Mon 26 Apr 99 16:32
MARTY LEIPZIG
From Earth to Moon

Although I know full well that Dr. Don can (and has) easily disposed of Herr Garard's specious claims; anytime I see someone bandying about my specialty in such a ridiculous manner as George's, it absolutely gets my knickers in a twist.

Time for a little massive retaliation.

I dug up a little paper I wrote back a few tens of years ago, dusted off the references and updated the thing. I firmly affirm that the best antidote for pseudoscience is a dose of science; so brace yourself all you pseudoscientists out there; this needle's sharp, barbed, and to the point.

I hope it hurts:

Theoretical selenology: geological constraints on the theories of lunar origins. - M.R. Leipzig, c. 1999

Since that time in 1609 when Galileo demonstrated that the Moon is a rocky astronomical body just like the Earth, there has been considerable discussion and theorizing on the not inconsiderable problem of the origin of the Moon. Before the Apollo 11, et al, landings, there were primarily three different and competing theories of lunar origin. These are:

1. The "Fission Theory", proposed by G.H. Darwin (the son of Charles of other "Origins" fame);

2. The "Capture theory", and

3. The "Co-accretion" or "double planet" theory.

These theories state, briefly, that (1.) the Moon was spun out of the Earth's crust and/or asthenosphere during an early era of rapid rotation of the earth; (2.) or that the Moon was formed elsewhere and later captured by the Earth's gravitational field; or (3.) the Earth- Moon system "grew up" together out of a primordial swarm of small "planetesimals".

So these were the competing theories before man reached the Moon. Since the heady days of the late 1960's and early 1970's Manned Lunar Missions, some 382 kilograms of lunar "geological" materials ("geological" in quotations as the prefix "geo-" refers only to Earth; therefore "seleonology" and "selenological" should be the preferred nomenclature) have been returned to Earth and studied in geological (ahem) laboratories the world over.

Before we can objectively examine each of the three pre-Apollo theories (and the post-Apollo theory that sprung from this data); we must first objectively examine the information that we have in hand regarding the selenochemistry, selenopetrology, and selenotectonic history the Moon.

A simple listing and brief explanation of each point follows:

1. The Moon has no substantial metallic (Iron, nickel, cobalt ("FeCoNi")-rich) inner or outer core.

The Earth has a substantial, and segregated, FeCoNi system of cores; a liquid outer core and a solid inner core (as is thoroughly geophysically evidenced by earthquake shear wave studies). The Moon, by dint of "moonquake" geophysical studies, does not possess a similar core or cores.

2. Rocks from the Moon indicate that they, in relation relative to Earth rocks, are depleted in volatile (C, K, Na, Cl) elements.

This difference in volatile chemistry is a very important item from the standpoint of lunar history; particularly in it's relationship to the Sun. Solar radiation would have quickly depleted all the volatiles floating around the early solar system through being dissipated through the influence of the solar wind. This lack of volatiles also impacts the type, rate and volume of both intrusive and extrusive igneous activity possible on the Moon.

3. Oxygen isotopic ratios of both Earth and Moon are virtually identical.

In rocks of the same clade (comparing 'apples to apples' or 'anorthosites with anorthosites', as it were), the oxygen isotope ratios of samples from ancient shield areas of the Earth and those from the Moon display a remarkable concordance; even though it is better with older Earth rocks when comparing the older lunar material; as would be expected. But even when rocks younger than 3.0 GA are compared to concordia for lunar rocks greater than 4.0 GA; the oxygen isotopic ratios agree within a few percent.

4. Lunar rocks are enriched in refractory (less reactive, more stable) elements.

As noted in point #2, this is a direct consequence of the depletion through solar radiation of volatile elements. These are the remainder of elements left after a selective "solar winnowing" of protolunar, pre-condensation material. It also supports the idea that the Sun had ignited it's thermonuclear furnace before the Moon had formed.

5. The age of Moon rocks are between 3.0-4.6 GA.

This is a fortuitous match with the oldest terrestrial rocks which are very rarely older than 3.5 GA (rocks from Greenland at 3.65 GA hold the current record) due to both the Earth's dynamic geological history and the Moon's rather tranquil history. The Moon thusly provides evidence about the early history of the Solar System not available here on Earth.

6. Crustal thickness on the Moon and general lithotopes.

The Moon's crust averages 68 kilometers thick; which varies from essentially 0 km thickness under the Mare Crisum ("Sea of Crises") to 107 km thick north of the crater Korolev, on the "farside" of the lunar surface. Below this crust is a "mantle" (asthenosphere or selenosphere) and probably a small (approximately 2% by mass, or 300 km in radius) undifferentiated core. Unlike the Earth's mantle, the mantle of the Moon is only partially molten (a rheid solid). Oddly enough, the center of mass of the Moon is offset from it's geometric center by about 2 km, towards Earth.

Further, there are two primary types of terrain found on the Moon: the scarred, cratered and very old lunar highlands, and the relatively smoother and younger lunar maria. Maria are immense impact craters (astroblems) which later filled with basaltic lava. Finally, most of the surface of the Moon is covered with regolith, an admixture of fine dust and debris produced from impacts (impactites and cosmic debris).

7. The Moon has no magnetic field, although some lunar rock samples display remnant magnetism.

Lacking a substantial core, or core systems, the Moon also lacks a magnetic field. Strangely enough, some lunar samples do display a remnant magnetism (i.e., preferred orientation of magnetic minerals). This indicated one of three possibilities: A. the Moon did, at one time, have a magnetic field, B. the magnetism is remnant from Earth materials or C. the magnetism is remnant from some other cosmic body. As A is the most unlikely (due to relative core size) B and C also have problems due to the fact that rocks heated above the Curie (cf. Neel) point have their magnetic compasses "reset". It is conceivable, though, that B and C are both correct and the remnant magnetism seen in lunar samples is of extra-lunar origin.

8. The Moon has virtually no atmosphere.

A fairly obvious fact; as the Moon's gravity is too feeble (approximately 1/6'th that of Earth's) to hold gasses. Although recent probe studies (the Lunar Prospector Project) has evidenced the presence of water on the Moon, in deep craters near the Moon's permanently shaded south and north poles. The origin of this water is still highly conjectural.

9. The Moon has a very low density.

This is a result again of the Moon lacking a metallic core of any consequence; as is also evidenced by lunar seismic studies. The material gathered from the Moon are predominantly lightweight silicate and metal oxide minerals; in fact, asteroids are orders of magnitude more enriched in free metals than the Moon.

10. The Moon cooled quickly after initial condensation.

The slow cooling of early molten planetary material generally causes mineralogic and elemental segregation at different temperatures as the magma cools slowly over time. Due to it's relative surface area, low mass and low density, the Moon cooled relatively rapidly thus preventing coalescence of a metallic core and dynamothermal mineralogic differentiation.

11. Lunar rock samples have similar, though not identical, geochemistries.

This implies a similar, although non-identical, genesis for both lunar and Earth materials. Also, it indicates that thermal and dynamic histories of the two bodies have varied widely over the span of geological time.

12. There is no evidence of orogeny (mountain-building), subduction, manifest plate tectonics or vulcanism on the Moon.

The first three phenomena are fairly easily understood through the lack of differentiation of the Moon and it's lack of an internal heat engine of any significance. The vulcanism referred to here is vulcanism of the explosive or Strombolian (Peleen) type. Surely, volcanics have played a role in the selenological history of the moon; but all these are related to impacts of meteors, resulting in flood (mare) basalts, not rhyolite/andedacitic lavas and their associated landforms.

There are a series of other lunar facts available which are not selenological (cf. geological) in nature; but are rather related to orbital physics or are cosmological. These are:

1. The Moon is receding from the Earth.

Gravitational forces between the Earth and the Moon produce some fascinating effects. The most obvious are the lunar tides. Gravity works in such a manner that the Moon's gravitational attraction is stronger on the side of the Earth nearer the Moon and weaker on the opposite side. Now, since the Earth is not perfectly rigid (nor are the oceans of Earth), it is elongated along that plane. This is manifested as two small "bulges" on the Earth (and much more noticeably in it's oceans), one in the direction of the Moon, and one directly opposite.

But the Earth isn't perfectly fluid, either. The Earth's rotation offsets these bulges slightly ahead of the point that directly beneath the Moon. This force is off-centered between the Earth and the Moon, and produces a torque on the Earth and an acceleration on the Moon. The net result is that there is a transfer of rotational energy from the Earth to the Moon, slowing the Earth's rotation by 0.0015 milliseconds per year and raising the Moon to a higher orbit by approximately 3.8 centimeters per year.

2. The Moon rotates in orbital synchronicity with the Earth.

As a result of point #1, the asymmetric nature of the gravitational interaction between the Earth and the Moon results in the Moon's orbital synchronicity, which means that it is locked in phase in it's orbit so that the same side is always facing Earth. As the Earth is being slowed by the Moon, so was, in the distant past, the Moon's rotation slowed by the Earth; although the effect on the Moon from the Earth was much stronger (inverse square law, again). When the Moon's rotation rate was slowed to match it's orbital period, there was no longer an off-center torque on the Moon and a stable situation was achieved.

3. The orbit of the Moon "wobbles".

Due to it's slightly elliptical orbit, the Moon appears to "wobble" so that a few degrees of arc of the "far-side" can be seen from Earth. This is probably a remnant of the commotion and upheaval in the era when the Moon was formed. This also is explained the wide variations in orbital inclinations, eccentricities, rotational periods and spin axis directions observed among the inner terrestrial planets of the Solar System.

4. The Earth-Moon system are of the same age.

This is evidenced by the concordance of Rb/Sr ages of the oldest Earth rocks (metamorphosed granitic rocks found in Greenland) and those from the highlands areas of the Moon.

5. The orbit of the Moon lies in the same orbital plane of other planets in the Solar System, not around the equator of the Earth.

Other moons in the Solar System orbit around their planet's equator; in the same fashion of planets all orbit the Sun in the plane of the Sun's equator. The Moon alone is unique because it does not orbit around Earth's equator, it orbits in the plane of the Sun's equator; rather like a small independent planet than a moon.

6. Relatively speaking, the Moon is small.

The Moon is 81 times less massive than the Earth. This is a simple fact and consequence of celestial mechanics and known to a high degree of precision. Therefore, plate tectonic movements, if any, were slight on the Moon, and with less "insulating cover", the Moon cooled much more quickly than more massive bodies. There was less chance for crustal and asthenospheric differentiation, therefore, the Moon is more inhomogeneous.

7. Finally, there is the question of conservation of angular momentum.

There appears to be an excess of angular momentum for the simple Earth-Moon system. Part of this can be accounted for my orbital peculiarities, such as the aforementioned "wobble" and eccentricities. But, working backward, the amount of angular momentum gave an early Earth-Moon system a rotational period of just eight hours. This is not enough for one theory, and far too much for the others. About more later.

Well, there you have, in a brief manner (!), a synopsis of what is known, from a geological (pardon the convention), physical, cosmic and celestial standpoint in the lines of fact and evidence about the Moon.

So, back to those previous theories of the origin of the Moon, what can be said?

Theory #1, the Fission Theory, is rendered invalid by the differences in core size, distribution, lack of metallics on the Moon, loss of volatiles, and particularly, angular momentum. For fission of a Moon-sized chunk of the Earth to occur, the Earth would have to have a two hour rotational period, not eight hour, as noted above.

Bang goes that theory.

Theory #2, the Capture Theory, claims that the Moon was formed elsewhere in the cosmos and was captured by the Earth during a close encounter. Unfortunately, there is no known mechanism which could have dissipated enough of the Moon's kinetic energy for it to be captured. Further, if the Moon formed somewhere other than in the local system, why, save and except for the FeCoNi content and a few other assorted abnormalities, are the earth and Moon as similar in composition as they as the are?

Bang goes theory #2.

Theory #3, the Binary Accretion Theory, is dealt a death knell through the lack of FeCoNi materials on and in the Moon. How can the Earth and Moon accrete from the same chemistry cloud of pre-planetary debris, and have such differing ferrous chemistries? The answer is, it can't, it didn't, and it don't.

Bang goes theory #3.

Seems that we've run out of theories.

Well, when you have loads of data and a paucity of theories, what is it that science does? It evolves new theories which explain the data, both the old and the new.

And in 1984, that's precisely what happened.

This new theory, dubbed the "Giant Impact" (or "Big Whack") theory, takes into account every of the previously mentioned lunar facts and ties them up rather nicely in one neat package.

And it goes something like this:

It is generally accepted that the early Solar System of some 4.6 GA past was not as neat, orderly nor quiet place as it is now. It was a period of violence, not filled with swarms of orderly 10 km diameter planetesimals accreting directly into the four inner planets, but rather teeming with throngs of embryonic planets accreting from the material left from the condensation of the local system. These would have a huge range of size, mass and velocities in closely spaced orbits. The final accretionary stage would be one of equilibrium after initial accretion of often rather large bodies, punctuated with the occasional huge impact where bodies of near comparable size colliding with one another at high velocities. The chaos of the era explains the wide variation in orbital inclinations, ellipcities, rotational periods and spin axis precessions in the inner planets and their moons; as previously noted.

The glancing, not direct, impact of a present-day Mars sized body with the early Earth at 4.6 GA would have vaporized the impacting planetesimal and much of the Earth's primordial crust and upper asthenosphere. Some of this material rained back down on Earth, but much was lost, accelerated to escape velocity by the sheer force of impact; to settle into orbit as a ring of hot gas and vaporized rock, around the Earth. Eventually, thanks to gravity-aided celestial mechanics, the Moon accumulated from the cooling material in this ring.

The Big Whack explains all the above cited facts. Briefly, and specifically: the material liberated by the impact would be from the crust and upper asthenosphere, and therefore deprived of the FeCoNi which would reside deeper in the inner and outer cores of the early differentiated Earth. The vaporized material would be acted upon by solar wind and winnowed of lighter volatile elements (C, Na, Cl, S) and preferentially enriched in refractory elements; which is precisely what is noted in comparative mineralogy of the Earth-Moon system.

Also, the huge amount of angular momentum brought in by the impactor would go directly into the orbiting debris, although the early Earth would also get a fair boost. It is from calculations involving this angular momentum we know the mass of the Mars size impacting bolide.

Further, part of the Moon's material is composed of matter from this impacting body, so while the chemical compositions and distribution of elements are similar between the two, they are not identical, due to the inclusion of extra-systemic (impactor) chemistry.

The Moon's orbit, which is moderately inclined with respect to the equator of the Earth, reflects the trajectory of the intruding body as it struck the Earth; and the resultant coalescence of the Moon is normal to the Sun's equator (a unique situation in the Solar System).

Finally, major early collisions of huge planetesimals help to explain other oddities seen in the Solar System; such as the high FeCoNi content of Mercury and the inclination of the orbital plane of Uranus.

*30*

So, George, the Earth did not "throw rocks into space to create a moon". Rather, the early Earth was impacted by a Mars sized projectile, had a portion of it's early crust and upper mantle ejected into orbit, along with vaporized impactite, to form a ring around the Earth. This material was winnowed by solar activity, and the Moon later condensed from this material.

Muck about in a scientist's backyard and you're bound to come up with mud on your face.

So there.

... "Around the sun we go, the moon goes round the Earth."