On alien moon, hints of early Earth?

He adds that volcanism on Titan is likely driven by radioactive decay in the moon's rocky core. Water ice is thought to be present in a slushy layer between the surface and the moon's frozen interior. As a result, the heat from radioactive decay could supply the energy to drive some interesting organic chemistry in the presence of water. These are conditions thought to replicate those of an early Earth.

Indeed, Dr. Lorenz says he finds Titan more promising in this regard than Jupiter's moon Europa, which captivated planetary geologists with evidence of an active icy crust overtopping an ocean of liquid or slush.

Grinspoon gingerly takes the line of reasoning one step further, suggesting Titan could host simple life forms.

"The safe way to talk about Titan is that we'll learn about pre-biotic chemical evolution," he says. "The more enticing but more out-on-a-limb way of talking is to wonder if there's some kind of life there today."

Prior to October's flyby, he wasn't a believer. But the early results from Cassini "have made me more open to that possibility" of extant life, he adds.

Huygens could help begin to shed light on many of these issues. The craft carries a suite of instruments to measure Titan's winds, tiny particles in its atmosphere, the amounts of solar radiation ricocheting around within the atmosphere, and the composition of its gases. The probe will also track the atmosphere's density with altitude, and yield basic data on the surface it finds.

Indeed, the projected landing site represents a bonus, JPL's Buratti notes. The probe will be parachuting toward an area that hosts both light and dark features as seen from Cassini. The hope is that the probe will shed enough light on surface composition that the information can be used as "ground truth" for what the orbiter's instruments see.

Scientist expect to receive the probe's full batch of data relayed through the orbiter by 2:07 p.m. Pacific Standard Time Friday.

"We're keeping our fingers crossed," Grinspoon says. "This is rocket science, and anything can happen."

• At least 139 moons orbit seven planets in our solar system. More than half of these satellites have been discovered since 2000 - many of those from images taken by the Voyager 1, Voyager 2, and Cassini spacecrafts.

• Jupiter has the most moons, 62. Mercury and Venus have the fewest, 0. Jupiter also sports the largest and, so far, the smallest moons in the solar system. With a diameter of 3,280 miles, Ganymede is bigger than Mercury; S/2003 J9 and S/2003 J12 are less than a mile across.

• Saturn's Titan, where the Huygens probe is scheduled to touch down Friday, is the second-largest moon in the solar system.

• Neptune's moon Triton is covered with nitrogen and methane frost, which makes it the coldest place in the solar system (-391 degrees F.).

• Pluto's only moon, Charon, is more accurately the other half of a double-planet system. The orbits of the two are so carefully synchronized that from one side of Pluto, Charon never changes position in the sky. It's never seen from the other side.

Source: National Aeronautics and Space Administration

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