The Huygens probe has captured an image of what may be the first drop of liquid ever observed on an extraterrestrial surface.

The photo is evidence that liquids may exist on the surface of other planets and moons, not just frozen lakes. And liquid is more likely habitat for extraterrestrial life.

Among the pictures snapped by the Huygens probe after landing on Saturn's moon Titan in 2005, one appears to show a dewdrop made of methane that briefly formed on the edge of the probe itself (indicated by arrow at bottom of image on right). Scientists think heat from the probe caused humid air to rise and condense on the cold edge of the craft.

Though Huygens may have helped produce it, the methane drop is still the first liquid directly detected at a surface anywhere beyond Earth.

Like Earth, Titan has clouds, lakes and river channels, and it may be the only other place in the solar system where liquid evaporates from the surface and returns as rain. "Aside from Earth, it's the most exciting world there is," said lead author Erich Karkoschka of the University of Arizona in Tucson.

The Cassini space probe, which took data from above the moon after separating from the Huygens lander, detected what scientists believe are lakes of liquid methane on Titan's surface. Microbes that eat methane thrive on Earth, and scientists think pools of methane could be comfortable homes for similar organisms on Titan.

Because Titan's current atmosphere is a lot like the early Earth's, the lakes could be a lab for studying the origins and early evolution of life.

Astronomers have speculated since they found methane in the atmosphere in 1983 about whether the moon's methane rain falls in violent thunderstorms, light drizzles or some other form. So far, no one has caught it on camera.

The hundreds of images snapped by Huygens, from the time it hit the atmosphere until its power ran out an hour after it landed, revealed only faint, wispy clouds that looked nothing like rain clouds, Karkoschka said.

None of the images showed evidence that it had rained during the previous few years, according to an analysis to be published in the journal Icarus. And some images suggested that Titan’s lower atmosphere was full of small dust particles, which would have been cleared out by rain.

But the scientists noticed light splotches in some of the pictures that hadn't been there moments before. Some of them had spots that initially looked like raindrops because of their uniform size and smooth edges, but analysis showed they were most likely electronic imprints created by cosmic rays.

However, Karkoschka said, "One of those spots was so big that it really cannot be a cosmic ray." He concluded that it was a real, short-lived dewdrop, so close to the camera that it must have condensed on a cold metal shield designed to protect the camera lens from direct sunlight.

Robert West, a planetary scientist at the Jet Propulsion Lab in Pasadena, California, thinks the dewdrop is "a cute observation," but he's more interested in the lack of rainfall. "There are reports in the literature that concluded there is a drizzle going on near the surface," he said. "The fact that Huygens didn’t find anything is significant."

— Lisa Grossman for Wired.com

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Image: The dewdrop, which condensed on the edge of the Huygens Probe, is in the bottom left corner of the right-hand image, marked by a long arrow. The other splotches (shorter arrows) turned out to be noise from cosmic rays./JPL, University of Arizona