Re: Amazing facts from NASA

In article <454e1ca9$0$12118$88260bb3@xxxxxxxxxxxxxxxxx>,
Guy Fawkes <spare_the_rod@xxxxxxxxxxxxxxxxx> wrote:
Question: on what other planets/astroids etc. does helium-3 occur naturually
in mineable quantities? Why is it so abundant on the moon?

It's not actually particularly abundant on the Moon -- getting substantial
amounts of it would require processing huge amounts of lunar soil -- but
its abundance on Earth is essentially zero, so even the Moon's modest
supply of it looks like a lot.

Earth's helium is nearly all from radioactive decay in the interior --
every alpha particle becomes a helium nucleus when it slows down -- but
that source produces essentially exclusively He-4. There is the barest
trace of He-3 from lesser sources, detectable but not useful. The only
source of He-3 at the moment is the decay of reactor-produced tritium,
mostly in nuclear weapons. (Small amounts of tritium are used to boost
yields in fission bombs in particular, and it *must* be regularly purified
to remove the decay He-3, which has all the wrong properties.)

The Moon gets it from the solar wind. The helium in the lunar soil is
atoms implanted by the solar wind hitting the lunar surface. Most any
airless body should have some... but the Moon is a particularly favorable
case, a relatively large body with thick regolith (stirred by impacts, so
all of it gets to the surface in the long run), at modest temperatures
which permit hanging onto most of what arrives.

Mercury's surface is probably mostly too hot to hang onto much implanted
gas. Most asteroids are probably too small to hang onto regolith well, so
they likely don't build up a thick layer. And the solar wind is pretty
thin by the time it reaches the small bodies of the outer solar system.
Plus, of course, the Moon is more easily accessible than most of these
alternatives.

The *big* supplies of He-3 in the solar system are in the atmospheres
of the gas-giant planets. But mining it from there is a challenge.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | henry@xxxxxxxxxxxxx
.

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