Seven years worth of Cassini images of Enceladus have told us what many have long suspected: The intriguing moon does indeed have a subsurface ocean. Not that the presence of water on Enceladus comes as a surprise: The south polar region in the area of the famous ‘tiger stripes’ has long been known to be venting vapor and liquid water from its fractures. The question had become, is this a regional body of water, or is the Enceladus ocean global?
To find out, a team at Cornell University led by Peter Thomas, whose work was just published in Icarus, charted about 5800 surface features, contrasting images taken at different times and at different angles. Using a combination of dynamical modeling and statistical analysis, they sought to find the best values for the interior that would explain an apparent libration or ‘wobble’ (0.120 ± 0.014°) detectable in the imagery, a larger motion by far than would be expected if the surface of Enceladus were solidly connected with its core. The size of the libration tells us that the ocean is indeed global.
Image: What lies beneath… Now we learn that there is solid evidence for an ocean below this entire surface. Credit: NASA/JPL-CalTech.
Matthew Tiscareno, now at the SETI Institute after working on the Enceladus data at Cornell (he is a co-author of the just published paper), explains the significance of the finding for possible astrobiology:
“This exciting discovery expands the region of habitability for Enceladus from just a regional sea under the south pole to all of Enceladus. The global nature of the ocean likely tells us that it has been there for a long time, and is being maintained by robust global effects, which is also encouraging from the standpoint of habitability.”
Image: This illustration is a speculative representation of the interior of Saturn’s moon Enceladus with a global liquid water ocean between its rocky core and icy crust. The thickness of layers shown here is not to scale. Scientists on NASA’s Cassini mission determined that the slight wobble of Enceladus as it orbits Saturn is much too large for the moon to be frozen from surface to core. The wobble, technically referred to as a libration, reveals that the crust of Enceladus is disconnected from its rocky interior. Credit: NASA/JPL-Caltech.
We’ve come a long way since the first Cassini discoveries of the plume of water vapor, ice and organic molecules erupting from Enceladus’ south pole. It was in 2009 that we learned by measuring the saltiness of the geyser particles that their source could only be a reservoir of liquid, and by 2014 it was possible to analyze the gravitational pull of the Saturnian moon on Cassini itself, which showed that at least a regional sea must be present under the ice.
Now we have a global ocean to deal with, although the question of how it remains liquid is still unresolved. Are tidal forces from Saturn’s gravity generating more heat than we realize? Much work remains as we try to answer that question. In the near term, Cassini is scheduled for a close flyby at the end of October in which it will pass a scant 49 kilometers above the moon’s surface. This ‘deep dive’ through Enceladus’ active plumes will be the closest yet. With a global ocean spewing material into space, the case for further work at Enceladus is overwhelming.
The paper is Thomas et al., “Enceladus’ measured physical libration requires a global subsurface ocean,” published online by Icarus on 11 September 2015 (abstract).
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I’m pretty stoked for the active dive as well. It should be interesting to see what it finds, and the close-up pictures of Enceladus that it will take. If they find something really interesting, then it might help build the case for the Enceladus Life Finder Discovery-Class mission as well.
I wonder why the venting is just from the South Pole and nowhere else (such as the North Pole).
Does anyone know if the paper has determined the oceans must be fully liquid, or can they be slush instead, with local pockets of liquid? Is this determined by pressure alone, or by a combination of pressure and some sort of heating, e.g. gravity/flexing/friction etc.
The close flyby/dive should give us stunning resolution if the cameras are used. Do we know what that resolution would be?
Just nitpicking, in no way critique, but: Solid evidence of an ocean? figured it would be a fluid… ;)
Maybe it has something to do with the tidal forces. Could they be fragmenting the ice at the south pole, allowing water to continually well up through it?
Off Topic Rant: Sorry, I couldn’t help myself. Orion is now delayed until 2023, are you kidding me? Eight years away! In contrast there were only 7 years and 7 months between Alan Shepherd’s suborbital flight and the Apollo 8 flight around the moon .
The 30 mile altitude flyby through the plumes in late October is going to be very enlightening. Would be great to send a bespoke mission there but the maximum funding available for the foreseeable future is through New Frontiers programme at $1 billion which is a push for a lander. Andrew Le Page has proposed a sample return mission which could be done within this funding envelope and as Enceladus gets increasingly exciting sounds like worth the effort.
Solar cell/concentrator arrays can now work as far out as Saturn which saves money over RTGs and there is one Nasa lander concept that is costed at $1.5 billion . Limiting the instrumentation package , using only “off the shelf” technology and critically having any lander transmit direct to Earth without an expensive relay satellite ( as in the Nasa concept ) might just get this down to a billion. The big risk here is firstly that the landing on unknown terrain as Enceladus hasn’t been imaged to the 0.5m /pixel level necessary to choose a safe site as near to the “tiger stripes” as possible . The lander could do this whilst in orbit round the moon and/or use LIDAR ( laser equivalent of radar) as part of a “Hazard detection and avoidance system” as the lander descends. This system and its intelligent software is currently being perfected . Any overseas funding either direct or for instrumentation would help and be possible given the high profile mission and exciting potential science return. One final issue is that both the sun and Earth drop below the southern polar region in 2040 which will scupper both direct transmission and solar battery recharging. Though that sounds a long way away, with the limited launchers currently available a venus/Earth flyby trajectory is required to get to Saturn ( VEEGA) followed by three years braking and getting into position around Enceladus via multiple moon flybys over 3 years. With an 8 years plus transit that’s a total of 12 years or so. Run in times to Nasa missions as we know are not quick ( how long was it for New Frontiers ?) and designing missions takes time so before you know it 2040 will be on us ( or some of us !) . For me Drew LePage’s New Frontiers funded sample return seems the best bet and is very much in the spirit of this planetary programme . Much longer wait than New Horizons even though.
Hopefully, this gives NASA strong incentive to approve the Enceladus Life Finder Mission for the next Discovery mission slot instead of something like the last two snores: GRAIL and INSIGHT. Sure, mapping the moon’s gravity is better than no mission at all, but, aside from those who worked on the mission, did anyone care? Can I take a poll? Did anyone who read my comment get excited by aspect at all of the GRAIL mission?
@Don — GRAIL, not much excitement. Clementine, Lunar Prospector, Chandrayaan-1, quite a bit. Less excited by lunar structure and history at the moment than where the water is. And speaking of water, it would be great to see a mission to Enceladus to investigate that ocean.
@Joy — It illustrates the difference between a national priority focused on a clear goal, and what we’ve had since Apollo, at least as far as manned space flight is concerned; low national priority, politicized, and programs with vague and changing mission definitions. Reversion to the mean I’m afraid.
I may be in the minority here, but GRAIL gave us insights (boom boom) into the Moons geology and history, which is beginning to emerge as much more active, for much longer than we previously thouight. The Moon might have had active volcanism and even a wisp of atmosphere into geologically recent times, and GRAIL helpoed us better underastand how the flow of heat from the core outwards evolved, as well as giving us clues to mysteries like mascons, the extra thick crust on one side, and whther or not the Moon still has any internal activity.
I liked it, though it wasn’t headline stuff – it’s the kind of genuine science mission that gives us the knowledge to do more dramatic things, and informs the rest of our solar system studies. In GRAIL’s case it helps us understand how small worlds develop – something that will have bearing on both the history of smaller worlds like Ceres, and larger ones like Mars.
Plus (and here I’m going to be really in the minority) I really do find investigations into the solar system that aren’t related to looking for space aliens interesting. There’s a lot of beauty, complexity, and knowledge that has nothing to do with life. There’s a lot more to space exploration than this pressing need to find aliens.
Your polling question highlights our personal preferences. For me, I was excited about GRAIL and I have my reasons (as an amateur astronomer and astrophotographer the moon has always been like an old friend to me so any mission, including LADEE, gets me going), not least among them, how the GRAIL results fit into the long term drive to return us to the moon.
I’m confident that we would all find some degree of disagreement berween any of our ‘lists of missions we want to see’. It’s a shame that governments and private companies don’t have bottomless purses… then we’d all be happy with whatever choices are made. For the budget cost of GRAIL I think it did good.
John said on September 17, 2015 at 3:06:
“Plus (and here I’m going to be really in the minority) I really do find investigations into the solar system that aren’t related to looking for space aliens interesting. There’s a lot of beauty, complexity, and knowledge that has nothing to do with life. There’s a lot more to space exploration than this pressing need to find aliens.”
I know what you are saying and agree here. Yes, looking for alien life is exciting and important, but studying the worlds in themselves is just as valuable and contains much of its own excitement and beauty.
I was very disappointed when the Viking missions to Mars were declared failures by many because they did not unambiguously detect signs of life on the Red Planet. This ignored and insulted the many incredible successes those two robotic missions did conduct on that alien world, including the very first close up images of the Martian surface.
It also left me shaking my head why they did not provide the means to analyze the surface composition properly, if for no other reason that to help scientists better understand and contrast the inorganic from the organic material for the biology experiments. I guess even in the 1970s we were still pretty confident that Mars had some kind of life, even if just microbes.
The apparent EXCESS heat generating mechanism must account for BOTH the global ocean AND the apparent hydrothermal activity as well. Since tidal flexing is an “outside-in mechanism, from JUST A STANDPOINT OF LOGIC, I cannot see how it would produce hydrothermal activity(OBVIOUSLY, there may be a SCIENTIFIC mechanism which I am not aware of which doed the trick). This is why it is SO ESSENTIAL that we determine whether Ceres has a subserface global ocean as well! My HOPE is, that the mechanism IS internal, but CANNOT BE EXPLAINED BY MERE PHYSICAL PROPERTIES! One would THEN need to invoke BIOLOGICAL solutions, like the SUSPECTED microorganisms “operating” the Oklo “natural” nuclear reactor!
Enceladus joins the elite club of Sol system worlds with confirmed global subsurface oceans of liquid water, which of course increases the moon’s chances for having life forms:
@Brett September 16, 2015 at 16:23
‘Maybe it has something to do with the tidal forces. Could they be fragmenting the ice at the south pole, allowing water to continually well up through it?’
I would think the pull at the equator is greater than that of the poles which would tend to open cracks on the surface and suppress cracks underneath at the equator. At the poles I would have thought cracks would open underneath and tend to close cracks on the surface, but heat from inside the moons could escape through these internal polar cracks which would weaken the ice above them allowing the movement of water to the surface to escape in plumes.
@Harry R Ray September 17, 2015 at 9:22
‘This is why it is SO ESSENTIAL that we determine whether Ceres has a subserface global ocean as well!’
Any heat in Ceres would have a difficult time escaping through the upper dust layer on the surface, it is around 100 to 1000 times more insulative than ice! I suspect there is a water layer on Ceres with a fair amount of gas. Ceres offers a great chance of having life as everything needed is there, rock, water, organics, ammonia etc. If life arose locally it would have most likely have spread amongst the asteroids early on, Ceres has probably exchanged material with every asteroid in the belt giving numerous chances to exchange biological material as well. A rover and submersible would be much easier to achieve on Ceres than one to Europa or Enceladus by a long way.
A side effect of the geisers venting into space is that Enceladus is loosing mass to the rings of Saturn. Over the billions of years of loosing mass, the moon must have gotten smaller, I wonder if that rate is significant.
Io looses matter by vulcanism too to the surrounding environment and pluto looses a big amount of it`s atmosphere to space of which some lands on the poles of Charon. All these bodies must have been bigger then they are now, i wonder by how much as they might have looked mightily different in the distant past.
“Any heat in Ceres would have a difficult time escaping through the upper dust layer on the surface, it is around 100 to 1000 times more insulative than ice! ”
Reports of “plumes” on Ceres (see http://www.nasa.gov/jpl/herschel/ceres-20140122) gives support to the possibility of a sub-surface ocean.
If Ceres is “active” I would not be surprised to find depressions/sinkholes where recent plumes have occurred.
Is it possible that Occator is a “subsidence depression/sinkhole” rather than a crater?
Near vertical “crater walls” of irregular shape look more like the result of subsidence than impact.
If significant subsurface water/ice escaped in a plume subsidence could easily occur. Not sure how the effect of lower gravity would impact on the size of subsidence depressions on Ceres but on earth these are usually limited to kilometre size or less (and on Earth are usually caused by erosion in limestone).
“Cracks” that appear on the floor of Occator also look more like what one would expect in a subsidence depression rather than a crater.
I expect we should get definitive answers in coming months on the existence of a subsurface ocean on Ceres.
Rafik: If you turn out to be right(and I HOPE you DO), the EXPECTED AMOUNT of radioactive materials left over after four plus billion years could NOT generate enough heat to maintain a global ocean over geological time! There are only two alternatives that could make up the difference. The OBVIOUS one(and the one which the scientific community will rush to judgement on) is that the INITIAL MASS of radioactive elements in Ceres’ core was much greater than expected. The reason for this will prove VERY DIFFICULT TO FORMULATE! the second alternative, and the one I favor for BOTH Enceladus(to get back ON TOPIC here) AND Ceres, is a more EFFICIENT HEAT GENERATING MECHANISM(i.e. nuclear reactor) produced by the EXPECTED amount of available radioactive materials RIGHT NOW!
@Rafik September 18, 2015 at 8:38
‘If Ceres is “active” I would not be surprised to find depressions/sinkholes where recent plumes have occurred. Is it possible that Occator is a “subsidence depression/sinkhole” rather than a crater?
I think it is to large for a gas expulsion but there may be many more smaller expulsions.
‘Near vertical “crater walls” of irregular shape look more like the result of subsidence than impact.’
The subsidence is a function of the angle of repose which is more dependant on the geometry of the particles than anything else and a bit on how deep the dust column was before impact.
“Cracks” that appear on the floor of Occator also look more like what one would expect in a subsidence depression rather than a crater.’
Craters could also appear with cracks in like on Mercury.
@Harry R Ray September 18, 2015 at 9:29
‘Rafik: If you turn out to be right(and I HOPE you DO), the EXPECTED AMOUNT of radioactive materials left over after four plus billion years could NOT generate enough heat to maintain a global ocean over geological time! ‘
It could if it was insulated by the constant rain of dust and heat addition through the occasional deep impact.
‘…is a more EFFICIENT HEAT GENERATING MECHANISM(i.e. nuclear reactor) produced by the EXPECTED amount of available radioactive materials RIGHT NOW!’
Unless fairly recently lifeforms have got smart enough to start collecting enough U238/U235 to form their own little heat reactor ‘hive’. The problem with a nuclear reactor process is that the more you bring it together the quicker the heat is lost through fission.
Talk of going to Enceladus to retrieve samples of that alien moon to see if it harbors life:
A collection of new items on Enceladus, including Cassini’s last flyby of the alien moon and a report that the geysers are not geysering as much as they were when first detected ten years ago: