After a six-year odyssey the Galileo Probe plunged into Jupiter's atmosphere at 2:04 p.m. PST on December 7. During the first two minutes of entry, the craft experienced temperatures twice as hot as the Sun's surface and deceleration forces as great as 230 g's as it was slowed by the atmosphere. The Galileo Orbiter, which entered orbit around Jupiter a few hours after the Probe's descent, recorded 57.6 minutes of data from instruments before the Probe fell silent.

Because of an unexplained 53-second delay at the start of transmission, direct atmospheric measurements started deeper in the atmosphere than originally planned (0.35 bars instead of 0.1 bars). Playback of early data from the Orbiter confirmed that all six scientific instruments onboard the Probe operated properly.


Three hours before entry, the Energetic Particle Instrument (EPI) measured the radiation in previously unexplored inner regions of Jupiter's immense magnetosphere. It detected a new intense radiation belt between the planet's thin ring and its uppermost atmosphere. About 10 times as intense as Earth's Van Allen belts, the region contains high-energy helium ions of unknown origin.


As the plunge into the atmosphere began, the Atmosphere Structure Instrument (ASI) measured temperature, pressure, and density throughout the Probe's descent. Initial results show upper atmospheric densities and temperatures that are significantly higher than predicted by most models. An additional source of heating besides sunlight appears necessary to account for them. Lower in the atmos-phere, temperatures were close to those expected. The vertical variation of temperature in the 6-15 bar pressure range (about 90- 140 kilometers below visible clouds) indicates the deep atmosphere is dryer than expected and is convective.

ASI also measured vertical wind speeds in the lower reaches of the atmosphere and provided evidence that the deep atmosphere is highly turbulent. Data transmission ended at an atmospheric pressure of 23 bars and a temperature of 305 F (152 C). The upward and downward winds appear to be much stronger than expected, requiring a revision of ideas about the escape of heat from Jupiter's interior.


Visibility in the atmosphere was much greater than expected in the immediate vicinity of the Probe entry site. The Nephelometer (NEP) instrument was designed to detect and characterize cloud particles throughout the descent in the immediate vicinity of the Probe by shining a laser beam across a short distance to a small mirror deployed just outside the spacecraft and measuring the scattered and transmitted light. Initial results surprised investigators. No thick dense clouds were found, contrary to expectations based on telescopic and flyby spacecraft observations of Jupiter and simple theoretical models. In fact, only very small concentrations of cloud and haze materials were found along the entire descent trajectory. Only one well-defined distinct cloud structure was found, which appears to correspond to a previously postulated ammonium hydrosulfide cloud layer.

Variation of the amounts of sunlight and infrared radiation with depth were measured by the Net Flux Radiometer (NFR) to aid in detecting cloud layers, understanding the power sources for winds, and detecting water vapor. On a clear day on Earth, the sky is quite bright in the direction of the Sun and less bright in other directions. On a very cloudy day, the sky is almost equally bright in all directions and determining the direction to the Sun is difficult. The NFR used this effect along with the Probe's spin to locate cloud layers on Jupiter. Large variations in sky brightness in different directions were measured until an abrupt drop-off occurred at a pressure level of 0.6 bars, indicating a layer that is most likely the ammonia clouds that form the uppermost cloud layer that we observe on Jupiter.

No other significant cloud layers were found. The hazy cloud layer detected by the NEP was not seen by the NFR experiment, nor was the cloud layer observed by the NFR seen by NEP, because the NEP measured cloud particles in the immediate vicinity of the Probe while the NFR measured clouds at a greater distance away. The simplest explanation for the results from the two cloud-detecting experiments is that the Probe entered through a relatively clear area with only patchy clouds.


Heating of the ammonia cloud layer by energy escaping from the interior of Jupiter also appears to be occurring and may account for the observations of Jupiter's winds. Once again the cloud structure at the Probe entry site appears to be very different than atmospheric modelers expected. Previous studies of Jupiter's cloud motions show a very unusual wind system consisting of strong alternating east-west jetstreams. The origin of these streams is not clear, because we cannot see structure below the uppermost clouds. The Doppler Wind Experiment used the Doppler effect to evaluate the vertical variation of winds. Initial results from this experiment indicate that the winds below the clouds are 540 kilometers per hour (330 miles per hour) and do not decrease with depth as most models had predicted. One implication is that Jupiter's winds do not appear to be produced by heating from sunlight or condensation of water vapor_the heat sources that power winds on Earth. A likely mechanism for powering the winds now appears to be the heat escaping from Jupiter's deep interior.


The Lightning and Radio Emission Detector searched for optical flashes and radio waves emitted by lightning discharges. No optical lightning flashes were observed in the vicinity of the Probe, but many discharges were observed at radio frequencies at distances about an Earth-diameter away from the craft. Bolts are much stronger than Earth's, but radio wave intensity suggests lightning activity is 3-10 times less than on Earth. This initial analysis implies that lightning activity on Jupiter is very different than on Earth.


Jupiter's composition has been thought to closely resemble the "primordial" solar system chemistry and scientists looked to direct measurements to provide clues to planetary formation and evolution processes, including the addition of materials from asteroid and comet collisions. Initial results from the Neutral Mass Spectrometer (NMS) suggest the atmosphere has less water than expected. Carbon, in the form of methane, is also less abundant, as is sulfur as hydrogen sulfide. Noble gas concentrations differ from expectations as well, including a notable depletion of neon. Little evidence for organic molecules was found. The Helium Abundance Detector found significantly less than solar abundances of helium, which may be a result of the element raining out at depths in the atmosphere as Jupiter evolves chemically over time. Ideas about the formation and evolution of Jupiter will be revised in light of these results.


One important question that arises from these as well as other observations is whether the Probe's entry location is representative of most other regions of Jupiter. A simple explanation for many unexpected results is that the Probe apparently entered a rather unusual location on a quite nonuniform world.

Groundbased telescopic observations of the Probe entry site (6.5 N, 4.5 W) immediately before the Probe arrived show a region near the edge of an infrared "hot spot" where clouds appear to be much thinner or absent. Thus many of the unexpected results from the Probe instruments may reflect an entry location that is not typical of much of the jovian clouds.


This summary of scientific findings from the Galileo Probe Mission is the result of a quick, preliminary analysis of the data. Much additional work will be done in the coming months and years. Scientific and popular publications will report further on the Galileo Probe Mission results. Additional information on the status of the data reduction and the latest results will be listed on the World-Wide Web at URL