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November 17, 1999
Galileo Probe Results Suggest Jupiter Had an Ancient, Chilly Past
Jupiter's history may be much older and colder than previously believed, according to newly released findings from NASA's Galileo spacecraft published in the November 18 edition of the journal Nature.
"This new information might shake up our view of how the solar system formed," said Dr. Tobias Owen, astronomy professor at the Institute for Astronomy of the University of Hawaii, Honolulu, HI, and a scientist on the Galileo probe neutral mass spectrometer instrument team. When Galileo arrived at Jupiter on December 7, 1995 and dropped a probe into the atmosphere of the huge, gaseous planet, the mass spectrometer measured the chemical composition of Jupiter's atmosphere.
In Jupiter's atmosphere, the spectrometer detected surprisingly high concentrations of argon, krypton and xenon, three chemical elements called noble gases because they are very independent and don't combine with other chemicals. Tiny traces of these gases are found in the air we breathe on Earth, and argon is sometimes used like neon in advertising signs.
The discovery of these gases in such high quantities at Jupiter raises questions about how they got there. "In order to catch these gases, Jupiter had to trap them physically by condensation or freezing," Owen said. This process, he said, requires extremely cold temperatures of about -240 degrees Celsius (-400 degrees Fahrenheit), colder than the surface of Pluto, the planet farthest from the Sun. Planetesimals (small objects orbiting the Sun) in the Kuiper Belt beyond Pluto would be this cold, but Jupiter is more than six times closer to the Sun and thus is much warmer. For this reason, Jupiter could not have been the site where the three noble gases were originally trapped.
"This raises some intriguing possibilities," Owen said. "One explanation suggests that Jupiter was formed out in the area around the Kuiper Belt and was dragged inward to its present location. Another possibility is that the solar nebula, a huge cloud of gas and dust from which our solar system formed, was much colder than scientists believe. A third hypothesis proposes that the solid materials that brought these noble gases to Jupiter began forming in the original huge, interstellar cloud of gas and dust even before it collapsed to form the solar nebula. That would make these icy materials older and more primitive than we had expected."
"If either of the last two hypotheses turns out to be correct, it would suggest that giant planets can form closer to their stars than current theories predict," Owen said. "This could help explain the new observations of planetary systems around other stars, in which such close-in giant planets are relatively common."
"These new Galileo probe results provide new insights into how planets form in the solar system and around other stars," said Galileo project scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory, Pasadena, CA.
"Measuring the composition of Jupiter's atmosphere was a primary scientific objective of the probe, because we knew it could change our understanding of Jupiter's formation and evolution," said Galileo probe project scientist Dr. Richard Young of NASA Ames Research Center, Moffett Field, CA. "These latest probe results have done exactly that, and the measurements are the sort that could only have been obtained by in-situ measurements from an entry probe."
Owen's co-authors on the Nature article are: Drs. Paul Mahaffy and Hasso Niemann of NASA's Goddard Space Flight Center, Greenbelt, MD, Drs. Sushil Atreya and Thomas Donahue of the University of Michigan, Ann Arbor, MI, Dr. Akiva Bar-Nun of the University of Tel Aviv, Israel, and Dr. Imke de Pater of the University of California, Berkeley. Although the data were collected by the Galileo probe in December 1995, careful and thorough analysis was necessary in Earth laboratories to verify the findings.
When it dropped 156 kilometers (97 miles) through Jupiter's atmosphere, the Galileo probe relayed data back to the main Galileo spacecraft more than 209,215 kilometers (130,000 miles) overhead for storage and transmission to Earth. The probe operated longer than expected during its descent deep into the atmosphere, but was finally overcome by Jupiter's high temperatures and pressures.
The Galileo spacecraft, meanwhile, has been orbiting Jupiter and its moons for nearly four years, beaming back to Earth thousands of pictures and a wealth of scientific data. Its two-year, primary mission ended in December 1997, but it was followed by the current, two-year extended mission.
Artist's concepts of the Galileo spacecraft and probe can be accessed at http://www.jpl.nasa.gov/pictures/solar/gllartist.html. Further information and images about the Galileo mission can be accessed at http://galileo.jpl.nasa.gov.
The Galileo Project is managed by Jet Propulsion Laboratory and the Galileo atmospheric probe is managed by Ames Research Center.