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By Root 1438 0

is picked up by an array of radio telescopes near Socorro, New Mexico. Great. If Titan has a rocky or icy surface, a radar pulse reflected off its surface should be detectable on Earth. But if Titan were covered with hydrocarbon oceans, Muhleman shouldn’t see a thing: Liquid hydrocarbons are black to these radio waves, and no echo would have been returned to Earth. In fact, Muhleman’s giant radar system sees a reflection when some longitudes of Titan are turned toward Earth, and not at other longitudes. All right, you might say, so Titan has oceans and continents, and it was a continent that reflected the signals back to Earth. But if Titan is in this respect like the Earth—for some meridians (through Europe and Africa, say) mainly continent, and for others (through the central Pacific, say) mainly ocean—then we must confront another problem:

The orbit of Titan around Saturn is not a perfect circle. It’s noticeably squashed out, or elliptical. If Titan has extensive oceans, though, the giant planet Saturn around which it orbits will raise substantial tides on Titan, and the resulting tidal friction will circularize Titan’s orbit in much less than the age of the Solar System. In a 1982 scientific paper called “The Tide in the Seas of Titan,” Stanley Dermott, now at the University of Florida, and I argued that for this reason Titan must be either an all-ocean or an all-land world. Otherwise the tidal friction in places where the ocean is shallow would have taken its toll. Lakes and islands might be permitted, but anything more and Titan would have a very different orbit than the one we see.

We have, then, three scientific arguments—one concluding that this world is almost entirely covered with hydrocarbon oceans, another that it’s a mix of continents and oceans, and a third requiring us to choose, counseling that Titan can’t have extensive oceans and extensive continents at the same time. It will be interesting to see what the answer turns out to be.

What I’ve just told you is a kind of scientific progress report. Tomorrow there might be a new finding that clears up these mysteries and contradictions. Maybe there’s something wrong with Muhleman’s radar results, although it’s hard to see what it might be: His system tells him he’s seeing Titan when it’s nearest, when he ought to be seeing Titan. Maybe there’s something wrong with Dermott’s and my calculation about the tidal evolution of the orbit of Titan, but no one has been able to find any errors so far. And it’s hard to see how ethane can avoid condensing out at the surface of Titan. Maybe, despite the low temperatures, over billions of years there’s been a change in the chemistry; maybe some combination of comets impacting from the sky and volcanoes and other tectonic events, helped along by cosmic rays, can congeal liquid hydrocarbons, turning them into some complex organic solid that reflects radio waves back to space. Or maybe something reflective to radio waves is floating on the ocean surface. But liquid hydrocarbons are very under-dense: Every known organic solid, unless extremely frothy, would sink like a stone in the seas of Titan.

Dermott and I now wonder whether, when we imagined continents and oceans on Titan, we were too transfixed by our experience on our own world, too Earth-chauvinist in our thinking. Battered, cratered terrain and abundant impact basins cover other moons in the Saturn system. If we pictured liquid hydrocarbons slowly accumulating on one of those worlds, we would wind up not with global oceans, but with isolated large craters filled, although not to the brim, with liquid hydrocarbons. Many circular seas of petroleum, some over a hundred miles across, would be splattered across the surface—but no perceptible waves would be stimulated by distant Saturn and, it is conventional to think, no ships, no swimmers, no surfers, and no fishing. Tidal friction should, we calculate, be negligible in such a case, and Titan’s stretched-out, elliptical orbit would not have become so circular. We can’t know for sure until we start getting radar or near-infrared