Shortly after the Moon formed, an asteroid smacked into its southern hemisphere and gouged out a truly enormous crater, the South Pole-Aitken basin, almost 1,500 miles across and more than five miles deep.

"This is the biggest, deepest crater on the Moon -- an abyss that could engulf the United States from the East Coast through Texas," said Noah Petro of NASA's Goddard Space Flight Center in Greenbelt, Md. The impact punched into the layers of the lunar crust, scattering that material across the Moon and into space. The tremendous heat of the impact also melted part of the floor of the crater, turning it into a sea of molten rock.

Planetary scientists have been puzzling for years over the honeycomb patterns and flat valleys with squiggly edges evident in radar images of Saturn's moon Titan.

Now, working with a "volunteer researcher" who has put his own spin on data from NASA's Cassini spacecraft, they have found some recognizable analogies to a type of spectacular terrain on Earth known as karst topography.

Flowing lava can carve or build paths very much like the riverbeds and canyons etched by water, and this probably explains at least one of the meandering channels on the surface of Mars.

These results were presented at the 41st Lunar and Planetary Science Conference by Jacob Bleacher at NASA's Goddard Space Flight Center, Greenbelt, Md. Whether channels on Mars were formed by water or by lava has been debated for years, and the outcome is thought to influence the likelihood of finding life there.