The latest work involving Cassini’s Cosmic Dust Analyzer (CDA) gives us fresh information about Saturn’s intriguing moon Enceladus and the likelihood of an internal ocean there. You’ll recall that plumes of water vapor and grains of ice have been found spewing from the ‘tiger stripe’ fractures at the moon’s southern pole, feeding material to Saturn’s E ring. Three times during the spacecraft’s passes through the plumes of Enceladus in 2008 and 2009, the CDA measured the composition of plume grains. Some of these icy particles struck the detector moving at speeds of up to 17 kilometers per second, vaporizing them on impact. The particle constituents could then be separated for close study.
What we find is that the ice grains most distant from Enceladus are poor in ice and match the composition of Saturn’s E ring. But move closer to the moon and large, salt-rich grains begin to appear. If the plumes came only from surface ice, we would expect little salt rather than the ‘ocean-like’ composition of the salt-rich particles. That’s an interesting result indeed, says Frank Postberg ( Universität Heidelberg), lead author of a study being published in Nature on June 23. “There currently is no plausible way to produce a steady outflow of salt-rich grains from solid ice across all the tiger stripes other than the salt water under Enceladus’ icy surface,” the scientist adds. And co-author Sascha Kempf (UC-Boulder) expands on the idea:
“The study indicates that ‘salt-poor’ particles are being ejected from the underground ocean through cracks in the moon at a much higher speed than the larger, salt-rich particles. The E Ring is made up predominately of such salt-poor grains, although we discovered that 99 percent of the mass of the particles ejected by the plumes was made up of salt-rich grains, which was an unexpected finding. Since the salt-rich particles were ejected at a lower speed than the salt-poor particles, they fell back onto the moon’s icy surface rather than making it to the E Ring.”
Image: Water vapor jets spewing from Saturn’s icy moon Enceladus. (Credit: NASA/JPL/Space Science Institute).
The layer of water these researchers are talking about would lie perhaps as much as 80 kilometers below the surface crust, presumably kept in a liquid state by the tidal forces created not only by Saturn but by several neighboring moons, along with the heat of radioactive decay. A crack in the outermost crust exposes some of this water to space, causing flash-freezing into salty ice grains along with accompanying water vapor. The scientists’ calculations show that the liquid ocean must have a sizable evaporating surface or the plumes would freeze over. About 200 kilograms of water vapor are lost every second from the plumes, along with smaller amounts of ice grains.
Enceladus is but a single moon, but what it seems to be telling us is that icy bodies orbiting gas giants may sustain large amounts of liquid water, a finding of obvious interest to astrobiologists. As for Enceladus itself, the paper’s abstract notes that “Whereas previous Cassini observations were compatible with a variety of plume formation mechanisms, these data eliminate or severely constrain non-liquid models and strongly imply that a salt-water reservoir with a large evaporating surface provides nearly all of the matter in the plume.”
The paper is Postberg et al., “A salt-water reservoir as the source of a compositionally stratified plume on Enceladus,” published online 22 Jun 2011 in Nature (abstract).
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What I find more interesting is that this allows us to look for life in Enceladus’ ocean easily, without needing to actually land, drill and and immerse a probe in it. If there is life in the ocean, there will be microscopic life. That life will be ejected in the plumes too. If we can trap those, even at the level of organic molecules, we can do basic detection for biased stereoisomers. Dyson may once have suggested looking for orbiting “fish”, but looking for the remains of [abundant] living things from an orbiter might be a lot easier.
this is good news. I hope that we can check fast if there is life .
This is very encouraging. Ultimately, we’ll need an outer system probe to drill through a thin section of ice and observe the ocean directly. But I don’t know how far off this is logistically.
To me it seems a little too early to get excited about life’s prospects here. We need more than a deep ocean, we need a long-lived ocean. We also need an energy source beyond gentle tidal warming.
The Laplace resonances of Saturn’s moons are unstable and this must destroy their ability to provide sufficient heating for the ocean to last the millions of years necessary for much of interest to occur. Alternatively we can rely on augmentation by radioactivity, but given the very low density of this moon I am not sure such a high inventory of radionuclides is possible.
Given that Enceladus is too small for tidal heating to provide a core that is hot enough for a temperature gradient to allow the production of high energy chemicals, we would rely on particles cycling through the E ring via the tiger stripe geysers to energise the ecosystem. As above, I can’t yet see how this is a long-term situation.
If we can reasonably model these requirements, then testing for life on Enceladus should become a high priority. There is much to be said for the looking-for-your-keys-were-the-light-is-best approach, but first we need excuse of a small chance that we actually dropped them there.
Further to the points raised, the geology, chemistry..science of that space laboratory even if it has no bugs, is a beautiful fantastic carrot dangling in front of Earth scientists. Is this not the perfect challenge for the leaders in this type of mission ? JAXA how can you resist?
Forgive my sillyness…Cassini team, we implore you, calculate a trajectory for a nearly-dead Cassini probe sometime in the future that slingshots (via Titan?) back to near-Earth space. How grand to bring her home laden with salts and dirt from Saturn-space. I’m told I dream too much. Circumspice!
How much water are we talking about in relation to the amount of water in Earth’s oceans, and Europa’s?
“About 200 kilograms of water vapor are lost every second from the plumes, along with smaller amounts of ice grains.”
That’s a metric ton in five seconds; twelve tons per minute, 720 tons per hour, about 6.5 million tons per year. Enceladus’ mass is a whisker over 10^20 kilograms or 10^17 tons. So, it would take about 15 billion years for the geysers to blow all of Enceladus into space.
That’s just a silly BOTE calculation, but it’s worth noting that the geysers are big enough to have altered Enceladus mass over geologic time. Go back a couple of billion years, and — assuming mass loss has been constant, which is certainly an open question — and Enceladus would be about 14% more massive than it is today. Go all the way back to the early years of the Solar System and it would be almost a third more massive.
“Forgive my sillyness…Cassini team, we implore you, calculate a trajectory for a nearly-dead Cassini probe sometime in the future that slingshots (via Titan?) back to near-Earth space. How grand to bring her home laden with salts and dirt from Saturn-space. I’m told I dream too much. Circumspice!”
The anti-nuke forces who were so vocal back in 1997 might rear their ugly heads again, especially after so many NASA folk swore up and down that Cassini was going out into deep space forever and the recent nuke crisis in Japan.
Personally I think it is a great idea to say nothing of a relatively cheap method of sample return. Plus we could see what kind of battering a spacecraft takes in Saturn space.
Ad astra per asperum!
I really like the idea of the nearly-dead Cassini sample return to Earth. Great think-outside-the-box! Is it really possible to do? It seems it would require a lot of delta-vee…. unless we started orbit and trajectory modifications now…
If there really is so much concern about the Plutonium, then line up a lesser mass object, maybe there is something appropriate in the E-ring and smack Cassini into it…a line drive into a near-Earth orbit…heh. Supposedly I’m meeting Bob Mitchell at the Shuttle launch on 8 July. Something to chat about at least.
Published online 3 October 2011 | Nature | doi:10.1038/news.2011.569
Saturn’s moon has never-ending winter
Millions of years of snowfall on Enceladus boost promise of subsurface ocean.
A 100-metre thick layer of snow has muted many of the surface features in this area of Enceladus.NASA/JPL / Paul Schenk
Jets of water vapour and ice shooting from the south pole of Saturn’s moon Enceladus have been active for up to 100 million years, boosting the odds that the moon harbours a liquid ocean beneath its icy surface, a study suggests. If the existence of such an ocean is confirmed, Enceladus will become one of the most promising places in the Solar System in which to search for signs of extraterrestrial life.
Paul Schenk, a planetary scientist at the Lunar and Planetary Institute in Houston, Texas, and his colleagues based their findings on high-resolution images of Enceladus recorded by the Cassini spacecraft, combined with a model of jet activity. Schenk described the study at a joint meeting of the European Planetary Science Congress and the American Astronomical Society’s Division for Planetary Sciences1 in Nantes, France, on 3 October.
Full article here: