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Water on the Moon
Juramike
post Jul 9 2008, 08:38 PM
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Recent space.com article describes H2O detected in Apollo samples of volcanic glass beads:
http://www.space.com/scienceastronomy/080709-moon-water.html

The authors estimate this implies a concentration of 260 ppm H2O in the lunar magma. [The article states this is close to the level of H2O in some Earth magma environments]

-Mike





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dvandorn
post Jul 9 2008, 10:32 PM
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Well, it makes a certain amount of sense. If the Moon was created in the Big Whack, you'd have to think that the bodies involved (proto-Earth and the impactor) would have a fair amount of water built up already. I always found it unsatisfying to believe that *none* of that water would end up in the mass that accreted into the Moon. Small amounts, to be sure -- but I always felt that *some* water ought to have been incorporated.

Seems my gut-level feeling was correct.

-the other Doug


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Juramike
post Jul 10 2008, 03:41 AM
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Water in the Moon
Water in Mercury's atmosphere.
Ice on Mars
0.002% H2O in the atmosphere of Venus
And oh yeah, water was detected on the Sun (I hadn't realized this until today): http://solar-center.stanford.edu/news/sunwater.html
(Link to Science article here)

So I suppose that makes Io the driest place in the Solar System.

Wow. blink.gif

-Mike


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nprev
post Jul 10 2008, 01:01 PM
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Likewise wow, and I'll be damned. blink.gif (Well, that probably was a given in any case...) Cool as all get-out, Mike, and mind-expanding; thanks!


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Juramike
post Jul 10 2008, 10:54 PM
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Long, long ago. When things started to outgas and cool and condense:

Was there a one-time transient lakes or seas on the Moon? blink.gif blink.gif

Or snow? blink.gif

Why not?


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dvandorn
post Jul 11 2008, 04:19 AM
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Why not? Three words: global magma ocean.

The energy of the final accretion of the Big Whack impact fragments melted the entire "crust" of the Moon. Any water that outgassed as vapor from the interior at that time would have been so highly energized by the high temperatures of the magma ocean that it would likely have had enough energy to escape out of the Moon's gravity well. Heck, some of that energy, combined with the chemical composition of the outgassing lunar crust and mantle, might well have dissociated some of the water molecules into other chemical forms.

By the time the surface cooled to the point where liquid water could actually exist (assuming a temporary atmosphere that would provide enough pressure for liquid water), the vast majority of the water that was "releasable" would already have been released, lost from the lunar gravity field, or altered into other chemical forms. So, the largest remnant of water would have been retained in the mantle. (Remember, the Moon's crust and mantle must have remained molten for long enough for a fairly high degree of differentiation, in which the relatively iron-rich magmas sank into the mantle and the relatively light aluminous feldspathic magmas floated to the top. It's not like the crust cooled to hardness in a matter of days or weeks after the final phases of that initial accretion.)

As for snowflakes, that's a different matter. At the very least, a fair amount of the water vapor that remained unaltered as it escaped the Moon's gravity had to have coalesced into tiny icy particles, which might have amalgamated into structures reminiscent of snowflakes. Some of that ice may well have settled back onto the Moon, and considering how long such particles might have hung around the Earth-Moon system, there's a chance some of it might well have accumulated at the lunar poles, in permanently shadowed locations. However, the time it took for the Moon and Earth to sweep up ice particles was probably less than the time from the Moon's crustal cooling to the end of the Late Heavy Bombardment, which probably scattered most of the accumulated ice back into space, or at least redistributed it into non-shadowed locations.

My feeling about the amount of water postulated for the lunar mantle is that, even during the most extensive periods of mare volcanism, the total amount of water available for liberation would have been so small (and the surface conditions now so near to a perfect vacuum) that deposition of liquid water would have been impossible, and only a tiny fraction of the liberated water would have ended up deposited in polar shadows.

Hopefully, all that made some degree of sense... smile.gif

-the other Doug


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Juramike
post Jul 11 2008, 05:30 PM
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QUOTE (dvandorn @ Jul 11 2008, 12:19 AM) *
Hopefully, all that made some degree of sense..

-the other Doug


Makes sense. I can imaging a vent spewing out small amounts of "condensables" like S, H2O, CO2? which then eventually sublime and escape away in the vacuum. (I still have to imagine some type of transient thin atmosphere as the surface magma cooled, but while the tail end of the outgassing was still occuring).

I did find an interesting report regarding a possible second lunar differentiation and magma episode (nearside maria?) during a hypothetical core overturn. (Pleasantly referred to in the article as "The Big Burp".)

http://www.universityofcalifornia.edu/news/article/5102

Presumably this could have generated a second venting episode at the tail end of 3.9-3.6 GYA.

I find it fascinating that both Titan and Earth's moon had a hypothetical core overturn scenario (different materials, but a similar dance) as well as some of the planets (Mars is mentioned in the article).

-Mike


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nprev
post Jul 12 2008, 12:55 AM
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I hope that LRO will dedicate some time to monitoring areas where transient lunar events (TLEs) seem to happen. The crater Aristarchus (damn...spelled that right the first time after checking on Google! smile.gif ) has a long history of these, and IIRC somebody actually got a spectrum of one back in the 60s that indicated a rich CO2 content. According to Wikipedia (yes, I know), Apollo 15 detected alpha-particle enrichment over the crater, and Lunar Prospector confirmed radon emissions.

Radon ain't great stuff, but its presence means uranium, which in turn might imply decay heat, which in turn might suggest enough local heating to produce some interesting "geo"chemistry and a source of volatiles...valuable stuff on the Moon any way you cut it.


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dvandorn
post Jul 12 2008, 04:36 AM
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I'm unsure of that radon report -- the Moon doesn't follow the same rules as worlds that accreted more slowly and melted from the inside out.

The Moon melted more from the outside in. Some decent information developed from study of lunar volcanic glasses, and the gasses and traces of volatiles found on and in the glasses, strongly indicates that the Moon's core is an unmelted, undifferentiated mass of chondritic material. There seems to be a still-molten lower mantle in which the core moves *separately* from the outer mantle and crust; The Moon's core actually seems to move and librate within at different rates, and with different motions, that the outer mantle and crust, even today.

But in a system where the entire body didn't melt, very heavy elements like uranium would sink to the bottom of the lower mantle and then stop. That inevitably means that uranium and other heavy radioactive elements would be located non-uniformly. I can imagine quite well enough local melting from radioactive "hot spots" to keep the lower mantle liquid and create just enough sluggish movement in the mantle to allow the progression of gasses up through to the surface. But in widely scattered spots.

See -- the Moon, to me, is still a puzzling place where the rules seem to have been turned upside-down, a place that still offers an awful lot for us to learn about how planetary bodies form. There have been many tantalizing hints of processes we'd never, ever have suspected. The old "been there, done that" attitude about studying the Moon is just not a rational attitude, to me... *sigh*...

-the other Doug


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kenny
post Mar 22 2014, 03:44 PM
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The BBC has a fairly detailed report on doubts cast over previous suggestions of lunar water, from this week's LPSC.

BBC News report - moon water doubts
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Vultur
post Mar 22 2014, 05:40 PM
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Interesting.

But that seems only to relate to hydrogen abundances in minerals, not affecting the LCROSS results relating to polar ice in craters.
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marsbug
post Apr 2 2014, 08:56 PM
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The ice thought to be in the shadowed polar craters is usually attributed to cometary impacts or the solar wind combining with oxygen in lunar rocks. The bbc article is talking about chemically bound hydrogen in magma that solidified billions of years ago. The hydrogen may be from ( and so indicitive of) water abundance in ancient lunar rocks, but may alsio be from other sources like the solar wind or other H bearing materials. Lastly, to quote that article:

QUOTE
They demonstrated that it was possible to start with any water composition in the magma and, by varying only the degree of crystallisation and the chlorine content, reproduce all the features seen in a diverse range of apatite from the Moon.
Bolding mine.

It sounds to me less like a confirmed number for ancient lunar water that is less than previously thought, and more like there is now reason to doubt any estimate of ancient lunar water based on apatite H content. Less than the terrestrial levels of hydration would fit better than terrestrial levels, but it doesn't sound like they have a really firm idea of how much.

Plus: The Moon is (apparently) 95 million years younger than the rest of the solar system, although some dating methods disagree.

So we may actually have more questions about the Moon than ever, which will please lunar geologists!


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