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Moon, Earth water traced to same source: ancient meteorites

May 10, 2013
Courtesy of Case Western Reserve University
and World Science staff

The moon’s wa­ter, like Earth’s, came from small, prim­i­tive me­te­orites in the first 100 mil­lion years or so of the so­lar sys­tem, re­search­ers have found.

They say the ev­i­dence—from moon dust brought home to Earth by the crews of Apol­lo 15 and 17—dis­pels a the­o­ry that com­ets de­liv­ered the wa­ter molecules.

The tell­tale sign is in the pres­ence of an al­tered form, or iso­tope, of hy­dro­gen, in some wa­ter molecules, the sci­en­tists ex­plained. They found that lev­els of this iso­tope, called deu­ter­i­um, are al­most iden­ti­cal in the wa­ter of Earth, moon and the me­te­orites, which are of a type called car­bo­na­ceous chon­drites.

But com­et wa­ter was far dif­fer­ent in this meas­ure­ment, said the in­ves­ti­ga­tors, who re­ported their find­ings in the May 9 early on­line is­sue of the jour­nal Sci­ence.

The moon is thought to have formed from a pancake-shaped cloud of de­bris that arose af­ter a gi­ant ob­ject hit the early Earth 4.5 bil­lion years ago. Sci­en­tists have long as­sumed the heat from such an im­pact would make hy­dro­gen boil off in­to space, so that the moon must have started off dry. But NASA space­craft and new re­search on sam­ples from the Apol­lo mis­sions have shown that the moon ac­tu­ally has wa­ter, both on and be­neath its sur­face.

The new study re­in­forces an idea that the moon’s wa­ter has been there all along, or nearly so, said Al­ber­to Saal, a ge­o­chem­ist at Brown Uni­vers­ity in Prov­i­dence, R.I. and the stu­dy’s lead au­thor. “The sim­plest ex­plana­t­ion for what we found is that there was wa­ter on the proto-Earth at the time of the gi­ant im­pact,” he said “Some of that wa­ter sur­vived the im­pact, and that’s what we see in the moon.”

Or, the fledgling moon and Earth were show­ered by the same family of car­bo­na­ceous chon­drites soon af­ter they sep­a­rat­ed, said James Van Or­man of Case West­ern Re­serve Uni­vers­ity in Cleve­land, and a co-au­thor.

The re­search­ers stud­ied trapped vol­can­ic glass in moon dust, re­ferred to as a melt in­clu­sion, in which crys­tals of a min­er­al called ol­i­vine keep wa­ter locked in. Re­search from 2011 led by Er­ik Hauri of the Car­ne­gie In­sti­tu­tion in Wash­ing­ton, D.C.—who al­so par­ti­ci­pated in the new work—found the melt in­clu­sions have lots of wa­ter, as much as lavas form­ing on the ocean floor. 

Wa­ter orig­i­nat­ing from dif­fer­ent places in the so­lar sys­tem has dif­fer­ent amounts of deu­ter­i­um, the sci­en­tists ex­plained. Things formed clos­er to the sun tend to have less deu­ter­i­um. The car­bo­na­ceous chon­drites are be­lieved to have orig­i­nat­ed in the as­ter­oid belt near Ju­pi­ter and to be among the so­lar sys­tem’s old­est ob­jects. Most com­ets, on the other hand, formed in the icy Oort Cloud, more than 1,000 times fur­ther out than Nep­tune. 

There’s still the ques­tion of how that wa­ter was able to sur­vive such a vi­o­lent col­li­sion, Saal said. “Our work sug­gests that even highly vol­a­tile el­e­ments may not be lost com­pletely dur­ing a gi­ant im­pact,” said Van Or­man.


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The moon’s water, like Earth’s, came from small, primitive meteorites in the first 100 million years or so of the solar system, researchers have found. They say the evidence—from moon dust samples returned by crews of Apollo 15 and 17—dispels a theory that comets delivered the water molecules. The telltale sign is in the presence of an altered form, or isotope, of hydrogen, in some water molecules, the scientists explained. They found that levels of this isotope, called deuterium, are almost identical in the water of Earth, moon and the meteorites, which are of a type called carbonaceous chondrites But comet water was far different in this measurement, said the investigators, who reported their findings in the May 9 early online issue of the journal Science. The moon is thought to have formed from a pancake-shaped cloud of debris that arose after a giant object hit the early Earth 4.5 billion years ago. Scientists have long assumed the heat from such an impact would make hydrogen boil off into space, so that the moon must have started off dry. But NASA spacecraft and new research on samples from the Apollo missions have shown that the moon actually has water, both on and beneath its surface. The new study reinforces an idea that the moon’s water has been there all along, or nearly so, said Alberto Saal, a geochemist at Brown University in Providence, R.I. and the study’s lead author. “The simplest explanation for what we found is that there was water on the proto-Earth at the time of the giant impact,” he said “Some of that water survived the impact, and that’s what we see in the moon.” Or, the proto-moon and proto-Earth were showered by the same family of carbonaceous chondrites soon after they separated, said James Van Orman of Case Western Reserve University in Cleveland, and a co-author. The researchers studied trapped volcanic glass in moon dust, referred to as a melt inclusion, in which crystals of a mineral called olivine keep water locked in. Research from 2011 led by Erik Hauri of the Carnegie Institution in Washington, D.C.—who also participated in the new work—found the melt inclusions have lots of water, as much as lavas forming on Earth’s ocean floor. Water originating from different places in the solar system has different amounts of deuterium, the scientists explained. Things formed closer to the sun tend to have less deuterium. The carbonaceous chondrite meteorites are believed to have originated in the asteroid belt near Jupiter and are thought to be among the solar system’s oldest objects. Most comets, on the othe hand, formed in the icy Oort Cloud, more than 1,000 times further out than Neptune. There’s still the question of how that water was able to survive such a violent collision, Saal said. “Our work suggests that even highly volatile elements may not be lost completely during a giant impact,” said Van Orman.