Hubble finds one more oddity on an already strange moon

UW-Madison scientist Frederick Roesler and his colleagues have found evidence of glowing hydrogen gas on Jupiter's moon Io, first discovered in 1610 by Galileo Galilei. (Photo courtesy of NASA/Jet Propulsion Laboratory.)

Jupiter’s moon Io, whose strange surface is defined by active volcanoes, lakes of molten sulfur and vast fields of sulfur dioxide snow, has revealed another oddity to scientists: caps of glowing hydrogen gas at the moon’s poles.

Using a new instrument aboard the Hubble Space Telescope, the Space Telescope Imaging Spectrograph, UW–Madison scientist Frederick Roesler and his colleagues have found evidence of glowing hydrogen gas, something never before seen on the moon first discovered in 1610 by Galileo Galilei.

The discovery was a surprise, said Roesler, a UW–Madison professor of physics who reported his findings at a meeting of the American Astronomical Society.

“It adds to the mystery. What is the hydrogen’s origin? Why is it glowing? We’re not sure,” said Roesler who’s studied Io, Jupiter’s third largest and closest Galilean moon, for many years.

Roesler said the new observations may indicate that Io’s poles are swathed in a frost of molecular hydrogen sulfide, a toxic gas that requires temperatures on the order of minus 130 F. to freeze. Or, said Roesler, there might be other hydrogen-bearing frosts concentrated at Io’s poles.

Alternatively, the glowing hydrogen gas seen by Roesler in the Jovian moon’s atmosphere may be the result of a large electrical current flowing between Jupiter and Io where hydrogen atoms are propelled to the moon from the hydrogen-rich Jovian atmosphere.

In any case, the discovery of hydrogen in Io’s atmosphere adds another unusual feature to a moon chock-full of strange phenomena, including lakes of liquid sulfur, active volcanoes with plumes 200 miles high, and fantastic extremes of surface temperature.

The observations were made last October using one of Hubble’s new instruments, the Space Telescope Imaging Spectrograph.

Although resolution of Io’s latest mystery will most likely require further observations, scientists’ best guess at the moment is that hydrogen is somehow concentrated at the poles in the form of hydrogen sulfide frost which evaporates off the surface. The hydrogen sulfide molecules, Roesler explained, may then be broken up into their constituent hydrogen and sulfur atoms with the hydrogen atoms made to glow by ultraviolet light from the sun.

The discovery, according to Roesler, is of further interest to scientists because the hydrogen glow seen by Hubble seems to have no relationship to glowing belts of oxygen and sulfur atoms observed in the equatorial regions of the Jovian moon.

“It behaves completely differently from the oxygen and sulfur glows,” said Roesler, explaining that finding new features of Io, while initially complicating scientists’ picture of the moon, may ultimately help them untangle its mysteries.

To make the new discovery Roesler and his colleagues, including UW–Madison physics Professor Frank Scherb, Ron Oliversen of NASA’s Goddard Space Flight Center in Greenbelt, Md., and Warren Moos of Johns Hopkins University needed to cut through an obscuring corona of glowing hydrogen that surrounds the Earth.

The new Space Telescope Imaging Spectrograph, installed last February during NASA’s last servicing mission to the orbiting telescope, is able to sort through the soup of chemical compounds in space and in planetary atmospheres by its ability to localize and identify different atomic species through the characteristic signatures they emit in different ultraviolet wavelengths.