August 13, 1997
The Galileo Probe craft made history's first entry into Jupiter's atmosphere in December 1995 and penetrated into what appears to be the region of Jupiter's large-scale interior circulation. The wind speed findings, just completed after detailed analysis, indicate that Jupiter's whole atmosphere circulates, perhaps down to its liquid metallic hydrogen core.
"The winds below Jupiter's cloud tops do not die off as they would if driven by solar heating, but persist and increase to gale force at depths extending 100 miles below the clouds," said Al Seiff, probe investigator at NASA Ames Research Center, Moffett Field, CA. Seiff was principal investigator on the probe's atmosphere structure experiment during the most difficult atmospheric entry in the solar system. (Jupiter contains 71 percent of all the planetary mass in the solar system. The planet's massive gravity produced intense heating by atmospheric friction -- with temperatures hotter than the surface of the sun -- during the probe's descent.)
"We have dipped into the planet's internal motions and have the first clue about the circulation of gases in the deep interior," he said.
Probe instruments made two independent measurements of the winds. Principal investigator David Atkinson, University of Idaho, conducted the Galileo Probe wind experiment using radio measurements as well as data from Seiff's atmospheric structure experiment, obtaining data from eight miles above the cloud tops to 80 miles below. The probe measured winds that increased from 192 mph to 400 mph during descent. Wind speed was 192 mph at eight miles above the cloud tops, rose to 391 mph at 28 miles down, and stayed between 380 and 390 mph down to about 80 miles, the end of wind measurements.
Independent of the radio tracking, Seiff measured the winds using two accelerometers on board the craft that reported speeds from 400 mph to 450 mph. W.M. Folkner, Jet Propulsion Laboratory, measured wind speeds by tracking the probe from Earth during the first 28 miles of descent. He used the Very Large Array of Earth-based radio telescopes to measure the Doppler shift of the probe's incredibly faint radio signal as it came to Earth from deep in Jupiter's atmosphere a half a billion miles away.
The final details on Jupiter wind speeds from Atkinson's data are as follows: Winds were 232 mph at the cloud tops, rose to 293 mph at seven miles down; to 313 mph at 10 miles down; 360 mph at 17 miles down; 391 mph at 28 miles down and to around 390 mph to the end of measurement at 60 miles.
Both Seiff's and Folkner's data are in essential agreement with Atkinson's findings. The findings eliminate the long-held idea that jovian winds are confined to a 35-mile-deep weather layer containing Jupiter's brightly-colored, east to west wind-driven clouds.
The probe also measured atmospheric pressures of one half the pressure of Earth's atmosphere (.5 bars) at the top, to 21 times Earth's atmospheric pressure (21 bars) at the lowest level measured. Temperatures during the Probe descent ranged from a low of -230 degrees Fahrenheit at the top to a broiling 306 degrees Fahrenheit at the lowest level.
The results from Seiff and Atkinson's probe investigations are published in the August 14 issue of Nature.
The Galileo Probe's findings of Jupiter's huge winds may strengthen some existing theories about the gas giant's interior flow, according to Andrew Ingersoll, planetary meteorologist at California Institute of Technology and Galileo interdisciplinary scientist. Several scientists have proposed that the dense atmosphere of Jupiter has a pattern of cylindrical atmospheric vortices parallel to the axis of rotation, some extending completely through portions of the planet. Effects of this interior flow may be visible on the planet's surface as evidenced in its colorful belts and zones, Ingersoll said.
"The winds reached almost 400 mph at 30 miles down and stayed that way. This massive flow of dense atmosphere has tremendous momentum, and may account for centuries-long persistence of some jovian surface features," Ingersoll said. In a fairly well-mixed gas planet, we know of few forces that would drastically change this pattern as you go deeper.
The massive interior winds are probably driven by a balance of forces, solar heating at the top and internal heat elsewhere. Jupiter radiates 1.7 times as much heat as it absorbs from the Sun. The planet's surface radiates the same amount of heat at the equator and poles. Therefore, interior circulation is likely to move interior heat preferentially to the poles, balancing solar heating at the equator, he explained.
"We will be studying the parallel cylinders model to explain what we now know about interior atmosphere circulation. These cylinders may work like sets of counter-rotating gears moving heat and momentum from place to place," Ingersoll said.
The proposed atmospheric cylinders were first demonstrated in a series of laboratory experiments 25 years ago to chart atmospheric flow in a wholly gaseous planet. Friederich Busse, University of Bayreuth, Germany, and John Hart, University of Colorado, Boulder, used liquid-filled spheres with high rotation speeds and imposed interior-exterior temperature differences. The experiments showed that the convective and most other disturbances in these fast-rotating spheres of fluid almost always produced cylindrical vortices parallel to the test vessel's spin axis, called Taylor columns.
Jupiter is a giant gas planet, comprised mostly of hydrogen, that rotates on its axis once every 10 hours. Because its gravity is so huge, much of its hydrogen is compressed into a liquid metal. This dense "liquid" extends from Jupiter's central area to 80 percent of the planet's radius, Ingersoll explained. Rotating cylinders of atmosphere extending through Jupiter parallel to its spin axis would run into the dense metallic hydrogen and be cut off, or otherwise modified. This would make interior circulation more complex than if the planet were uniformly gaseous, he said.
The Galileo Probe mission was managed by NASA Ames Research Center, Moffett Field, CA, as part of the Galileo spacecraft mission conducted by the Jet Propulsion Laboratory, Pasadena, CA. The probe was built by Hughes Space and Communications Group, Redondo Beach, CA.