Index Home About Blog
From: Henry Spencer <>
Subject: Re: lunar resources (was Re: NASA's $3 billion ET Moonbase)
Date: Tue, 9 Jan 1996 19:34:55 GMT

In article <4cqjf0$> doyle@PROBLEM_WITH_INEWS_GATEWAY_FILE () writes:
>> > The moon is certainly our sibling as a child of the sun. and because of
>> > the common orgin I see it as our runt twin.
>> Definitely not a twin; not even the same species...
>The earth and the moon formed out of the same material. The earth hung on
>to its water and atmosphere because it was bigger. That is the difference.

No, sorry, that theory was thoroughly discredited by the Apollo samples.
The average composition of the Moon is *not* the same as the Earth's.  Its
density is much lower than Earth's, or indeed any inner planet's.  It is
iron-poor, and the iron it does have is less oxidized than Earth's.  The
lack of volatiles is far more severe than the smaller mass by itself can
explain; even water of hydration -- chemically bound to the rocks -- is
missing, and it's not just gases, even things like sodium and lead are
scarce on the Moon.  The Moon is not a twin of the Earth; the differences
are large enough that the two bodies must have had seriously different

In addition, the Moon's orbital angular momentum is very high, and its
orbit is not in the plane of Earth's equator (as one would expect if they
had accreted out of the same cloud, the classic "sister" theory).  (The
other classic theories were "daughter" and "wife"; they didn't fit the
facts any better, although for varying reasons.)

The accepted theory of the Moon's origin -- the only theory that appears
to explain the facts reasonably well -- is that the nearly-complete Earth
was struck off-center by a Mars-sized object, and the Moon is essentially
debris thrown up by that collision.  This was a minor event for the Earth,
but an utterly catastrophic one for the Moon, this being the necessary
difference in histories.

>> > It can tell us a lot about what the primative earth was like.
>> Not really.  The two would have diverged radically after the impact.
>Exactly the moon has remained mostly unchanged...

No, it hasn't.  It has evolved in its own way, by different mechanisms;
the lunar surface and interior of today are not those of the time of
its formation.
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |

From: Henry Spencer <>
Subject: Re: ? The Origin of the Moon
Date: Thu, 14 May 1998 14:05:20 GMT

In article <>,
John Beaderstadt  <> wrote:
>> I've heard that the moon may have once been part of the Earth, but
>> surely this would have left some evidence on the Earth
>I've heard that some people consider the size and shape of the Pacific
>Ocean to be evidence. I don't necessarilly believe, or disbelieve, it.

Before about 1960, the proponents of the "daughter" theory, in which
the Moon was the result of an early Earth fissioning, usually identified
the Pacific as the scar left from the fission event.

Then plate tectonics established that the Earth's surface rearranges
itself quite regularly -- so the current size and shape of the Pacific
are recent and temporary -- and the Apollo samples killed the "daughter"
theory of the Moon's origin.
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

From: Henry Spencer <>
Subject: Re: ? The Origin of the Moon
Date: Tue, 19 May 1998 00:49:17 GMT

In article <>,
Graham Nelson  <> wrote:
>Curious how the history of geology can be written in terms of
>the current fashionability of catastrophic explanations over
>gradual ones...

Not so much fashionability, as willingness to admit the possibility when
it seems to be the best fit to the facts.  Classical catastrophism has not
really made a comeback, but individual catastrophic events are now deemed
respectable when there is specific evidence for them.

>I'm intrigued that the proposition of a Mars-sized
>body crashing into Earth has been accepted, at least pro tem.
>It sounds so Velikovskian.

Unlike Velikovskianism, it has the merit of explaining a number of firmly
established facts which are otherwise pretty much inexplicable.  It might
not have been considered seriously, had it not been that all the other
lunar-origin theories had already gone down in flames.

>How many planets or planetesimals is the early solar system
>thought to have had?  How would it compare to the present state?
>(Presumably the answers are "don't know" and "don't know", but
>we must have some guesses.)  Do great big collisions naturally
>turn up in simulations?

Yes.  The best estimate right now is that growth by simple accretion and
related processes takes you only as far as Mars-sized objects at best, and
after that you get bigger objects mostly by collisions among those.  So
Earth is probably the result of quite a number of such collisions, and the
one that formed the Moon was merely the last (or close to last).
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

From: (Henry Spencer)
Subject: Re: Question about the Moon...
Date: Sun, 2 Aug 1998 22:07:40 GMT

In article <>,
Velovich <> wrote:
>Ok - there was a great deal of talk in the days where Apollo was just getting
>off the ground (sorry 'bout the pun...)  about whether or not  the moon was a
>piece of the earth.  What did we learn from Apollo?

"Lunar Sourcebook" by Heiken et al, a 736-page book, is a terse summary of
what we know about the Moon, with many references to other sources for
greater detail.  Almost everything in it comes from Apollo.

If what you're wondering about is specifically the *origin* of the Moon,
the Apollo samples shot down all three then-popular theories:  "daughter"
(piece of Earth), "sister" (formed in orbit around Earth at same time as
Earth), and "wife" (formed separately and then later captured).  The Moon
is too similar to Earth to have formed independently and too different to
have had exactly the same history.

The only theory which explains the evidence well is the giant-impact
theory, in which the proto-Earth was hit off center by a Mars-sized object
which merged with Earth, and the Moon is what's left of the debris that
was splashed off.  This accounts for both the many similarities and the
clear differences.  In particular, having the Moon condense out of molten
debris helps explain how it lost all its volatiles.  People are still
arguing about the details, but the basics look solidly established.
Being the last man on the Moon is a |  Henry Spencer
very dubious honor. -- Gene Cernan  |      (aka

Newsgroups: sci.astro,alt.sci.planetary
From: (Henry Spencer)
Subject: Re: Lunar Data Support Idea That Collision Split Earth, Moon
Date: Thu, 25 Mar 1999 17:25:16 GMT

In article <>,
Peter Munn  <> wrote:
>...This latest report is saying the material is not from the
>Earth's core, so presumably they're saying the collision was
>considerably off centre...

That has been clear from the start.  The Moon's average density is far too
low for its average composition to be the same as Earth's, and it's been
almost certain for some time now that the Moon is nearly straight mantle
rock, with little or no heavy core.  (LP has now tentatively confirmed a
rather small iron core.)  Also, the angular momentum of the impacting
body, preserved by an off-center impact, accounts for the Moon's orbit
being at a substantial inclination to Earth's equator.

>The problem I see is that initially all of the mass leaves the "surface"
>either at escape velocity or in an orbit, like all orbits, which ends up
>where it started, that is back down on the Earth again.  To get into a
>true orbit, there will need to be interactions between the material...

Probably correct.  The mass-particle model of the bodies involved is a
poor one in this case, because we're talking about large non-spherical
non-rigid objects, with all kinds of gravitational, collisional, and
gas-mediated interactions possible.

>But 5% just seems too high to me...

I haven't seen the numerical details, but I'm told that a Moon-sized
orbiting object seems feasible.  The initial orbit was probably quite
elliptical, and was circularized and then enlarged by tidal effects.

>...The impactor will
>have arrived in excess of escape velocity, so some will escape and much
>will go very high where it will be relatively sparsely scattered and so
>not interact much until it is back near the Earth...

Remember, though, that the flip side of this (hmm, wonder what fraction
of the audience doesn't know what a flip side is? :-)) is that it does
not take much of an interaction at very high altitude to turn a grazing-
impact trajectory into a not-quite-grazing elliptical orbit.

I'm not sure how well the details have settled out as yet.  But the big
reason why this theory of the Moon's origin is widely accepted is that its
problems lie only in the details.  It may need some work yet, but it fits
the basic facts.  The alternative theories are stone cold dead, killed
completely by the Apollo samples.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: Henry Spencer <>
Subject: Re: Asteroid impacts and Planetary Evolution (was Re: What if . . . 
Date: Mon, 5 Jan 1998 01:33:12 GMT

In article <>,
Ken Hayashida, MD <> wrote:
>> ... (The Moon is too like Earth to have originated
>> independently, but too different to have had exactly the same history.)
>I believe that the main difference in terran and lunar evolution is due
>to the mass of the two planetary objects.  Lunar evolution does not
>include radioisotopic heating which could drive the core temperatures
>high enough to create convection forces.  Those convection forces are
>responsible for volcanism and plate tectonics on Earth, Venus, and
>now potentially Mars.

Well, that's *a* difference... but note that there has been volcanic
activity of a sort on the Moon -- the maria are lava plains!  This is
beyond dispute; there are places where the lava has only partly drowned
earlier features.

The differences between Earth and Moon are well beyond what could be
accounted for by the size difference alone; hence the failure of the
pre-Apollo theories suggesting that the Moon formed in orbit around the
Earth, or split off from the Earth.  In particular, one of the most
striking features of the lunar samples is the near-total lack of water and
other volatiles -- not even water of hydration is present, even though
it's common in Earth's mantle and crust.  Any theory of lunar evolution
has to explain how the Moon got very thoroughly baked early in its
evolution, while the Earth did not get similarly baked at a similar stage,
despite their isotope ratios being so similar that they clearly had a
common origin.  This killed *all* the pre-Apollo theories.  The
giant-impact theory deals with this reasonably well.

>Does anyone know if volcanism or plate tectonics is conjectured as the
>process on the Jovian moon Io?

Last I heard, there's lots of volcanism (both sulfur volcanism and plain
old-fashioned silicate-rock volcanism), but no particular sign of any
plate tectonics.  Mind you, Io's surface is so young that it's quite
possible that the evidence has simply been erased.

>> The significant issue, actually, is not the ejection of the Moon, but the
>> arrival of a Mars-sized impactor, most of which merged with the Earth.
>Well, isn't the issue the mass which is ejected compared to the mass
>which merged?

The Moon's mass is simply too small to be an issue.  Unless substantial
amounts of mass got ejected at beyond escape velocity, the primary issue
in the change of Earth's gravity is the arriving mass, period.

>Supposing that some ejected material accreted into the moon, is
>it really possible to guess how much material was ejected?

The key question is how much of the ejected mass eventually re-accreted
onto the Earth.  Last I heard, we don't understand the details of the
impact process well enough to set good limits on this.

>If an object the size of Mars collided with a planet, that suggests to me
>that a significant change in the gravitational force of planet would
>occur (due to the added mass).  Am I wrong?

I'm afraid so.  Mars is only about 1/10 of the mass of the Earth; the
change in gravitational force would be small.
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        |

From: Henry Spencer <>
Subject: Re: Asteroid impacts and Planetary Evolution (was Re: What if . . . 
Date: Mon, 5 Jan 1998 16:24:50 GMT

In article <>,
Ken Hayashida, MD <> wrote:
>NSSDC says that the lunar mass is estimated at 1% of Earth's.  I'm not
>sure how they can actually calculate this, is it from gravitational

Correct.  The Moon's gravitational attraction on the Earth is observable,
so the ratio was moderately well known even in pre-spaceflight days.
Radio tracking of early lunar spacecraft nailed it down very, very, very
precisely.  The Moon's mass is 0.0124532 of the Earth's.  (The data is
actually more precise than that -- the precision to which such results can
be had is really ridiculous -- but I had to do a quickie calculation to
get the ratio, and my calculator program only does six digits by default.)

>...Dr. Urquhart seems to surmise that two
>lunar masses of material were lost from the Earth-Moon system.
>So, I'd like to offer a parting question...what happened to all the
>material?  For years we've noted the iron-laden meteroites which
>occasionally impact Earth.  Could these meteroites actually be
>returning from whence they came?  Perhaps these are actually just
>remnants of the Earth's core which were ejected by this collision 4-5
>billion years ago?

Unlikely.  Objects ejected by the Moon-forming event would necessarily
have been in Earth-crossing orbits.  The lifetime of such objects can be
calculated (as a statistical distribution), and it's short; some hit
planets, most are perturbed by near-misses into Jupiter-crossing orbits
and ejected from the solar system.  It's not plausible that substantial
numbers of them would still be around after 4-5 billion years.

The general history of meteorites is moderately well understood.  Some of
the proto-asteroids got big enough to differentiate somewhat before being
broken up by collisions.  The nickel-iron meteorites are pieces of their
cores; other types of meteorites are pieces from other regions of their
interior.  (In fact, one strong bit of evidence for this picture is that
we *do* see pieces from all the likely regions.)  There is no need to
invoke an additional source for the nickel-irons in particular.

>Maybe that's why the darn things always end up dating around that time?

Much simpler cause for that:  the entire solar system formed around then.

>In addition, could the origin of Earth-crossing asteroids and even
>between Mars and Jupiter be related to this large impact?

The average lifetime of Earth-crossing asteroids is too short for that.
Some of the Earth-crossers are dead comets (there is at least one case of
a lost old comet being rediscovered as an Earth-crossing asteroid), the
rest are most likely perturbed from the main belt.

As for the main belt, there's no obvious way to get so much debris from
Earth-crossing orbits into near-circular main-belt orbits.
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        |

From: Henry Spencer <>
Subject: Re: Asteroid impacts and Planetary Evolution (was Re: What if . . . 
Date: Mon, 5 Jan 1998 16:43:19 GMT

In article <>,
Ken Hayashida, MD <> wrote:
>From looking at Venus, one can see that the planet has a run-away
>atmosphere.  Why didn't that happen on Earth?

This is fairly well understood.  Earth's CO2 was not lost; it is locked up
in carbonate rocks.  Those formed in Earth's oceans... which remained
stable, even in the early days of a thick CO2 atmosphere, because Earth
was cooler than Venus.  The key event in Venus's runaway-greenhouse
catastrophe was when its oceans boiled and the water was lost to space
(the isotope ratios in Venus's atmosphere show clear signs of this event).
It didn't happen on Earth because we're farther from the Sun; Earth's
oceans would still be liquid even if Earth had Venus's atmosphere.

>My current question is whether or not the
>giant impact may have stripped a significant amount of gases away from
>the planet's surface (during the impact itself).

Possibly.  Last I heard, there was still considerable debate about the
importance of such processes in Earth's early history.

>...Also, even if the gravitational force of Earth would be changed
>by a few percent, is that enough to alter the escape probability of
>light elements from the atmosphere?  (i.e. could hydrogen or helium
>be more likely to leave Earth's atmosphere at a slightly lower
>gravitational force?)

A small difference in Earth's gravity would make only a small difference
in the loss rates of light gases.  It's not necessarily linear, mind you,
but Earth's gravity is *much* too weak to hold such gases efficiently, so
a small change would have been, well, a small change.

>Only if the gravitational force's change had neglible impact on the
>atmosphere's ability to hold water of neutral pH.  If hydrogen leaves
>the atmosphere (due to lower g-force), then the aerosolized water
>becomes less acidic...

The significant issue in Earth's loss of water, over at least *much* of
its history, is not gravity but the structure of the atmosphere.  The
loss rate of water is very low today because Earth's upper atmosphere is
extremely dry, and it's dry because the stratosphere is very cold and the
water condenses out before reaching high altitudes.  (The water-vapor
content of rising tropical air drops by *several orders of magnitude*
over only a few kilometers of height at the top of the troposphere.)

By the way, hydrogen doesn't acidify water.
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        |

Index Home About Blog