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.
NEW RADIATION BELT
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
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.
CLEAR TO PARTLY CLOUDY?
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.
WINDS AT DEPTH UNEXPECTEDLY
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.
LIGHTENING ON JUPITER DIFFERS
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.
NMS YIELDS SURPRISES IN
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.
HOW TYPICAL IS THE ENTRY
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.
MORE DATA TO COME
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