No rings round Rhea

This afternoon I set myself a challenge – take a paper published on the astronomy arXiv today and blog about it by the end of the day. Luckily for me an intriguing non-detection in our own Solar System popped up.

Back in 2008 it was widely reported that the Cassini spacecraft had detected a ring system around Saturn’s fifth moon, Rhea (that’s it in the images above). These rings weren’t directly seen, but rather something seemed to be absorbing the electrons, which are normally trapped by Saturn’s magnetosphere, in the region surrounding the moon. Three narrow rings of icy material were the most likely explanation; this was the first time such phenomena had been observed around anything other than a planet and caused a lot of interest and excitement.

The authors of the paper published today decided to search for these moon-rings directly. They used Cassini’s narrow angle camera to take deep, high resolution, pictures of Rhea. And they found……..nothing.

One of the Cassini images from Tiscareno et al. (2010) of the region surrounding Rhea. The black lines in the identical right-hand image show where rings should be seen if they were present. Rhea itself is off the image, to the left.

So, does this mean that there aren’t rings around Rhea after all? Well, it certainly looks that way. The new data rule out both narrow rings or a broad dusty cloud of material as the likely cause of the electron absorption.

Ok, no rings. So, does this mean that the 2008 observations were wrong? No, something is definitely absorbing the electrons near the moon, but the interpretation for why this is happening needs rethinking. Sometimes not seeing what you think should be there is more exciting than actually detecting it!

ResearchBlogging.orgMatthew S. Tiscareno, Joseph A. Burns, Jeffrey N. Cuzzi, & Matthew M. Hedman (2010). Cassini imaging search rules out rings around Rhea Geophys. Res. Lett. 37, L14205 (2010) arXiv: 1008.1764v1

Images courtesy of NASA & Tiscareno et al. (2010)

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Pluming marvelous

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Good news – the number of places in the Solar System to go for a swim increased today with the discovery of the signature of liquid water on Enceladus, one of the moons of Saturn.

This discovery was made by flying the Cassini satellite through one of the giant plumes of ice particles that erupt from cracks in its icy surface as shown in the images below (the second one has a bonus bit of Saturn included too).

(Credit: NASA/JPL/Space Science Institute)

What Cassini actually detected were negatively charged water ions; these are seen on Earth where water is in motion e.g. in waterfalls, so finding them here is a good indicator that liquid water exists under the exterior ice. If that’s true, then there must also be an internal heat source to keep it that way, and maybe even life swimming around out there too.