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Probe Mystery Solved: Jupiter as Wet (and Dry) as Earth
Questions And Answers
 The unexpected dryness at its entry site was one of the Galileo Probe Mission's more perplexing mysteries (see "The Probe Story: Secrets and Surprises from Jupiter," The Galileo Messenger, Issue 38, April 1996). Was Jupiter globally dry? Was the water everyone expected to see somehow locked in its interior? Or was the weather on Jupiter as varied (and interesting) as on Earth? These were the questions Andy Ingersoll of Caltech and the Galileo Science Team raised at the Galileo Press Conference at the Jet Propulsion Laboratory on June 5 in von Kármán Auditorium. The panel of researchers (pictured above) was most pleased to report that the latter model of a planet with a richly complex and dynamic weather system was the correct one.
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Probing Hot Spots
Since the Probe entered at a hot spot, the Galileo Science Team selected other such areas to investigate--like the hot-spot area shown in the SSI image and in the NIMS view below. Analysis from the output of the SSI (visible light) camera, NIMS, PPR, and the Ultraviolet and Extreme Ultraviolet Spectrometers (UVS/EUV) proved very fruitful.
Different atmospheric constituents (both chemical composition and particle size) react differently to different wavelengths of electromagnetic radiation. The NIMS image scanned the same hot spot as SSI, but in many different infrared (IR) wavelengths. The series of such images yields what amounts to a three-dimensional picture of the atmosphere, because different constituents are concentrated at different levels.
Because water absorbs IR, NIMS can generate water maps. Those that NIMS PI Bob Carlson displayed showed both the rare, very dry areas (like the Probe entry zone), with about 1% relative humidity, and more common water-saturated, very wet and likely rainy areas.
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Understanding Jovian Weather
Fundamentally, Jovian weather resembles Earth's, but Jupiter has no solid surface. Its weather is driven by heat (as on Earth), but the source is different; instead of the Sun warming a surface, and the surface heating the air above it, heat in Jupiter is from deep down (as indicated by Probe data).
The atmospheric processes that Toby Owen of the Probe NMS Team described resemble Earth's: a rising column of heated Jovian air (mostly a hydrogen-helium mix) cools with expansion as it rises, then condenses out clouds (water droplets low, ice crystals higher) and rain (or snow). But on Jupiter, the rising air also condenses out hydrogen sulfide as a higher, colored (but wispier) cloud layer and ammonia as the highest cloud layer. Seen from Galileo, the region of updrafts is cloud-topped, and very bright. Above the clouds, the air at the top is very cold, very clear, and dry of all volatiles (which condensed out below).
What Goes Up . . .Comes Down
 The high, clear air rolls outward, sinks, compresses, and heats up. Nothing condenses on the way down--the warming air is dry and soaks up any volatiles, so the region of subsiding air is clear. These areas are dark in visible light, but bright in the infrared. The videos that Glenn Orton of the PPR and Probe NEP Teams brought from NASA's 3-m IR telescope in Hawaii showed Jupiter glowing through its hot spots like a colossal cosmic jack-o-lantern.
While the Jovian atmosphere has no real bottom, far below the under-side of the water cloud deck is a layer of mixing, where the torrid air is enriched with heavier elements.
The global atmosphere circulates on a gargantuan scale (almost 500,000 km about the equator): regions of rising air spread around the planet as a system of bright, latitudinal zones. Regions of sinking air spread around the planet as a system of dark belts, studded with even darker hot spots, clear all the way down to the dark, hot mixing level.
The boundary between the belts and zones is very turbulent; images of these areas through time show wind strength and direction as clouds are blown about. Ashwin Vasavada of Caltech showed such videos of thunderheads moving toward a dark hot spot. His narration of an animated flight between cloud decks around a hot spot was especially interesting.
It was into such a dark, dry hot spot that the Galileo Probe dropped in December 1995. While the area around it was as dry as any desert on Earth, the distant lightning it detected hinted that wetter, stormier weather lay beyond.
Building the Atmosphere
The balance of constituents in the mostly hydrogen-helium atmosphere (carbon, sulfur, etc.) closely match that in comets, so Toby Owen suggested that the Jovian atmosphere, originally so similar in composition to that of the Sun, has been enriched over the eons by cometary bombardment. (Through the impacts of Shoemaker-Levy 9 in 1994, we see this process happening even now.)
And More
The Jovian Auroras
 Also like Earth, Jupiter has auroras. The images that Andy Ingersoll displayed showed a ragged curtain of light circling the north pole where charged particles, streaming down powerful magnetic field lines, slammed into atmospheric molecules in a high, glowing ring some 500 km above the 1-bar level.
Life on Jupiter?
Is there life on Jupiter itself?
Toby Owen echoed the consensus opinion that, because there are only clouds and no surfaces, complex molecules would have no place to collect and begin to evolve, but would be cooked when pulled down to the hot mixing level.
What Does it All Mean?
Understanding Jovian weather is of great value in understanding Earthly weather. Just as medical doctors must understand the comparative anatomy of other animals to make sense of human anatomy, so must meteorologists understand the comparative meteorology of other worlds to make sense of terrestrial weather. Without studying the weather and climate on other planets, there is no way to know just what is peculiar to the Earth, or what may be universal. Jupiter, Ingersoll reflected, is more like the Earth than we thought. But Jupiter is also so much more! From its broiling, roiling bottomless depths; through multiple cloud decks; to frigid, aurora-lit heights; through endless cloud canyons, searingly dry voids, and centuries-long downpours, Jupiter is an ideal meteorological laboratory.
Our evolving comprehension of this mammoth, kaleidoscopic treasure trove owes so much to the recent, ongoing contributions of the Galileo Orbiter and Probe and also the Hubble Space Telescope, the IRTF on Mauna Kea, Hawaii, and many other instruments and researchers of the Jupiter Watch. But we also have so many more questions. Andy Ingersoll said it best, "We need more probes!"
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