More early findings from Galileo probe.

While some astronomers ponder the latest findings about giant planets orbiting nearby stars (see p. 52), others are puzzling over the first data from the interior of our very own behemoth. On Dec. 7, a probe from the Galileo spacecraft plunged kamikaze-style into Jupiter. Science News reported some of the earliest analyses last month (SN: 12/23 & 30/95, p. 420).

This week, at NASA's Ames Research Center in Mountain View, Calif., investigators presented a slew of findings from the 57-minute mission.

Some of the results may force scientists to revise their thinking about how the planets matured, says Richard E. Young, Galileo project scientist at the center.

According to a widely held theory, comets bombarded the youthful planets, delivering a multitude of organic compounds, noble gases, and other atoms and molecules. As a result, the chemistry of the planets differs substantially from that of the primitive solar nebula from which they arose. But the quantitative nature of that difference and the relative importance of comets remain matters of debate, notes planetary scientist Tobias C. Owen of the University of Hawaii in Honolulu.

Data from the probe's mass spectrometer indicate that several elements, including carbon, oxygen, and sulfur, have abundances closer to solar values than previously thought. This suggests that scientists don't fully understand how the planets evolved, Owen says.

In contrast, another instrument on the probe measured only half the helium abundance of the sun's atmosphere. Researchers suspect helium formed droplets that rained down on Jupiter's core, a region the probe never reached.

A few of the other surprising findings may have more to do with the unusual nature of the probe's entry site-drier and more cloudfree than 98 percent of the planet's visible surface-than with the global character of Jupiter. For instance, the probe failed to detect the three distinct cloud layers thought to inhabit Jupiter's upper atmosphere. Scientists believe that below the visible cloud tops of ammonia reside layers of ammonium hydrosulfide and water vapor, but they found only wisps of gases and some evidence of water droplets.

"It's like the probe landed in Arizona or Death Valley rather than the Amazon rain forest," comments Gordon L. Bjoraker of NASA's Goddard Space Flight Center in Greenbelt, Md. "Unfortunately, if you want to look for clouds, this is not the place to land."

After correcting for a calibration error, probe scientists now say that the abundance of water on Jupiter is about the same as that indicated by the concentration of oxygen on the sun. That's about 10 times as much water as the researchers had previously estimated but only half the amount indicated by the Voyager spacecraft and one-tenth the amount anticipated by some scientists in the aftermath of Comet Shoemaker-Levy 9's 1994 crash into Jupiter.

Intriguingly, measurements of the Jovian winds indicate that they blow less strongly at the visible cloud tops-about 100 meters per second from west to east-than in deeper layers. The stronger winds, of 150 m per second, persisted as the probe descended, suggesting that heat escaping from deep within the planet, rather than sunlight striking Jupiter's topmost layers, drives the circulation, Young says.

The probe found that lightning occurs on Jupiter at only one-third to one-tenth the average terrestrial rate. Detectors recorded few optical flashes but about 50,000 radio signals consistent with lightning, indicating that the disturbances came from sources several thousand kilometers distant from the probe's equatorial entry site.