Many, many thanks to Anne Verbiscer for fascinating us at our meeting on October 13 with the story of the discovery of Saturn’s largest ring by her, Michael Skrutskie (UVA) and Douglas Hamilton (University of Maryland)!
In February of this year, Anne and her colleagues used Spitzer’s infrared camera, called the multiband imaging photometer, to examine a patch of sky far from Saturn and just inside the orbit of Phoebe, one of Saturn’s moons, at a distance of approximately 215 Saturn radii from the planet. The astronomers thought that Phoebe might be orbiting within a belt of dust that had been ejected over millions of years from its minor collisions with comets and micrometeoroids — a process similar to that around stars with dusty disks of planetary debris. When Anne and the team reviewed their images collected with the Spitzer telescope, just such a band of dust appeared in the images as they expected.
The reason the ring has not been spotted until now is that it neither scatters nor reflects visible light to any significant degree. The particles within the ring are composed of ice and dust and are extremely diffuse. The unique ability of the Spitzer Space Telescope to see deep into the infrared part of the spectrum unobscured by Earth’s atmosphere was required to detect the faint infrared glow from the ring.
The discovery may help solve a long-standing mystery associated with Iapetus, another of Saturn’s moons. Iapetus has a strange appearance — one side is bright and the other is quite dark. It is logical to conclude, given that we now know of the existence this newly discovered ring, that particles from the ring have bombarded Iapetus over many millennia, impacting the leading face of the tidally-locked moon with particles from the ring, leaving a darkened region. This region is referred to as “Cassini Regio,” named for Giovanni Cassini, who first observed Iapetus in 1671.
Anne described for us the process of submitting a proposal for the telescope time and the need to obtain the data before Spitzer consumed all of its cryogenic coolant which was used to keep the telescope’s detectors at a temperature only a few degrees above absolute zero. This amount of cooling is required to minimize thermal noise in the infrared detectors and thus increase the telescope’s ability to detect faint infrared light from distant sources. Fortunately for Anne and her team, the data was collected prior to the end of Spitzer’s coolant supply in May of 2009. Spitzer has since that time been operating in a warmer mode — still able to peer into the infrared and collect useful data, but with less sensitivity and at shorter wavelengths.
Thanks again to Anne for sharing the story of her discovery with us. Congratulations to Anne, Michael Skrutskie and Douglas Hamilton on their marvelous discovery of Saturn’s largest ring with the Spitzer Space Telescope, a mere 400 years after Galileo first aimed his modest telescope at the sky and observed the planets.
Regards,
Jim Browder
Richmond Astronomical Society