An Aug. 11 article about meteorites incorrectly described isotopes of nitrogen and hydrogen. An atom of nitrogen 14 contains 7 protons and 7 neutrons. Nitrogen 15 has 7 protons and 8 neutrons. A hydrogen atom has no neutrons. Its isotope deuterium has one.
The Sun regrets the error.
No, the meteorite is not stamped "Made on Mars."
But the Martian origin of the rock that NASA scientists say holds evidence of long-ago life on the Red Planet is probably the least controversial aspect of their extraordinary scientific claims.
"I believe 80 percent of the involved scientists [those working with meteorites] would agree" that the NASA rock is from Mars, said Dr. E. Julius Dasch. He is a NASA geologist and meteorite expert who remains skeptical of the life-on-Mars findings announced last week.
For more than a decade, it turns out, meteoriticists have conducted increasingly sophisticated chemical and isotopic examinations on meteorites. And the tests have found a growing number of close matches between some meteorites and the rocks or atmosphere of both Mars and the moon -- and sharp differences from Earth's chemistry.
There aren't many of these objects. Of the 20,000 or more meteorites collected by scientists over the years, only 12 have been identified as having come from Mars, and 12 from the moon.
The rest, so far, are believed to have originated in the asteroid belt -- a band of rocky debris circling the sun between the orbits of Mars and Jupiter.
The asteroids are probably the rubble of a planet that failed to form or broke apart because of nearby Jupiter's huge gravitational influence.
The number of meteorites available for study got a big boost in 1969 when Japanese scientists discovered concentrations of them on the surface of unusual blue Antarctic ice.
They're found in areas where dense ice from deep in the polar cap, which normally creeps slowly to the sea, has instead been forced to the surface by some obstacle, perhaps a mountain range.
Exposed to the cold, dry air, the ice ablates, or turns to vapor, exposing meteorites that fell thousands -- or millions -- of years earlier.
As more ice rises and ablates, more meteorites pop up and accumulate, like bonbons at the end of a conveyor belt.
Since the Japanese discovery, more than 10,000 meteorites have been recovered, most through the Antarctic Search for Meteorites program (ANSMET) run by the National Science Foundation, NASA and the Smithsonian Institution.
Dasch, who took part in a 1993 ANSMET expedition, said one meteorite field he saw held 70 space rocks in an area the size of a football field. They ranged in size from a match head to a bowling ball.
The ANSMET expeditions gather up hundreds each year, the numbers limited only by the size and budget of the expedition.
"You could bring back 10,000 a year if you wanted to," Dasch said.
Using sterile techniques similar to those designed for handling moon rocks, the scientists collect and pack the rocks. They're shipped frozen in sealed containers to the Johnson Space Center in Houston.
There they are processed in facilities built for lunar rocks -- sealed off from water, oxygen and industrial pollutants that might alter their chemistry.
The realization that some meteorites may have come here from Mars or the moon began in the 1970s. Scientists using radioactive dating techniques discovered that the rock in a small class of meteorites was too young to have originated as asteroids, which solidified with the rest of the solar system about 4.5 billion years ago.
"They were, like, 1.5 billion years old, way off," said Dr. Michael Meyer, exobiology program manager at NASA headquarters in Washington.
These dating methods measure how much of one unstable radioactive element may decay to become another, such as when rubidium gradually turns into strontium.
Because the decay happens at a known rate that begins when the rock first crystallizes, the relative amounts of those elements in a rock act as a precise clock that reflects its age.
After much debate, scientists began to agree that these meteorites must have solidified in more recent volcanic processes. And the only place that could happen, apart from the Earth, was on a terrestrial planet, like Mercury, Venus or Mars, or a rocky moon, like Mars' Phobos or Deimos. The giant outer planets are made almost entirely of gas.
Pinpointing the origin of these meteorites, then, became a matter of matching their chemistry to the known chemical profiles of the candidate planets.
Moon rocks retrieved by Apollo astronauts a generation ago are still available for direct comparisons with meteorites suspected of having lunar origins. There is little scientific debate about the dozen such meteorites found so far.
For the others, less direct chemical comparisons had to be made.
"Nitrogen clinched it for the Martian meteorites," Meyer said.
The 1976 Viking missions to Mars tested the thin Martian atmosphere. One of those measurements recorded the ratio of two isotopes of nitrogen -- nitrogen 14 (with 14 neutrons per atom) and nitrogen 15. Isotopes are varieties of the same element, with the same chemical properties, but a different number of neutrons.
Viking found that the Martian atmosphere has 60 percent more than Earth's of the heavier nitrogen 15 relative to nitrogen 14.
"It's a good debate that's going on now as to why there are these differences we see," Meyer said.
Differences in the size, origins and history of the planets are probably involved. Contributions from falling comets may play a role.
Scientists extracted gases trapped in glasses that formed within the rock while it was molten. The gas might have been trapped during volcanic activity, or perhaps when the rock was melted briefly by the impact of the asteroid or comet that blasted it off the planet millions of years ago.
"Just like sometimes if you freeze water very fast, it's cloudy because it traps air inside," Meyer said. "In the same way, the formation of glasses [in the molten rock] can trap atmospheric air from wherever they are."
There are other indicators, too. Scientists can check ratios of hydrogen and its two-neutron twin, deuterium, and compare them with known ratios on Mars. Those have been determined by remote spectrographic measurements of the Martian atmosphere.
Carbon is another indicator. Martian meteorites show higher amounts of the carbon 13 isotope than objects on Earth.
The meteorite that NASA has been crowing about recently was found in Antarctica in 1984, at a place called Allan Hills. But it was initially misidentified. Its Martian origins weren't established until Dr. David W. Mittlefehldt, a Lockheed-Martin geochemist working at Johnson, spotted the error in 1993 and ran some tests.
The rock's complement of heavy isotopes of oxygen flagged its Martian origin, Meyer said.
NASA geochemist David McKay and his team got their hands on the specimen in August 1994, but it was six more months before they saw anything of interest there.
"Then we started using some new tools that didn't exist five years ago," McKay said. A year ago, they began to see what they have interpreted as chemical and fossil evidence of life on Mars 3.6 billion years ago.
After that, he said, "I spent many nights until midnight in the lab, too excited to go home."
Pub Date: 8/11/96