The CAPS instrument is designed to detect charged gas (plasma), but its measurements in the plume revealed tiny ice grains whose signatures could only be present if they were electrically charged.
The particles have both positive and negative electrical charges, and the mix of the charges varied as the Cassini spacecraft crossed the plume.
"What are particularly fascinating are the bursts of dust that CAPS detects when Cassini passes through the individual jets in the plume" says Jones. “Each jet is split according to charge though”, adds Arridge, "Negative grains are on one side, and positive ones on the other."
scientists say the organics ‘taste and smell’ like some of those found in a comet.
"A completely unexpected surprise is that the chemistry of Enceladus, what's coming out from inside, resembles that of a comet," said Hunter Waite, principal investigator for the Cassini Ion and Neutral Mass Spectrometer at the Southwest Research Institute in San Antonio.
"Enceladus is by no means a comet. Comets have tails and orbit the sun, and Enceladus's activity is powered by internal heat while comet activity is powered by sunlight. Enceladus's brew is like carbonated water with an essence of natural gas," said Waite.
Saturn's curious moon Enceladus appears to have the same chemical makeup as a comet, according to the latest results from the Cassini probe. That's a big surprise, as Enceladus should have formed in very different conditions from those of comets.
As well as water vapour, the INMS detected carbon dioxide, methane and a range of more complex organic chemicals such as propane.
"The organics are clearly there in abundance beyond what we expected," says INMS lead scientist Hunter Waite of the Southwest Research Institute in San Antonio, Texas, US. "And the composition is very like the composition of a comet."
That is rather puzzling because comets are thought to have formed far from the Sun, out in the region of Uranus and Neptune, says INMS co-investigator Roger Yelle of the University of Arizona in Tucson, US. Enceladus, on the other hand, is thought to have grown within the "Saturnian subnebula" - the cloud of gas that coalesced into Saturn and its major moons.
"The temperature and pressure should have been very different, so you should get different gases," Yelle told New Scientist.
The first line of evidence comes from compounds that vaporise readily, called volatiles. LCROSS found spectral signs of volatiles containing carbon and hydrogen – likely methane and ethanol – as well as others such as ammonia and carbon dioxide. "It appears that we impacted into a very volatile-rich area," LCROSS principal scientist Tony Colaprete told the conference.
A number of instruments included in the ROSETTA payload are aimed at characterising the cometary dust properties. An important issue will be the nature of these dusts that will include organic (CHON), mixed CHON-silicate, and silicate dust, and ice grains, that are probably arranged in more or less heterogeneous fluffy agglomerates. The dust can be completely amorphous, partially structured and crystalline and these grains may coexist at small scales. Laboratory analysis of the morphological, crystallographic and chemical properties of cometary grain analogues produced in controlled laboratory conditions will play a critical role in the success of the ROSETTA mission. The results obtained by these laboratory analysis can be used to test and calibrate the instruments that will be sent to comet 46P/Wirtanen by this mission. The interpretations of returned scientific data will strongly benefit by the comparison with data recorded by the instruments in ground tests for different classes of analogues previously well characterised with various analytical techniques in the laboratory. In this paper we describe our facilities to produce and characterise elemental carbon, hydrogenated carbon, silicate and mixed carbon-silicate grains by laser bombardment and arc discharge techniques. We use field emission scanning electron microscopy and transmission and analytical electron microscopy to identify the textures, morphologies and grain compositions and structures in analogue samples. Spectroscopy (from far ultraviolet to far infrared) is used to study their optical responses. Some analogue properties, e.g. grain texture and morphology, are similar to those detected in chondritic interplanetary dust particles that may include solid debris from periodic comets.
"Cassini detection adds to Enceladus liquid water story"
There seems little doubt that Saturn's moon Enceladus hides a large body of liquid water beneath its icy skin.
The Cassini probe, which periodically sweeps past the little moon, has returned yet more data to back up the idea of a sub-surface sea.
This time, it is the detection of negatively charged water molecules in the atmosphere of Enceladus.
On Earth, such ions are often seen where liquid water is in motion, such as waterfalls or crashing ocean waves.
There are no "rollers" on the moon but it does have a very active region near its south pole where water vapour and ice particles shoot through cracks in the surface and rise high into the Enceladian sky.
"We see water molecules that have additional electrons added," explained Andrew Coates from University College London's Mullard Space Science Laboratory.
"There are two ways they could be added - from the ambient plasma environment, or it could be to do with friction as these water clusters come out of the jets, like rubbing a balloon and sticking it on the ceiling," he told BBC News.
Cassini has already detected sodium in the plumes - a signature of the dissolved salts you would expect to find in any mass of liquid water that had been in contact with rock deep within the world for a long period.
The latest observations were made using the Cassini plasma spectrometer (Caps). The instrument was originally flown to acquire data on Saturn's magnetic environment, by measuring the density, flow velocity and temperature of ions and electrons that enter the instrument.
It was never envisaged that Caps would also end up sampling jets at Enceladus and adding to what has become a very compelling story.
"While it's no surprise that there is water there, these short-lived ions are extra evidence for sub-surface water and where there's water, carbon and energy, some of the major ingredients for life are present," said Dr Coates.
"The surprise for us was to look at the mass of these ions. There were several peaks in the spectrum, and when we analysed them we saw the effect of water molecules clustering together one after the other."
Dr Coates and colleagues report the Caps data in the journal Icarus. The measurments were made as the probe plunged through the plumes of Enceladus in a close fly-by in 2008.
Caps found not just the negatively charged water ions but hints of negatively charged hydrocarbons, too. Positively charged hydrocarbons at Enceladus have already been identified by Cassini's Ion and Neutral Mass Spectrometer (INMS).
Where Caps has definitely seen negatively charged hydrocarbons is at Saturn's largest moon, Titan. There, it found colossal ions, some measuring more than 13,000 amu (an amu is roughly the mass of a single hydrogen atom).
"If you have a methane and nitrogen atmosphere and you bombard it with particles from the Saturn's magnetosphere and ultraviolet light from the Sun, you can cook up really large molecules," explained Dr Coates.
"They get bigger as the altitude decreases. They are the source of Titan's haze and also maybe the source of the dunes on the surface as they rain down."
Cassini is a joint venture between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (ASI).
http://news.bbc.co.uk/2/hi/science/nature/8495663.stm
Even in the absence of tidal heating, a conductive Enceladus is marginally capable of differentiating in the presence of long-lived radionuclides, and is certain to differentiate if significant amounts of the short-lived nuclide 26Al are present at the time of its formation (Schubert et al. 2007).
By rights, Enceladus, this moon of Saturn should be frozen over. We are a billion miles from the Sun...
Wal Thornhill wrote:According to the usual geological arguments, Enceladus’ plumes require some form of internal heating. Of course, NASA is quick to exploit any suggestion of subsurface liquid water on another body in the solar system as a reason for further missions to look for signs of life. But a source for that heat is not apparent. Susan Kieffer, a geology professor and planetary scientist at the University of Illinois at Urbana-Champaign, observed, "This tiny satellite should be cold and inactive, like our own moon. But it isn't." Modeling the maximum heat available from tidal distortion and radiogenic heating from a rocky core fails by an order of magnitude to explain the energy of the plumes. It also fails to explain the concentration of heat at Enceladus’ south pole. So, what if Kieffer’s intuition was correct and Enceladus is “cold and inactive?” Could the measured heat and the energy to drive the plumes come from space?
[...]
Both Saturn and Enceladus have surprisingly ‘hot’ south poles. Planetary scientists can see no connection. The vast disparity in size and lack of appreciable atmosphere on Enceladus renders comparisons seemingly pointless. However, more surprising was the discovery that Saturn has a hot spot at its north pole, which has been in darkness for 13 years. But a greater mystery within that enigma is a hexagonal feature inside Saturn’s auroral discharge. I explained in January, “The polar hot spot and long-lived hexagonal feature result from a continuous electric current flowing from the Sun into the pole of Saturn. The hot spot will remain for as long as the Sun shines electrically.” Now, what do we find encircling the ‘hot’ south pole of Enceladus but another polygonal feature.
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A team from the Mullard Space Science Laboratory working on the Cassini-Huygens mission have found negatively charged water ions in the ice plume of Enceladus.
Their analysis of data gathered during the spacecraft's plume fly-throughs in 2008 provide evidence for the presence of liquid water.
The spacecraft's plasma spectrometer, used to gather this data, also found other species of negatively charged ions including hydrocarbons.
One of the unusual characteristics of the E-ring is that it is predominately composed of micron sized grains with a distinct blue color. When particles are ejected into space they are subject to the gravitational forces that control the orbits of the moons and planets; however, small particles are greatly affected by other forces such as light pressure from the Sun, and electromagnetic fields around Saturn. Micron sized particles seem to be privileged. The gravitational and size-dependent non-gravitational forces balance for micron size particles. Only micron-sized particles can survive for long periods of time and spread out over the large expanse of the E-ring.
The geysers are not just liquid water like the steam condensate from Old Faithful. The detected particles are not frozen ice crystals clattering against a microphone diaphragm. They are ionized water molecules and other ionized species detected by the plasma spectrometer. Plasma. Micron-sized charged particles moving in electric currents from Enceladus's "geysers" are the source of Saturn's watery blue E-ring.
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