Plus one Ring: Minus one Belt

Saturn still shines in the early evening sky for July viewing. It’s getting low, though – not quite 40 degrees up at sunset – so get your viewing in while you can.

Here’s one thing you won’t see. Last year, NASA’s Spitzer Space Telescope discovered an immense new ring around Saturn, far beyond any of the existing rings. It stretches from about 6 million kilometers from the planet to nearly 20 million km, about the distance of Saturn’s distant moon Phoebe. For comparison, Saturn’s more familar ring system, out to the F ring, reaches only to about 120,000 km.

Phoebe, the moon surrounded by the new ring, is a weird moon because it doesn’t orbit in Saturn’s ring plane. Stranger, it orbits backward – “retrograde”, or in the opposite direction from the other moons and the rings. The new large ring, too, is tilted at 27 degrees to Saturn’s other rings, and its particles, too, have retrograde orbits like Phoebe.

The material in the large ring is so sparse that although it would span about a degree in our sky if we could see it, it’s far too faint to see visually with any telescope. Even Cassini, looking up close and personal, couldn’t see it; it took Spitzer’s infra-red camera to detect it.

There’s one more interesting thing about this immense ring: what happens to the particles in it. Some of them smack into Phoebe, giving that moon the heavily cratered appearance Cassini photographed early in its mission. But some of the smallest particles, driven by the effect of sunlight as it’s absorbed and re-radiated, end up falling in toward Saturn – a trajectory that leads them straight into the leading edge of the next moon in, Iapetus.

Remember Iapetus, the strange moon that’s dark on its leading edge but light on the other side? Ring particles from Saturn’s largest ring may finally explain how Iapetus got that yin/yang look.

Venus still hangs in the July early evening sky, though it’s getting lower, and it’s joined by Mercury after the first week of the month. On the night of July 12-13 look for Mercury on the edge of the Beehive cluster (M44). Mars is there too, looking small and distant just off Leo’s hind foot between Saturn and Venus.

Jupiter rises around midnight, so if you stay up until the wee hours, you can get a good look at it. But if you look, you might notice something strange. Those equatorial bands that are the most obvious features you can see on Jupiter, the features that jumped out at you even in your first look through a yard-sale Tasco ... well, one of them is gone!

That’s right, Jupiter’s southern equatorial belt (SEB) has disappeared. The Great Red Spot (GRS) is still there, and in fact it’s looking much redder than we’re used to seeing ... but that may be partly because the dark reddish band in which it usually sits is no longer there. What a strange sight!

But it’s a great opportunity, too. Normally the GRS is a bit subtle and hard to see, since it’s about the same color as the band surrounding it. But now it should really stand out.

Mind you, we’re talking about a belt of clouds that normally measures two or three earth diameters by ... well, so much bigger than the earth that it’s hard even to compare them. That’s a lot of atmospheric to suddenly go missing.

This isn’t the first time Jovian belts have disappeared. Usually it’s a somewhat gradual event and observers can watch it happen; but this time, it happened as Jupiter was hidden on the other side of the sun, so it was a surprise when it popped out of the sun’s glare missing a belt. In the past (once in the early 90s, and once in the 70s), a new belt has grown gradually starting from a prominent white spot. So keep your eyes and your telescope on Jupiter over the next few months and you might have the chance to see the birth of a new SEB.

If you do stay up for Jupiter, take a look at Uranus and Neptune too, both in the morning sky this month. Pluto, too, is almost perfectly placed for observing – for some value of perfect that includes being in the heart of the Milky Way, right at the edge of a bunch of stars of similar magnitude in the cluster M24. But give it a try anyway – a good Plutocrat loves a challenge!