Io's big sodium cloud looms even bigger.

Io's big sodium cloud looms even bigger

The large, crescent-shaped cloud of sodium atoms that stretches out into space from Jupiter's moon Io, fed by Io's volcanic eruptions, has intrigued astronomers since its discovery in 1973 from Earth. Studies of the fog-lamp-colored cloud have indicated that traces of it extend out into space from Io as far as 30 times Jupiter's radius, or about 2.1 million kilometers.

Now a group of researchers has observed the could again, finding evidence of its sodium atoms more than 32 million km out from Jupiter.

It is "possibly the largest permanently visible feature in the solar system," says astronomer Michael Mendillo of Boston University.

New observations by Mendillo and two co-workers, made with a ground-based telescope, reveal the cloud has an angular width of about six degrees, a portion of the sky, as viewed from Earth, equivalent to that of a dozen full moons placed side by side.

The sodium atoms are carried away from Io by processes associated with Jupiter's rapidly rotating magnetosphere, or magnetic field. The researchers, in fact, don't call the feature a could at all, but a "magneto-nebula."

Mendillo, together with Boston University colleague Jeffrey L. Baumgardner and graduate student Brian C. Flynn, reported their results last week at the American Geophysical Union meeting in Baltimore.

The group photographed the cloud on Jan. 25 from McDonald Observatory in Fort Davis, Texas, using a 100-millimeter telescope. Mounted on the telescope was a filter and a light-intensifying detector that allowed the astronomers to measure the small amounts of sunlight reflected from sodium atoms even at great distances from Jupiter.

Sodium, however, is only the most conspicuous part of the chemical cloud surrounding Io. "Now," says Mendillo, "we can pay attention to materials with fainter spectral lines, produced by other elements." Other observations have detected both sulfur and oxygen in Io's cloud.

In addition, there remains the tantalizing possibility of using the equipment to look at additional parts of the solar system, for, say, atomic clouds outside other planets. "Some folks have suggested that we look at other targets, such as Saturn," Mendillo says.

The telescope and filter used by the researchers cost a total of about $ 50,000, says Mendillo, who makes a point of citing the sodium image as a reminder that significant scientific findings can result from a modest investment.

"In the era of the Hubble Space Telescope, from which we all expect exciting results, it is encouraging to realize that ground-based astronomical instruments of all sizes can still play an important role in space science," he says. "This is particularly important to university-based research groups where the training of graduate students can proceed along time-scales much shorter than those associated with major space missions."