Underground Sea Detected On Icy Enceladus
Saturn’s icy moon, Enceladus, is one of the brightest objects in our solar system. Covered in water ice, the tiny (barely more than 300 miles in diameter) moon reflects almost 100% of the sunlight which strikes it. Because of this, Enceladus’ surface temperature is an extremely frigid -201° C (-330° F). Until fairly recently the icy satellite was thought to be cold and dead. (Image Credit: NASA)
In 2005 NASA’s Cassini Spacecraft discovered water vapor and ice spewing from vents near Enceladus’ south pole. Researchers at the time theorized the presence of an interior reservoir of water and subsequent observations of the jets showed them to be relatively warm, compared with other regions of the moon, and to be salty. All solid arguments for there being liquid water below the surface, and with this the possibility of extraterrestrial microbial life.
Taken by Cassini in November 2009 as the spacecraft flew past Enceladus and through the jets, the above image shows dramatic plumes, both large and small, spraying icy particles from many locations along the south pole of the moon. More than 30 individual jets of different sizes can be seen. (Image Credit: NASA/JPL/Space Science Institute)
On yesterday, April 3, 2014, researchers at NASA’s Jet Propulsion Laboratory (JPL) announced that new data gathered from Cassini and NASA’s Deep Space Network providing the first geophysical measurements of the internal structure of Enceladus is consistent with the existence of a hidden ocean inside the moon. The new findings from the gravity measurements are published in today’s (Friday, April 4) edition of the journal Science.
“The way we deduce gravity variations is a concept in physics called the Doppler Effect, the same principle used with a speed-measuring radar gun,” Sami Asmar of JPL, a coauthor of the paper in Science, said in a news release from JPL. “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.”
Cassini has flown near Enceladus 19 times. Precise trajectory measurements were made during three flybys, from 2010 to 2012. The gravitational tug of a planetary body alters a spacecraft’s flight path, and variations in the gravity field, such as those caused by mountains or differences in underground composition, can be detected as changes in the spacecraft’s velocity. The technique of analyzing a radio signal between Cassini and the Deep Space Network can detect changes in velocity as small as less than one foot per hour, or 90 microns per second. With this precision, the flyby data yielded evidence of a zone inside the southern end Enceladus with higher density than other portions of the interior.
Inside Enceladus: Illustration
“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,” the paper’s lead author, Luciano Iess of Sapienza University of Rome, said in the JPL news release. “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.”
The gravity measurements suggest a large, possibly regional, ocean about 6 miles deep covered by an ice shell about 19 to 25 miles thick. It’s not known for certain whether the subsurface ocean supplies the water plumes near the moon’s south pole, however scientists think it’s a real possibility. The fractures may lead down to a part of the moon that is tidally heated by the moon’s repeated flexing as it follows its eccentric orbit around Saturn.
Before Cassini reached Saturn in July 2004 there was no short-list which included this small icy world among the most likely places in our solar system to host microbial life. That’s changed. What else do we not know, and won’t know until we go? And what else can we, as a species, decide is a more important endeavor in which to spend a small fraction of our time and resources in discovering. If you agree, The Planetary Society is hosting a petition to reject the White House’s cut to Planetary Science at NASA. Signing up is quick and easy. And for non-US citizens, there’s also a form to email President Obama asking that he rejects the cuts and works towards keeping scientific research alive for the future of all humanity.