Moon rocks fall up?

From the News Services in today's Washington Post: "A chunk of the
moon that landed on Earth as a meterorite contains a new mineral ...."
Two things about this perplex me, and neither is addressed in the
article.

A meteor arrives on Earth. If it *did* come from the moon, how would
they know? Did it file a flight plan before departure? Or does it have
"Made on the Moon" stamped on it?

How would a rock from the moon come to fall to Earth anyway? Do moon
rocks *normally* fall up?


--
Harlan Messinger
Remove the first dot from my e-mail address.
Veuillez ôter le premier point de mon adresse de courriel.

Harlan Messinger wrote:

A meteor arrives on Earth. If it *did* come from the moon, how would
they know? Did it file a flight plan before departure? Or does it have
"Made on the Moon" stamped on it?

Lunar rocks have distinctive features (cemented impact debris),
and they are collected mostly on the Antarctic ice cap in places
where there are no terrestrial rocks.

Lunar materials can be distinguished from other meteorites by
oxygen isotope ratios.

How would a rock from the moon come to fall to Earth anyway? Do moon
rocks *normally* fall up?

Rocks are easily ejected off the moon by impacts. They can go
into orbit around the earth or sun, and many eventually hit the earth.

There are also meteorites that come from Mars (the SNC meteorites),
which have likely been ejected into solar orbit by a similar process.

Paul

"HM" == Harlan Messinger writes:

HM From the News Services in today's Washington Post: "A chunk of the
HM moon that landed on Earth as a meterorite contains a new mineral
HM ...." Two things about this perplex me, and neither is addressed
HM in the article.

HM A meteor arrives on Earth. If it *did* come from the moon, how
HM would they know? Did it file a flight plan before departure? Or
HM does it have "Made on the Moon" stamped on it?

In a sense, yes, it does have "From the Moon" stamped in it. I have
not followed up on this particular meteorite, but meteorites have
different isotopic ratios than terrestrial rocks. Washington
University maintains a fairly lengthy description of the analysis of
lunar meteorites at
URL:http://epsc.wustl.edu/admin/resources/moon/howdoweknow.html.

HM How would a rock from the moon come to fall to Earth anyway? Do
HM moon rocks *normally* fall up?

Moon rocks no more normally fall up than did the astronauts when they
visited.

The Moon is hit by stuff still orbiting in the inner solar system.
(Much of this stuff can be viewed as the remnants of the solar
system's formation. The Earth is hit even more frequently, but most
of the stuff burns up in the Earth's atmosphere.) If a meteorite hits
the Moon with a sufficient velocity, some of the "spray" can have a
velocity exceeding the Moon's escape velocity. It's exact trajectory
will then depend upon the velocity in the original impact and the
direction. Given that the Earth is so big and so close to the Moon,
though, it is not surprising that some of the "spray" finds its way to
Earth.

In addition to lunar meteorites, a dozen or so Martian meteorites are
known and there is a strange meteorite that some have hypothesized may
be from Mercury.

--
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sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

"Paul F. Dietz" wrote in message
...
Harlan Messinger wrote:

A meteor arrives on Earth. If it *did* come from the moon, how would
they know? Did it file a flight plan before departure? Or does it have
"Made on the Moon" stamped on it?

Lunar rocks have distinctive features (cemented impact debris),
and they are collected mostly on the Antarctic ice cap in places
where there are no terrestrial rocks.

Lunar materials can be distinguished from other meteorites by
oxygen isotope ratios.

How can we know that they can't be from somewhere far away in the universe
that happens to produce minerals similar to the ones on the moon? After all,
aren't there only two celestial bodies about whose mineral composition we
any idea, beyond the gross indications yielded by spectrometry?


How would a rock from the moon come to fall to Earth anyway? Do moon
rocks *normally* fall up?

Rocks are easily ejected off the moon by impacts. They can go
into orbit around the earth or sun, and many eventually hit the earth.

It hadn't dawned on me that they would reach escape velocity like that. Hmm.


There are also meteorites that come from Mars (the SNC meteorites),
which have likely been ejected into solar orbit by a similar process.

I reiterate my question.

"Joseph Lazio" wrote in message
...
"HM" == Harlan Messinger writes:

HM From the News Services in today's Washington Post: "A chunk of the
HM moon that landed on Earth as a meterorite contains a new mineral
HM ...." Two things about this perplex me, and neither is addressed
HM in the article.

HM A meteor arrives on Earth. If it *did* come from the moon, how
HM would they know? Did it file a flight plan before departure? Or
HM does it have "Made on the Moon" stamped on it?

In a sense, yes, it does have "From the Moon" stamped in it. I have
not followed up on this particular meteorite, but meteorites have
different isotopic ratios than terrestrial rocks.

So, presumably, do rocks from Mars, Venus, Mercury, Pluto, or any of the
other solid bodies circling the sun or any other star in the universe. I
understand how we determine that a meteor is not from *Earth*. That's not
part of my question.

Harlan Messinger wrote:

How can we know that they can't be from somewhere far away in the universe
that happens to produce minerals similar to the ones on the moon? After all,
aren't there only two celestial bodies about whose mineral composition we
any idea, beyond the gross indications yielded by spectrometry?

If it's from outside the solar system, isotope ratios will
be radically different than here, so that can be ruled out.

Inside the solar system, the oxygen isotopes are strong evidence,
as it the minerology, lack of water, shocked minerals, etc.


There are also meteorites that come from Mars (the SNC meteorites),
which have likely been ejected into solar orbit by a similar process.

I reiterate my question.

The SNC meteorites are much younger than the moon or asteroids.
Also, one of them had gas inclusions that had noble gases whose
isotopes matched those measured by mass spectrometers on the Viking
landers (they may have found these in more since then). The SNC
meteorites all fall on the same line on the oxygen isotope plot,
which is evidence they came from the same body.

Paul

"Harlan Messinger" writes:

"Paul F. Dietz" wrote in message
...
Harlan Messinger wrote:

A meteor arrives on Earth. If it *did* come from the moon, how would
they know? Did it file a flight plan before departure? Or does it have
"Made on the Moon" stamped on it?

Lunar rocks have distinctive features (cemented impact debris),
and they are collected mostly on the Antarctic ice cap in places
where there are no terrestrial rocks.

Lunar materials can be distinguished from other meteorites by
oxygen isotope ratios.

How can we know that they can't be from somewhere far away in the
universe that happens to produce minerals similar to the ones
on the moon?

Obviously we can't! However, it is more parsimonious to assume that
an object whose chemical composition and isotope ratios almost perfectly
match those of the Moon came from the Moon, than from somewhere else
entirely outside the Solar System that by some wild random chance
happened to be chemically and isotopically identical to the Moon...


After all, aren't there only two celestial bodies about whose mineral
composition we any idea, beyond the gross indications yielded by
spectrometry?

Three (the Moon, Mars, and Venus). And a number of different methods were
used to acertain chemical and isotopic composition in each case.


How would a rock from the moon come to fall to Earth anyway? Do moon
rocks *normally* fall up?

Rocks are easily ejected off the moon by impacts. They can go
into orbit around the earth or sun, and many eventually hit the earth.

It hadn't dawned on me that they would reach escape velocity like that. Hmm.

The Moon has a relatively low escape velocity --- it is only about 2.4 kps,
as opposed to roughly 11 kps for the Earth. Since the Moon's escape velocity
is only about 0.22 of the Earth's the kinetic energy required to escape it
is only about 0.048 of that required to escape the Earth, i.e., not quite 5%.

Furthermore, the fact that Newton's Law of Gravity is a conservative force
implies that lunar escape velocity is the _minimum_ speed that any unpowered
incoming object can hit the Moon with --- and since the Earth/Moon system
is deep within the Solar "gravity well," most of the objects that hit the
Moon do so at well over lunar escape velocity. (In the "worst case" scenario
that the impacting object is moving in the opposite direction around the Sun
and in a highly eccentric orbit so that it hits the Earth near perhelion,
the impact velocity can approach 70 kps, or over _30 times_ lunar escape
velocity!) Give that objects can strike the Moon at well over lunar escape
velocity, and given that escaping from the Moon is fairly easy to begin with,
is it so surprising that some of the bits and pieces knocked off might be
moving away from the Moon _almost_ as fast as the object that came in, and
therefore be moving fast enough to escape the Moon?

Finally, the Moon is still well within the Earth's "dominant region of
attraction," so any chunks knocked off the Moon have a significant chance
of winding up in a bound orbit around the Earth that crosses the Moon's orbit
about the Earth so that and subsequent close passes by the Moon and/or
orbital perturbations by the Sun may have a fair chance of deflecting them
onto orbits that hit the Earth --- albeit they are more likely to be ejected
from the Earth/Moon system. However, even those chunks that are ejected from
the Earth/Moon system will most likely wind up in orbits around the Sun that
are similar enough to the Earth's that they have a good chance of eventually
hitting the Earth on some future close passage.


There are also meteorites that come from Mars (the SNC meteorites),
which have likely been ejected into solar orbit by a similar process.

I reiterate my question.

Same principle. Objects impact Mars at well over the martian escape velocity,
and Mars is likewise a relatively small planet with an _almost_ negligible
atmosphere. Since _some_ of the debris from the impact will be moving
almost as fast as the impacting object itself, which came in at well over
escape velocity, the debris will leave Mars at more than martian escape
velocity. The debris will _not_ however, be moving at more than _solar_
escape velocity (since most of the incoming objects were themselves in
orbit around the Sun), and hence the debris will remain in solar orbit,
and may eventually be deflected by a later close encounter with Mars or
perturbations by Jupiter's gravity into an Earth-crossing orbit that will
eventaully impact the Earth if given enough time.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

"Harlan Messinger" wrote in message
...

How can we know that they can't be from somewhere far away in the universe
that happens to produce minerals similar to the ones on the moon?

We cannot.
But it's a good model (that 'lunar' meteorites are from the moon).
The oxygen isotope ratios give strong clues, the elemental compositions are
good fits, the morphological texture of the grains, etc.etc.

They quack like a moon rock, walk like a moon rock, although we cannot prove
it, they are likely to be moon rocks. They could be produced in another way,
but golly gosh, there's a big honking source of lookalike material in orbit
about us and Occam's Razor says that that's the likeliest source.
(Note, likeliest, not proven, but then science isn't about absolute truth,
just better and better models that agree with the available data)

The SNCs are even better constrained, as vesicles within contain gas that
has a composition that is essentially Martian atmosphere (which is well
known from Viking n).

After all,
aren't there only two celestial bodies about whose mineral composition we
any idea, beyond the gross indications yielded by spectrometry?

Well, Venusian regolith has been passed through a variety of nuclear
spectrometers.
(Venera, VeGa)

It hadn't dawned on me that they would reach escape velocity like that.
Hmm.

Impact gun work over the last few decades has shown that this is a viable
mechanism. There is plenty in the public domain (Journal of Impact
Engineering for starters) about the techniques, and plenty in the planetary
journals about the postulated process.

From sunny Leiden,

-James Garry

"Paul F. Dietz" wrote:

Harlan Messinger wrote:

How can we know that they can't be from somewhere far away in the universe
that happens to produce minerals similar to the ones on the moon? After all,
aren't there only two celestial bodies about whose mineral composition we
any idea, beyond the gross indications yielded by spectrometry?

If it's from outside the solar system, isotope ratios will
be radically different than here, so that can be ruled out.

What is the basis for believing the ratios on the moon to be unique in
all the universe? Are they identical for every rock *on* the moon?
Does every rock on Earth have the same isotope ratios as every other
rock on Earth? (I'm not being a crank--if there are perfectly good
answers to these questions, I'm prepared to accept that. They just
seem really unobvious to me.)


Inside the solar system, the oxygen isotopes are strong evidence,
as it the minerology, lack of water, shocked minerals, etc.


There are also meteorites that come from Mars (the SNC meteorites),
which have likely been ejected into solar orbit by a similar process.

I reiterate my question.

The SNC meteorites are much younger than the moon or asteroids.

How can that be known? Unless one presupposes the conclusion, aren't
we starting with a situation where we have no idea whatsoever what the
conditions are in which the rock formed? If so, wouldn't that mean we
have no basis for judging its age?

Also, one of them had gas inclusions that had noble gases whose
isotopes matched those measured by mass spectrometers on the Viking
landers (they may have found these in more since then). The SNC
meteorites all fall on the same line on the oxygen isotope plot,
which is evidence they came from the same body.


--
Harlan Messinger
Remove the first dot from my e-mail address.
Veuillez ôter le premier point de mon adresse de courriel.

"James Garry" wrote:

"Harlan Messinger" wrote in message
...

How can we know that they can't be from somewhere far away in the universe
that happens to produce minerals similar to the ones on the moon?

We cannot.
But it's a good model (that 'lunar' meteorites are from the moon).
The oxygen isotope ratios give strong clues, the elemental compositions are
good fits, the morphological texture of the grains, etc.etc.

They quack like a moon rock, walk like a moon rock, although we cannot prove
it, they are likely to be moon rocks. They could be produced in another way,
but golly gosh, there's a big honking source of lookalike material in orbit
about us and Occam's Razor says that that's the likeliest source.

We have a sample size of three (not two, I've been informed), out of
potentially quadrillions or more. How do we know that 50% of the
planetoids in the universe don't have exactly the same composition as
the moon? I'm exaggerating, but you catch my drift. Is there a basis
for assuming the isotope patterns around the universe to be completely
random rather than largely confined to a few specific paradigms?

(Note, likeliest, not proven, but then science isn't about absolute truth,
just better and better models that agree with the available data)

The SNCs are even better constrained, as vesicles within contain gas that
has a composition that is essentially Martian atmosphere (which is well
known from Viking n).

Ah. OK.



--
Harlan Messinger
Remove the first dot from my e-mail address.
Veuillez ôter le premier point de mon adresse de courriel.