Every two hours, a meteorite at least the size of a softball plunges from the heavens through the atmosphere and slams somewhere into the surface of Earth.
In fact, 20 tons of these objects reach Earth’s surface every day, though most are invisible, tiny grains. Only a tiny fraction of the larger ones are ever found, as most fall unseen into oceans or rural fields and forests, blending invisibly into the earthly environment.
The few that have been found throughout history, about 12,500 of them, are prized by collectors and especially by scientists. The prosaic-looking extraterrestrial rocks have revealed clues to how and when the solar system was formed, what it is made of and even how life may have stirred into being.
Until Aug. 7, however, meteoritics–the study of meteorites–had been something of a sleepy scientific backwater far outshone by astronauts, space probes and giant, high-tech telescopes.
On that day meteoritics spectacularly captured science’s center stage with assertions by a group of NASA and university researchers who had been studying a single potato-sized meteorite from Mars. Group members announced they believed they found evidence that primitive forms of life once thrived on Mars, the first sign of life anywhere else but Earth.
“All of a sudden, it’s like somebody hit the accelerator pedal,” said Allan Treiman, a meteoritic scientist at the Lunar and Planetary Institute, an independent space research and education center in Houston, “and we’re zooming down the highway faster than most of us are comfortable with.”
The community of scientists who study meteorites is so small worldwide that almost all of them know one another. Chicago and its Field Museum of Natural History, with a collection of 1,500 meteorites, is one of the four or five centers worldwide for meteoritics.
University of Chicago geochemists, working with Field Museum meteorite samples, have even managed to recover actual stardust, the only matter known to science to have come from outside the solar system.
Meteorites in the past have catastrophically changed life globally. A giant meteorite or series of meteorites is pretty much the accepted culprit for killing off dinosaurs 65 million years ago.
Meteoritic studies over the last three decades have helped explain how the solar system was formed from dust and gases from dead stars swirling in space 4.5 billion years ago.
The bulk of that matter massed into a dense core that ignited into a solar furnace, becoming the sun. Most of the leftover material formed the planets revolving around the sun.
Material labeled as asteroids, however, orbits the sun in an area between Mars and Jupiter, having never succeeded in combining to form a planet. Uncounted billions of fragments called planetismals–rocks ranging in size from several hundred miles wide to grains of sand–form the asteroid belt.
Asteroids that fall into erratic orbits crossing into the path of Earth eventually get caught in Earth’s gravity. They become meteors; most of them vaporize as they fall into the atmosphere, the most brilliant appearing as “shooting stars.”
Those that survive the fall are called meteorites. Until the early 1980s, most scientists believed all meteorites came from asteroids.
A tiny few, however, stymied science. While almost all could be dated as being 4.5 billion years old, some were far younger rocks, ranging from 1.5 billion years to 180 million years old.
In 1980, researchers at the National Aeronautics and Space Administration’s Johnson Space Center in Houston were looking at a “younger” meteorite found in Antarctica. They found its mineral composition to be nearly identical to rock samples brought back from the moon by the Apollo lunar missions.
Since then, scientists have established that 12 meteorites in the worldwide collection came from the moon. Most probably, they were dislodged and hurled toward Earth by other meteorites slamming into the moon’s surface.
The discovery of the first moon meteorite sent scientists back to look at a handful of other, anomalous “young” meteorites that some researchers had suspected came from Mars.
Heating up a meteor in the early 1980s, scientists found gases trapped in pockets that were identical in composition to the Martian atmospheric gases measured by the 1970s Viking spacecraft missions to Mars.
In the last 30 years, sophisticated new analytical tools developed for the space program have allowed scientists to “decode” contents of meteorites in ways never before possible.
Their work has been vastly expanded since the discovery in the early 1970s by Japanese and American scientists that Antarctica is the best meteorite-hunting area in the world.
With 98 percent of that continent covered with ice, any rock sitting atop an ice sheet hundreds or thousands of feet thick is almost certainly a meteorite. Ever since then, annual National Science Foundation expeditions have been bringing home new meteorites by the hundreds, adding 10,000 more meteorites to the 2,500 in collections in 1970.
These developments allowed meteoritics to establish the age of the solar system at 4.5 billion years. They also have revealed what elements and gases were present in the swirling dust before the sun and planets were formed.
In Houston, Allan Treiman has spent 10 years studying all 12 known Mars meteorites, about 175 pounds of rock in all.
“Never has so much been learned by so many from so little,” Treiman jokes about Mars meteorites. Treiman’s work is independent from the researchers who said this month they have found Martian organic fossils.
Treiman looks for traces of clay, minerals and rust in the Mars meteorites, trying to understand the nature of the groundwater there before the planet froze.
At the Field Museum in Chicago a young Indian-born, American-educated geologist also has been hard at work analyzing the now-famous Allan Hills Mars meteorite from Antarctica. Her eventual findings may contribute to the ultimate proof of whether the recent fossilized life theory is true or false.
Meenakshi Wadhwa, 28, is acting curator of the Field’s meteorite collection. She spent weeks living in a tent in Antarctica with a team hunting meteorites on glacial ice in 1992 and 1993.
Since she joined the museum last October, she has been working to establish the age of carbonate globules within the Mars meteorite named Allan Hills 84001 because of where it was found in 1984 in Antarctica.
Dating the carbonate globules is important, because the supposed fossilized life forms found inside the meteorite are within those globules.
“They (the Martian life theorists) assume an age for these carbonates at 3.6 billion years, that I don’t think is at all well-established,” says Wadhwa.
Mars at 3.6 billion years ago was warm enough and hospitable enough to support life if it existed. Shortly thereafter, however, its atmosphere collapsed, and Mars’ surface climate has been too cold ever since to allow any life form to survive.
If Wadhwa’s excruciatingly exacting rubidium strontium age dating of the globules eventually show the carbonates to be younger than 3.6 billion years, it will seriously challenge the Martian life theory.
“These certainly are reputable scientists,” says Wadhwa of the scientists who announced the Martian life evidence this month. “I know all of them, and they have done very careful work.
“It’s in their interpretations of the work where the controversy occurs. It is still a work in progress.”
