Europa’s geysers play hard-to-see

SAN FRANCISCO — If Europa is venting its watery interior into space, it’s doing so stealthily. Follow-up observations of Jupiter’s icy moon failed to confirm the existence of powerful geysers spotted with the Hubble Space Telescope last year, researchers reported December 19 at a meeting of the American Geophysical Union. The fruitless search, when combined with other recent results, suggests that Europa’s plumes erupt weakly or sporadically, if at all.

The stakes are high for indisputably observing plumes on Europa because the moon’s surface ice shell conceals a vast ocean that represents one of the most potentially habitable extraterrestrial environments in the solar system. Geysers would make the contents of that ocean, including water, minerals and perhaps even life, accessible to future spacecraft. “We’re holding our breath to see if and when they get another observation,” says Cynthia Phillips, a planetary scientist at the SETI Institute in Mountain View, Calif. “Until then, the geysers are in the intriguing but unproven category.”

The first sign of geyser activity emerged in 2013 when scientists analyzing December 2012 Hubble data detected ultraviolet radiation at the telltale frequencies emitted by hydrogen and oxygen atoms above Europa’s southern hemisphere. Space scientist Lorenz Roth from the Southwest Research Institute in San Antonio and colleagues concluded that the moon was venting water into space (SN: 1/25/14, p. 6).

The discovery opened up the tantalizing possibility that Jupiter’s most intriguing moon routinely expels samples of its shielded ocean (SN: 5/17/14, p. 20). Researchers were hopeful that follow-up observations would reveal active geysers similar to the plumes on Saturn’s moon Enceladus, which have been erupting continuously since they were discovered by NASA’s Saturn-orbiting Cassini probe in 2005.

Yet nature has not made things so simple, Roth says. Multiple Hubble measurements made in January, February, November and December 2014 showed no signs of water surrounding Europa, he reported at the meeting. Some of those observations took place when the moon was at the same spot in its orbit as it was during the original detection.

Other evidence also casts doubt on the idea that Europa consistently propels water into space. A reanalysis of data obtained by Cassini as it whizzed by the Jupiter system en route to Saturn in 2001 revealed no signs of geysers, researchers reported at the meeting December 18. The findings also appear in a recent paper in the Astrophysical Journal. Study coauthor Amanda Hendrix, who works in Los Angeles for the Tucson-based Planetary Science Institute, says Cassini’s ultraviolet sensor detected fewer neutral gas molecules around the moon than would be expected if Europa were venting water.

Despite the disappointing recent results, many scientists at the meeting expressed confidence in the one-off Hubble detection. “I think there was definitely something going on there,” Hendrix says. She says that the typical output of Europa’s plumes may be too feeble to detect from faraway instruments such as Hubble and Cassini. Phillips speculates that Europa’s geysers may resemble the volcanoes on Jupiter’s moon Io, which erupt intensely but sporadically, more than the consistent plumes of Enceladus.

Roth and his team plan to make about 45 follow-up observations with Hubble in 2015 in search of the elusive geysers. In the meantime, Europa enthusiasts are wasting no time considering the implications of the Roth team’s initial finding. NASA is organizing a February meeting to discuss whether the proposed Europa Clipper spacecraft could include an instrument that would scan the debris from potential geysers for signs of life. “People are still very, very excited about this,” Phillips says. “They’re taking it very seriously.”