Gravity measurements made in six flybys of the moon by the international Cassini spacecraft show its 16 day-orbit of Saturn is marked by stretching and deformation of its crust. Cassini has explored Saturn and its moons since 2004, most dramatically parachuting a European probe onto its methane ice surface in 2005.
" Titan is highly deformable over time scales of days," concludes the Science journal discovery report led by Luciano Iess of Italy's Università La Sapienza in Rome. A subsurface ocean, likely water laced with ammonia and perhaps sulfur, lurking within about 60 miles of its surface best explains this deformation, suggests the study team.
The largest of Saturn's moons, Titan is about 3,200 miles across, making it one of the largest moons in the solar system, most remarkable for its thick orange atmosphere rich in methane and other organic chemicals, blowing over a frozen world chilled by -289 degree surface-temperatures.
"The bottom line is that - on its own - the Iess et al. result is not the "smoking gun" proving there is an ocean, but when you add it to all the other circumstantial evidence, it seems almost certain that an ocean is there," says planetary scientist Francis Nimmo of the University of California, Santa Cruz, who was not part of the study.
Nimmo commented by email on the overall impact of the finding:
"Titan's orbit round Saturn is not quite circular, so the distance between Saturn and Titan is constantly changing. As a result, the gravity force Titan feels from Saturn is also constantly changing, so Titan gets slightly squeezed and stretched every orbit. The Cassini spacecraft has now managed to detect this regular change in Titan's shape. Detecting this change is an enormous technical achievement, because Cassini is not in orbit around Titan - it just zooms past every so often, making measurements as it goes.
The amount by which Titan's shape changes depends on how rigid it is - a Titan made of (for example) diamond would be so strong that Saturn's gravity wouldn't affect it, and so its shape wouldn't change. The big surprise about the Iess et al. result is that Titan is weaker than anyone expected.
It could be that Titan is solid throughout, but just soft and squishy. But a much more likely explanation is that Titan has a liquid ocean below its icy surface. A liquid ocean allows the ice shell above to flex more vigorously in response to Saturn's gravity, so the presence of an ocean can easily explain the Iess et al. observation that Titan's shape changes so much.
Although they can't rule out a solid, squishy interior, Iess et al. favour the ocean explanation, and so do I. That is because there is a lot of other circumstantial evidence in favour of an ocean. No single piece of evidence absolutely requires an ocean, but the Iess et al. result adds one more piece to the jigsaw puzzle - and all the pieces are pointing towards the presence of an ocean.
An ocean is important because it tells us a lot about how Titan has cooled (or failed to cool) over billions of years. Because the ocean is sandwiched between two layers of ice, it is probably not a habitable environment. But it adds one more body to the growing list of icy moons which have ice-covered oceans, and reminds us that these ocean-bearing moons might be common in solar systems other than our own."
For more on the report, we separately asked study lead author Luciano Iess (LI) and a co-author, planetary scientist Jonathan Lunine (JL) of Cornell a few questions by email, about their results. Luckily enough, they agree on things -- here are their (lightly-edited) replies:
Q. The study concludes that "A simple water or ammonia-doped water ocean overlain by a thin (<100 km) shell is favored" by your results. How deep could it go?
LI: Our measurements tell nothing about the depth of the ocean. It can be 10 km, 100 km or more. The large tides tell us that the outer shell is highly deformable, and this can only occur if it is decoupled from the silicate core. Only a liquid layer can provide this decoupling.
JL: The technique is sensitive to the thickness of the upper (ice) lid, and not to the thickness of the ocean itself. So the crust atop the ocean should be less than, roughly 100 km in thickness, but we don't know how deep into the interior the bottom of the ocean extends.
Q. Would this ocean serve as a reservoir for methane or other organic atmospheric constituents on Titan?
LI: Yes. It makes much easier to explain why Titan's atmosphere is so rich in methane, a molecule that is dissociated very quickly. The methane in the thick atmosphere of Titan has to be replenished continuously.
JL: Yes, it could well do so. Other workers have been modeling this for a number of years, under the assumption that an ocean exists.
Q. Is tidal flexing alone enough of an energy source for the melting required to keep an ocean liquid, or is interior heat required?
LI: Tidal dissipation is not enough to explain the presence of a liquid layer. Very likely water exists at the liquid state because there is heat produced in the interior. If the thermal conductivity in the outer shell is small, an ocean may form.
JL: In principle, yes the flexing could maintain the liquid state. In practice, though, this would result in so much dissipation of orbital energy that Titan's orbit would have circularized rapidly. (There is no other moon large enough to maintain the eccentricity of Titan's orbit). Two other sources of energy--the original heating during assembly of Titan from smaller pieces, and the decay of radioactive elements in the rocky core--are actually sufficient to maintain the ocean in a liquid state.
Q. Do you see this as a long-term feature of Titan? Did Titan always have an interior ocean? (Might it ever have had a surface ocean, since it now has lakes, we asked Lunine)
LI: Probably yes, as Titan's interior was hotter in the past. However, the favored models of the interior, as emerging from gravity measurements, entail a core made up by hydrated silicates. These rocks cannot exists if the temperatures exceeds 800-1000 K, so the interior of Titan should have been always relatively cold.
JL: Yes, I do think it is persistent. In computer models of the evolution of Titan's interior, the moon has been cooling off from early times to the present, and so if a subsurface ocean exists today then it would have been a persistent feature--even bigger early on--throughout Titan's history. Indeed, as you note, very early on it might have extended all the way to the surface.
Of course, we know of hundreds of lakes and three large seas in the (primarily north) polar region of Titan--imaged by Cassini radar and the near-infrared camera-spectrometer. They are most likely mixtures of liquid hydrocarbons--ethane and methane--which are stable as liquids at the surface temperature of Titan. In the case of one such feature the ethane was directly detected by Cassini's spectrometer.
Q. What do you see as the most important part of your results?
LI: The search for water in the solar system has been always an important goal in planetary exploration. Water certainly does not imply life, but it is difficult to think to life without water. Titan is a very interesting and unique object in the solar system, probably the body most similar to the Earth. It was known that it hosts a thick atmosphere rich in methane, hydrocarbon lakes and a hydrological cycle (based on liquid hydrocarbons). We knew already that the outer icy shell was made mostly of water ice. Now we know that water is abundant also at the liquid state.
JL: We have detected the effect of Saturn's tides on Titan for the first time--and the results strongly suggest the presence of a subsurface liquid layer, which (because of its position and extent) must be mostly water. Titan joins the select club of icy moons with underground seas.
Jupiter's large moons, Europa and Ganymede, are thought to also possess sub-surface oceans.