June 24, 2009
One of Saturn’s moons, Enceladus, appears to be missing some sodium. The new observations made at the W. M. Keck Observatory suggest that the plumes of gas and ice seen exploding from the moon are not fueled by geysers erupting from a salty ocean just beneath Enceladus’ surface. The conclusion has left scientists uncertain about the tiny moon’s interior.
“We really expected to see sodium in the data giving evidence right then and there of a possible ocean on Enceladus,” said astronomer Mike Brown of the California Institute of Technology. To the astronomers’ surprise, no spectral feature appeared in the data.
The team of international astronomers, led by Nick Schneider of the University of Colorado, used the High Resolution Echelle Spectrometer, or HIRES, at Keck Observatory to analyze the sunlight reflected by Enceladus. The calculated upper limit of sodium present in the moon’s vapor plume was 30 times lower than expected, the astronomers report in the June 25Nature.
Observing little to no sodium in the plume vapor and ice grains erupting from the southern pole of Enceladus is “quite surprising” because other, less active bodies in the Solar System have sodium signatures that are much easier to distinguish, said planetary scientist John Spencer of the Southwest Research Institute in Boulder, Co., who was not involved in the study.
Sodium atoms are among the most easily detected and most common elements in the Solar System. Their non-existence in the Enceladus’ jets suggests the moon is very different from Jupiter’s moons Io and Europa, the Moon and Mercury, which all show sodium signatures in their thin atmospheres, Brown added.
The vapor and ice particles found in the jets are also thought to be the source of Saturn’s outermost ‘E’ ring. The jets, and salt in the ring, could therefore be coming from a much deeper, salt-based ocean. Brown said that deep caverns might allow water to evaporate slowly, which would mean it would contain little sodium—much like water evaporating from Earth’s oceans. The vapor could turn into a jet because it would leak out of fractures in the moon’s icy crust and then into the vacuum of space.
The scenario is possible, but the new observations could also mean that the low-salt vapor escaping from Enceladus comes from evaporation of a low-salt, liquid reservoir or from warm ice sublimation—the transition from solid to gas with no liquid phase, Brown explained. The plume could also originate from the decomposition of molecular cages called clathrates, which might release small amounts of sodium.
“In essence, we still don’t understand the origin of the ice and vapor jets and therefore how the salt particles get into space,” he added.
Enceladus is a “very different beast,” and theorists need to do more work to understand how vapor and ice grains form inside Enceladus and then get out to the ‘E’ ring, Spencer said.
“Basically, this result shows we don’t really know what’s going on within this little moon. It also shows us that ground-based telescopes, such as Keck, are essential to Solar System studies,” Brown said. “Now we astronomers have to put our heads together to figure out how we are going to learn more about Enceladus.”
The W. M. Keck Observatory operates twin 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i. The two telescopes feature a suite of advanced instrumentation including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and a world-leading laser-guide-star adaptive optics system. The Observatory is a scientific partnership of the California Institute of Technology, the University of California and NASA. For more information please call 808.881.3827 or visit http://www.keckobservatory.org.