The University Record, November 22, 1999

Jupiter’s atmosphere indicates icy beginnings

By Karl Leif Bates
College of Engineering

Galileo image of Jupiter’s Great Red Spot. Photo courtesy of NASA
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 from 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 College of Engineering. He was joined in this 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, 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 that 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. However, 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 says.

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, says Donahue, the Edward H. White II Distinguished University Professor Emeritus of Planetary Science.

The Galileo probe, about the size of a Volkswagen Beetle, detached from the main Galileo spacecraft in summer 1995 and made a fiery one-hour plunge into Jupiter’s windy atmosphere 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 that was passing by high above.

Because of the difficulty of beaming data over the hundreds of millions of miles from Jupiter to Earth, particularly with Galileo’s crippled antenna, 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 took several more years for the team to comb through the data in what Donahue calls “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 says.

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 says.

“If Jupiter had migrated inward, it would have had to come from way out there, 40 or 50 astronomical units,” Atreya explains. 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 says. 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 four trillion miles from the sun. “There may be more to the solar system than we know about,” Donahue notes.

Atreya and Donahue 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 says. “We certainly would like to know, is this same thing true of Saturn, Uranus and Neptune?”