Flying Over Ontario Lacus

When Cassini first arrived at the Saturn, in 2004, we knew almost nothing about the surface of its largest moon, Titan. We didn't know if it was covered in oceans of liquid methane, or in drifts of waxy organic snow; we didn't know if it was smooth or if it had hills and mountains. We know a lot more now. We know, like Earth, it not only has hills and mountains (although of water ice rather than rock), but it also has vast fields of dunes (of tarry organics rather than sand) and rivers and lakes (of liquid ethane, propane and methane, rather than water).

And now the Cassini science team have produced this terrific short video showing what it's like to fly around the shoreline of Ontario Lacus, the largest lake in Titan's southern hemisphere. It's amazing in its own right, but if you've read The Quiet War or Gardens of the Sun, you'll understand why I'm knocked out by it.



At about 15000 square kilometres, Ontario Lacus is a little smaller than its terrestrial namesake, Lake Ontario (or about three-quarters the size of Wales). Like Lake Ontario, it has a meandering shoreline fretted with bays, inlets, and beaches; there's a river that feeds into it via a delta that looks exactly like deltas formed by rivers on Earth. And like terrestrial lakes, Ontario Lacus is undergoing seasonal changes, too.

Titan's years, like Saturn's, are about 29 years long. When Cassini arrived, it was summer in Titan's southern hemisphere. Now, the days are dwindling down to autumn. Cassini first imaged Ontario Lacus in 2004; since then, its shoreline has receded by about 10 kilometres. And in four years of measuring the lake's depth by radar, its level has gone down by about a metre. For although the summer temperature in the southern hemisphere is minus 180 Centigrade, that's warm enough to allow evaporation of liquid methane. But now the temperature is dropping, that evaporation will cease. Soon, perhaps, the evaporated methane will condense into clouds and fall as winter rains, and run down the hills in rivers, and replenish the lakes...