Deep, Dark Waters of Titan
Just when you thought it was safe to go back in the water, NASA finds another ocean for us to worry about, this time on Saturn's moon, Titan.
Funny thing about oceans in the solar system, other than Earth's deep waters, they seem to be popping up where we least expect them: in the outer solar system. Should we change our preconceptions and start expecting them there?
Rewind to my childhood, when my fascination with space, and in particular the planets and moons of the solar system, was only tantalized by the distant, fuzzy images collected by Earth-based telescopes.
Looking through my small telescope, Jupiter's four Galilean moons were sparks of light, or at best, fuzzy disks showing blotches of light and dark and perhaps a little color taken "close up" by the Pioneer spacecraft. This left a lot to the imagination–which was actually part of the fascination. Who might have suspected, looking at the blurry whitish fuzz-patch of Europa, as seen by Pioneer, that there was an ocean hidden under the icy crust tens of miles deep and containing more liquid water than all of Earth's?
When the Galileo spacecraft eventually obtained images that alluded to the existence of Europa's oceans, scientists explained how heat generated through tidal flexing by Jupiter was the mechanism that kept the waters fluid. Voyager had already revealed that the moon Io experienced heating that powered its many active volcanoes.
And now there is evidence for another big ocean out there, on Saturn's moon Titan. Titan? Did you hear me right? That outer solar system moon whose atmosphere is so cold that ice is as strong as steel and methane—the stuff that spews out of your gas stove—is in a liquid state? That Titan?
From what the Cassini spacecraft has reported to us, the answer is very likely yes, buried deep under Titan's surface. You may ask, how has Cassini detected this hidden ocean's presence? Europa's ocean was inferred by the patterns of cracking in its icy crust, similar to cracks seen on Earth in floating sheets of sea ice. But Titan sports no cracks that we can see.
You probably know something about the tides in Earth's oceans caused by the gravitational pull of the moon. (Oh, does it seem like I just completely changed the subject? Read on, I'm getting there.)
The effect of the moon "stretching" the ocean's waters raises two liquid bulges and as Earth's surface rotates into and out of these bulges, we experience the rising and falling of the tides.
The moon actually raises Earth's waters by as much as two feet and in fact Earth's crust itself rises by nearly 20 inches! Earth is stretched oh-so-slightly into the shape of a football.
The Cassini spacecraft has measured similar tidal deformation in Titan's shape caused by the pull of Saturn's gravity. The extremely sensitive measurements were made during several close fly-bys of Titan over a five-year period. Based on the strength of Saturn's gravity, were Titan a more-or-less solid object, its tidal bulges shouldn't be a departure of more than about three feet. However, Cassini's measurements show a stretching on the order of 30 feet!
The inference is that there is a liquid layer beneath Titan's crust–and since the composition of outer solar system objects is heavy on water ice, probably liquid water.
Exactly how far below the surface and how deep this ocean is we don't know yet, and while the crust above it is likely quite icy, what surface is to be found at the ocean's floor—icy or rocky–also is unknown. We're a bit fuzzy on those details right now, but it leaves a lot to the imagination.