|
|
The Planetary Society Blog
By Emily Lakdawalla
Spokes in Saturn's rings caused by thunderstorms on Saturn?
Dec. 27, 2006 | 11:05 PST | 19:05 UTC
Several weeks ago I received an email from Geraint Jones at the Max-Planck-Institut für Sonnensystemforschung with a copy of a paper he'd just had published in the journal Geophysical Research Letters, "Formation of Saturn's ring spokes by lightning-induced electron beams." It's an interesting paper: the main idea is that the spokes on Saturn's rings could be caused by thunderstorms on Saturn itself! One of the key observations about spokes that has been difficult to explain is why they disappear and reappear seasonally; Jones and his coauthors suggest a tricky mechanism for this, involving the latitudinal motion of Saturn's major storm activity with the changing seasons.
To review, this movie shows spokes in Saturn's B ring, as seen by Voyager 1: Spokes in the B ring movie
Credit: NASA / JPL |
Spokes are linear features that appear to cut across the rings, a fact that was utterly perplexing when the spokes were first spotted in Voyager images in 1980. It is now widely accepted that spokes appear as a result of electrostatic levitation of very tiny ring particles (less than 0.6 microns in size) above the plane of the rings, but there is still much debate on what gives the electrostatic charge to the particles to cause them to levitate. The hypothesis I've heard most often is that it's caused by meteoroid impacts on the rings, but the paper states that spokes grow and develop over a period of an hour or more and are most common in the morning sector of the rings (corresponding to local times of 3 to 7 am on Saturn), neither of which facts can be explained by meteorid impacts.
One key observation that Jones focuses on in the paper is the fact that the shape of spokes cannot be explained solely by a single, split-second event like a meteoroid impact. To follow this argument, begin with a Voyager image of the spokes.Spokes in Saturn's B ringThis view of spokes in Saturn's B ring was captured by Voyager 2 on August 22, 1981.Credit: NASA / JPL |
The spokes generally look to be radial to Saturn -- that's why they're called "spokes," after all, they look like the spokes on a bicycle wheel. But that's because our view of them is foreshortened. Jones reprojected this image to what it would have looked like from a viewpoint over Saturn's north pole. From such a viewpoint, the rings would appear circular.Spokes in Saturn's B ring (reprojected)This Voyager view of spokes in Saturn's B ring has been reprojected as though it were being viewed from above Saturn's north pole. This view makes it clear that the spokes are mostly not oriented radial to Saturn. The rings rotate counterclockwise in this view. Keplerian shear causes spokes' inner tips to move faster around the rings than their outer tips, making an originally radial spoke tilt into the shape of a forward slash (/). But at least one spoke in this view is tilted in the opposite direction. The "corotation distance" is the position within the rings at which ring particles revolve around Saturn at the same angular rate at which Saturn itself rotates. Credit: NASA / JPL / Geraint Jones |
Looked at this way, it's obvious that the spokes are definitely not radial to Saturn. Instead, they're tilted. Most of the spokes are tilted in the direction of a forward slash (/). This is the direction you'd expect from Keplerian shear. Objects in orbit move faster the closer they are to the body they're orbiting. As a result, the inner part of Saturn's rings orbits Saturn much faster than the outer part of the rings, so any radial structure would get sheared out with the inner part moving ahead and the outer part lagging behind. Here's a diagram of the spoke formation process:Ring spokes' changing shapesSpokes in Saturn's rings form by sustained generation along a central axis, combined with Keplerian shear that extends the spoke ahead and behind (t1, t2, and t3) over an average period of about 1.8 hours, pivoting about an "apex" near the corotation distance (where a ring particle's orbital speed is the same as the rotation rate as Saturn). Following this "active" period, the whole spoke's bow-tie morphology is gradually distorted by Keplerian shear (t4). Credit: Grün et al. (1983), courtesy of Geraint Jones |
This diagram works well if all spokes start out as lengthening elliptical blobs. But the Voyager image above contains at least one spoke that's actually oriented backwards with respect to the direction of tilt you'd expect to develop from Keplerian shear.
How does that work? Jones and his coauthors argue that if the spokes are caused by thunderstorms in Saturn's atmosphere, then you can make a spoke of pretty much any initial shape -- including one that appears to be oriented "backwards" -- all you need is a thunderstorm with that initial shape. Incoming cosmic rays trigger upward-moving "electron avalanches" above the thunderstorms. The billion billions of electrons in the avalanche are channeled along Saturn's magnetic field lines from the location of the storm, up and out from Saturn and then collide with the rings, ionizing the tiny ring particles and creating a spoke. Here's a pretty picture:
How thunderstorms create Saturn's ring spokes(a) A cosmic ray particle triggers an electron avalanche above a thunderstorm. (b) The electrons propagate along Saturn's magnetic field lines, and on striking the rings induce rapid negative charging of ring grains and high-energy bremsstrahlung emission. The ring atmosphere is also ionized, dissociated, and excited by electron impact. Auroral emission emanates from excited gases, including in the X-ray range. (c) Large ring particles lose their small grain regolyths, which are repulsed by the ring to form a spoke. Credit: Geraint Jones et al., Geophysical Research Letters, 2006 |
Since a thunderstorm can be active for several hours, the spoke-inducing electron avalanches can also continue for hours. The coolest element of this model is this: because the electrons propagate along magnetic field lines, which have a fixed, linear geometry, a thunderstorm at one latitude on Saturn will always produce a spoke in the rings at one fixed radial location. Thunderstorms at lower latitudes produce spokes closer to Saturn; at higher latitudes, the storms "map" to a location on the rings at greater distance from Saturn. Now, spokes are only readily visible if they occur in the more opaque part of the rings, especially the B ring. The latitudes of Saturn where storms would "map" to the B ring are from 43° to 52°N and 38° to 46°S, so spokes in the B ring would form only when there is thunderstorm activity at those latitudes.
I think it's a cool model, and it's fairly straightforward to test: use Cassini to try to observe spokes and the correct conjugate location on Saturn's atmosphere at the same time, and see if they occur in the same time at the right places. I asked Jones what observations Cassini would need to perform, beyond the obvious choice of using the cameras to look for spokes and storms. He said that "The RPWS instrument could provide information about the positions of thunderstorms, particularly their longitudes. Work by the RPWS team has shown that events such as the 'Dragon Storm' of early 2005 are sources of Saturn Electrostatic Discharges, or SEDs. Also the coming and going of SEDs in the RPWS data are closely correlated to the positions of candidate storms, which during the Cassini mission have mostly clustered at latitudes around 35 degrees south," which is just equatorward of the latitude where thunderstorms could generate spokes in the B ring (they would generate spokes in the C ring instead, where they'd be too faint to see).
Also, I was curious how much searching Cassini would have to perform to be lucky enough to catch a spoke / thunderstorm pair. Jones said that this was wandering beyond his field of expertise, but he gave it a try: "...it appears that trying to positively detect thunderstorms on Saturn is more challenging than on Jupiter. Because of reflected sunlight from the rings illuminating the planet's dark side, night-time storms can't as easily be observed on Saturn as on Jupiter. At Jupiter, lightning has been observed on the dark side and correlated with candidate cloud formations on the dayside. So, I guess there would have to be quite a large set of positive examples of candidate cloud formations and spokes to confirm a link."
Jones cautioned me at the end, saying that while he thinks this model is a "very promising line of inquiry, we're not sure that thunderstorms on Saturn can indeed release electrons into space; matters such as the depth in the atmosphere that they occur come into play."
All very interesting, and it's a story I wouldn't have followed up on unless Geraint Jones had made the effort to send his paper and help to explain parts of it to me, for which I'm grateful!
|
|
[ what is this? ]