Another possible piece of evidence for a Rhea ring
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 (a subject I'll write more about later) 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."
NASA / JPL / SSI / Paul Schenk
Equatorial UV-bright spots on Rhea
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."
Intriguing indeed! More on Paul's color maps shortly...
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