Emily LakdawallaSep 13, 2005

DPS: Central transit of Earth as seen from Saturn

There were a few talks at the Division of Planetary Sciences meeting dealing with a rare and fortuitous event that happened on January 13, 2005. On that day, if you were standing on Saturn or its moons, you would have seen a central transit of Earth across the disk of the Sun. Apart from being cool, it provided a unique opportunity for Earth-based observers to study how the surfaces of Saturn's moons and rings reflect sunlight at precisely zero phase. "Phase" is a measure of the angle between the Sun, a target, and the observer. As phase approaches zero, the observer sees what's called an "opposition surge," where the target brightens much more than you expect it to.

I missed the first talk about the opposition observations, but the speaker, Anne Verbiscer, was kind enough to seek me out and give me a version of her talk and even gave me one of the key images she showed in it. Here it is:

Opposition surge of Saturn’s rings
Opposition surge of Saturn’s rings Three views of Saturn captured on different dates with the 2.2-meter telescope at the Calar Alto Observatory in Spain. Phase angles are measured in degrees. On January 13, 2005, Earth crossed the disk of the Sun as seen from Saturn, so Earth-based telescopes saw Saturn and its rings and moons with a "phase angle" (Sun-target-observer angle) of zero degrees. The globe of Saturn does not change much in the three views, but the rings flash into brilliance as the phase angle goes to zero. This effect is called "opposition surge" and was also seen on some of Saturn's moons, especially Enceladus.

We see three different views of Saturn on different dates. The globe of Saturn hardly looks different at all at these small differences in phase angle. But look at the rings! With the minutest difference in phase angle -- from 0.13 degrees down to 0.02 degrees -- the rings suddenly flash into brilliant light. Anne told me that she saw similar effects on Saturn's icy satellites, especially Enceladus. The size of the opposition surge she saw was "drastically higher, 40% higher, than previously published values."

I'm not an astronomical observer myself so Anne had to explain to me what causes the opposition surge. One explanation is "shadow hiding" -- at any non-zero phase angle, the particles in the rings cast shadows across other particles in the rings, so that darkens the surface. But at zero phase, you see no shadows at all, and the surface looks brighter. But Anne said that "it doesn't work" to explain all of the observed opposition effect. You need something more, like coherent backscatter: that is, constructive interference of the light being reflected from the rings and Enceladus. Anne was just showing me data from the Calar Alto observatory, but she told me that she'd also sent out an alert via Sky & Telescope, and a lot of amateur astronomers had participated in a worldwide observing campaign to gather photometry data during the transit. I'm looking forward to results that include the amateur observations. I think it's neat how much amateurs can contribute to astronomy, even in the space age.

These transit events don't happen very often at all. Anne told me that the next transit of Earth as seen from Saturn is in 2019, but it's not a central one, so you don't see the phase angle go to zero. The next central transit is 2049, which is after most current observers will have retired!

At DPS, I did see a talk given by Dick French on the same event as observed from Hubble. The most interesting thing I saw in his results was that the opposition effect changed with wavelength, but only in some of the rings. The A ring displayed changes in the strength of the opposition effect with color; the C ring did not.

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