New research using highly precise measurements from NASA's Cassini spacecraft has concluded that Saturn's biggest moon is likely hiding a global water ocean 100 kilometers beneath its surface.
NASA's Cassini spacecraft has flown by Saturn's Titan moon more than 80 times since entering Saturn's orbit in 2004. Titan is bigger than the planet Mercury, and is the only moon with a real atmosphere that is even denser than the Earth's. It has Earth-like weather such as rain and snow, as well as geological features such as valleys, deserts, and plains, and it is the only object except Earth to experience standing bodies of liquid.
There are many observations of this kind, although they barely scratch the surface of one of the most mysterious and intriguing bodies in our solar system. "In contrast, information on the moon's deep interior is scarce," planetary scientist Luciano Iess wrote in the latest issue of Science.
There are no geologists on Titan's surface to measure its seismic waves, and without a detectable internally generated magnetic field, all of the known information regarding Titan's interior has resulted from careful analysis of its orbit, gravity, rotation, and topography. Rigorous analysis, however, can reveal exciting things.
Titan travels around Saturn in an elliptical orbit. It experiences the greatest gravitational pull as it approaches the pericenter, its closest point in orbit, and the least gravitational pull at the epicenter, the farthest point in orbit. Tides rise from these variations, squeezing the moon's surface and causing it to flex. In turn, this tidal flexing distorts Titan's gravitational field, affecting the speed at which Cassini can approach and recede from the moon as it flies by. Titan's changing gravitational field also affected Cassini's velocity, measured by the spacecraft's onboard equipment.
As the moon's interior is less dense, its surface flexes more throughout its orbit, resulting in greater distortions in the moon's gravitational field. Titan makes a full trip around Saturn in just 16 days, which allowed Iess and colleagues to use Cassini's velocity over the course of six different instances when it flew by Titan. The researchers used this velocity to estimate the changes in Titan's shape throughout its orbit, and calculated that if it were composed entirely by rock, Titan would have bulges in its surface measuring up to one meter high. In an interesting twist, Cassini's measurements indicate that Titan's bulges are actually ten times that height.
The researchers combined these findings with data from previous research, concluding that Titan's interior likely hides a sub-surface global ocean located beneath an icy shell. "Cassini's detection of large tides on Titan leads to the almost inescapable conclusion that there is a hidden ocean at depth," said Iess.
The fact that the hidden ocean's waters lie beneath a sheet of ice, however, does not favor life, as most experts agree that life is most likely to spring where water comes in contact with rock. On the other hand, the models Iess and his colleagues used cannot tell whether the floor of the moon's sub-surface ocean is made up of ice or rock, which means life is not completely out of the question. At this point, planetary scientists are mainly interested in the ocean's role in maintaining Titan's diminishing atmosphere.
"The presence of a liquid water layer in Titan is important because we want to understand how methane is stored in Titan's interior and how it may outgas to the surface," said Cassini team member Jonathan Lunine. "This is important because everything that is unique about Titan derives from the presence of abundant methane, yet the methane in the atmosphere is unstable, and will be destroyed on geologically short timescales."
