Earlier this month a paper was published showing evidence of rings around Saturn’s moon Rhea. This would be the first case of rings or other natural material orbiting a planet’s moon, though asteroids and Kuiper belt comets have been observed to have natural satellites. The Cassini project issued a press release announcing the results. The press release is titled “Saturn’s Moon Rhea Also May Have Rings” and includes phrases like “this is the first time rings may have been found around a moon”. The careful wording stems from the nature of the observation and the lack of a visual confirmation of the rings (so far, at least). Among Cassini’s dozen instruments are charged particle detectors that measure the energy and abundance of electrons in Saturn’s magnetosphere. Moons plowing through the magnetosphere usually leave a wake in the magnetosphere – a region downstream of the moon that is relatively depleted in charged particles. On one of Cassini’s close flybys of Rhea, however, the Magnetospheric Imaging Instrument (known as MIMI) detected localized regions near Rhea with fewer electrons, indicating that some material near that region absorbed those electrons. The most intriguing aspect of the MIMI measurements is that the instrument detected dips on each side of the moon at locations consistent with the electron absorption being produced by a circular ring around Rhea. Rhea itself, while large, is a relatively unremarkable moon (putting aside the issue of its possible rings).
The next most intriguing aspect of the discovery is that no images show any material orbiting Rhea. That doesn’t mean there aren’t rings. Cassini’s cameras (like all cameras) see the surfaces of things. The more surface area something has, the easier it is to see it with a camera. MIMI, however, indirectly measures the mass of an object. The more massive it is, the better it is at absorbing electrons. So the objects that absorbed MIMI’s electrons must be relatively large. To have enough dust particles to produce the observed absorptions, those particles would have been detected by Cassini’s cameras. The puzzle is compounded because larger particles would naturally produce dust as a byproduct of meteoroid bombardment on the larger particles. We are left with a mystery wrapped in a conundrum. Future observations of Rhea are planned. Images with greater sensitivity may reveal the rings. Cassini’s dust detector will sample the dust population near Rhea during Cassini’s extended mission. Stay tuned.