Researchers hypothesized the presence of an internal water reservoir in 2005 when Cassini discovered water vapor and ice spills coming out of "vents" near Enceladus' south pole. Gravity data collected on Cassini's flybys of Enceladus now allow us to confirm this
The Cassini spacecraft orbiting the planet Saturn and a network of radio telescopes operated by NASA to maintain contact with spacecraft at the edge of the solar system (the Deep Space Network) have revealed evidence of the existence of a large underground ocean of liquid water beneath the icy surface of Saturn's moon Enceladus. . NASA estimates that, at least in terms of the conditions, bacteria could have developed there.
Researchers hypothesized the presence of an internal water reservoir in 2005 when Cassini discovered water vapor and ice spills coming out of "vents" near the moon's south pole. First data from geophysical measurements of the internal structure of Enceladus are consistent with the existence of a hidden ocean inside the moon. The findings of the gravity measurements were published in the April 4 edition of the journal Science.
"The way we measure changes in the gravitational field is through the Doppler effect, the same principle used in a radar gun to measure speed," says Sammy Esmer of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, and a co-author of the paper. "When a spacecraft flies by Enceladus, its speed is affected by a summation that depends on the changes in the gravitational field that we are trying to measure. We see the change in speed as a change in radio frequency that we receive at our ground stations on Earth as they cross the solar system."
Gravity measurements suggest the existence of an ocean at a depth of about ten kilometers, under an ice shell 30-40 kilometers thick. Evidence of a subsurface ocean makes Enceladus one of the most likely places in the solar system to host microbial life. Before Cassini arrived at Saturn in July 2004, no one included this icy moon, barely 500 km in diameter, on such a list.
The ocean provides one possible explanation for why water breaks out of the cracks we see at the South Pole," said David Stevenson of the California Institute of Technology in Pasadena, one of the authors of the paper.
The Cassini spacecraft flew by Enceladus 19 times. Three approach flights, 2010-2012, yielded accurate track measurements. The gravitational pull of a planetary body, such as Enceladus, changes the spacecraft's flight path. Changes in the gravitational field, such as those caused by mountains on the surface or differences in composition underground, can manifest as changes in the speed of the spacecraft, as measured from Earth.
The technique of analyzing the radio signals between Cassini and the Deep Space Network can detect small changes of less than 90 percent per hour (XNUMX microns per second)). With this precision, the flyby data yielded evidence of a higher density in the south polar region than other parts of the Moon.
In the South Pole region there is a surface depression that causes a decrease in the local strength of gravity. However, the magnitude of the decrease is smaller than expected given the size of the depression, leading researchers to conclude that the impact of the depression was partially offset by a high-density feature in the region, below the surface.
"The gravity measurements carried out by Cassini show a negative gravity anomaly at the south pole which is not as large as we expected from this depression, as detected by the integrated camera" said the lead author of the article Luciano Les from the University of Rome. Hence the conclusion that there must be denser material deep in the compensating moon on the missing mass. It is likely that this is very liquid water that is 7% denser than the ice density and the intensity of the anomaly gave us the size of the water reservoir. "
There is no certainty that the subsurface ocean supplies the plume of water that is regularly ejected to the area near the southern cuvette of Enceladus. If so, that could be a good explanation. The fragments may lead to a part of the moon heated by tidal waves resulting from Enceladus' eccentric orbit around Saturn.
"Much of the excitement over the discovery of the Enceladus water plume stems from the possibility that a wet environment could be a favorable environment for microbial life." Les said.
The material in these jets contains salty water and organic molecules, the basic chemical substances for every life form" says Linda Spilker, the chief scientist of the Cassini project at JPL. The discovery expands our perspective on the "life zone" within our solar system and in the solar systems of other stars. This new verification of the origin of the jets in the liquid water ocean advances our understanding of this intriguing environment."