Cassini provides further evidence of underground ocean on Enceladus
Sen—The international Cassini spacecraft has found further evidence of a large underground ocean on Saturn's icy moon Enceladus. The new data, from Cassini’s radio science experiment, provides the first geophysical measurements of the internal structure of Enceladus, consistent with the existence of a hidden ocean inside the moon.
Cassini first discovered plumes of ice and water vapour jetting from its "tiger stripes"—fractures at the moon's south polar region—in 2005. Subsequent observations of the jets showed them to be relatively warm compared with other regions of the moon and to be salty—strong arguments for there being liquid water below the surface.
On three separate occasions in 2010 and 2012, the spacecraft passed within 100 km of Enceladus, twice over the southern hemisphere and once over the northern hemisphere. During the flybys, Cassini was pulled slightly off course by the moon’s gravity, changing its velocity by just 0.2 to 0.3 millimetres per second.
Sami Asmar of NASA's Jet Propulsion Laboratory (JPL) a co-author of the new research paper explained "As the spacecraft flies by Enceladus, its velocity is perturbed by an amount that depends on variations in the gravity field that we're trying to measure. We see the change in velocity as a change in radio frequency, received at our ground stations here all the way across the solar system."
Artist’s impression of the possible interior of Enceladus based on Cassini’s gravity investigation. Image credit: NASA/JPL-Caltech.
The technique of analyzing a radio signal between Cassini and the Deep Space Network back on Earth can detect changes in velocity as small as less than one foot per hour (90 microns per second). The gravity measurements suggest a large, possibly regional, ocean about 6 miles (10 kilometres) deep, beneath an ice shell about 19 to 25 miles (30 to 40 kilometres) thick.
The south pole area has a surface depression that causes a dip in the local tug of gravity. However, the magnitude of the dip is less than expected given the size of the depression, leading researchers to conclude the depression's effect is partially offset by a high-density feature in the region, beneath the surface.
While the gravity data cannot rule out a global ocean, a regional sea extending from the south pole to 50ºS latitude is most consistent with the moon’s topography and high local temperatures observed around the tiger stripes.
"The Cassini gravity measurements show a negative gravity anomaly at the south pole that however is not as large as expected from the deep depression detected by the onboard camera," said the paper's lead author, Luciano Iess of Sapienza University of Rome. "Hence the conclusion that there must be a denser material at depth that compensates the missing mass: very likely liquid water, which is seven percent denser than ice. The magnitude of the anomaly gave us the size of the water reservoir."
"Material from Enceladus’ south polar jets contains salty water and organic molecules, the basic chemical ingredients for life," said Linda Spilker, Cassini's project scientist at JPL. "Their discovery expanded our view of the 'habitable zone' within our solar system and in planetary systems of other stars. This new validation that an ocean of water underlies the jets furthers understanding about this intriguing environment."