The tiny moon of Mars named Phobos appears to be a captured asteroid that looks for all the world like a piece of the oil-shale-rich Colorado Rockies in orbit around the red planet.

The surface of Phobos was described today as looking remarkably like a class of meteorites that strike the earth and are called "carbonaceous chondrites," which are rich in coal tar and the kinds of long-chain hydrocarbons found in the oil out of the rock on Phobos, he might be able to pump as much as 500 million barrels.

The meteorites that Phobos so resembles are also believed by scientists to be among the most pristine bodies left in the solar system. Phobos appears to have undergone none of the heating and reheating that the earth, the nearby planets and even the moon went through billions of years ago when they lost their original composition.

"There is very clearly a case for Phobos to be an asteroid captured by Mars," Cornell University's Dr. Joseph Veverka today told the annual meeting of the American Association for the Advancement of Science.

The evidence for this comes from a close approach last week of the orbiting Viking I spacecraft, which passed as close as 60 miles from Phobos and photographed it from a distance of 410 miles. The close approach allowed scientists to estimate the mass and density of the tiny moon and the photograph allowed them to match up its color and reflectivity with other bodies in the solar system.

Phobos is about half as dense as Mars itself, which came as a surprise to scientists.

Its color and reflectivity are very much like that of the larger asteroids.

If Phobos is a lost asteroid made up of carbonaceous chondrites, it would also be rich in amino acids, so much of which are found in meteorites that a scientific argument raged until recently about whether these meteorites had biological origins.

"If you were standing on Phobos you'd be able to smell it,' chief Viking scientist Gerald B. Soffen said at a press conference here today. It would be very rich in oil-like hydrocarbons.

The finding that Phobos appears to be a captured asteroid could settle a debate that has lasted for years on the origins of Phobos and Deimos, the two Martian moons named for the Greek gods for fear and dread.

The asteroid belt lies between the orbits of Mars and Jupiter, and its wanderings and great size often perturb one of the smaller asteroids out of the belt. The tough question to answer about the moons of Mars is: how did the planet capture them?

"You can't just throw a rock near a planet and have it go into orbit," said Dr. Robert Clayton of the University of Chicago, one of the world's leading experts on meteorites. "Most rocks that come that close to a planet hit them and make the holes we call craters."

In analyzing the craters spread across the surface of Phobos, Cornell's Dr. Veverka estimated that the surface has not been disturbed for at least 2.5 billion years and possibly as long as 4.6 billion years, the age of the solar system. Phobos is the inner and larger of the two Martian moons, measuring 13 miles across and 11.8 miles from top to bottom.