DPS: New stuff from New Horizons at Jupiter
Posted by Emily Lakdawalla
10-10-2007 14:40 CDT
I'm already on my way home from the Division of Planetary Sciences meeting in Orlando, which will continue there for the rest of this week. Overall, it was not quite as exciting as some of the previous DPS meetings I've been to. Most of the presentations were refinements or updates of things I've seen before, rather than new results; they were interesting, but not Earth-shattering.
A case in point was the New Horizons results from Jupiter; there was a special session yesterday (Tuesday) and the first scientific publications from the encounter are coming out in Friday's issue of the journal Science. But most of the stuff they presented -- spectacular though it is -- I had already seen at the icy satellites conference in Boulder in August (you can read about that here). There were a couple of new items, though
Amy Simon-Miller presented on "Jovian Winds and Waves from New Horizons." She showed various interesting features in the moving bands and belts of Jupiter's clouds, but some of the coolest were what she called "mesoscale gravity waves" near Jupiter's equator. They are like ocean waves in that the waves propagate through the medium of Jupiter's atmosphere with a speed that is different from the speed of the moving atmosphere itself. Their wavelength is small by Jupiter standards, only 300 kilometers from peak to peak, striping Jupiter's equator with a faint fishbone pattern. And they move fast -- by tracking wave features, Amy found that they move 100 meters per second faster than the atmosphere itself is moving. She found the waves in images taken by New Horizons' color imagers, MVIC and LEISA, in observations that were designed not for science but for instrument calibration; the science was a bonus.Another Jupiter atmospheric discovery from New Horizons was lightning strikes near both the north and south poles, in a presentation made by Hal Weaver. Lightning has already been observed on Jupiter's night side, but mostly at lower latitudes; there was a striking north-south asymmetry to the lightning strikes. Three days after the flyby, New Horizons pointed back at a crescent-phase Jupiter to look for lightning, taking 16 images, and spotted several little convective clouds lit from within by multiple lightning bolts. They saw a total of 17 strikes, of which 12 were at latitudes above 60 degrees, with approximately the same rate of strikes in both hemispheres. The important thing about polar lightning is that because the Sun doesn't light these areas very strongly, it seems that the convection that creates the clouds that host the lightning can't be driven by solar energy, but must instead be driven from below. The basic puzzle about Jupiter's atmosphere is how the inputs of solar and internal heating drive the complex flow of the atmosphere, so this is an important new detail for atmospheric scientists to plug in to their models. These papers on New Horizons' Jupiter encounter were given in the context of a much longer series of observations from Earth-based spacecraft intended to support the New Horizons flyby, which fortuitously caught Jupiter undergoing a rapid and dramatic "global upheval," Over the past year, bands have been changing color, storms have been forming and disappearing, dark regions spreading, so that Jupiter hardly appears the same from one night to the next. I'm afraid that I didn't know enough about Jupiter's atmosphere going in to this meeting to be able to follow the talks on these changes very well, but I did spend some time at yesterday's poster session speaking to the man who's been developing the best records on these changes: Christopher Go, who is not a scientist, but an amateur astronomer with an 11-inch telescope and a dedication to going out and observing and photographing Jupiter whenever possible. His work has been so valuable to Jupiter astronomers that he was invited to collaborate with one (Imke de Pater) on her observations of Jupiter with the Hubble Space Telescope, and he's now collaborating with Kevin Baines on further atmospheric observations. There were a few new items in Mark Showalter's presentation on Jupiter's rings as seen by New Horizons. His observations had been designed to look for new moonlets in the ring system, and he did not find any, which rules out there being any moons larger than about 1 kilometer but smaller than Metis (44 kilometers) and Adrastea (16 kilometers). Now, at Saturn, Cassini has busily been discovering tinier and tinier ring-moons, moons that are five, two, down to even one kilometer in diameter; so the fact that they're missing at Jupiter demands explanation. Showalter presented a simple model, that there were once many smaller moons but that there was an active erosional process going on that whittled all the moons down at an even rate, a process that has effectively obliterated all the smallest ones, leaving behind only a couple of cores of the largest ring moons. However, he didn't offer an explanation for why the same process has apparently not happened at Saturn.
The most amusing part of Showalter's presentation was when he compared images of the rings from Voyager and Galileo to images of the rings from New Horizons. If you look at every Voyager and Galileo image, he said, you see "quadrant asymmetries," meaning that one side of the rings appeared brighter than the other, which was deeply puzzling to ring scientists. But in every single New Horizons image of the rings, the rings are symmetric in brightness. "We can breathe a sigh of relief that a problem that we completely didn't understand is now gone, so the problem is solved," he joked. Of course, the fact that the weird phenomenon has apparently disappeared over a very short time frame really just adds to the puzzle.
John Spencer presented the Io volcanism results, which I have covered extensively before; I'll only add this nifty map of places where there have been surface changes on Io since the volcanic moon was observed by Voyager and Galileo, and a diagram that he presented on how differences in the solar illumination angle can make lava flows on Io disappear and reappear.I'll finish the New Horizons stuff by mentioning a noteworthy comment that Alan Stern made at their press briefing: "a dirty little secret on the Jupiter encounter is that we returned more data from Jupiter [40 Gigabits] than we will from Pluto." This is because Jupiter is close enough to Earth that they were able to turn back and relay some of the data in the middle of the encounter, freeing up space on the recorders for more encounter data. Pluto is so far away from us that the data transmission rate is going to be incredibly low, so mid-encounter data playbacks wouldn't free up any significant amount of space and would only take away observing time; so they will fill their recorder only once at Pluto. By the time they turn back to Earth, relay the data, and empty the recorders, Pluto will be far, far away in the distance. (I do wonder, though, if it will still be visible as a mote of light, permitting another Voyager-like solar system family portrait that actually includes Pluto this time.)
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