Posted by: Paul A. Forsyth | April 3, 2014

An Ocean on Enceladus

The Cassini spacecraft, in orbit around Saturn, has found evidence of an ocean of liquid water underneath the thick ice of the moon Enceladus.

In 2005, Cassini saw geysers of salty water spewing from the southern polar region of the moon. This evidence of subsurface liquid water led scientists to attempt calculations of the density of different regions of the Enceladus based on small variations in its gravitational pull on Cassini. From Ars Technica:

The data that allow us to understand Enceladus’ internal structure came from measuring changes in Cassini’s speed as it flew close to the moon. When passing the denser parts of the moon, it sped up by a few extra thousandths of a meter per second. That minute change was tracked through recordings of the radio signals that Cassini was sending to NASA’s Deep Space Network station.

The geysers of Saturn’s moon Enceladus are salty, which probably means that …
In making such tiny measurements, scientists had to filter out other factors that could influence Cassini’s speed. These include pressure on the spacecraft from sunlight, the nudge from heat radiating from its nuclear-powered electrical generator, and the drag of the particles it strikes as it passes through the south polar plumes.

[Dr. Luciano] Iess and his colleagues have produced a model of the internal structure of Enceladus using the measurements. They conclude that there is a core that is roughly 200km in diameter; above that lies a 10km-thick layer of liquid water, which is followed by 40km of ice crust.

Cassini detected the ice geysers (shown below) in 2005, but it took the next eight years to make the observations necessary to determine the density of Enceladus. These days, Cassini orbits Saturn about once every 7 days, but the orbiter has a lot of things to study — Saturn itself, the rings, some of Saturn’s other 60+ moons and numerous moonlets — and can only devote so much time to flybys of Enceladus. Also, apparently, when Cassini is making gravitational measurements, “it needs to point its antenna towards Earth, but in doing so all its other instruments face away from Enceladus.” Cassini has made 19 flybys of Enceladus since 2004, only three of which were used to make gravitational measurements.

Enceladus geysers

Wired says that, on at least one other flyby, Cassini flew through one of the water jets to sample the chemical composition of the plumes, finding traces of carbon and nitrogen in the water. More from Wired:

NASA’s philosophy on finding life beyond Earth is “Follow the water,” so all these moons are potential places to explore. Given that it shoots free samples from the interior up into space, some scientists are now saying that Enceladus should get priority for future life-finding missions.

“Enceladus has the most accessible extraterrestrial habitable zone,” said planetary scientist Carolyn Porco, leader of Cassini’s imaging team … . “This place is really where we should be going.”

Cassini was launched in 1997. It’s plutonium power source will likely run out of juice in 2017 (the spacecraft does not have solar panels, since Saturn is so far from the Sun). Before complete loss of power, NASA plans to crash the orbiter into Saturn’s atmosphere, so as to avoid any potential later impact of the spacecraft on one of the moons (which conceivably could lead to biological contamination).

Maybe by then a follow-up mission will have been launched.

Cassini images and artwork courtesy JPL and NASA.

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