Starship Asterisk*

makc wrote:who cares? maybe some little green people

http://www.planetary.org/blog/article/00002137/ wrote:
Another possible piece of evidence for a Rhea ring
Oct. 5, 2009 | 11:30 PDT | 18:30 UTC

<<One of the more exciting discoveries made by Cassini at Saturn is the possible presence of a ring around one of its moons, Rhea. Evidence for a ring around Rhea comes from the Magnetospheric Imaging Instrument (MIMI); Cassini saw symmetric drops in the flow of electrons around Rhea, and one possible explanation for such a pattern is the presence of big chunks of ice in Rhea's orbit that physically block the free flow of electrons through Saturn's plasma environment. Unfortunately, the particles would be too big to scatter light the way Saturn's dusty rings do, but too small to be individually visible to Cassini's cameras, so there didn't seem to be any way for Cassini to confirm or refute this intriguing idea.

Today at the Division of Planetary Sciences (DPS) meeting, which takes place all week in Puerto Rico, Paul Schenk presented another indirect piece of evidence for the possible presence of a Rhea ring. He was studying the color properties of Saturn's moons and, as he explains in his blog, he noticed an unusual "narrow set of small ultraviolet-bright spots on Rhea." That is, there was a set of spots that was bright as seen through the ultraviolet filter on Cassini's cameras, compared to their appearance through longer-wavelength filters. Paul continues: "Normally this is not a cause for excitement, as fresh crater rims have this signature, but these were lined up along a great circle trace very close to Rhea's equator. This alignment is not a random coincidence. No other satellite has comparable features."

• • Equatorial UV-bright spots on Rhea
    Credit: NASA / JPL / SSI / Paul Schenk
  This image covers the entire equator of Rhea, Saturn's second largest moon. It is a map of Rhea's infrared reflectance divided by its ultraviolet reflectance; brighter areas are "redder" and darker areas are "bluer." There is a line of dark spots running across 75% of Rhea's equator, tiny spots that are relatively bright in ultraviolet wavelengths, an indicator of fresh impacts. Their coincidence with the equator is unusual, and may indicate the impact of a short-lived ring onto the surface of Rhea.

Paul goes on to say that "This feature is only a few kilometers across, but its linear pattern across nearly 3/4ths of Rhea's circumference and alignment within 2 degrees of the equator indicate it is quite plausibly material from Rhea's proposed ring system that has struck the surface of Rhea. A higher resolution color observation (the one that started this entire project) suggests that this material would be composed of discrete but incoherent packets of ring material that hit the surface at scattered intervals along the equator. These and the other observations make an intriguing story but one that requires a lot more work to fully understand.">>

They would have been the first rings ever observed around a moon: three narrow bands of icy debris encircling Saturn's second-largest satellite, Rhea. Space physicists announced their existence in March 2008. But a more definitive search finds that they simply aren't there. And that raises the question of what exactly the first team saw.

In a rendition (left) of a Cassini image of space near Rhea (which is just off the image to the left), any rings should appear where the black lines are in the identical right-hand frame. None could be detected. Credit: NASA/JPL/Space Science Institute; (inset, right) Tiscareno et al., Geophysical Research Letters (2010)

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The original ring detection was based on plasma measurements the Cassini spacecraft made while orbiting Saturn in 2005. In a 2008 paper in Science, space physicist Geraint Jones of University College London and colleagues described how some unseen solid material around Rhea, which is less than half the size of our moon, seemed to be absorbing energetic electrons that are trapped in Saturn's magnetosphere. That absorption appeared to be creating electron shadows, much as a ring creates a light shadow when it passes in front of a star. At Rhea, the stunningly symmetrical electron shadows—three on either side of the moon—implied three narrow ringlets embedded in a broad disk of icy debris in the moon's equatorial plane. That, said Jones at the time, was "the only reasonable explanation we've been able to come up with."

But Rhea's supposed rings bothered ring specialists from the start. Such rings weren't likely to form because just the right sort of grazing impact on Rhea would be required, they said. And the rings would be quickly destroyed by the tug of Saturn's gravity and worn down by eroding small impacts.

Ring specialist Matthew Tiscareno of Cornell University was one of the skeptics. So in late 2008 and early 2009, he and three colleagues took a closer look around Rhea using the visible-light camera on board Cassini. The camera had views of Rhea backlit by the sun—which would light up dust inevitably associated with any rings—as well as other views with the sun behind the camera, when objects up to boulder size would be most easily detected. But nothing showed up. The Cassini search was good enough to detect several orders of magnitude less material than needed to account for the observed electron shadows, the researchers say. "We are now ruling out the possibility that [the electron shadows] are due to rings of solid material," Tiscareno says.

Jones's team concedes. "We can't argue with the conclusions of Tiscareno and colleagues," Jones says. But if not rings, then what? "What we saw is clearly real," he says. "It's probably due to interactions between Rhea and the surrounding magnetosphere." Nothing like the Rhea electron shadows has been seen elsewhere, so Jones doesn't have a clue yet what those interactions might be. Fortunately, Cassini has more Rhea flybys planned during its recently extended mission.

[url=http://photojournal.jpl.nasa.gov/catalog/PIA06578][b]Rhea in Natural Color [i](Feb 2005)[/b][/i][/url] [s][/s] [b][i](Credit: NASA/JPL/SSI/Cassini)[/i][/b]

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Something unknown is causing a strange, symmetrical structure in the charged-particle environment around Rhea, Saturn's second-largest moon. But contrary to 2008 reports, it's not a system of rings.

Using NASA's orbiting Cassini spacecraft, a team of astronomers led by Cornell research associate Matthew Tiscareno searched for narrow rings, broad rings and any material from dust to giant boulders that might be orbiting the 1,500 km- (950 mile)-wide moon.

They report their non-detection in the July 29 issue of Geophysical Research Letters.

The research contradicts earlier suggestions that Rhea has a system of narrow rings embedded in a broad circumsatellite disk or cloud. That 2008 announcement was based on a sharp, symmetrical drop in electrons detected around the moon by Cassini during a 2005 flyby.
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Putting the hypothesis to the test, they positioned Cassini to view the moon at what would be edge-on to the rings, where the greatest amount of material would be within its line of sight. The researchers took 65 images between 2008 and 2009, some at high phase angles (toward the unlit side of the moon) to detect micron-sized particles that diffract light forward; and others at low phase angles (toward the lit side of the moon) to see larger objects that absorb and reflect light back toward the sun.

Looking for larger objects was more difficult, because the light reflected back from such objects would be faint compared to the light reflected back from nearby Rhea. The researchers used a series of short exposures added together to bring out any potential faint objects -- but still found none.

Using the charged-particle data from the 2008 report, they also refined previous calculations of the size and abundance of ring particles that would be necessary to explain the odd observations.

• Geophysical Research Letters 37 L14205 (29 July 2010) DOI: 10.1029/2010GL043663