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Claims of a watery moon in question

Aug. 6, 2010
Courtesy of Science
and World Science staff

Re­cent stud­ies re­port­ing that the Moon has un­ex­pectedly high amounts of wa­ter seem to be wrong, a group of sci­en­tists says.

Based on a new anal­y­sis of lu­nar sam­ples from NASA’s Apol­lo mis­si­ons, this team is pro­claim­ing our pale lit­tle com­pan­i­on world “essen­tial­ly” wa­terless.

Many sci­en­tists be­lieve the Moon formed out of a huge crash be­tween the Earth and some oth­er ob­ject. That would sug­gest lu­nar rocks share a si­m­i­lar sog­gy his­to­ry as their earth­en breth­ren. 

But the sci­en­tists found vastly dif­fer­ent “fin­ger­prints” of wa­ter con­tent be­tween Earth and Moon rocks based chlo­rine found in those rocks.

Zachary Sharp of the Uni­vers­ity of New Mex­i­co in Al­bu­quer­que and col­leagues meas­ured the com­po­si­ti­on of dif­fer­ent forms, or iso­topes, of chlo­rine in lu­nar vol­can­ic rocks. They found the range of chlo­rine iso­topes con­tained in those sam­ples to be 25 times great­er than what is found in rocks and min­er­als from Earth and from me­te­orites. 

Since chlo­rine is very hy­dro­phil­ic, or at­tracted to water, it serves as a good in­di­ca­tor on Earth of lev­els of hy­dro­gen, a com­po­nent of wa­ter.

The researchers main­tain that, if lu­nar rocks had in­i­tial hy­dro­gen con­tents any­where close to those of Earth rocks, then the sep­ar­ati­on of chlo­rine in­to so many dif­fer­ent iso­topes would nev­er have hap­pened on the Moon. 

The Moon probably has one ten-thousandth to one hundred-thousandth as much hy­dro­gen as Earth, sug­gesting a dras­tic­ally low­er wa­ter con­tent, they pro­posed. Hy­dro­gen ac­counts for two out of every three atoms in wa­ter.

Sharp’s group pro­poses that re­cent cal­cul­ati­ons of high hy­dro­gen con­tents in some lu­nar sam­ples are not typ­i­cal, and that those sam­ples are likely the prod­uct of spe­cial pro­cesses in­volv­ing in­tense heat. Water-ice found in some lu­nar sur­face sam­ples probably comes from comets, they added.

The re­search­ers con­sid­ered that pro­cesses in­volv­ing so­lar winds, un­ique to the Moon, might ex­plain the chlo­rine com­po­si­ti­on, but dis­missed this idea af­ter mim­ick­ing the ef­fects of so­lar winds us­ing an ion beam in a base­ment lab­o­r­a­to­ry.

The dif­fer­ence in wa­ter re­mains “ar­guably, the most dra­mat­ic ge­o­chem­i­cal distincti­on be­tween the Earth and Moon,” the team wrote, re­port­ing their find­ings in the Aug. 6 is­sue of the re­search jour­nal Sci­ence.


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Recent studies reporting that the Moon has unexpectedly high amounts of water seem to be wrong, a group of scientists says. Based on a new analysis of lunar samples from NASA’s Apollo missions, this team is proclaiming our pale little companion world “essentially” waterless. Many scientists believe the Moon formed out of a huge crash between the Earth and some other object. That would suggest lunar rocks share a similar soggy history as their earthen brethren. But the scientists found vastly different “fingerprints” of water content between Earth and Moon rocks based chlorine found in those rocks. Zachary Sharp of the University of New Mexico in Albuquerque and colleagues measured the composition of different forms, or isotopes, of chlorine in lunar volcanic rocks. They found the range of chlorine isotopes contained in those samples to be 25 times greater than what is found in rocks and minerals from Earth and from meteorites. Since chlorine is very hydrophilic, or water-repellent, the researchers say it serves as a sensitive indicator of levels of hydrogen, a component of water. They maintain that, if lunar rocks had initial hydrogen contents anywhere close to those of Earth rocks, then the separation of chlorine into so many different isotopes would never have happened on the moon. The moon probably has one ten-thousandths to one hundred-thousandths as much hydrogen as Earth, suggesting a drastically lower water content, they proposed. Hydrogen accounts for two out of every three atoms in water. Sharp’s group proposes that recent calculations of high hydrogen contents in some lunar samples are not typical, and that those samples are likely the product of special processes involving intense heat. Water-ice found in some lunar surface samples, probably comes from comets, they added. The researchers considered that processes involving solar winds, unique to the Moon, might explain the chlorine composition, but ruled this out after mimicking the effects of solar winds using an ion beam in a basement laboratory. The difference in water remains “arguably, the most dramatic geochemical distinction between the Earth and Moon,” the team wrote, reporting their findings in the Aug. 6 issue of the research journal Science.