Electrically Charged Particles Found in Enceladus' Plumes
Written by Nancy Atkinson
Plumes of Enceladus. Credi: NASA/JPL
A team of planetary scientists working on the Cassini-Huygens mission has discovered tiny, icy particles in the plume from Saturn’s moon Enceladus that offer a tantalizing glimpse of the interior of this enigmatic world. The spectrometer on Cassini, the Cassini Plasma Spectrometer (CAPS) discovered a surprise: the ice particles are electrically charged.
Cassini has been exploring Saturn and its moons since 2004. Enceladus is 500 kilometers (300 miles) wide and Cassini’s suite of instruments has found the moon to be active, with jets near its southern pole that spew gas and water thousands of kilometers out into space. During two particularly close flybys of the moon in 2008, skimming only 52 and 25 km from the surface at around 15 km per second (54,000 km per hour), the CAPS instrument on the spacecraft was pointed to scoop up gas as it zoomed through the plume.
Observations from the Cassini Plasma Spectrometer (CAPS) made during the Cassini flyby of Enceladus on 12th March 2008, superimposed on Cassini’s path. As the spacecraft passed the moon, CAPS detected streams of charged particles in individual jets within the plume; negative particles are shown in this view. Each ribbon in the image gives an indication of the measured particle energy per charge: high energy particle fluxes are shown nearest Enceladus, and lower energy particles are farthest. The red points marked on Enceladus show the locations of known jet sources found by other Cassini instruments. Credit: MSSL-UCL.
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. These grains, probably only measuring a few nanometres across (billionths of a meter – 50, 000 times thinner than a human hair), fall into a size range between gas atoms and much larger ice grains, both of which were sampled directly during previous Enceladus flybys. The particles have both positive and negative electrical charges, and the mix of the charges varied as the Cassini spacecraft crossed the plume.
Dr. Geraint Jones and Dr. Chris Arridge, both from University College London’s Mullard Space Science Laboratory, present the results for the CAPS team at the European Week of Astronomy and Space Science conference at the University of Hertfordshire.
Jones and Arridge suggest that the grains may be charged through so-called triboelectric processes, through bumping together in the vent below Enceladus's surface before they emerge into the plume. This provides important hints to the conditions in the vents, and in turn may help with understanding conditions in the interior.
Jones and Arridge are intrigued by what their discovery reveals about Enceladus: “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”.
Arridge said that perhaps, as these charged grains travel away from Enceladus, their paths are bent by electric and magnetic fields in Saturn's giant magnetosphere. In this way Saturn's magnetosphere acts as an enormous mass spectrometer for the plume particles, allowing scientists to constrain their masses. Arridge has begun modelling the paths of these newly-discovered particles.
Ionised gas (plasma) in Saturn’s magnetosphere flows past Enceladus at over 80000 km per hour. Arridge’s results show that for this enormous mass spectrometer to work and for these dust particles to reach Cassini, this river of plasma must be significantly slowed down, in and near the plume, to speeds of less than 3200 km per hour. This slowing of the plasma is a result of the plume injecting particles into the plasma stream – making the whole flow slow down in a similar effect to when cars join a busy motorway. These new results provide further evidence that the material in the Enceladus plume has a huge influence on the moon’s surroundings.
Future Cassini flybys will help further understand the processes that occur at Enceladus and in its vicinity. William Herschel could not have suspected that the tiny point of light that he found in 1789 would turn out to be such an exotic place.
Source: RAS
Filed under: Saturn
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April 21st, 2009 at 7:24 pm
"In this way Saturn’s magnetosphere acts as an enormous mass spectrometer for the plume particles, allowing scientists to constrain their masses."
How kind of Saturn – we couldn't bring the lab instruments out to it so it provided one for us. Handy!
April 21st, 2009 at 7:57 pm
Ah, finally an intelligent post sans 17th century occult gravitation.
April 21st, 2009 at 8:28 pm
"As the spacecraft passed the moon, CAPS detected streams of charged particles in individual jets within the plume."
Streams of charged particles in individual jets within the plume. And the particles in Saturns plasma stream slow down when interacting with Enceladus plume. Individual jets come and go, or move, but the region as a whole is persistent. Note the huge difference between the predicted and the observed temperature of Enceladus.
April 21st, 2009 at 8:36 pm
Oops. Hit submit by mistake.
So to continue, here's that link to Enceladus temp map:
http://www.nasa.gov/mission_pages/cassini/multimedia/pia06432.html
Kind of odd, to NASA anyways, that the heat is localized at the South pole. The particle streams, charge seperation, anomolous hot spot and very fine particles among other things, indicate that those are electric discharge vortices, rather than subsurface eruptions.
April 21st, 2009 at 8:43 pm
You know what this thread needs? A post containing a 2 page-, 8 paragraph-long rant about how that person is not ranting. It does seem like a feature in every thread on Universe Today lately.
April 22nd, 2009 at 12:46 am
solrey says
"…indicate that those are electric discharge vortices, rather than subsurface eruptions"
I was thinking along similar lines. Enceladus is very close to Saturn, and its rings, and may interact with Saturns huge magnetic field rather like some vast van de graff generator with fine ionised particles of ice being pulled off the surface through lines of magnetic flux.
April 22nd, 2009 at 6:56 am
Ice particles in a nearly vacuum emerging into a magnetic field. If you did this experiment on earth, triboelectric effects would charge the particles, and they would segregate by charge in the magnetic field.
Now if we only knew what really caused triboelectric effects.
April 22nd, 2009 at 7:27 am
@ Dave Fenton,
You are right. The only reason the unnamed ranter keeps posting is because people keep responding. I don't fault him, I do blame the responders.
April 22nd, 2009 at 7:48 am
I think the causative effect is a current flux through the moon, and the ice particles (by the description these are nanoparticles) are electrically sputtered off the surface. Enceladus is part of an electric circuit with Saturn. A charged particle flux, as has been described, is the flow of electricity, and all circuits must complete. I suspect that in the flow of charged nanoparticles will be found lone electrons and ions comprising a current. I also think the oppositely charged particles will be found to be flowing in opposite directions, as they would in a circuit. There is no reference to a measurement of the flow direction, it is assumed to be outward, and I have not seen that the direction of flow is measured.
April 22nd, 2009 at 9:34 am
@billdavis
While a backflow of positive ions toward the surface is likely happening, I think what's been detected is the Langmuir sheath/double layer, of the plasma glow discharge. The + ions in the positive sheath are probably drifting toward the surface.
Enceladus and Saturn's electric connection seems very similar to what's happening between Io and Jupiter, only with lower current density.
April 22nd, 2009 at 11:08 am
Isn't this a "plasma fountain", AKA "plasma plume", which I said was like the Ice Volcanoes on Enceladus in the BAUT thread here:
http://www.bautforum.com/universe-today-story-comments/35489-new-view-space-weather-cold-fronts.html
April 22nd, 2009 at 5:39 pm
It's been well demonstrated that liquid water will "freeze" at room temperature in a strong electric field. Enceladus has a surface made primarily of water ice. A strong electric current could flow through the frozen crust and not melt it. Water is an effective conductor of electricity. The current flowing through Enceladus, entering via the northern third (roughly), travels along the surface, producing the Lichtenberg-like scarring of grooves and fissures, and exits the south pole along the "tiger stripes", spaced and aligned via Biot-Savaart law in conjunction with Enceladus' weak magnetic field. The "plumes" are actually cathode glow discharge arcs at high points along the "fissures", which act as wire, or line, "coronal discharge" filaments. Why does Enceladus have a magnetosphere? It's a "load" in a circuit.
April 22nd, 2009 at 9:09 pm
Of course the particles are charged. Enceladus' plumes have been giving off radio emissions to the extent that discharges can be co-related to changes in the rotational speed of Saturn's rings (ring-speed is determined by "listening" to the radio frequencies- http://www.spaceref.com/news/viewpr.html?pid=22197).
The aurora found at the poles of many planets are also "electrical", as can be seen by their filamented structure. I guess it's no wonder we find charged particle-plumes at Enceladus' pole. I think it's high time people stopped imagining frozen volcanoes (did anyone really take that seriously?) and started looking at more sensible explanations.