101 Geysers on Enceladus (and What They Imply)

by Paul Gilster on July 31, 2014

I’ve mentioned before the irony that we may discover signs of robust extraterrestrial life sooner around a distant exoplanet than right here in our own Solar System. The scenario isn’t terribly implausible: Perhaps we come up empty on Mars, or find ourselves bogged down with ambiguous results. As our rovers dig, we still have Europa, Enceladus and other outer system possibilities, but probably face a wait of decades before we could build and fly the missions needed to identify life.

Meanwhile, the exoplanet hunt continues. While we’ve had many a setback — the Space Interferometry Mission will always stand out in this regard, not to mention the inability to follow through with Terrestrial Planet Finder, Darwin and other high-end concepts — it’s just possible that within the next few decades, a space-based observatory will detect a solid biosignature from an exoplanet’s atmosphere. Even the James Webb Space Telescope should be able to detect the transmission spectrum of an Earth-class planet transiting a dim red dwarf star. Future instruments will be able to take atmospheric characterization down to an Earth 2.0 around a Sun-like star.

Then again, maybe the outer Solar System will prove so enticing that we do decide to make it a priority. We could be seeing this happen right now. Every new piece of evidence from Cassini helps to build the case that Enceladus is an attractive proposition for the life search, the latest news being that the Saturn orbiter has identified 101 distinct geysers erupting on the moon’s surface. We first detected geysers of ice particles and water vapor at Enceladus’ south pole almost a decade ago. Now we have a map of geysers erupting from the so-called ‘tiger stripe’ fractures coincident with surface hot spots.

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Image: This two-image mosaic is one of the highest resolution views acquired by Cassini during its imaging survey of the geyser basin capping the southern hemisphere of Saturn’s moon Enceladus. It clearly shows the curvilinear arrangement of geysers, erupting from the fractures. From left to right, the fractures are Alexandria, Cairo, Baghdad, and Damascus. As a result of this survey, 101 geysers were discovered: 100 have been located on one of the tiger stripes , and the three-dimensional configurations of 98 of these geysers have also been determined. The source location of the remaining geyser could not be definitively established. These results, together with those of other Cassini instruments, now strongly suggest that the geysers have their origins in the sea known to exist beneath the ice underlying the south polar terrain. Credit: NASA/JPL-Caltech/SSI.

The reason this is so exciting is that the hot spots that Cassini’s heat-sensing instruments found in the south polar region are only a few tens of meters across. That means they’re too small to be produced by the kind of frictional heating that would be caused by the repeated flexing of Enceladus due to tidal effects from Saturn. Frictional heating could have accounted for the geyser phenomena by turning surface ice into vapor and liquid, but it now appears that we’re dealing with water from the ocean below being exposed by opening and closing of the fractures.

Carolyn Porco (Space Science Institute) is leader of the Cassini imaging team, and lead author of a new paper on the Cassini findings:

“Once we had these results in hand we knew right away that heat was not causing the geysers but vice versa. It also told us the geysers are not a near-surface phenomenon but have much deeper roots.”

The source of the material forming the geysers of Enceladus is thus found to be the sea that exists under the ice shell, a sea that Cassini’s gravity data on the moon has confirmed. This news release from CICLOPS (Cassini Imaging Central Laboratory for Operations) has more, including the results of a second paper in which the authors report that the brightness of the combined geyser plume as viewed by Cassini changes periodically during the moon’s orbit of Saturn. In most respects, the brightness variations track the expected tidal venting cycle.

But not entirely. What would be expected from the opening and closing of the fractures does not predict when the plume begins to brighten, a finding that could implicate the spin rate of Enceladus. Francis Nimmo (UC-Santa Cruz) is lead author on the second paper:

“It’s an interesting puzzle. Possibilities for the mismatch include, among other effects, a delay in the response of the ice shell, which would suggest tides are heating the bulk of the ice at the south pole, or subtle changes in the spin rate of Enceladus.”

That last remark points to the possibility that the liquid water under the Enceladan ice may be global, even if deeper under the south pole region. So we have yet another reason for fascination with a moon whose salty sea, known to contain organic compounds, is spouting geysers and, possibly, reaching the surface on occasion as a liquid. We have a potentially habitable environment under the ice that periodically offers up samples to nearby spacecraft.

Enceladus is too good a target to resist, and it’s worth remembering mission concepts like Life Investigation for Enceladus (LIFE), developed by Peter Tsou. LIFE could launch in the early 2020s, reaching Saturn in 2030 with the help of gravity assists along the way, capturing material from the Enceladus geysers with an aerogel collector like the one NASA used in its Stardust comet mission. With a final gravity assist at Titan, LIFE would then bring its samples back to Earth in 2036.

I’m remembering, too, NASA astrobiologist Chris McKay’s exhortation that the venting of water and organics into space is ‘an open invitation to go there.’ The German Aerospace Center (DLR) has likewise been exploring Enceladus mission concepts, envisioning a lander that would drill through the ice. Enceladus Explorer would use an ice drill probe to melt its way into a water-bearing crevasse to look for microorganisms, on the theory that any life in the plumes would have been destroyed by sudden exposure to space. Thus the need to probe the ocean itself.

So the ideas for sampling Enceladus for life are out there and they’ll doubtless increase as Cassini continues to demonstrate how potent an astrobiological target this moon is. Which concept should we choose, and for that matter, which should we choose between Enceladus and Europa in terms of life-seeking mission destinations for spacecraft that can be flown in the near future? Both have legitimate claims on our attention, and the possibility of plumes on Europa itself (see Water Vapor Detected Above Europa) may change the equation. Will these enticing moons motivate us to reach them before a near-term space telescope finds the first biosignatures around an exoplanet?

The papers are Porco et al., “How the Geysers, Tidal Stresses, and Thermal Emission Across the South Polar Terrain of Enceladus are Related,” The Astronomical Journal Vol. 148, No. 3 (2014), 45 (abstract) and Nimmo et al., “Tidally Modulated Eruptions on Enceladus: Cassini ISS Observations and Models,” The Astronomical Journal Vol. 148, No. 3 (2014) 46 (abstract). On the LIFE mission, see Tsou et al., “LIFE: Life Investigation For Enceladus: A Sample Return Mission Concept in Search for Evidence of Life,” Astrobiology Vol. 2, No. 8 (September 12, 2012). Abstract available.

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{ 8 comments… read them below or add one }

ericsect July 31, 2014 at 15:00

I am still trying to figure out how the liquid water ocean became established in the first place, although if there were orbital resonances in the past, or if Enceladus and her sisters periodically move in and out of resonances, that could explain it. How about the idea that… Enceladus somehow interacts with Saturn’s magnetic field; electric currents get generated in the rocky-metallic core (or salty ocean) and inductive heating takes place to melt the ice? If so, could mean a lot more icy moons with liquid water oceans.

Joncr July 31, 2014 at 18:28

Assuming the ice surrounding the hole created by a geyser is warm enough to become slushy (and too thin or too deep to position a vehicle on it) then perhaps a simple probe, attached to a line, might be propelled from the vehicle onto this slushy area. The line would be reeled back to the vehicle while the probe scraped up slush for analysis.

Not nearly as useful as a direct probe of the internal ocean. Doing that, though, requires a crust thin enough to get a drill through.

Alex Tolley July 31, 2014 at 20:13

Going back to micro sats for a moment, could a spinning cone offer a simple way for a small probe to fall into a plume with minimal effort. I’m thinking of the Benford’s spinning ablation cones in microwaves. Just substitute the plume for the microwave beam and have the cone be heavy enough to track down to the plume source. As for finding life – there are simple throwaway microscopes , e.g. Foldscope to provide unambiguous images of cellular forms, plus a host of simple molecular detectors for Earth type life. I would also consider some sort of simple lure (light?) for larger orgnisms. Send a swarm to Enceladus, and see if any strike pay dirt. The cost should be comparatively low as the probes and detection methods are almost trivially simple and basic.

Eniac July 31, 2014 at 23:42

I think drilling is a very difficult prospect for a space mission, seeing as it requires massive consumables and is prone to failure. Sinking a probe through miles of ice by melting requires a lot of energy, but could conceivably be achieved with a nuclear reactor or suitable radioisotope heating elements. Until such time as we are ready to develop and fly such high power nuclear technology, it seems picking up traces of organics from the geyser ejecta is the only viable approach to looking for life on Enceladus.

Harry R Ray August 1, 2014 at 9:37

Microbes (if there ARE any) being destroyed in the venting process MAY turn out to be a very good thing! Cassini can get as close to the holy grail without touching it as possible. but only if we get INCREDIBLY lucky! Heres the scenario. For this to be possible, there must ALSO be a very RUDIMENTARY form of multicellular life, as well as the microbes. When this multi-celled life DECOMPOSES, Methyl Mercaptan (CH3HS) is produced. The microbes then INGEST it, and it is released into space when the microbes are distroyed. CH3SH can only* be produced by either natural BIOLOGICAL or by non-natural industrial proceces! The compound is SIMPLE enough to detect with Cassini’s spectrometer in sufficient (aye, there’s the rub). The LACK of any announcement means that, if it’s there, the amounts are too low to be conclusive. BUT: next year (if all goes well), just before Cassini enters its “Grand Finale” phase, it is scheduled to make ONE MORE PASS through the guysers, AND, it will do so at a MUCH LOWER ALTITUDE then ever before! I ,foe one, am holding my breath for any positive results! *should this near-miracle occur, expect MANY CHALLENGES to this ASSUMPTION from the scientific community.

ljk August 1, 2014 at 10:35

We could nuke Enceladus from orbit to make a hole for a probe. It’s the only way to be sure. :^) Hey, a Russian scientist seriously suggested this for Europa less than a decade ago.

Behold Enceladus: Cassini Maps 101 Geysers on Tiny Saturn Moon

By Paul Scott Anderson

Saturn’s moon Enceladus is already known as one of the most intriguing places in our solar system, and now new findings from the Cassini spacecraft have been published, which will only add to our fascination with this little world.

Before Cassini, Enceladus was expected to be little more than a frozen ball of ice and rock, being so distant from the sun. But this moon held surprises, the kind which would make Enceladus a much more interesting place, and a new prime target in the search for possible life elsewhere in the solar system.

In 2005, Cassini made its first discovery of something amazing – water vapor geysers spewing out from the surface. Geysers? How could there be something like that on this tiny cold moon? But there they were; since then many images have been taken and Cassini has even passed directly through some of them, sampling the spray as it did so. The plumes contained water vapor, ice particles, salts and organics. They were found to originate from deep fissures called “tiger stripes” at the south pole of the moon, which were warmer than the surrounding icy terrain. So what did this mean? Could there be water somewhere below the surface, like on Jupiter’s moon Europa? The new results presented today support that incredible idea – the fissures allow water from a subsurface sea to make its way to the surface, when then explodes out into space as huge plumes of water vapor which then freeze into ice particles.

The new findings are the result of the previous seven years of study of the geysers; Cassini scientists have now produced a detailed map of the known geysers, all 101 of them! They have been published in two new papers in the online edition of The Astronomical Journal (abstract/download here).

http://m.iopscience.iop.org/1538-3881/148/3/45/

“Once we had these results in hand, we knew right away heat was not causing the geysers, but vice versa,” said Carolyn Porco, leader of the Cassini imaging team from the Space Science Institute in Boulder, Colorado. “It also told us the geysers are not a near-surface phenomenon, but have much deeper roots,” she added. She is also the lead author of the first paper.

Mapping the locations of the plumes helped scientists to better pinpoint where they originate, which, as theorized, turned out to be below the outer icy crust of Enceladus. Pathways through the ice (the fissures) should be able to remain open, allowing liquid water from deeper inside the moon to escape to the surface. By analyzing the gravity data from Cassini, it was determined that the source of the plumes must be the subsurface sea.

Full article here:

http://www.americaspace.com/?p=64962

To quote:

See also this excellent summary of these findings on the CICLOPS website. As Carolyn Porco so eloquently summarizes:

“As we contemplate the approaching end of Cassini’s travels around Saturn, we dream of the day, hopefully not far in the future, when we can return to Enceladus to answer the question now uppermost in the mind: Could a second genesis of life have taken hold on this small icy moon of a hundred and one geysers? For we now know this: if life is indeed there, it is there for the taking.”

Enzo August 1, 2014 at 17:04

@Eniac

I agree that drilling on any icy moon is very far in the future.
However, since the geysers spray the material into space a sample
return with an aereogel like material should be possible.
A difficulty would be preserving the samples for the long return
trip. And, of course, it would be a very long mission.

It’s funny how NASA is concentrating its search of life on Mars
where they neither organic compounds nor liquid water have been
found when they have Enceladus with both detected already.

ljk September 9, 2014 at 13:02

Living On The Edge – The Icy Plains Of Enceladus (part 3)

By Leonidas Papadopoulos

“We must believe then, that as from hence we see Saturn and Jupiter; if we were in either of the Two, we should discover a great many Worlds which we perceive not; and that the Universe extends so in infinitum”.

- Cyrano de Bergerac – “A Voyage to the Moon”, (1656)

In his ‘Voyage to the Moon’, which is considered one of the best examples of early science fiction, 17-century French satirist and dramatist Cyrano de Bergerac satirized the politics and religious beliefs of his day, while also contemplating an infinite Universe that was populated with an infinite number of worlds.

Taking a cue from this fictional story, NASA’s Voyager, Galileo and Cassini missions have helped to reveal more than 300 year later the true magnificence and beauty of the moon systems of all the gas giant planets in the outer Solar System, while also opening our eyes to the intriguing possibility for life on some of these fascinating worlds.

The second part of this article focused on Jupiter’s moon Europa, whose underground ocean is considered a potential cradle for life. Yet, even more equally fascinating worlds await us as we journey further out in the Solar System. Approximately a billion and a half kilometers from the Sun, two of Saturn’s 62 moons, Enceladus and Titan, are also intriguing astronomers with their potential to host potentially habitable environments as well.

Full article here:

http://www.americaspace.com/?p=67223

To quote:

Yet, the fascinating and mystifying Enceladus beckons. “As we contemplate the approaching end of Cassini’s travels around Saturn, we dream of the day, hopefully not far in the future, when we can return to Enceladus to answer the question now uppermost in the mind: Could a second genesis of life have taken hold on this small icy moon of a hundred and one geysers?” asks Porco. “For we now know this: if life is indeed there, it is there for the taking.”

The only thing that stops us from returning again to this fascinating small world in the outer Solar System, is simply our own decision not to.

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