Galileo Probe Results Suggest Jupiter had an Ancient, Chilly Past

Kathleen Burton

NASA Ames Research Center, Moffett Field, CA

(Phone: 650/604-1731, 650/604-9000) kburton@mail.arc.nasa.gov

Jane Platt

Jet Propulsion Laboratory, Pasadena, CA

(Phone: 818/354-0880) jane.platt@jpl.nasa.gov

RELEASE: 99-75

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 the descent probe of NASA’s
Galileo spacecraft published in the Nov. 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 Dec. 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.

The spectrometer detected in Jupiter’s atmosphere higher than expected
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 Earth’s atmosphere, 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
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,” he said.

“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,” he said.

“If either of the last two hypotheses proves 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, CA. 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 descended deeper into the atmosphere than expected, 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. The
Galileo Project is managed by the Jet Propulsion Laboratory, Pasadena, CA;
the Galileo atmospheric probe is managed by NASA Ames Research Center,
Moffett Field, CA. Further information and images about the Galileo
mission to Jupiter are available on the Internet at:

http://www.jpl.nasa.gov/galileo

-end-