Enceladus Rains Water on Saturn

by Nancy Atkinson on July 26, 2011

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At least four distinct plumes of water ice spew out from the south polar region of Saturn's moon Enceladus. Credit: NASA/JPL/Space Science Institute

It’s raining on Saturn! Well, kind of. Actually, not really. But there’s some really cool news about Saturn, Enceladus and water – great topics, all. The bubbly water shooting from the moon Enceladus is responsible for the “mystery” water that was found in Saturn’s upper atmosphere several years ago. Observations with the Herschel space observatory has shown that water ice from geysers on Enceladus forms a giant ring of water vapor around Saturn.

Astronomers from the ESA’s Infrared Observatory discovered the presence of trace amounts of water in Saturn’s atmosphere back in 1997, but couldn’t really find an explanation for why it was there and how it got there. Water vapor can’t be seen in visible light, but Herschel’s infrared vision was able to track down the source of the water vapor.

Enceladus expels around 250 kg of water vapor every second, through a collection of jets from the south polar region known as the Tiger Stripes because of their distinctive surface markings. Much of the ice ends up in orbit around Saturn, creating the hazy E ring in which Enceladus resides.

But a small amount reaches Saturn – about 3% to 5% of Enceladus’s ejected water ends up on the home planet of Saturn.

Phil Plait, The Bad Astronomer figured out that a decent rain shower on Earth is 7,000,000,000,000 times heavier than the rainfall on Saturn. So, not a lot of water makes it to Saturn.

But the fact that a moon is having an effect on its planet is unprecedented, as far as we know.

“There is no analogy to this behaviour on Earth,” said Paul Hartogh, Max-Planck-Institut für Sonnensystemforschung, in Germany, who led the collaboration on the analysis of these results. “No significant quantities of water enter our atmosphere from space. This is unique to Saturn.”

The running theory is that Enceladus has a liquid subsurface ocean of Perrier-like bubbly (and maybe salty) water. No one knows yet how much water lies beneath the moon’s surface, but it is thought that the pressure from the rock and ice layers above combined with heat from within force the water up through the Tiger Stripes. When this water reaches the surface it instantly freezes, sending plumes of ice particles hundreds of miles into space.

The total width of the torus is more than 10 times the radius of Saturn, yet it is only about one Saturn radius thick. Enceladus orbits the planet at a distance of about four Saturn radii, replenishing the torus with its jets of water.

The water in Saturn’s upper atmosphere is ultimately transported to lower levels, where it condenses. But scientists say the amounts are so tiny that the resulting clouds are not observable.

Again, despite its enormous size, this torus has it has escaped detection until now because of how water vapor is transparent to visible light but not at the infrared wavelengths Herschel was designed to see.

“Herschel has proved its worth again. These are observations that only Herschel can make,” says Göran Pilbratt, ESA Herschel Project Scientist. “ESA’s Infrared Space Observatory found the water vapour in Saturn’s atmosphere. Then NASA/ESA’s Cassini/Huygens mission found the jets of Enceladus. Now Herschel has shown how to fit all these observations together.”

Read the team’s paper here.

Source: ESA

About 

Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

Anonymous July 27, 2011 at 3:32 AM

wouldn’t the water dry out ? or eventually will?

Armagh Planetarium July 27, 2011 at 8:32 AM

Yes, eventually, but the quantities of water escaping can’t be that great so I imagine this will go on for a really long time (billions of years).

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Torbjörn Larsson July 27, 2011 at 1:19 PM

Enceladus is also a topic of the Origins 2011 conference, that Planetary Society features a report from. [You can find the link to the first part of the report from there.]

The sample problem of the plumes has evidently started to be considered by the space community:

“A talk by Luther Beegle from JPL addressed some estimations of the ability of current instruments to detect possible extant life within the liquid water reservoir of Enceladus. [...]

In his estimation, he also considered spatial heterogenity and the stochastic nature of samples. His main conclusion is that, without significant advances in detection and quantifiaction techniques, the only feasible approach to detecting possible life would be to carry out a sample return mission.”

I don’t think that is the optimal mission profile by any means, so I put that as a challenge over there and do it here too:

*** Sample return missions, while valuable,* would cost more and take more time. ***

Assuming that more samples would help with quantification (“stochastic nature of samples”), how do you make more than one fly-through on the cheap (low delta-v)? Aerocapture with Titan for a Saturn orbiting Enceladus-dipping mission?

The response so far seems to be sending a cluster mission to ensure sampling a plume. However, it doesn’t seem that plume delivery is a problem. Nor does it adress the challenge, since it suggests a variant of sample return.

Since putting the original challenge I learned that there is a difference in aeorobraking, which intends to take an elliptic orbit to a circularized, and aerocapture, that uses one pass (obviously) to take an hyperbolic trajectory to an elliptic orbit. [Wikipedia]

Wikipedia also gives me this reference on an oldish study of Titan aerocapture.

It is only an abstract, but looks promising:

“The orbiter uses aerocapture, a form of aeroassist, to replace an expensive orbit insertion maneuver with a single guided pass through the atmosphere. Key environmental assumptions addressed in this study include: the uncertainty in atmospheric density and high frequency atmospheric perturbations, approach navigation delivery errors, and vehicle aerodynamic uncertainty. [...]

The Monte Carlo analysis verifies that a high-heritage, low L/D, aeroshell provides sufficient performance at a 6.5 km/s entry velocity using the Hybrid Predictor-corrector Aerocapture Scheme guidance. The current mission design demonstrates 3-sigma success without
additional margin, assuming current ephemeris errors, and is therefore not dependent on the success of the Cassini/Huygens mission. However, additional margin above 3-sigma is expected along with the reduced ephemeris errors in the event of a successful Cassini mission.”

So maybe we could do a quite intriguing aerocapture, sample, return (heh), sample, … sequence.

Is that feasible?

a decent rain shower on Earth is 7,000,000,000,000 times heavier than the rainfall on Saturn

Ha, that is one trick of the english language, because I would like to say “more massive” considering differences in surface gravity. :-D

BTW, Plait did an error on Avogadro’s number, and changed his estimate to ~ 4*1012.

————-
* The last time UT described the plume I voiced the concern that, considering the limits of a low-mass (< 100 au) low resolution mass spectrometer, the organics in the plume looks like pristine cometary. I.e. without the patterns of consumption and waste production et cetera that life would make.

However, the new models for Enceladus' plumes puts up an alternative scenario for water origination that I think (perhaps thought already then) would predict this even in case of an extinct/extant biosphere.

The gas loaded heated water that is forced up from the very deep original water source can dissolve ice close to the surface. These scattered ice water reservoirs originates the water what is then ejected. If so, there is massive dilution with pristine ices going on.

Torbjörn Larsson July 27, 2011 at 1:42 PM

But wait! There is more!

I want to drum on my cultural drum, as I learn more and more how active swedes are on the scene of space physics. Göran Pilbratt is now naturalized in the Netherlands, but has long worked on Corot and other missions.*

And there are many more like him that I can find on the web, telling of involvement in space missions.

* A fun fact to make it worth for you: if you google translate this, Pilbratt tells of how Corot started out with observing a lot of space junk.

Among them a 30 cm piece from a chinese exploded craft that passed ~ 150 m [!] from Corot. Could have been that military experiment that was so criticized at one time as it spread tens of thousands of pieces into Earth orbit.

“May [your enemies] have an interesting life”, indeed.

Torbjörn Larsson July 27, 2011 at 1:42 PM

But wait! There is more!

I want to drum on my cultural drum, as I learn more and more how active swedes are on the scene of space physics. Göran Pilbratt is now naturalized in the Netherlands, but has long worked on Corot and other missions.*

And there are many more like him that I can find on the web, telling of involvement in space missions.

* A fun fact to make it worth for you: if you google translate this, Pilbratt tells of how Corot started out with observing a lot of space junk.

Among them a 30 cm piece from a chinese exploded craft that passed ~ 150 m [!] from Corot. Could have been that military experiment that was so criticized at one time as it spread tens of thousands of pieces into Earth orbit.

“May [your enemies] have an interesting life”, indeed.

Gadi Eidelheit July 27, 2011 at 2:37 PM

Nice to know, but that is only a small fraction of the water/ice at Saturn rings. Also the sentence “But the fact that a moon is having an effect on its planet is unprecedented, as far as we know. ” must have a correction. The moon has great effect on Earth, just to mention keeping its AXIS in the irght angle and the tide and law, probably you meant transferring materials from a moon to a planet.

Torbjörn Larsson July 27, 2011 at 10:24 PM

Among large effects moon-planet, the Io-Jupiter flux tube.

Btw, besides making a mass correspondence, the abstract notes that they measure non-Keplerian dynamics which implies the water ice is undergoing molecular collisions which ties in with that it is widely spread*. It is more dynamical “hot” than the near Keplerian rings, I take it.

———————-
* Appreciable amount of molecular collisions in space. That sounds like an interesting environment!

William Sparrow July 27, 2011 at 11:26 PM

Gadi,
I had the same thought with respect to the Moon’s effect on tides. The statement should have been “the fact that the Moon is having THIS effect on the planet is unprecedented, followed by the statement that a transfer of water from moon to a planet is unprecedented. In any event, an interesting article. Enceladus continues to intrigue me.

Gadi Eidelheit July 27, 2011 at 2:37 PM

Nice to know, but that is only a small fraction of the water/ice at Saturn rings. Also the sentence “But the fact that a moon is having an effect on its planet is unprecedented, as far as we know. ” must have a correction. The moon has great effect on Earth, just to mention keeping its AXIS in the irght angle and the tide and law, probably you meant transferring materials from a moon to a planet.

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