Under the Moon
By David Grinspoon
Published: January 13, 2005
SINCE its discovery in 1655, Titan, Saturn's giant moon, has been an object of great mystery and speculation. We may now be a day or so away from clarifying many of those mysteries.
What has Titan been hiding? Although it is larger than Mercury and Pluto, and easy to spot with even a backyard telescope, little is known of its surface. The only moon with a thick atmosphere (mostly nitrogen, like our own), it is perpetually shrouded in dense smog, which has long obscured our view.
What's more, Titan has methane clouds and probably rain, perhaps producing the only open seas in our solar system other than those on Earth, though these would be seas of liquefied natural gas. The prospect of hydrocarbon lakes on Titan also suggests that much of what gives our planet its unique character -- surf, waterfalls, storms, lightning, rivers and tides, may also be acting, in some form, on the surface of Titan.
But what about that certain something that really makes Earth, well, Earth: life itself? We know that Titan was once more like our planet -- hotter and watery in its youth. We know, too, that Titan's global smog bank is a smorgasbord of organic molecules -- the stuff of life -- that must continually be drizzling down on the surface.
This manna makes Titan a prime exploration target for astrobiology -- the study of life beyond Earth. If terrestrial life started from a primordial soup of complex organics floating in young seas, then the deeply frozen surface of Titan (minus 290 degrees Fahrenheit) is a primordial snow cone loaded with the same promising molecules.
Thus Titan seems like an analog of the environment of Earth on the eve of life, a place where we can study the type of complex organic chemistry that at least once somehow became alive. Could the moon be home to primitive organisms that emerged during its warmer period? Might some of these still be playing ''survivor'' in underground seas or near-surface lakes melted by volcanic heat or the occasional energetic comet collision?
We just might find out. Last July 1, after an intrepid seven-year trek from Earth, NASA's Cassini spacecraft reached Saturn, swinging within 12,000 miles of the roiling cloud tops before hitting the brakes hard with a 97-minute blast of its main engine. This harrowing maneuver slowed the spacecraft so that Saturn's gravity could pull it into orbit like a horseshoe hitting the post.
Cassini's close fly-by of Titan in October allowed the first decent surface images of the moon, made by using cloud-penetrating radar and infrared light. These show us that Titan is an active world with a youthful, still evolving surface.
How can we tell? The key clue is a dearth of the circular impact craters that stipple the landscapes of all worlds with ancient surfaces. Most moons, including Earth's, have been geologically dead for eons, as they lack the internal energy to produce the upheavals that would erase the scars of long-ago comet or asteroid collisions.
Titan, however, is sparsely cratered, which tells us the surface is young and, like Earth's, undergoing some kind of makeover. A variety of surface features are seen but none are obviously recognizable, giving us, for the moment, a planetary Rorschach test. Some scientists see hints of possible ice volcanoes. Others see streaks and curving boundaries suggesting blowing dust and flowing liquid, cracks from Titan-quakes and signs of large-scale erosion. Large dark areas first hinted at seas, but these sites seem to have the wrong reflective properties for bodies of open liquid. Nothing is universally accepted among the scientists now obsessing over these new images. It is rare for us to have this quality and quantity of images and still not really have a clue as to what is going on down there beneath the enveloping haze.
All this may be about to change.
Christiaan Huygens, the 17th-century Dutch astronomer who discovered Titan, believed that it was inhabited, and speculated on how the creatures there would have adapted to the perpetual winter experienced at 890 million miles from the sun. Now the European-built Huygens probe, which separated from Cassini on the night of Dec. 24-25, is about to take the plunge.
Huygens is expected to reach Titan on Friday morning and drop through the smog and the clouds and the thick atmosphere to land, or crash, or perhaps even splash on the surface. Scientists don't know if the probe will hit solid ice, organic slush or some kind of liquid, so it is built to float just in case. During the two-hour descent, it will be taking photos and sampling the air, haze and light. With some luck, it could survive on the surface for half an hour, snapping pictures and testing the ground until its batteries die out or Cassini recedes beyond range of its transmitter.
This is a precious opportunity. In recent years we have proved there are scores of planets in our own galactic neighborhood, which tells us that surely there are trillions out there in the wider universe. Each of these, in our current ignorance, is a potential home for some kind of life. But most of them are out of reach for us and our nonremote descendants -- the laws of physics and the (truly) astronomical distances to the stars will see to that. To tease out what we can about the universal laws of planetary birth and evolution, and the range of possible biological habitats our universe has to offer, we are left only with the planets orbiting our sun, and their moons.
Among these, Titan is the only clouded, gas-enshrouded solid world in our solar system we have not yet visited. So tomorrow's probe entry is the last time we will first burst through the clouds of an unknown planet, parachute to the ground and see, during the descent, the approach and -- if we're lucky -- after the landing, the secret vistas of another alien place.
Drawing (Drawing by David Sandlin)