Hubble measures deuterium on Jupiter.

According to the Big Bang theory of cosmology, only the three lightest elements-hydrogen (along with its much rarer isotope, deuterium), helium, and lithium-were forged in the first few seconds after the birth of the cosmos.

Deuterium plays a special role because the amount generated in the Big Bang traces a key quantity in cosmology-the density of ordinary matter, such as protons and neutrons, present when the universe began.

Because stars burn deuterium into helium, the abundance of this isotope dwindles over time. To measure the original allotment, astronomers must use their telescopes like time machines, peering back to the era when the universe was only about 1 billion years old. Determining exactly how much deuterium has been depleted since then is also useful: It indicates the rate at which stars lived and died in different parts of the cosmos.

Several teams of astronomers have measured the abundance of deuterium early in the universe, but their numbers don't agree (SN: 5/18/96, p. 309). In contrast, a newly reported determination of deuterium abundance on Jupiter matches the value measured by a probe that parachuted into the planet late last year.

Lotfi Ben Jaffel and Alfred Vidal-Madjar of the Institute of Astrophysics of Paris and their colleagues used the Hubble Space Telescope's Goddard high-resolution spectrograph to measure the ratio of deuterium to hydrogen in Jupiter's atmosphere. Although hydrogen atoms far outnumber deuterium, Jaffel and his collaborators succeeded in detecting the faint deuterium emission by directing the spectrograph to look along the limb of the planet. From this vantage, the instrument looks through a greater depth of atmosphere, maximizing the number of deuterium atoms it sees.

Last month, at a conference on the spectrograph held at NASA's Goddard Space Flight Center in Greenbelt, Md., the group reported that Jupiter has about 6 deuterium atoms for every 10,000 hydrogen atoms. This number agrees with the value measured by a device, carried on the Galileo spacecraft, that plunged into Jupiter on Dec. 7.

Jupiter and the rest of the solar system became isolated from the surrounding interstellar gas and dust about 4.5 billion years ago, when the sun was born.

Although stars have continued to burn deuterium since then, no such depletion should have occurred on Jupiter. In addition, Jupiter's gravity prevents hydrogen and deuterium from escaping. For these reasons, the planet's current abundance should represent the amount the planet had at the birth of the solar system.

"This is a good example of what I like to call cosmic archaeology," says Tobias C. Owen of the University of Hawaii in Honolulu. "The deuterium-to-hydrogen ratio is like an ancient artifact that we have unearthed by investigating a tomb-Jupiter-that has been undisturbed for 4.5 billion years."

Although Jupiter's deuterium abundance now seems secure, measurements of the isotope elsewhere in the universe remain inconclusive. In the Milky Way, astronomers have measured deuterium abundance only in nearby stars, those within about 160 light-years of Earth, notes George Sonneborn of NASA's Goddard Space Flight Center in Greenbelt.

Most spectrographs now in operation record only near- and mid-ultraviolet light, and they can't detect the weak deuterium emission from distant stars. A spectrograph now scheduled for launch in late 1998 can scan the sky in the far ultraviolet, where deuterium emission is stronger. The instrument, known as FUSE (Far Ultraviolet Spectroscopic Explorer), should enable astronomers to measure deuterium in some of the oldest and most distant reaches of the galaxy, Sonneborn says.