Byexamining the chemistry of 29 chunks of rock that formed billions ofyears ago, probably in close proximity to our planet, two Purdueresearchers, Michael E. Lipschutz and Ming-Sheng Wang, have clarifiedour understanding of the conditions present in the vicinity of theancient Earth's orbit. Because direct evidence for these conditions islacking in terrestrial samples, the scientists believe that thecomposition of these so-called enstatite chondrite (EC) meteoritescould offer a window into the planet's distant past.
"Whathappened to these rocks most likely happened to the Earth in its earlystages – with one great exception," said Lipschutz, a professor ofchemistry in Purdue's College of Science. "Shortly after the earlyEarth formed, an object the size of Mars smashed into it, and the heatfrom the cataclysm irrevocably altered the geochemical makeup of ourentire planet. These EC meteorites, however, are likely formed ofmatter similar to that which formed the early Earth, but they were notinvolved in this great collision and so were not chemically altered.They might be the last remaining pristine bits of the material thatbecame the planet beneath our feet."
The research appears intoday's (Sept. 27) edition of a new journal, Environmental Chemistry,which solicited the paper. Lipschutz said the journal's editorial boardincludes F. Sherwood Rowland and Mario Molina, who received the Nobelprize for their discovery that Earth's protective ozone layer wasthreatened by human activity.
Lipschutz and Wang initially setout to increase our knowledge of EC meteorites, one of many differentmeteorite classes. Meteorites come from many different parts of thesolar system, and a scientist can link one with its parent object bydetermining the different isotopes of oxygen in a meteorite's minerals.Chunks of the moon, the Earth and EC meteorites, for example, have verysimilar isotopic "signatures," quite different from those of Mars andother objects formed in the asteroid belt. The variations occurredbecause different materials condensed in different regions of the diskof gas and dust that formed the sun and planets.
Bits of thesematerials orbit the sun, occasionally falling to earth as meteorites.But there is one place on our planet that meteorites accumulate and arepreserved in a pristine fashion – the ice sheet of Antarctica.
"Overthe millennia, many thousands of meteorites have struck the Antarcticice sheet, which both preserves them and slowly concentrates them nearmountains sticking through the ice, much as ocean waves wash pebbles tothe shore," said Lipschutz. "These stones have come from many differentparts of the solar system and have given us a better picture of theoverall properties of their parent objects."
By examining theirmineralogy, scientists have determined that about 200 of theseAntarctic stones are EC meteorites that formed from the same localbatch of material as the Earth did more than 4.5 billion years ago. Butthere is additional information that the chemistry of these ECs canoffer on the temperatures at which they formed. To obtain thisinformation, however, required Lipschutz to analyze chemicals in themeteorites called volatiles – rare elements such as indium, thalliumand cadmium.
"Volatiles in meteorites can give unique informationon their temperature histories, but only 14 of them had ever beenanalyzed for these elements," Lipschutz said. "Naturally, we want toknow the story behind the formation of objects in our own neighborhood,so we set out to increase that number."
In this study, theresearchers gathered samples taken from another 15 EC meteorites thathad, for the most part, landed in Antarctica tens of thousands of yearsago. Using a unique method involving bombardment of the samples withneutrons, chemically separating the radioactive species and countingthem, the researchers were able to determine the amounts of 15volatiles that together offered clues to each rock's heating history.
"Volatilescan act like thermometers," Lipschutz said. "They can tell you whetherthe temperature was high or low when the rock formed. We tested twodifferent kinds of ECs, and the oldest, most primitive examples of eachkind had very similar volatile contents – which means their temperatureat formation was similar. These rocks have essentially recorded thetemperature at which the early Earth formed, and we now know that thiswas much lower than 500 degrees Celsius."
The two different kindsof EC meteorites, known as ELs and EHs, were found in the Purdue studyto have condensed at low temperatures like the Earth. However, the twogroups are controversial because scientists have not been able to agreeon whether they originated from a single parent object or two differentones. Unfortunately, Lipschutz said, the data from the 29 ECs theyanalyzed were insufficient to settle the issue.
"There are stillquite a few unanswered questions about the earliest periods of theEarth's history, and this study only provides one piece of the puzzle,"he said. "But aspects of this study also show that ECs differsubstantially from other meteorite types that came from much fartherout in the disk, in the region of the asteroid belt."
ForLipschutz, who had an asteroid named for him on his 50th birthday inhonor of his many studies of meteorites, their parent bodies and theearly history of the solar system, deeper answers may lie farther awaythan Antarctica.
"If we understand how our solar system formed,we might be better able to understand the processes at work in othersolar systems, which we are just beginning to discover," he said."Probing the asteroid belt could give us clues to these processes."
This research was funded in part by NASA.
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