Galileo Views Earth With an Alien’s Eye : Science: Spacecraft glimpses the forces that drive the Northern Lights.

When the Galileo spacecraft swept past Earth earlier this month it revealed what it would be like to be aboard an alien spacecraft visiting the planet, and it collected clues that may help solve a number of scientific mysteries, scientists said Wednesday.

The spacecraft provided a glimpse into the dynamic forces that drive the spectacular northern lights and the role of moisture in creating the hole in the ozone layer over the South Pole. The craft also revealed the existence of a huge basin on the far side of the moon that was caused by a collision so powerful that it made the moon wobble and could have threatened to rip it apart.

When the spacecraft looked at Earth it saw a planet teeming with biological life, and it picked up some “odd” radio signals, but it detected no clear signs of intelligent life.

The craft passed within 600 miles of the Earth on Dec. 8 to use the planet’s gravity to pick up a little speed on its long journey to Jupiter, so the flyby was a bonus that allowed scientists to see just how well their instruments are working. And by all accounts, everything aboard Galileo is working “beautifully,” said project manager William J. O’Neil.

Most of the information collected by the craft about the Earth and its immediate environment was already known to the scientists. But the encounter gave them a chance to see what alien creatures might be able to learn if they sent a craft like Galileo close by the Earth.

The first question the aliens would want to answer is whether the planet has life, and Galileo answered it.

“I can tell you that life abounds on the Earth,” based on the Galileo data, said Robert W. Carlson of the Jet Propulsion Laboratory, which is running the mission for the National Aeronautics and Space Administration.

But the flyby demonstrated how easy it would be to send a spacecraft past a heavily populated planet and not detect evidence of intelligent life.

Instruments aboard the craft were able to determine the precise composition of the Earth’s atmosphere, revealing gases that are produced by biological processes.

The presence of an abundance of oxygen, for example, is “a very strong indication that you have biological activity” on Earth, said project scientist Torrence V. Johnson, also of JPL. “But intelligent life? We don’t see anything obvious, other than something odd going on in the radio spectrum” that could tip off aliens to the fact that intelligent beings have created devices on Earth, such as radar receivers and microwave transmitters, that emit various signals at radio frequencies. The craft did not have a receiver that could pick up actual radio programs.

The spacecraft was close enough to Earth for a while to capture images of a large city, but at that time it was over Antarctica and the South Atlantic. So when Galileo swept over the South Pole and across the cloud-covered continents of the Southern Hemisphere, there were clues in the form of radio signals, but no unambiguous signs of human life.

If, however, Galileo’s “aliens” had known just what to look for, they might have detected human presence. There is a faint image, which could be a human-built structure, in one of the photos transmitted back by Galileo. The object may be a scientific research station at the South Pole.

“That is possibly the only direct evidence of the . . . people who occupy this planet,” said Michael J. S. Belton of the National Optical Astronomy Observatories, leader of the imaging team.

The scientists reported that the unique trajectory that sent Galileo through the Earth’s near environs like a visitor from outer space provided them with a number of discoveries.

Among the most startling is the confirmation that a giant depression near the moon’s south pole, about the size of the Western United States, from the Mississippi to the Pacific, was caused by a violent impact with another body when the moon was very young. Many scientists have suspected that for some time, but since most of the depression is hidden on the dark side of the moon, it has not been possible to confirm it.

Galileo approached from the dark side and its instruments revealed that the chemical composition of the depression is consistent with theories that it was caused when an object about 100 miles in diameter slammed into the moon.

The impact would have been so powerful that it would have caused the moon to wobble and reorient itself on its rotational axis, said James Head of Brown University, a member of the imaging team.

“It was a big one,” he said. The impact, he quipped, would have been sort of “like a small state coming through space” and slamming into the ground.

“It would have been a heck of a show to watch from Earth,” he added.

Scientists will study the data to see if the impact, which created a basin 1,200 miles in diameter, nearly blew the moon apart.

Since Galileo’s course carried it over the Earth’s South Pole, scientists were eager to see if it could help them understand the menacing hole that appears in the ozone layer over Antarctica each spring. Ozone helps protect the Earth from ultraviolet radiation from the sun, and the annual appearance of the hole has given rise to concern that chemicals induced by humans into the atmosphere may be destroying the protective shield.

One of the missing pieces of the puzzle, said JPL’s Carlson, is the amount of moisture in the upper atmosphere over Antarctica.

“How much water vapor is there?” he said. “It’s hardly known,” yet water at that elevation is a key part of the chemical cycle that breaks down the ozone.

Galileo’s near infrared mapping spectrometer can measure moisture, and Carlson believes data collected by the spacecraft may help scientists create models that will further explain the dynamics of the ozone problem because it should spell out how much moisture is in the upper atmosphere.

And there was a little bonus. When the craft zipped over the South Pole it found a cloud, rich in water ice, about 10 miles above the ground. Such clouds are believed to play a significant role in ozone depletion, and “we are seeing one of the culprits,” Carlson said.

Galileo also taught scientists something about the auroral activity that creates the northern and southern lights.

As the Earth travels around the sun, it constantly plunges through a field of charged particles that blow off of the sun at a million miles an hour. Some of those particles become trapped in the Earth’s magnetic field, which directs them toward the polar regions. As the particles zip down through the atmosphere, they strike and interact with other particles, creating the spectacular lights.

Louis W. Frank of the University of Iowa, leader of the space plasma science team, said scientists believe the driving mechanism is dependent on dynamic forces in the magnetic envelope that encompasses the Earth as it passes through the solar wind.

“We believe there is an explosive region” in the tail of the envelope, he said. The rarefied gases in that region mysteriously “explode,” he said, sending particles zipping out and into the magnetic field that channels them toward the poles.

“Galileo, fortunately, went right through that region,” Frank said. “As luck would have it, we saw the plasma (the thin mixture of gas and dust that makes up the interplanetary medium) disrupt. It’s just fantastic.”

During the “explosion,” as he described it, there was “a total loss of particles” in the suspected region, according to data returned by Galileo. The discovery supports the contention that the explosions send the particles on their way, resulting in the auroral lights.

Scientists will continue analyzing their data from the Earth flyby for many months. The next “stop” on Galileo’s long journey is on Oct. 29, 1991, when it will become the first spacecraft to encounter an asteroid. It will fly past Gaspra at a distance of about 1,000 miles.