Friday, April 1, 2011

Drifting Crust

Titan is the largest moon of Saturn, the only moon that is known to have a dense atmosphere and the only known object in the Solar System other than Earth with stable bodies of surface liquid. With a diameter of 5150 kilometres, Titan is the second-largest moon in the Solar System as it is slightly smaller than Jupiter’s moon Ganymede. However, when placed together with Ganymede, Titan will actually appear larger because Titan’s dense and opaque atmosphere extends many kilometres above its surface and increases its apparent diameter. NASA’s Cassini spacecraft is currently in orbit around Saturn and it frequently makes flybys of Titan.

On Titan, the average surface temperature is roughly minus 180 degrees Centigrade and the surface atmospheric pressure is 1.45 times the atmospheric pressure at sea-level on Earth. For every square meter of Titan’s surface area, the overlying atmosphere 7.3 times more massive in comparison to the Earth’s. The surface gravity of Titan is one-seventh the surface gravity of the Earth such that when combined with the dense atmosphere, Titan’s gravity is sufficiently low to allow humans to consider flying through the atmosphere on their own strength by flapping artificial wings strapped to their arms!

Beneath an icy crust that has a thickness of perhaps a hundred kilometres or so, Titan is believe to have a global subsurface ocean of liquid water. The presence of a subsurface ocean dynamically decouples the crust of Titan from its much more massive interior bulk, thereby lowering the effective moment of inertia of the moon’s crust. This allows the global circulation of air within Titan’s thick atmosphere to drag and torque the entire crust around such that the crust does not rotate at exactly the same rate as the rest of Titan.

Like the other large moons of Saturn, Titan’s rotation is synchronous, which implies that Titan rotates once with each orbit around Saturn. However, because Titan’s crust is decoupled from its interior by the subsurface ocean, it allows the crust to be freely dragged around by the movement of air in Titan’s thick atmosphere. Surface features imaged by Cassini during one flyby are observed to be offset by as much as a few tens of kilometres when imaged in subsequent flybys. The entire surface of Titan shifts by one-third of a degree each year as the winds in Titan’s thick atmosphere freely torques the entire crust. Therefore, surface features on Titan will be noticeably offset in images of the same locations that were taken on different dates.

Having an atmosphere which pushes around the entire surface of a moon is not something that is new. In fact, the same thing happens on the Earth where the length-of-day changes by about one millisecond over the duration of a year because of winds speeding up and slowing down in the atmosphere. However, that is a tiny amount when compared to Titan because the Earth is much more rigid and more massive than Titan, and the Earth’s atmosphere is less dense than Titan’s atmosphere. On Titan, it seems that the entire world has to be considered, from its thick atmosphere to its icy crust to its interior ocean, just to explain the length of its day and the locations of its surface features. This makes Titan a world that is probably no less complex and dynamic as the Earth.