Lunar water may have come from comets - scientists

The image of the moon is courtesy of NASA.

Water on the Moon came in large part from comets which bombarded the lunar surface in its infancy, a study published on Sunday suggests.

For decades, the was thought to have been as dry as it was void of life and atmosphere.

This assumption, though, has been revisited after findings by last year of significant traces of frozen water in a permanently shadowed crater.

Astrophysicists led by James Greenwood of Wesleyan University in Connecticut analysed rock samples collected during the Apollo expeditions, looking in particular at variations in hydrogen isotopes in a water-loving mineral called apatite.

The signature, they say, points to three potential sources: from the sub-surface lunar mantle, from protons brought by the "solar wind" of particles blasted from the Sun -- and from comets.

The isotope measurements in the apatite were similar to those previously found in three well-known comets: Hale-Bopp, Hyakutake and Halley.

Comets have been described as frozen reservoirs of water orbiting the Sun, for they contain vast quantities of ice in their heads.

Under a "giant impact" theory dating back to the 1970s, the Moon was formed from part of Earth, after our planet collided with a or planet some 4.5 billion years ago.

"Significant delivery" of cometary water occurred after the Moon-forming event, suggests Greenwood's team.

Comets also provided Earth with some of its lavish endowment of water as well as key chemicals to kickstart life, according to some hypotheses.

The paper is published online by the journal Nature Geoscience.

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Jan 10, 2011
Questions, questions, questions:
1. So comets existed 4.5 billion years ago[infancy of moon]. Why are they still around because as far as I know no comet lasts for more than a few hundred years at best.
There is no Oort cloud that we can detect with current technology and even if the reports of vapoury objects out there in the outer reaches of space is true, there is far too few to give the right number and size of comets required to show up here.

2. How long would water last on the moon? 4.5 billion years? With such a low gravity? Maybe there's a way to keep the water trapped that I've not considered. Or maybe it's only on a side that never gets any sun?

3. Just how do we know that comets provided earth with some of it's water? Is there any substantiating evidence for this? Or is this simply a guess-o-fact?
4. Just how many miracles does it take? First the giant impact [for which no evidence exists] then the comet[s] smashing into the moon?

Jan 10, 2011
OK, the real reason that the water has to have come from comets is that with the giant impact theory, any and all water pesent at any point in time should have been vapourised by the impact heat. This was a satisfactory state of affairs whilst the moon was supposedly dead and dry. Now suddenly there's water on the moon so an adhoc explanation has to be found to keep the giant impact theory intact.

Have fun guys.

Jan 10, 2011
as far as I know no comet lasts for more than a few hundred years at best.

Halley's comet (and all others) would beg to differ.

2. There is no side of the moon that never gets any sun (only a side of the moon that always faces earth and one that doesn't). Although there may be parts that are shelterd from direct sunlight. with no atmosphere to disperse the heat any part that is continually in shadow (e.g. an overhang or a cave) is permanently below freezing point for water. Sublimation can still lead to loss of water.

3. Prevalence of isotopes in comets/asteroids and earthbound water (though that ratio points more to asteroids than to comets)

4. None. Miracles don't occur beause we live in a real universe. If you look the composition of the moon and the earth they are rather similar - leading to a theory about a common origin (e.g. by giant impact). It would be highly unlikely that two so similar objects just happened to be caught in a newtonian lock by chance.

Jan 10, 2011
Why are they still around because as far as I know no comet lasts for more than a few hundred years

I think we all know that as far as your knowledge goes that it barely registers on the nano-scale.

Jan 10, 2011
Regions of the moon in permanent shadow have temperatures around 40 Kelvin so there is almost no energy to kick a water molecule free. It takes a miracle for enough energy to be concentrated in one molecule it to break free (humor) - roughly a 20 sigma event, so most remain for billions of years.

It is very likely that the impactor that created the moon came from L4 or L5 - these are stable enough to have accumulated a lot of mass when planets were forming, and the best models of the moon formation involve a low-speed collision implying something in a similar orbit to earth's orbit.

The are many comet-like bodies in the Kupier belt, with Pluto being one of the biggest known. These orbits are stable over long periods (billions of years) until passing stars nudge them into resonance with inner planets, destabilize orbits and letting them trickle in.

You could find these things out for yourself, Kevin, by keeping an open mind while reading.

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