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No Ice on the Moon
eldavojohn writes "In 1994, there was speculation that there might be a southern ice cap on the moon — something our exploration of it could take advantage of. Unfortunately, recent evidence has come to light revealing that this probably isn't true." From the article: "If there is any ice at the South Pole, it probably comes from tiny, scattered grains that probably account for only one or two percent of the local dust, the authors suggest. "Any planning for future exploitation of hydrogen at the Moon's South Pole should be constrained by this low average abundance rather than by the expectation of localized deposits at higher concentrations," the paper says soberly. The research involved sending a radar signal from the Arecibo telescope in Puerto Rico. The signal hit the southern lunar region and the reflection was picked up by the Green Bank Telescope in West Virginia." Well, it looks like we're going to have to hit Hoth before we hold that kegger on the moon."
These types of stories seem to go back and forth. First the moon had no water/ice/hydrogen, then they thought there might be some subsurface ice/water, then maybe ice at the south pole, now nothing. I wouldn't be surprised if another study came out revealing that the moon was actually made of ice with a couple feet of moon rock and dust on top. If nothing else, the constantly changing ideas of our moon's makeup is keeping plenty of scientists employed.
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Note that no water doesn't mean no hydrogen. You can produce hydrogen from most rocks too. It takes too much energy, I hear the peanut gallery cry? Nah, energy on the moon is abundant -- there's no atmosphere to filter the sunlight, and all you need is time to wait. So what if it takes fifty times as much free energy as breaking up water?
Not as impressive as Arecibo though. I was expecting more like an array but it really is just one giant dish.
It is *just* one big dish.. but it's also the world's largest full steerable telescope (aricebo isn't fully steerable). Also, it's one of very few off-axis paraboloid telescopes. (One of the nice things about this is the collection unit doesn't block any of the light that would be incident on the reflector.)
For impressive arrays, check out the VLA, ALMA (soon), or SKA (later). I was at the VLA last summer as part of my research (I do astronomy), it is very impressive. I was able to go into the dishes.. they're huge.
1% (speculated proportion of water in the dust) seems like a lot to me. Separating that 1% of water from the dust would probably be more cost effective than bringing it up from Earth.
Wasn't there also talk about ice in deep craters situated where the sun can never shine on it?
...at least that's what I recall. It sounds kinda plausible.
The theory was (and I hope I have this right) that cometary ice must impact the moon from time to time - so there is water there from time to time - but whenever the sun shines, in the absence of an atmosphere, the water will evaporate (sublimate?) away quite quickly during the lunar day - then freeze out of the atmosphere during the night.
This mechanism would generally keep whatever water molecules there is up there moving around...*UNTIL* (by chance) it lands somewhere where there is never any sunlight - inside a cave or a deep crater. At that point it must settle - and there is no longer a mechanism to move it around again. With no atmosphere to scatter sunlight, permenantly dark places will be profoundly cold.
It follows then that whatever water there is will always end up in these relatively rare places EVENTUALLY - so given enough time, all of the moon's water would end up stashed away in just a few easy-to-predict places.
Furthermore, we'd never be able to see those places from earth-bound or low orbit telescopes because any place we can see must also collect sunlight at some point in the lunar orbit.
www.sjbaker.org
This study does *not* indicate that there is no ice in permanently shadowed areas of the Moon. Before this study came out, there were two widely cited lines of evidence for ice:
1) Lunar Prospector found elevated hydrogen near the poles using a neutron spectrometer
2) Clementine bi-static radar (and later ground-based measurements) found backscatter effects that looked like ice
Most planetary geologists weighted (1) more heavily. There was always a lot of argument about interpretation of (2).
Now this study comes along and fairly definitively throws out (2) by showing the data has another explanation. Fine. From (1) we still have solid evidence of hydrogen near the poles, and most geologists would agree that the likeliest explanation of that hydrogen is water ice deposits in permanently dark crater interiors (the only places cold enough for ice to be stable in a vacuum).
So the main impact of this study is to suggest (but not prove) that ice, if present, is not found in clumps of centimeter scale or larger. And the 1% concentration figure they cite is a *lower* bound, not an upper bound. That is, ice could be more concentrated than 1% in some regions below neutron spectrometer resolution (e.g. kilometer scale) and we would have no way to know.
The meaning of terms depends on context and audience. If you refer to radio waves as "light", you will be misunderstood not only by laymen, but also by people who know about the physics of "light" in many contexts. Furthermore, when talking about blocking, wavelength matters, and it makes a big difference whether it's "(visible) light" or "radio waves". In this context, "radio waves hitting the dish" is the correct usage, while "light hitting the dish" is sloppy, unnecessarily imprecise, and misleading. If you want to be generic about it, the correct term is "electromagnetic radiation hitting the dish", not "light hitting the dish".