Measurements returned by NASA's Galileo probe into Jupiter have provided dramatic new evidence about circulation processes within the planet's atmosphere and prompted scientists to propose radical new theories about Jupiter's original formation.

The new concepts arise from the probe's successful parachute-borne descent into Jupiter on Dec. 7, 1995. The probe made the first quantitative measurements of the Jovian atmosphere below its outer clouds, reaching a region below where heat from the Sun can penetrate. This means the probe sampled the upper part of what is believed to be JupiterUs well-mixed, relatively uniform Rinterior atmosphere.S

Several members of the probe scientific team announced new mid-term findings today at a meeting of the American Geophysical Union in Baltimore. "The returns from the probe's scientific instruments have sparked a lively worldwide scientific debate about theories of planetary formation and about internal mechanisms in the huge Jovian atmosphere," according to Dr. Richard Young, Galileo probe scientist at NASA's Ames Research Center, Mountain View, CA.

Prior to the probe mission, the leading theory of Jovian weather assumed that, like on Earth, most action occurs in the thin, cloudy, solar-heated exterior region -- the so-called "skin of the apple." Winds within Earth's 100-mile-deep atmosphere are primarily the result of differential sunlight at the poles versus the equator, and heat released due to water condensation.

According to mission scientists, Galileo probe data strongly suggest that circulation patterns in Jupiter's cloud tops and its interior (which runs 10,000 miles deep) are part of one continuous process. Dr. David Atkinson of the University of Idaho continues to report persistent Jovian wind velocities of over 400 mph. The probe detected no reduction in wind speed, even at its deepest levels of measurement, approximately 100 miles below Jupiter's clouds.

Galileo scientists regard this finding as confirmation that the main driving force of Jupiter's winds is internal heat radiating upward from the planet's deep interior. The strength of the Jovian winds and the fact that they do not subside with depth is very significant, according to Dr. Andrew Ingersoll of the California Institute of Technology, Pasadena, CA.

"This may be evidence that Jupiter has high-speed wind currents extending thousands of miles deep into its hot, dense atmosphere," Ingersoll said. Such interior currents are believed by probe scientists to be the source of the dramatic banded appearance of Jupiter's cloud tops.

The most difficult probe finding for scientists to explain continues to be the extreme lack of water detected in the Jovian atmosphere. Pre-probe mission scientific estimates based on planetary formation theories, data from the earlier NASA Voyager spacecraft flybys of Jupiter and observations from the impacts of the fragments of Comet Shoemaker-Levy 9 with Jupiter forecast Jovian water levels at or well above those found in the Sun. However, probe scientists report that Jupiter is extremely dry -- with water levels (based on oxygen content) at one-fifth to one-tenth of the solar amount.

This finding is now well established, having been confirmed by analysis of data from five of the probe's science instruments. For example, the virtual absence of Jovian water clouds and the low relative frequency of lightning are all consistent with dry atmospheric conditions. Where is the water that should remain from Jupiter's formation in the same primitive nebula of gas and dust that spawned the Sun and the other planets? Several theories have been proposed.

According to one theory, Jupiter's true total water levels are probably at or above solar, with the bulk of Jovian water trapped in the planet's deep interior. According to this view, Jupiter began as a solid, rocky/icy proto-planet that grew to 8-10 times the mass of the Earth by gathering up ice grains and dust in the original primordial cloud. This process may well have concentrated water ice in the solid body, trapping it in the core while drying out surrounding regions.

As the solid body of the proto-Jupiter became larger, it attracted the already-dried-out surrounding lighter gases, mixing them with its existing atmosphere. This atmosphere would contain carbon and other gases that were originally locked in the core but had escaped as methane, ammonia, hydrogen sulfide and other volatiles as the core heated up. This process would produce a gas mixture similar to that found by the Galileo probe. It also would explain the enhanced carbon, sulfur and nitrogen levels found on Jupiter, which are significantly enriched relative to their abundance on the Sun.

In fact, this water-locked-in-the-Jovian-interior theory explains many of the measurements made by the probe. However, "there are problems with this new view, as there are with all the other current theories," said Dr. Tobias Owen of the University of Hawaii. "The primary one being, how does the ice stay in the hot planetary core while carbon-containing gases escape?"

An alternative theory suggests that the probe entered the Jovian atmosphere in an area comparable to the Earth's desert regions. This theory is supported by Earth-based telescopes and other spacecraft that observed extreme dryness at the probe's entry point on Jupiter's north equatorial belt. This theory holds that, like on Earth, Jupiter's atmosphere is heated by the Sun at the equator, causing air to rise until clouds form and water is lost. The dry air then may flow north and south, descending in "desert" regions. If a large enough downdraft exists, it might be sufficient to explain the dryness that the Galileo probe encountered.

However, several scientists find fault with this "huge downdraft" theory, doubting that such a massive downdraft and continued dryness could exist at the depth and pressure levels to which the probe descended. While such a downdraft might explain the observed dryness, its persistence down to 20 times Earth's atmospheric pressure is very hard to explain, according to Ingersoll.

"This explanation is particularly difficult when considering that Jupiter emits more heat from its interior than it receives from the Sun," he said. "This up-flowing interior heat should block a huge, deep downflow of dry air. It should evenly mix Jupiter's atmospheric water vapor at this pressure level, preventing the existence of a very dry region such as that found by the probe."

One possibility, Owen responds, is that "perhaps Jupiter's interior heat comes out only in certain regions where ascending currents bring up hot material from the planet's interior, like the heat escaping from the Earth's interior" in volcanoes and mid-ocean floor spreading zones.

A variation on the dry-region theory has been advanced by Young and others. "Jovian water distribution may vary radically over large latitude regions, with much of Jupiter's water being concentrated at high latitudes where most of the planet's lightning has been detected," he said. "More of Jupiter's interior heat is also emitted at high latitudes. Unfortunately, at the moment, we can't put all of this into a mechanism to explain how major parts of Jovian water could be concentrated uniquely at these high latitudes."

The Galileo probe successfully accomplished the most difficult planetary atmospheric entry ever attempted. It relayed a total of 61 minutes of unique science data to the Galileo orbiter passing 100,000 miles overhead for subsequent transmission to Earth. The probe descended about 400 miles into the Jovian atmosphere, taking measurements down to a level corresponding to 20 times Earth's atmospheric pressure. The Galileo orbiter has since embarked on a two-year tour of Jupiter and its moons.

Additional information on the Galileo probe, including a discussion of the craft's science instruments and a non- technical summary of the first scientific papers on the probe mission that were published in the May 10 issue of Science magazine, can be found on the Internet at the following URL:

http://ccf.arc.nasa.gov/galileo_probe/

The Galileo probe is managed by NASA's Ames Research Center, Mountain View, CA. Hughes Space and Communications Co., El Segundo, CA, designed and built the probe. Lockheed Martin Hypersonic Systems (formerly General Electric), Philadelphia, PA, built the probe's heat shield. NASA's Jet Propulsion Laboratory, Pasadena, CA, built the Galileo orbiter spacecraft and manages the overall mission.

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