Enceladus is a small, lovely, and bewitching icy moon of the beautiful ringed planet Saturn–an enormous world that circles our Sun in the cold and relatively dark outer domain of the four giant planets. Saturn has 62 known moons, and most of them are small dancing moonlets of ice. However, Enceladus is a mid-sized moon, about 500 kilometers in diameter. For several years, planetary scientists have believed that Enceladus contains a subsurface reservoir of liquid water–perhaps even a global ocean–hidden deep beneath its frozen crust. In April 2014, a team of planetary scientists using NASA’s Cassini spacecraft and Deep Space Network announced that they had uncovered further evidence that Enceladus harbors a large, secret ocean of liquid water beneath its thick icy coating–furthering scientific interest in the moon as a possible abode for extraterrestrial microbes.
Planetary scientists considered the possibility of an interior reservoir of life-loving liquid water on Enceladus back in 2005, when Cassini detected water vapor and ice shooting out from vents–the so-called tiger stripes–near the moon’s south pole. The new observations provide the very first geophysical measurements of the internal structure of Enceladus, consistent with the existence of a hidden ocean deep inside the moon’s secretive heart. Where there is liquid water, there is the possibility–though not the promise–of life as we know it to exist. Findings describing the study’s gravity measurements are published in the April 4, 2014 issue 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. 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,” noted Mr. Sami Asmar in an April 3, 2014 NASA Jet Propulsion Laboratory (JPL) Press Release. Mr. Asmar, a co-author of the study, is of the JPL located in Pasadena, California.
The discovery of a large interior ocean sloshing around beneath Enceladus’s frigid, icy coating hoists it up into the highest tier of candidate life-bearing bodies in our Solar System–along with Europa of Jupiter, another ice-coated moon that also contains a secret subterranean ocean. Both of these icy, small moons demand much closer study, planetary scientists suggest.
“I don’t know which of the two is going to be more likely to have life. It might be both; it could be neither. I think what this discovery tells us is that we just need to be more aggressive in getting the next generation of spacecraft both to Europa and to the Saturn system once the Cassini mission is over,” commented study co-author Dr. Jonathan Lunine in the April 4, 2014 Space.com.
The Saturn System
Saturn is the smaller of the two gas-giant planets inhabiting the outer domain of our Solar System–Jupiter is the largest. With its stunning system of magnificent rings, bright moons of ice, and a host of dancing, tumbling, icy moonlets, Saturn is arguably the most beautiful denizen of our Sun’s lovely family. Until 2004, no spacecraft had visited Saturn in about a generation: Pioneer II had taken the very first long journey to the ringed-planet back in 1979, when it flew past it, snapping the first close-up and personal pictures of this strange, giant world. Voyager 1 had its own close encounter about a year later, and in August 1981 Voyager 2 had its own productive–albeit brief–visit.
At last, on July 1, 2004, NASA’s Cassini spacecraft whizzed into Saturn orbit, and started to snap some very revealing pictures.
Cassini is currently doomed to meet its fate in a dramatic grand finale in September 2017, when it is destined to plummet headlong down into Saturn’s thick, gaseous atmosphere.
Enceladus sports the highest albedo of any moon dwelling in our Solar System–beautifully showing off a dazzling white surface coating of highly reflective ice. It also possesses an extremely active geology, which has rendered its surface almost completely free of craters. This is because Enceladus is continually being resurfaced by the gushing eruptions of ice from its numerous geysers, which are the source of bright, fresh snow that keeps the surface of this enchanting, distant moon sparkling and smooth.
The geyser eruptions on Enceladus are apparently the result of its close proximity to its enormous parent planet, Saturn, according to information obtained from the Cassini spacecraft. The geyser-jets emanating from the surface of Enceladus have been likened to adjustable garden hose nozzles. The nozzles are most open when the moon is farthest away from Saturn, and almost closed when it is nearer to it. This is believed to be caused by the way Saturn squeezes and then releases the icy moon with its enormous gravity.
Cassini detected the jets spewing from a very bright plume back in 2005. The water ice is laced with organic particles and shower out from several thin fissures that have been playfully nicknamed tiger stripes by planetary scientists who noticed an obvious similarity between the fissures tearing through the south pole of the moon, and the stripes that characterize a tiger’s coat. The warm vents that are responsible for the tiger stripes shoot out enormous plumes of water vapor, ice, and organic particles hundreds of miles into interplanetary space. This offers scientists a way to observe the subsurface ocean from a great distance.
An Ocean For Enceladus
The gravity measurements reported in the new study indicate a large–and possibly regional ocean–approximately 10 kilometers (6 miles) deep, secreted beneath Enceladus’s frosty, bright, icy shell that is about 30 to 40 kilometers (19 to 25 miles) thick. Before Cassini reached the Saturn system in July 2004, planetary scientists did not think that Enceladus would be one of the most likely worlds in our Solar System to host living tidbits.
“This then provides one possible story to explain why water is gushing out of these fractures we see at the south pole,” noted study co-author Dr. David Stevenson in the April 3, 2014 JPL Press Release. Dr. Stevenson is of the California Institute of Technology (Caltech) in Pasadena, California.
Cassini has now whizzed by Enceladus 19 times. A trio of close flybys, dating from 2010 to 2012, provided some very precise trajectory measurements. The gravitational tug of a body, such as Enceladus, changes a passing spacecraft’s flight path. Alterations in the gravity field, such as those that are the result of variations in underground composition or mountains on the surface, can be observed as alterations in the spacecraft’s velocity, as measured from Earth.
The technique for analyzing a radio signal between Cassini and the Deep Space Network can spot alterations in velocity as tiny as less than one foot per hour. With this kind of high precision, the information derived from the flyby provided evidence of a region secreted inside the southern end of the moon that sported a higher density than other portions of the interior.
The Deep Space Network (DSN) is NASA’s international array of giant radio antennas that supports interplanetary space missions, such as Cassini, as well as a few that orbit Earth. The DSN, operated by the JPL, provides radar and radio astronomy observations that improve scientific understanding of our Solar System and the Universe.
The south pole region of Enceladus shows a surface depression that causes a dip in the local pull of gravity. However, the magnitude of this dip is not as great as predicted when taking into account the size of the depression. This finding suggests to the planetary scientists that the depression’s influence is partially offset by a high-density feature in this area, which is hidden beneath the surface.
“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. Hence the conclusion that there must be a denser material at depth that compensates the missing mass: very like liquid water, which is seven percent denser than ice. The magnitude of the anomaly gave us the size of the water reservoir,” explained the paper’s lead author, Dr. Luciano Iess, in the April 3, 2014 JPL Press Release. Dr. Iess is of Sapienza University in Rome, Italy.
However, it is still uncertain that the subsurface ocean is the source of the water plume shooting out of surface fractures close to the south pole of Enceladus– although the team of planetary scientists thinks that this is a real possibility. The slender fissures may lead down to a hidden part of the moon that is warmed tidally by its repeated flexing, as it follows an eccentric orbit around its parent planet.
A great deal of the excitement about the Cassini mission’s discovery of the water plume on Enceladus stems from the possibility that it originates from a watery environment that might be friendly to the evolution of microbial life.
“Material from Enceladus’ south polar jets contains salty water and organic molecules, the basic chemical ingredients for life. 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,” explained Dr. Linda Spilker in the April 3, 2014 JPL Press Release. Dr. Spilker is Cassini’s project scientist at JPL.
The habitable zone surrounding a star is that “Goldilocks” region where it is not too hot, not too cold, but just right for liquid water to exist in its life-friendly liquid state.
Interestingly, planetary scientists announced in December 2013 that they had spotted plumes of water vapor shooting from Europa’s south polar region as well. Therefore, that icy Jovian moon’s subsurface ocean could also be sampled during flybys, too–perhaps by a future mission called the Europa Clipper.
Currently, NASA is developing the Europa Clipper as a concept mission. Recent estimates forecast the mission’s cost at approximately $2 billion. This is rather expensive during these hard-hit economic times, and so a scaled-down version might have the best chance of materializing, NASA officials have said.
But Enceladus and Europa aren’t the only icy moons in our Solar System hiding subsurface oceans of precious liquid water. Jupiter’s giant moon Ganymede–the largest moon in our Sun’s family–also has one. But Ganymede’s ocean is stuck between layers of ice, while the seas of Enceladus and Europa are in contact with rocky ocean floors. This makes possible an enticing array of fascinating chemical reactions, the researchers say.