AMATEUR NATURALIST

The fascinations of Titan

Posted July 15, 2012, at 3:36 p.m.
Saturn looms behind its moon Titan.
Cassini spacecraft photo courtesy of NASA
Saturn looms behind its moon Titan.

Saturn’s moon Titan is so big that on a clear night you can actually see it with high-power binoculars, even though it’s around 800 million miles from us. It’s about 3,200 miles in diameter, bigger than the moon (2,160 miles), bigger even than Mercury (3,032 miles), and looks like a tiny pin of light kind of hanging off to the side of the planet.

Part of its brightness is due to the lighting up of its orange smog by the sun. Titan is encased, in other words, in an atmosphere, which is true of only a handful of places in our solar system, including Earth and Venus. In 2004 the Cassini spacecraft started taking close-up pictures of it, and in 2005 dropped a probe (named Huygens, after the 17th century astronomer who first identified the pin of light) to get details of what the atmosphere is made of, exactly, and what that bemisted surface is like that a person’s actual feet might touch, in one spot at least. The findings then and up to now are increasingly, well, fascinating.

Not only does Titan have this thick atmosphere, heavier than Earth’s and made mostly of nitrogen, but it has clouds and rain made of hydrocarbons. These are mainly methane and ethane, which here on Earth are gases but in the distant-sun cold of Titan (around minus 290 degrees Fahrenheit) are liquid. One large cloud toward the northern hemisphere expands and contracts with seasons (one round of which lasts about 30 Earth years), creating regular weather patterns of rainy and dry spells.

The methane rain appears to fall into methane lakes, most of them near the poles and some of which are very large. Ontario Lacus in the south polar region is about four-fifths as big as its Earth-namesake Lake Ontario. It has defined shorelines, and being normally just a few feet deep it appears to shrink into sort of methane mudflats in the dry season. The planetary scientists recently determined there also are lakes nearer the more desertlike middle parts of Titan, where roll different kinds of massive, Earth-like dunes made of water-ice sand. One strange thing about these methane (as well as ethane and propane) lakes is that they exist at all. In standard Earth-based atmospheric chemistry, methane would be expected to convert naturally into more complex hydrocarbons. But Cassini’s findings indicate the fairly stable bodies of liquid are replenished by other chemical mechanisms such as rainfall and aquifers — and theoretically, even microbes metabolizing hydrogen and acetylene into methane. And there appear to be rivers.

It also is generally thought, after a research report published this summer, that there is an underground ocean inside Titan. The evidence comes from above: Cassini measures a bending of Titan’s surface in response to Saturn’s enormous gravity during its orbit. If Titan’s insides were solid, it would not bend like this, so there must be a large amount of liquid in there, which the scientists think is actual water.

All this implies that Titan — although it’s even colder than Maine in winter, is smoggier than Beijing’s worst days and has liquid methane on its surface instead of liquid water — has terrains, weathers and climates with sort of sympathetic parallels to ours here on Earth. Because of its chemistry, Titan could even harbor living beings, or might do so in a far-future evolutionary time. Sentience is the real prize.

What does all this mean to us down here on Earth, who see only photos of orange haze taken by spacefaring robots or, at nearest to our actual gray matter, a pin of light off to the side of Saturn? In some ways it amounts to no more than a set of novel surprises popping up out of the mist. Or a bit more, a challenge to what we called knowledge before Cassini. Or beyond its own beyond, a discovery that there are more things in heaven and earth than is dreamed of in our science. To put it in the words of Thoreau, who was familiar with this kind of ambiguous revelation: “my desire to bathe my head in atmospheres unknown to my feet is perennial.” That feels just about exactly like what is driving our fascination with cold, distant, hazy, bright Titan so far. “We are all children of the mist.”

Dana Wilde’s collection of writings on other worlds, “ Nebulae: A Backyard Cosmography,” is available from Booklocker.com in electronic and paperback formats. Please go to www.dwildepress.net/nebulae for more information.

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