Images from the itinerant spacecraft Galileo have revealed the moon's largest known impact basin -- a suspected but never-before-seen dark region 1,200 miles across blasted out by an asteroid-like object 100 miles wide that slammed into the moon, probably making it wobble and lurch.
The spacecraft, launched 14 months ago into a long, looping path, zoomed past Earth and the moon 11 days ago. It sent back pictures showing the pockmarked basin of what once must have been "a chunk out of the bottom half of the moon," on the side never visible from Earth, according to Brown University scientist James W. Head III.
The robot craft also captured images of Earth. "We don't see anything in the pictures that would indicate intelligent life," said Torrence V. Johnson of NASA's Jet Propulsion Laboratory in Pasadena, Calif., which is managing the mission.
Scientists noted, however, that the spacecraft was not designed to detect life. For one thing, its camera could not show small enough detail to reveal civilization.
But, said Robert W. Carlson, leader of the Galileo infrared mapping team at JPL, if he were a scientist across the cosmos at Arcturus, a red giant star, and were using Galileo data to study an alien planet, "I'd say life abounds on Earth."
This is because Galileo's instruments detected an abundance of nitrous oxide (laughing gas), methane and oxygen, which all are "biological products" that indicate the presence of living organisms.
The craft's instruments also captured new data from inside Earth's magnetic "tail," made a recording of the whistling sounds generated by lightning, filmed the first movie of Earth spinning in space, and produced what scientists called a "key piece of data" needed to study the depletion of Earth's protective ozone layer.
Galileo, which has traveled about 664 million miles since it was launched in October 1989, returned to use Earth's gravity to increase its speed as it plies a winding, five-year, 1.3 billion-mile course to Jupiter. It traveled first to Venus for a gravity assist; in October it is scheduled to make the first encounter of a spacecraft with an asteroid, and a year from now it will return to steal more speed from Earth.
In the encounter scientists call "Earth One," Galileo swooped past the moon and to within 600 miles of this planet's surface, sending back about 2,600 frames of images and 58 billion bits of data as its instruments continued to collect information on the Earth-moon system for seven days.
The results are valued not only for what they tell about Earth, but also to test and adjust Galileo's instruments in preparation for their work at Jupiter, scientists said.
Galileo's snapshots of the moon contribute "important complementary data . . . toward understanding the total global geology of the moon," said planetary geologist Head.
The Galileo images provide clear evidence of a huge basin, suspected since the Apollo astronauts saw hints of it from lunar orbit, that extends from the moon's south pole up into its far side, Head said. That portion of the moon was in darkness, however, every time the astronauts saw it. Galileo saw it in sunlight.
The impacting object would have been about 100 miles in diameter, "like a small state coming through space," and would have caused Earth's satellite to "wobble and possibly reorient its axis," he said. Scientists will look for radial cracks and other clues to "how close it came to cracking up the moon."
The object dug deep into the lunar surface, exposing the layer of molten material beneath the crust, "like a window on the interior," he said.
The largest lunar basin previously documented, he said, was 800 miles across.
Galileo's analysis of the moon's surface composition also adds to evidence that there was volcanic activity on the moon tens of millions to hundreds of millions of years earlier than scientists once believed, Head said.