Galileo Probe Mission Science Summary

Date: 10 May 1996

The Most Difficult Atmospheric Entry in the Solar System was Successfully Accomplished!

During the parachute-descent phase of the mission, the Galileo Probe successfully studied the atmosphere of Jupiter with seven scientific experiments and radioed its findings to the Galileo Orbiter, which was 215,000 km (134,000 miles) overhead. The Galileo Probe and Orbiter separated on July 13, 1995 and both arrived at Jupiter on Dec. 7 on slightly different trajectories. The Probe, which did not include a camera, transmitted data for 57.6 minutes until about 200 km (125 miles) below the visible cloud tops, where the communication system failed due to the high temperatures.

Telescopic observations from Earth helped determine the appearance of the Probe entry site at the time of entry and have monitored its changing appearance over time. Such observations are vital for placing the Galileo Probe measurements -- taken at one location on the planet -- in the context of Jupiter as a whole. The initial results show the entry site to be a highly variable and dynamic region of Jupiters atmosphere. The Hubble Space Telescope image on this page shows the appearance of Jupiter in October 1995.

A New Intense Radiation Belt

Jupiter's radiation belts are so intense that the Galileo Orbiter must orbit quite high above Jupiter's cloud tops to avoid exposing its electronics to this damaging radiation.

The EPI discovered a new intense radiation belt between Jupiter's ring and the uppermost atmospheric layers. The radiation is approximately 10 times as strong as in Earth's Van Allen radiation belts, and includes high energy helium ions of unknown origin.

With further analysis, these discoveries will increase our understanding of Jupiter's magnetosphere and of its high frequency radio emissions. Many other celestial objects (e.g. stars, galaxies, and pulsars) have extensive magnetic fields and trapped radiation, so directly measuring the particularly strong magnetosphere of Jupiter can provide us with new understanding of the nature of these other objects as well.

Measurements of Temperature, Pressure, and Vertical Winds Reveal Several Discoveries

Initial results include finding that upper atmospheric densities and temperatures are significantly higher than expected. An additional source of heating beyond sunlight appears to be necessary to account for this result. At deeper levels the temperatures and pressures are close to expectations. The vertical variation of temperature in the 6- 15 bar pressure range (about 100-150 km below visible clouds) indicates the deep atmosphere is dryer than expected and is convective.

The ASI's initial results have various important implications. Our ideas about the abundance and distribution of water on Jupiter will need to be reconsidered. The ASI measurements will increase our understanding of the escape of Jupiter's internal heat -- a power source for its dynamic atmosphere. In addition, because of the convective nature of the lower levels of the atmosphere, the deep atmosphere must be well mixed, and composition measurements obtained by other instruments must be representative of the deeper levels of Jupiter's atmosphere as well.

Visibility in the Atmosphere is Much Greater Than Expected in the Immediate Vicinity of the Probe Entry Site

To scientists' surprise, no thick dense clouds were found, in contrast to expectations based on telescopic and flyby spacecraft observations of the planet and theoretical models. In fact, only very small concentrations of cloud and haze materials were found along the entire descent trajectory. One well-defined distinct cloud structure was found, and this appears to correspond to a previously postulated ammonium hydrosulfide cloud layer.

One important question which has arisen from these as well as other observations is whether the Probe's entry location is representative of most other regions of Jupiter. The cloud structure at the Probe Entry site appears to be very different than expected for Jupiter as a whole. Models of cloud formation on Jupiter may have to be revised.

Dense Cloud Detected Some Distance Away from the Probe Entry Site

Large variations in the brightness of the sky in different directions were noticed until an abrupt drop-off in the variation occurred below a pressure level of 0.6 bars, indicating a cloud layer which is most likely the previously postulated ammonia cloud layer -- believed to be the uppermost cloud layer on Jupiter. No other significant cloud layers were found-- in particular, the tenuous cloud layer detected by the NEP (See "Visibility in the atmosphere"...section) was not seen by this experiment. Moreover, the cloud seen by the NFR was not seen by NEP!

The explanation for this apparent contradiction is that the NEP measures cloud particles in the immediate vicinity of the Probe while the NFR measures clouds over a long distance. The simplest explanation for the results from these two cloud-detecting experiments appears to be that the clouds are patchy and that the Probe went through a relatively clear area.

The NFR also measured variations of infrared "thermal" radiation. Heating of the NFR's cloud layer by heat escaping from the interior of Jupiter appears to also be occurring and may affect the nature of Jupiter's winds.

Strong Winds Persist to Great Depth

Initial results from the Doppler Wind experiment indicate that the winds below the clouds blow at 700 km/hour (435 mph) and are roughly independent of depth. Winds at the cloud tops monitored by the Hubble Space Telescope are of similar strength. These results have profound implications.

It now appears that winds on Jupiter are probably not produced by heating due to sunlight, or by heating due to condensation of water vapor -- two heat sources which power winds on Earth. A likely mechanism for powering the winds now appears to be the heat escaping from Jupiter's deep interior.

Lightning on Jupiter Very Different than on Earth

On Earth we are accustomed to lightning discharges between the clouds and the ground. However, lightning discharges within clouds are by far the most common. On Jupiter, where no solid surface exists, lightning is believed to be occurring within the water clouds.

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 Galileo Probe, but many discharges were observed at radio frequencies.

The form of the radio signals indicates that the discharges were far away (roughly one Earth diameter away), and that the lightning bolts are much stronger than Earth's. Radio wave intensity suggests lightning activity is 3-10 times less common than on Earth.

Initial analysis implies that lightning on Jupiter is very different than on Earth. The unusual form of the radio signals from the lightning indicates more work on lightning discharge physics on Jupiter is needed. Ideas of water cloud distribution, precipitation, and heat escape from Jupiter may need revision.

Composition Surprises and Puzzles

The Neutral Mass Spectrometer (NMS) experiment's objective was to accurately determine the composition of the atmosphere. Initial results indicate the atmosphere has much less oxygen -- mainly found as water vapor in Jupiter's atmosphere -- than the Sun's atmosphere, implying a surprisingly dry atmosphere. On the other hand, the amount of carbon -- mainly found as methane gas -- is highly enriched with respect to the Sun, while sulfur -- in the form of hydrogen sulfide gas -- occurs at greater than solar values. The abundance of nitrogen -- in the form of ammonia gas -- is still pending. The abundance of neon -- a Noble or "inert" gas -- is highly depleted. Little evidence of organic molecules was found.

According to these first results, the six most abundant elements occur in widely varying proportions relative to values in the Sun. Planetary scientists had expected oxygen to be enriched relative to the solar value due to impacts by comets and other small bodies over the 4.5 billion year history of the solar system. The helium abundance was expected to be somewhat lower due to the internal evolution of Jupiter. Accounting for these results provides challenges and opportunities for refining our ideas about the formation and evolution of Jupiter and the solar system. The role of meteorology in producing the dryer atmosphere must also be considered.

The Probe Apparently Entered a Rather Special Location on a Complex World

Starting in the spring of 1996 after completing the transmission of the Galileo Probe data, the Galileo Orbiter will commence detailed observations of Jupiter's satellites, atmosphere, and magnetosphere. These atmospheric observations combined with telescopic monitoring from Earth will enable a generalization of the Galileo Probe's findings to other regions of the planet.

There are More Results to Come!

In June 1996 the Galileo Orbiter will begin 18 months of very detailed imaging and other observations of Jupiter's four large intriguing moons and atmosphere along with further measurements of the magnetosphere.

NASA's Ames Research Center near Mountain View, California managed the Galileo Probe project and conducted science and engineering studies enabling this most difficult atmospheric entry. Hughes Space and Communications Company built the Galileo Probe. NASA's Jet Propulsion Laboratory in Pasadena, California managed the overall Galileo Project and built the Galileo Orbiter.