To Contents
Page
After a 6-year journey replete with nail-biting trials and eye-popping triumphs in space, and after a frustrating 6-week delay back on Earth, the Galileo Probe's bounty of scientific data was finally presented, on January 22 at the Ames Research Center, to a fascinated public by a panel of Probe science investigators headed by Probe Scientist Rich Young (see photo). Their preliminary report summarized the condensed memory readout from the Orbiter's solid-state memory downlinked in December and January. In the months since that meeting, subsequent analyses have changed some of the scientists' earliest views on their data. Probe investigators are still waiting for the complete playback from the Orbiter's tape recorder, but that won't be finished until mid-April.
While the picture of Jupiter that has emerged is, generally, similar to what was expected, the details are sufficiently different to merit some serious rethinking on the origin and structure of the planet and its atmosphere.
The Probe was certainly well prepared for its brief but celebrated exploration of the Jovian atmosphere. Radio contact with the Orbiter was solid for almost an hour, and every instrument performed perfectly through the nominal mission.
The approach and entry were not without some surprises. The space between Jupiter's rings and atmosphere was expected to be fairly quiet, but Harald Fischer's energetic particle instrument discovered a new, powerful radiation belt here---populated by high-energy helium ions and ten times stronger than Earth's Van Allen belt.
Probe deceleration in the upper atmosphere as measured by Al Seiff's atmospheric structure instrument was greater than expected, indicating a much denser (100 times) and hotter (227°C) atmosphere 340 km above the 1-bar level. Unexpected, too, was a parachute deployment 53 seconds late and 26 km below the planned 0.1-bar level.
When the heat shield dropped off and the instruments started recording, Larry Sromovsky's net flux radiometer (NFR), designed to measure the energy balance between the Sun above and the planet below, showed variations in sky brightness that indicated scattered clouds. At this 0.4- to 0.6-bar level (-150°C to -130°C and 20 km above the 1.0-bar level), these were likely ammonia clouds.
Boris Ragent's nephelometer, which reads a reflected laser beam for cloud particles, saw none at this altitude, suggesting the ammonia clouds were distant, or at least scattered. Maybe 45 or 50 km further down, however, at about 2 bars (-70°C), it did record substantial concentrations of what were believed to be ammonium hydrosulfide clouds. While well defined, even these clouds were not nearly as thick as postulated; the NFR did not report them. Even further down, 60 to 80 km below 1 bar, at the 5- to 8-bar level where temperatures support liquid water (0 to 40°C), the nephelometer found no evidence for water clouds, though these should have been the thickest of all.
Glenn Orton's ground-based, infrared telescopic observations showed the Probe's entry site to sit on the edge of a prominent "hot spot." This broad patch of clearer, drier atmosphere looked to be a region of thinner, even absent clouds. This certainly confirmed the nephelometer readings and suggests that, at least in its upper atmosphere, Jupiter is a very heterogeneous planet. One of the principal tasks of the investigators will be to distinguish those data that measure local phenomena from those that measure the global.
Hasso Nieman's neutral mass spectrometer, which determines the composition of the atmosphere, also revealed the atmosphere to be drier than expected---much drier than predicted from Shoemaker-Levy 9 data, and even drier than predicted from Voyager data! Initial results suggested generally Sun-normal values for many other atmospheric constituents, but later work has changed that picture. Solar values would suggest little change in Jupiter's evolution from the original solar nebula, but increased concentrations of any element (besides hydrogen and helium) would suggest a history of cometary accretions. Concentrations of methane and hydrogen sulfide were greater than solar values. Ammonia values, even at this date, still puzzle researchers. Concentrations of the noble gases krypton and xenon were much greater than solar values, but isotopic ratios were near solar. Fewer organic molecules and substantially less neon than expected also characterized the Jovian atmosphere sample.
Ulf von Zahn's helium abundance detector measured the concentration of helium at 0.24 by mass, close to solar abundance. Low levels of helium indicate depletion in the atmosphere of Saturn, but this is not seen at Jupiter, probably because of Jupiter's larger size (three times more massive) and higher internal temperatures.
Lou Lanzerotti's lightning and radio emission detector looked for both optical flashes (from near discharges) and radio waves (from more distant ones). The expected thick cloud decks suggested lots of cloud-to-cloud bolts. In retrospect, considering the lack of water clouds, it's not surprising that no flashes were seen. On the other hand, the Probe did record the radio signatures of perhaps 50,000 strikes---up to an Earth's diameter away. These numbers translate to very powerful discharges but to only a third (or even a tenth) the occurrence rate of lightning on Earth. Fewer strikes are also consistent with fewer organic molecules (which the strikes generate).
Dave Atkinson's Doppler wind experiment tracked the Doppler shift in the Probe's radio signal to measure the speed of the Jovian winds. Wind speed at the cloud tops was thought to be 360 to 540 km/h, and this was expected to drop to zero at some point---if, as on the Earth, such winds are generated by sunlight and release of latent heat by condensation of water vapor. Not unexpectedly, Jupiter is not like the Earth. Winds are faster than expected, clocking 720 km/h below the cloud-top level, and show no tendency to slow with depth. Jovian winds are apparently generated by heat coming from below.
The helium abundance detector stopped recording at 14 bars, as designed, after 40 minutes of activity. At this time, signals from the nephelometer and net flux radiometer also became useless as they degraded to noise. After 48 minutes of recording, 110 km down, the instrument shelf temperature inside the Probe was much closer to the outside 15-bar temperatures of 100°C than the expected 50°C. The lightning detector and neutral mass spectrometer stopped sometime after this point.
Only Al Seiff's atmospheric structure instrument was still operating when radio transmission stopped after 57.6 minutes at the 23-bar level (152°C, 140 km down). This instrument measured the atmospheric temperature, pressure, and densities during the entire 160 km or so (20 km above 1.0 bar to 140 km below) of descent. During the entry phase, it showed hotter temperatures and higher densities than expected in the upper atmosphere, and numbers much closer to those expected in the lower atmosphere. Also consistent with Doppler data, it showed that the Probe dropped through a very turbulent atmosphere. And consistent with the other instruments, it measured a lapse rate or change of temperature with altitude that showed a very dry atmosphere in the 6- to 15-bar range and convective transfer of heat, which powers the wind systems and keeps the deep layers well mixed.
After its last transmission, the Probe, we imagine, continued to sink into the Jovian depths. Without a surface to hit, the Probe lost its Dacron parachute, its aluminum fittings, and even (by the 5000-bar level, 1700°C) its titanium shell to melting and evaporation. Ten hours after entering the atmosphere there would have been nothing left to see, and the Probe would have become a part of the planet that its sister Orbiter will be watching so closely.
The Probe science team eagerly awaits the return of the last bits of the taped Probe data set. These data, along with additional atmospheric data from the Orbital tour, will keep the team busy for years unwrapping the secrets of this mysterious giant.