batesk@umich.edu
734-647-7088
University of Michigan
Jupiter’s atmosphere gives clues on how solar system started
ANN ARBOR—A new analysis of data collected by the Galileo spacecraft’s suicide probe as it plunged into Jupiter’s roiling atmosphere
in December 1995 has stamped a huge question mark over the prevailing models of how our solar system began.
According to an international team of scientists writing in the Nov. 18 edition of the British journal Nature, the gas giant was found to
contain two to three times more of the heavy noble gases Argon, Krypton and Xenon than one would expect had it formed solely from
solar nebulae, the leftovers of the formation of our sun. It also had about three times more nitrogen than would be expected under the
prevailing models of how our solar system formed.
Where Jupiter now orbits, about five times the distance from the Earth to the sun (five astronomical units), it is much too warm to have
accumulated those gases in the quantities detected by the probe, according to Sushil Atreya, director of the Planetary Science Laboratory
at the University of Michigan College of Engineering. He was joined in the research by U-M planetary science colleagues George
Carignan and Thomas Donahue and a multidisciplinary team from the University of Hawaii, NASA’s Goddard Space Flight Center, Tel
Aviv University in Israel, and the University of California at Berkeley.
Jupiter is believed to have formed from the solar nebula and from a collection of small bodies called icy planetesimals, or micro-planets.
Most planetesimals, a class of objects which includes comets, are thought to have formed somewhere between the orbits of Uranus and
Neptune, 20 to 30 astronomical units from the sun. Even at that distance, however, the initial temperature of these icy bodies would have
been far too warm for them to trap the heavy noble gases and nitrogen in an icy form.
An alternative scenario involves the delivery of these gases to Jupiter by small icy bodies that lie beyond the orbit of Neptune in an area
called the Kuiper Belt, which is more than 40 astronomical units from the sun. But had the planetesimals somehow fallen from their
current orbits in the Kuiper Belt to Jupiter’s present orbit, the heavy noble gases and the nitrogen would have largely dissipated in the
warmer temperatures before they arrived at Jupiter. So neither theory accounts for what the probe actually found.
“The implications are enormous—how do you do that?” Atreya said.
Could it be, the team now wonders, that Jupiter was initially much farther from the sun and that it moved in to its present orbit more
recently? It’s either that, or the solar nebula was much, much colder than the models have estimated, said Donahue, the Edward H. White
II Distinguished University Professor Emeritus of Planetary Science at U-M.
The Galileo probe, about the size of a Volkswagen Beetle, detached from the main Galileo spacecraft in the summer of 1995 and made a
fiery 1-hour plunge into Jupiter’s windy atmosphere on Dec. 7, 1995. Before being burned and crushed on its descent, the probe took
atmospheric chemistry measurements with a mass spectrometer built by the Michigan team and Hasso Niemann of the Goddard Space
Flight Center. Its data was relayed to the orbiter passing by high above.
Because of the difficulty of beaming data over the hundreds of millions of miles from Jupiter to Earth, particularly with the crippled
antenna Galileo has, data from the probe was saved in compressed form in Galileo’s computer and stored in more detail on its tape
recorder. The initial results came from the computer, but the more detailed tape-recorded measurements took two months to be transmitted
to Earth.
And it has taken several more years for this team to comb through the data in what Donahue called “a lot of agonizing spectroscopy
analysis.” Sophisticated computer modeling was done to try to account for what the probe reported. “This is a complicated calculation,
even for the people who are experts,” he said.
But these findings, coupled with the recent discovery of planets in other systems that appear to be much larger than Jupiter and much
closer to their stars, may lend support to the idea that gas giants can migrate from one spot to another in their solar systems. “This is a
piece of information that’s got to be factored in,” Atreya said.
“If Jupiter had migrated inward, it would have had to come from way out there, 40 or 50 astronomical units,” Atreya said. And the
Kuiper Belt currently does not have enough mass to account for something the size of Jupiter being formed or the amount of heavy
elements now found within the planet.
“You have to characterize our understanding of how the solar system got started as sort of in a state of flux,” Donahue said. Some
theorists have even proposed that a Jupiter-sized object could be lurking undetected out in the Oort Cloud, a thin shell of comet-like
objects 4 trillion miles from the sun. “There may be more to the solar system than we know about,” Donahue said.
Both Michigan researchers would like to see additional probes sent to Jupiter and Saturn—another gas giant—to help figure out these
riddles. “Imagine working out the entire physics and chemistry of the Earth based on a single probe,” Donahue said. “We certainly would
like to know, is this same thing true of Saturn, Uranus, and Neptune?”
Contact: Karl Leif Bates, (734) 647-7088, batesk@umich.edu
or: Sushil Atreya, (734) 763-6234, atreya@umich.edu
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