THANKS to Voyager 2's visit to the outermost planet, Neptune, astronomers now know a little more about the innermost planet, Mercury.Technology designed to hear Voyager's faint radio "voice" from several billion miles away has enabled Duane Muhleman and his team to scan Mercury with earth-based radar. The California Institute of Technology planetologist calls this new capability an unexpected bonus from the 1989 Neptune mission. The radar pictures may seem unimpressive. They show a few bright splotches that may be craters or fractured impact basins. But the brightest spot, which lies at the planet's north pole, hints of deposits of water ice and perhaps other materials that may hold clues to Mercury's composition. Commenting on the report he made last week to an American Astronomical Society meeting in Palo Alto, Calif., Dr. Muhleman said his pictures have not yet yielded major new knowledge. What they have done, he added, is to stimulate "a sudden increase in the desire of planet scientists to do more experiments." Mercury observer Ann Sprague at the National Aeronautics and Space Administration's Ames Research Center at Moffett Field, Calif., calls this "a wonderful observation." She explains that it is "a total surprise" that has "stirred up interest within a [scientific] community that has turned its attention elsewhere." Seen from Earth, Mercury tends to be lost in the sun's glare. Moreover, what little astronomers have seen of the planet has seemed uninteresting. Mariner 10 sent back the only clear views of Mercury when it flew past the planet three times in 1974 and '75. These show a bare, cratered surface that looks much like Earth's moon. Planet scientist Robert Strom of the University of Arizona has pointed out that the similarity to our moon is only skin deep. He has explained that "the inside of Mercury is very different from our moon or anything else" in the solar system. Short of landing on the planet, scientists look for clues to Mercury's makeup in material emitted from below its surface. Dr. Sprague and her colleagues have found evidence of sodium and potassium that may have come from Mercury's interior. Now the radar pictures that suggest that materials have collected in a polar deposit indicate there may be more "outgassed" interior material as well as deposits from meteorites to study. Muhleman says it would be interesting to try to analyze that material with telescopic studies. However, Sprague, who specializes in such work, says it's hard to see how to do that from Earth. She explains this requires a large telescopic array to show the degree of detail needed. Meanwhile, Muhleman says he would like to make more use of the new radar capability. This consists of special receivers placed at the 27-antenna Very Large Array of radiotelescopes near Socorro, N.M. The array now can detect signals sent out by the 70-meter-diameter NASA space communication antenna at Goldstone, Calif. This, Muhleman says, represents "an entirely new concept" in radar astronomy.