Tidal flexure detected in Titan's surface suggests that a global ocean must lie beneath this moon's icy crust.
Last week a team of scientists, led by Luciano Iess (Sapienza University, Italy), presented evidence that a global ocean lies beneath the icy crust of the Saturnian moon Titan. They didn't detect all that water directly — it's deep down and completely hidden — but instead they inferred its existence by noting how much Titan's crust flexed as it orbits Saturn.
Well, to be honest, they didn't actually see the crust heave up and down either.
Instead, Iess and his collaborators determined something called the Love number, abbreviated k2, and it's about 0.6. This is a measure of how rigid Titan is and, in particular, how much its shape changes in response to Saturn's gravity. Careful tracking of Cassini's position on six close flybys of Titan allowed the researchers to map the moon's gravity field and, from that, to deduce its shape at various points of its 16-day-long orbit.
With a diameter of 3,200 miles (5,150 km), Titan is a world slightly bigger than Mercury. But it shares some orbital similarities with a smaller, more familiar object in our own sky. Like the Moon, Titan is gravitationally locked such that one hemisphere faces Saturn all the time and, ditto, it's in a slightly eccentric orbit. Consequently, from Saturn's perspective Titan appears to nod (librate) a little bit from side to side as it orbits, just as the Moon does as it circles Earth.
But, unlike our Moon, its icy surface bulges outward by as much as 30 feet (10 m) — more so when Titan is relatively close to Saturn, and less when it's farther away. This NASA animation shows what's going on.
If Titan were completely rigid inside and out, the Love number would be zero. So the giant moon must be a bit "squishy" inside, and the only reasonable explanation is an ocean of liquid water, anywhere from 30 to 60 miles down, that lets the icy crust deform more easily. "These high values of k2 definitely tell us that Titan shelters a deep ocean," notes Julie Castillo-Rogez, an icy-body specialist at the Jet Propulsion Laboratory. "I cannot think of any other explanation."
The finding by Iess's team seems to confirm what planetary scientists have long suspected. Some two decades ago Jonathan Lunine (now at Cornell University) and David Stevenson (Caltech) predicted that Titan must have a subsurface ocean on purely compositional grounds — but the idea was suggested even earlier. "I always give John Lewis and his students some credit for this," Lunine explains. "In the 1970s he talked about ammonia being present in the interior producing melts within the ice layer."
Titan joins the icy Jovian moons Europa, Ganymede, and Callisto as having subterranean sloshing. The report by Iess's team, was published online by Science, and NASA's press release offers a summary of the results.
What makes Cassini's observations so remarkable is how they're even possible. If you look at the list of instruments aboard the spacecraft (or those on any interplanetary spacecraft, for that matter), down near the bottom you'll invariably see "radio science". The instrument, in this case, is simply the radio transmitter itself.
Cassini has flown past Titan several dozen times since its arrival in 2004, but on six of those occasions from 2006 to 2011 all measurements were put on hold for about 48 hours so that a pure carrier tone could be transmitted to Earth. When Titan's gravity causes Cassini to speed up or slow down, subtle Doppler shifts appear in the signal's frequency. From these engineers can deduce incredibly tiny changes in the spacecraft's line-of-sight velocity — as small as 75 microns per second (0.0002 mile per hour).
That was not a misprint.
Consequently, just by passing by at close range, the spacecraft itself becomes a very sensitive probe of Titan's gravity field. The tiny velocity shifts have already been used by Iess and others to derive Titan's mass, density, and its moment of inertia (how mass is distributed throughout the interior); they published those results two years ago. And now they've got the final piece of the gravitational puzzle, k2.
What's still not known is what lies beneath the subsurface ocean. Most speculations envision a either a core composed of hydrated rock overlain by a "mantle" of high-density water ice, or a mishmash of rock and ice that never segregated into discrete layers.