Starship Asterisk*

The discovery of water vapour and ice particles erupting from Saturn's moon Enceladus fuelled speculation that an internal ocean was the source. Alternatively, the source might be ice warmed, melted or crushed by tectonic motions. Sodium chloride (that is, salt) is expected to be present in a long-lived ocean in contact with a rocky core. Here we report a ground-based spectroscopic search for atomic sodium near Enceladus that places an upper limit on the mixing ratio in the vapour plumes orders of magnitude below the expected ocean salinity. The low sodium content of escaping vapour, together with the small fraction of salt-bearing particles, argues against a situation in which a near-surface geyser is fuelled by a salty ocean through cracks in the crust. The lack of observable sodium in the vapour is consistent with a wide variety of alternative eruption sources, including a deep ocean, a freshwater reservoir, or ice. The existing data may be insufficient to distinguish between these hypotheses.

Saturn's moon Enceladus emits plumes of water vapour and ice particles from fractures near its south pole, suggesting the possibility of a subsurface ocean. These plume particles are the dominant source of Saturn's E ring. A previous in situ analysis of these particles concluded that the minor organic or siliceous components, identified in many ice grains, could be evidence for interaction between Enceladus' rocky core and liquid water. It was not clear, however, whether the liquid is still present today or whether it has frozen. Here we report the identification of a population of E-ring grains that are rich in sodium salts (0.5–2% by mass), which can arise only if the plumes originate from liquid water. The abundance of various salt components in these particles, as well as the inferred basic pH, exhibit a compelling similarity to the predicted composition of a subsurface Enceladus ocean in contact with its rock core. The plume vapour is expected to be free of atomic sodium. Thus, the absence of sodium from optical spectra is in good agreement with our results. In the E ring the upper limit for spectroscopy is insufficiently sensitive to detect the concentrations we found.

BMAONE23 wrote:I believe the first report was put together by Data from ground based observations by instruments that weren't designed to detect sodium at a level of around 2% or less at that distance and that the second report was based on Data gathered by the Cassini probe on a Fly Through with instruments better calibrated for detecting such minute ammounts of sodium.

Though I would just take that with a grain of salt.

http://www.planetary.org/blog/article/00002006/ wrote:
<<Enceladus was considered odd even before Cassini arrived. Some 30 years ago, Voyagers 1 and 2 found that its surface is virtually 100% reflective and nearly pure water ice. This moon averages only 313 miles (505 km) across — it's only 4% the diameter of Earth and has just 0.00002 as much mass. So how could an ice ball no larger than the British Isles be so active? What's going on inside it?

Three years ago, Cassini investigators led by ISS honcho Carolyn Porco (Space Science Institute) published a long article in Science to provide some answers.

But first, here's some useful background: An orbital resonance with nearby Dione forces Enceladus into a very slightly elliptical orbit. This, in turn, means Enceladus can't keep exactly the same hemisphere pointing toward Saturn; from the planet's perspective, the moon nods slightly side to side. The upshot is that Enceladus's icy crust flexes rhythmically during each orbit, causing frictional heat to build up inside. All that bumping and grinding is generating about 1015 ergs per second — equivalent to more than 100,000 hair dryers set on "high"! On paper, that's enough heat to create a subsurface ocean of global extent, the "lubricant" that decouples the flexing crust from the moon's deeper interior.

So what's this ocean like, and what's it got to do with all that south-polar spewing? Porco and her team proposed the jets must be powered by a not-very-deep reservoir of liquid water that boils spontaneously and violently once it finds a way to reach the the vacuum of space. These escape hatches are provided by four 80-mile-long cracks (dubbed "tiger stripes" but officially designated sulci) near the south pole.

Not everyone agrees with this "Cold Faithful" model. Others (most notably University of Illinois geologist Susan Kieffer) speculate that an ocean might not be needed at all. Instead, frozen compounds called clathrates, which lock up volatile molecules like methane, carbon dioxide, or nitrogen in water-ice crystals, could be decomposing to create the jets of gas and ice.

A few weeks ago I attended a meeting of geophysicists in Toronto, which included a session titled the "Great Enceladus Debates". It was an apt choice; to be perfectly honest, there is much disagreement about the moon's inner workings. At the meeting, James Roberts (Applied Physics Laboratory) explained there almost has to be a layer of liquid water down there somewhere — it's the only physically plausible way to channel so much heat out of the interior. But right now the orbit of Enceladus isn't nearly elliptical enough (by at least a factor of three) to sustain all that tidal friction indefinitely. However the subsurface ocean came to exist, Roberts says, it's likely to freeze quickly on a geological timescale. Fortunately, Cassini has been getting quite close to Enceladus recently. The craft brushed past the moon four times last year, and two more close flybys are planned for November 2nd and 21st.

Two new results, discussed in Toronto and published last week in Nature, put important constraints on the putative ocean. According to Frank Postberg and others, about 6% of the tiny E-ring particles swept up by Cassini's Cosmic Dust Analyzer are quite salty. They contain up to 1.5% sodium chloride (NaCl), about half the salt content of terrestrial seawater. So, the thinking goes, any liquid water inside Enceladus must have been in contact with deep-seated rocks for a long time — millions of years — to allow sodium and other elements to leach into the water.

But something else, published in the same issue of Nature, is at odds with the CDA results. Nicholas Schneider led a group of observers who used telescopes on Earth to search for sodium in the escaping plumes spectroscopically. They turned up nothing, not even a trace, even though atomic sodium has very strong emissions in yellow light. Schneider says that the lack of sodium in the plumes' vapor rules out the violent geyser model proposed by Porco's team, because if briny water were boiling explosively, both the gas and particles should contain salt.

Plume Vent Models

Several mechanisms could be driving the powerful jets of water vapor and ice particles coming from Saturn's moon Enceladus.

Several researchers now think it's more plausible that the gas is slowly evaporating in deep caverns below the surface — just like water escaping as vapor from the top of Earth's oceans. The gas collects in pressurized pockets until it can escape to space through fissures. "There's no debate over the ocean, only the source of the plumes," Schneider said in Toronto. And other explanations are still possible; clathrate-rich ice isn't out of the question, nor is melting caused by slabs of deep-seated ice rubbing against each other.

Meanwhile, all this talk of warm saltwater has really gotten the attention of astrobiologists. Enceladus appears to have a sustained source of heat, a suitable chemical mix, and liquid water. Throw in a subterranean environment protected from space radiation, and you've got what might be just the kind of place that's conducive to life.>>

Loco wrote:What causes the Tiger stripes? Perhaps large, whale-like creatures in the oceans beneath the ice have a mating ritual somewhat like Hummingbirds, but instead of spiraling downard at high speeds in a spiral, the bird pulling out of its dive inches from the ground, the whales rocket upwards, striking the ice with their foreheads, causing the cracks, and displaying to their potential mates that they have the capacity to withstand the multiple shock of matrimony. These whale-like creatures must mate for life, as there would be no other reason for their ice-cracking antics, as casual affairs have little reason for shock, unless the participant was totally overtoxified the night before the awakening, but I'm sure the whales do not consume alcohol even if they should, using the substance to alleviate the urges which cause the displays. Perhaps the whale-like creatures are themselves decorated, like the surface of their home planet, with Tiger stripes.

• In the earliest times all the animals lived together in peace. There was no killing and no fear. Tha, the First of the Elephants, made the First of the Tigers the master and judge of the Jungle, but when there was a dispute between two bucks, the First of the Tigers leapt on one of them and broke his neck. The smell of the blood made the people foolish and they ran about wildly. The First of the Tigers fled, and Tha made the trees and trailing creepers mark the killer with stripes, which the tiger wears to this day. In the place of the Tiger he made the Gray Ape master of the Jungle, but the Ape mocked the people, and there was no Law. Tha decreed that since the first master had brought Death, and the second Shame, the jungle people should henceforward know Fear, the fear of Man. The First of the Tigers went to find Man and break his neck, but when he found him, standing hairless on two legs in his cave, he was afraid, and ran away. Thenceforward there was fear in the jungle; each 'tribe', the buffaloes, the deer. the pig, feared the others, and all feared Man.

neufer wrote: <<It is a time of drought in the jungle, the rains have failed...

Cassini researchers find additional support in the moon's plumes.

Editor’s Note: A paper in the July 23 Nature reports on evidence for ammonia in the plume of water vapor and ice emanating from Saturn’s moon Enceladus, along with the probable detection of argon-40, raising the chances that Enceladus has (or had in the recent past) liquid water in its interior. Science News reported that finding in this story, originally posted in May