The weather report is in from Jupiter, and the skies seem to be hotter, windier, drier, and clearer—but much less electric—than most forecasters predicted.
According to measurements made by NASA's Galileo probe, the temperature in Jupiter's atmosphere ranges from -144°C at the top of the ammonia-cloud covered atmosphere to +152°C at 600 km into it. Atmospheric pressures range from one-third those on Earth to 22 times our planet's readings, and those forces compress the gases in some places to densities 100 times greater than previously postulated. The extremes of temperature and pressure stir up turbulent east-to-west winds reaching 640 km/hr, more than 50% stronger than most predictions.
Fig. 1. The Galileo probe's entry into Jupiter's atmosphere was designed to occur at a precise angle of 8.5° to the horizontal. At 7°, or 1.5° less than the necessary angle, the probe would have bounced off of the atmosphere like a billiard ball; at a descent angle of 10°, the probe would have been incinerated almost immediately.
Galileo's observations of the chemical composition of the gas giant confirmed that helium is abundant in the Jovian atmosphere. Planetary scientists have long theorized that hydrogen and helium make up 99% of Jupiter's atmosphere, and that those gases are present in ratios comparable to those on the Sun. Indeed, investigators found that Jupiter's atmosphere is about 24% helium, close to the Sun's 26%."The amount of helium in the Jovian atmosphere is close to the original amount that Jupiter gathered as it formed from the primitive solar nebula," said Richard Young, project scientist for the Galileo probe. The abundance of helium suggests that Jupiter's interior is much hotter than that of Saturn.
Galileo's observations produced some surprises. For example, Jupiter is much drier than anticipated. Evidence from the Voyager missions of the late 1970s had led researchers to believe Jupiter possessed twice as much water as the Sun. So when the Galileo probe found that Jupiter's atmosphere contains water concentrations merely equal to the Sun's, scientists were left wondering "where's the oxygen?"
The three-tiered cloud structure that many researchers had expected came out two tiers short. The probe detected an outer layer of ammonia ice crystals, but the expected middle layer of ammonium hydrosulfide was just a thin fog. As for water clouds, mere wispy clumps of water particles (like cirrus clouds) were discovered. The probe did find lightning—more than 50,000 electrical events during the 57-minute descent through the atmosphere—but the occurrence rate was just one-third to one-tenth of what we experience on Earth.
Planetary scientists suspect that the Galileo probe dropped into a "hot spot," a rare region where more radiation emanates from Jupiter's interior than at other points in the atmosphere. "If we had designed this entry point 20 years ago, we would never have believed we could possibly come in at this place on this complicated planet," said Boris Ragent of San Jose State University. "Apparently, we just descended in a very clear region of the Jupiter atmosphere, which we never could have expected at all." As Glenn Orton of the Jet Propulsion Laboratory noted, there was "a 1% chance" that Galileo could have entered one of the least cloudy areas on Jupiter. It did.
Galileo's findings are fine-tuning scientists' theories about the formation of Jupiter and the planetary evolution process as a whole. "The things that we are learning about Jupiter will not only be applicable to understanding our own solar system," said Richard Young of the NASA-Ames Research Center, "but they will almost certainly be applicable to understanding these new solar systems that we are just now beginning to discover."
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