Ices, Oceans, and Fire: Monday afternoon: New Horizons at Jupiter
Posted by Emily Lakdawalla
14-08-2007 11:34 CDT
Monday afternoon John Spencer gave a lengthy talk about the New Horizons encounter with Jupiter. I've posted extensively on this in the past: approaching Jupiter; Jupiter rotating; full frame on Jupiter; Jupiter cloud motion; Io erupting; Alice's view; retreating view of Io and Europa; early science results; Europa images; Tvashtar movie; etc, etc, etc. And in fact John Spencer posted here too, about approaching Jupiter and Io's eruptions. So I'll try to keep to things that were new to me.
John went over New Horizons' trajectory to Pluto, and mentioned that during the long cruise they will actually be doing a little bit of science on Uranus and Neptune. New Horizons will be way too far away to resolve either planet, but will be able to measure each planet's brightness at a variety of phase angles. From Earth, we pretty much only see Uranus and Neptune almost fully lit by the Sun (that is, from very low phase angles); New Horizons will get much higher phase views.
He mentioned that the goals for the Jupiter encounter were fourfold. First in importance was the gravity assist; second was for the science team to undergo a stress test ("at this," John remarked, "we were very successful.") Third was to calibrate the instruments; science came in last, but they still managed to accomplish a lot.
He showed some images from the Ralph MVIC multispectral imager that I hadn't seen before. Ralph could not be used much at Jupiter because it was too sensitive for the bright lighting conditions there; the detectors would be totally saturated, even with the shortest possible exposures. But there was an exception: one of the filters is a narrow-band one in a wavelength in which methane strongly absorbs light. Because Jupiter has a lot of methane in its atmosphere, and methane absorbs so much of the light, Ralph was able to capture a 3,000-pixel portrait of Jupiter in this methane band. Turns out the images was released on the mission website in May; I just hadn't noticed it before. Here it is in all its glory.The amazing images of Io's erupting plume Tvashtar have been extensively discussed here already. John pointed out something about them that I hadn't appreciated before. First, here's the picture under discussion: One thing scientists are interested in is: what's the size of the individual grains that are falling out of the Tvashtar plume? To get at grain size of dust and ice particles, the best thing to do is to view the dust and ice when they are back-lit (lit at high phase angles approaching 180 degrees). Now, New Horizons' instruments are way too sensitive for them to risk pointing at angles that are so close to the Sun, and anyway they didn't know the plume would be there when they planned the spacecraft observations. But look closely at that image. The top part of the plume is Sunlit at a reasonably high phase angle of roughly 120 degrees or so. But the bottom part of the plume, in Io's shadow, is backlit by reflected Jupitershine at a phase angle of 176 degrees -- nearly perfect for those grain size studies. John didn't have any conclusions from those studies to report yet, but was excited about the luck of the lighting geometry.
He discussed the cool Io eclipse images too:He compared the eclipse view to a Sunlit view with the same geometry (sorry, I don't have that one to show). He said, that every bright spot in the eclipse image corresponds to a dark spot in the visible-light image. All the dark spots glow at night -- but only around the sub-Jupiter point. On the other hand, there is very little correlation between the eclipse image and the hot spots seen in thermal infrared wavelengths, so "whatever is going on at the sub-Jupiter point is non-thermal," a remark that brought an interested "huh" sound from the audience. He also pointed out that the sub-Jupiter glows are produced in areas smaller than 30 kilometers across, so whatever process is making that glow has to be happening pretty close to the surface. He hypothesized that the gas being emitted by volcanoes (something volcanoes do even when they aren't erupting) is glowing near the surface; somehow, Jupiter's magnetospheric effects are penetrating that far down into Io's charged atmosphere.
Looking at the famous Tvashtar plume movie:John pointed out that while they could see the stuff in the plume falling down to the surface, none of the images showed anything in the plume moving upward. "This is a good indication that particles are condensing out of the plume" -- that is, the plume starts out gaseous and condenses solid particles (presumably, as the temperature falls). Also, if you try to track individual plume features, he notes that the trajectories do not appear to be ballistic. Instead plume material is sliding down the sides of the plume structure. "Lots of hydrodynamic effects going on here."
He noted that all of their temperature measurements indicated that the lavas were likely of ordinary basaltic composition; they see no evidence for higher temperatures requiring exotic lava compositions.
Commenting on the satellite search, he said "We did not find any satellites that we believe yet, down to 1 kilometer in size. But we found something more interesting. We found two ring clumps confined to a narrow bright arc. At first Mark Showalter thought these were collisional remnants from a recent collision, but we do not see them brightening at high phase angle" as they would expect from a collision, which would have produced dust. "So they may be confined arcs like at Neptune or in Cassini's G ring, related to orbital resonances with Metis."He closed his talk by showing some of the pretty pictures from the encounter, whose targeting was suggested by members of the public via unmannedspaceflight.com. This was his last image: With this image on the screen, he remarked, "now which of these satellites is the one you would fly a mission to?" This brought a laugh from the audience. Io is not one of the four targets being considered for the next flagship mission, but it would be an important target for either a Jupiter system orbiter or a Europa orbiter, because, as John pointed out, it's clear from New Horizons' data set that spectacular Io science is possible from quite a great distance away.
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