From: fcrary@rintintin.Colorado.EDU (Frank Crary)
Subject: Re: lunar resources (was Re: NASA's $3 billion ET Moonbase)
Date: 10 Jan 1996 04:21:11 GMT
In article <email@example.com>,
>> > A lot of the material in the metoerites is gone because of its journey to
>> > the earth.
>> There's still plenty of it when it hits, and most of it is unchanged by the
>> passage through the atmosphere (they tend to be cold when they land). A
>> quick look in my solar system atlas shows one meteorite weighing 60 tonnes;
>> another at 27 tonnes (those are single chunks on the ground). Compare that
>> to the 800 pounds of lunar rock from the Apollo program. I guess that we
>> do have a fair amount of direct samples of asteroids to help plan a mining
>These meteorites weighed a lot more before they hit the atmosphere.
This is not the case. Meteorites are moving so rapidly that
they spend very little time in the atmosphere. Alteration
from passing through the atmosphere is bimodal. If they
are large enough to reach the surface, they are almost
unaltered. The millimeter or less closest to the surface
is melted, but the rest of the rock is untouched. If
they aren't large enough, they don't reach the surface
at all. The transition between these two possibilities
is very small. If memory serves, around one half to two
centimeters in size. The meteorites we collect on the
surface did _not_ weight a lot more before they hit
>Also one thing scienctist look at very carefully is the element
>and isotobe ratios, you lose a lot of data coming through the
Is this way, for example, the SNC meteorites have isotope
ratios that exactly match those of Mars, and ones that
are nothing like terrestrial isotope ratios?
From: firstname.lastname@example.org (Henry Spencer)
Subject: Re: Chemists Find Tagish Lake Meteorite In A Class By Itself
Date: Sun, 10 Sep 2000 04:47:19 GMT
In article <NN3u5.12238$Oc.email@example.com>,
Robert Brown <firstname.lastname@example.org> wrote:
>Are there any meteorites that are thought to have originated on Earth, .e.g.
>rocks that were blasted via an impact into space, dwelt there for many
>millions of years, and then returned to Earth as a meteorite...
No, with the borderline exception of tektites. Tektites are tentatively
thought to be splashes from impacts, tossed into suborbital trajectories
(i.e., they spent essentially zero time in space); however, their origin
is still somewhat debated (although the old theory that they were from the
Moon was pretty much destroyed by their numerous differences from the
Apollo lunar samples).
Meteorites thrown off Earth would, no surprise, be in Earth-intersecting
orbits, and hence their expected lifetime is not long -- millions of
years, not billions. It doesn't take them a terribly long time, on
average, to end up hitting an inner planet, being thrown into the Sun, or
encountering Jupiter and being ejected from the solar system.
Meteorites are rarely actually seen to fall. Normally, rocks are
recognized as potential meteorites because they show up in places where
Earth rocks are not often found, e.g. the Antarctic ice sheet. Those
places tend to be of fairly recent geological origin, which means that
only meteorites which have fallen recently are recognized as such.
The bottom line from these two facts is that an Earth meteorite would have
to have been ejected from Earth relatively recently to be found and
recognized as such. That's a problem because Earth's thick atmosphere and
strong gravity make it quite difficult to get rocks off the planet. It
takes quite a large impact, and such impacts simply haven't happened much
since the early days of the solar system.
There are no Venus meteorites known either, for the same reasons. The
known planetary meteorites are from smaller planets with thin or no
atmosphere, where ejection is relatively easy. The one candidate that
hasn't been seen yet is Mercury... and it is hard to get meteorites out
here from so deep in the Sun's gravitational field. It should happen
occasionally, though, and if we keep looking it's likely that some will be
found (although recognizing them may be tricky, since we don't know much
about Mercury's surface composition).
>Finally, within the same context, what do you think would happen to the
>oxygen isotope ratios in a small (ALH84001-sized) rock subjected to the
>solar wind for 65 million years?
Nothing. The solar wind is too thin and too easily stopped to affect more
than a thin surface layer. (Remember, we have actual evidence on this,
from lunar rocks.)
Microsoft shouldn't be broken up. | Henry Spencer email@example.com
It should be shut down. -- Phil Agre | (aka firstname.lastname@example.org)