Scallops and Wisps

I don't know about you, but with Jupiter getting so low in the sky and most of the other planets hiding behind the sun, I've been doing most of my planetary observing on the web recently. At saturn.jpl.nasa.gov, to be specific: the home of the Cassini-Huygens probe, which entered Saturn's system last month, crossed the ring system and has been sending back some phenomenal pictures.

Pictures that I don't understand at all. What's up with the scalloped edges of the Encke gap, that narrow division at the outer edge of the A ring? Orbital dynamicists are feeling vindicated because one of the theories of the formation of that gap, involving a moon, Pan, orbiting within the gap itself. I haven't yet seen a good explanation of why it happens, though. There's also a narrow ring inside the already narrow Encke division. That's a bit more detail than we can see from the backyard!

The ring pictures also show density waves, concentric and spiral light and dark bands apparently caused by complex gravitational interactions between the ring particles and Saturn's extensive satellite system. The close-up of the F ring, that mysterious ringlet outside the main ring system which showed a "braided" appearance in Voyager photos, shows light and dark zones, ribbons and wisps running inward toward the small moon, Prometheus, which is one of the two thought to be responsible for the F ring's odd shape.

In addition, Cassini has shown that in addition to the ice and rock particles known to comprise the rings, there's also a fair amount of ... dirt. The dirt is apparently quite similar to the composition of Phoebe, one of Saturn's moons, which may lend credence to the idea that the rings represent the breakup of one or more Saturnian moons.

The probe's ultraviolet camera also detected a burst of oxygen at the edge of the ring system. Perhaps we saw a satellite collide with the rings! No one seems sure yet.

We've seen a few tantalizing pictures of Titan, Saturn's cloud-shrouded moon - but they don't show much yet. Stay tuned! We'll be seeing a lot more Titan information as Cassini's journey continues.

If you rise early (or stay up late), you can see Saturn for yourself, low in the predawn sky. Don't expect to see scalloped edges in the Encke gap, or even to see the gap at all, with the planet so low; but it's always a pretty sight. Venus, still in crescent phase, shines about ten degrees above Saturn.

Uranus, in Aquarius, reaches opposition on August 27th, when it will be magnitude 5.7, visible with the naked eye on a dark night. Neptune, a good binocular challenge at magnitude 7.8 in Capricornus, trails an hour behind Uranus, and reaches opposition on the 6th. Both are low in the southern sky, reaching only about forty degrees of elevation. Pluto, in Ophiuchus, transits in early evening at a somewhat higher elevation. Use a good finder chart and a telescope of moderate size (it helps to have 12" or more of aperture, but good observers have spotted Pluto in much smaller instruments) and you shouldn't have too much trouble. The most distant planet is slowing as it comes to the end of retrograde motion, so it won't appear to move very much against the background stars this month. This makes Pluto easier to find from one night to the next (or from one weekend to the next), but also makes its motion less obvious when you try to compare views from one night to the next to verify which one moved.

Early in August, Jupiter is low in the western sky at sunset, but it closes with the sun and disappears before month's end. Mercury and Mars are too close to the sun to be observable this month.