so close to its sun was impossible. That kind of close-in orbit is where small rocky planets might be found, not Jupiters that belonged much further out and were presumed to always be cold.

That was a soberingly appropriate introduction to the new era of exoplanets, and set the stage for 20 years of surprises and re-evaluations of long held theories and understandings.

While the presence of close-in hot Jupiters certainly remains one of the great puzzles of the exo-planet era, the most consequential exo-planetary revelation has likely been the discovery of many planets larger than Earth and smaller than the next largest planet in our solar system — icy, gaseous Neptune.

These “super-Earths’ and “sub-Neptunes” range greatly in size since Neptune has a radius four times greater than our planet. What’s so surprising about the presence of this class of planets is that they are not just common, they are by far the most frequently detected exoplanets to date.

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Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars, by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)

Perhaps most intriguing of all, however, is their absence in our planetary line-up.

It has long been predicted that the planetary make-up of our solar system would be typical of others, but now we know that is (again) wrong. As Mark Marley, a staff scientist at NASA’s Ames Research Center who studies exoplanets put it, the widespread presence of “super-Earths” elsewhere and their absence in our system “is telling us something quite important.” The work to tease out what that might be has just begun, and will likely keep scientists busy for some time.

“It certainly seems that the universe wants to makes these planets,” Marley told me. “And they’re surprising not only because nobody predicted their vast number but also because they have been intractable – very, very difficult to characterize. It seems like they want to keep their secrets close to the vest.”

How are these planets keeping their secrets – the ingredients of their atmospheres, in particular – from researchers?   Because many seem to be surrounded by thick clouds and layers of sooty smog, like Los Angeles on a very bad day. As a result, the spectroscopy normally used to read exoplanet atmospheres and determine what elements and compounds are present is of little use. The instruments can’t see through the thick film

This helps explain why many astronomers and planetary scientists don’t like the term “super-Earths.” The word implies that they are sized-up Earths, but there’s every reason to believe that very few fit into that category. Nonetheless, the name is so compelling that, for now at least, it seems to have stuck – with that addition of “sub-Neptunes.”

This artist's illustration represents the variety of planets being detected by NASA's Kepler spacecraft. A new analysis has determined the frequencies of planets of all sizes, from Earths up to gas giants. (C. Pulliam & D. Aguilar, CfA)
This artist’s illustration represents the variety of planets being detected by NASA’s Kepler spacecraft. A new analysis has determined the frequencies of planets of all sizes, from Earths up to gas giants. (C. Pulliam & D. Aguilar, CfA)

Despite the difficulties in characterizing these planets, some progress is being made. Researchers Leslie Rogers of Caltech and Lauren Weiss at Berkeley have separately, for instance, determined which super-Earths and mini-Neptunes are likely to be rocky like Earth and which are likely to be gaseous and icy like Neptune.  The cut-off is by no means precise or across-the-board, but it appears that once a planet has a radius more than 1.5 or 1.6 times the size of Earth, it will most likely have a thick gas envelope of hydrogen, helium and sometimes methane and ammonia around it.

Weiss, a Ken & Gloria Levy Graduate Student Fellow, described some other super-Earth/sub-Neptune characteristics that she and others have found. These exoplanets, for instance, very often have nearby companions in the same class. Many of these larger ones (above 1.5 Earth radii) also tend to be fluffy; quite big but not particularly dense. Weiss likens the least dense super-Earths to macarons – a light, French meringue-based confection (that is definitely not a macaroon.)

They may well have cores of iron and some inner rockiness, but they are so light that they have to consist in large part of hydrogen, helium, water and other gases. It is common to find super-Earths and even sub-Neptunes that have much larger diameters than Earth, but have less mass than Earth.

While some of the super-Earths and sub-Neptunes were, and still are being detected using ground-based radial velocity and other techniques, most were found by Kepler.  That means the field is very young because that early data came out only a few years ago.  But it represents such an important and compelling paradigm shift in astronomy and planetary science that a large and growing contingent of researchers has quickly assembled to search for and study these properly high-profile planets – their orbits, their planetary neighbors, their masses, and now to some extent the make-up of their atmospheres and cores. Some of the work involves observation, some theory and some modeling.

As Mark Marley pointed out, these planets are not giving up their secrets easily. And inevitably, given the great interest and limited data, conclusions and findings will be published that appear strong at the time, but are quickly eclipsed by new information.

Take, for instance, the announced interpretation in 2009, 2012 and 2013 of a sub-Neptune size “water world.” While the papers that introduced the possibility of a very wet exoplanet Gliese 1214b contained caveats, the news stories that went around the world reported that the first water world had apparently been discovered. Exciting news, for sure.

The planet Gliese 1214b was initially described as a possible "water world," and the idea caught the public imagination. But subsequent examination, and the characterizing of other super-Earths and sub-Neptunes, has led to a different conclusion: that the planet is most likely covered by a hydrogen/helium envelope and a thick film of sooty dust. (Artist rendering by L. Calçada, European Space Observatory.)
The planet Gliese 1214b was initially described as a possible “water world,” and the idea caught the public imagination. But subsequent examination, and the characterizing of other super-Earths and sub-Neptunes, has led to a different conclusion: that the planet is most likely covered by a hydrogen/helium envelope and a thick film of sooty dust. (Artist rendering by L. Calçada, European Space Observatory.)

But several years later, it is clear that the water world story was premature. The presence of water had never been confirmed for Gliese 1214b, but rather had been inferred by other limited measurements involving mass, radius, and the absence of spectral data, which were together interpreted to mean the possible, or even probable, presence of a steamy, wet atmosphere.

It still may be the case that the planet has abundant water. But follow-up investigation using the Hubble Space Telescope showed conclusively that the planet was covered in clouds of unknown make-up and origin, and that the presence of massive amounts of water could not be properly inferred from the data at hand.

Zachory Berta-Thompson of MIT was one of the key participants in the Gliese 1214b papers, and he agrees that the evidence today does not point to a water world.  “There was a very deep investigation of the GJ 1214b atmosphere with the Hubble, and if water was there it would have been detected,” he said.  (The lead author of that paper was Laura Kreidberg of the University of Chicago.)

“We used the data we had when the planet was discovered, and made calculations and inferences that made sense at the time,” Berta-Thompson said.  “But the field moves quickly and with the discovery of many other sub-Neptunes, we would draw other conclusions.”  Gliese 1214b, he said, is most likely a puffy planet (with an envelope of hydrogen and helium) rather than a water world.

This is not, it should be noted, a knock on the initial paper. If anything, it’s a knock on journalists (of which I have long been one) who highlighted the water world story. But primarily, the Gliese 1214b research is one of numerous examples of the exciting new science of super-Earths and sub-Neptunes playing out at very high speed, with inevitable potholes on a bumpy and terribly hard-to-navigate road.

 

Many Worlds will continue this discussion of super-Earths and sub-Neptunes on Friday, with an emphasis on thinking about whether they might be conducive or anathema to life.

 

 

 

 

 

 

 

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