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Posted on 10/11/2001 6:53:58 AM PDT by callisto
If you ever find yourself at a cocktail party of astrophysicists and don't know what to say, try this: "But what about the angular momentum?" No matter what the topic of conversation, you'll be guaranteed to sound erudite. Nearly every field of astronomy, from galaxy formation to star formation, has an "angular momentum problem." Nothing in the cosmos ever seems to spin or orbit at the rate it should.
The moon is no exception. It is the flywheel to end all flywheels; if its orbital angular momentum were transferred to Earth's axial rotation, our planet would come close to spinning apart. No other planetary sidekick wields such power, except for Pluto's cryptomoon, Charon. The moon's prodigious angular momentum is one reason that planetary scientists believe that it formed when another planet--no piddling asteroid but an entire Mars-size world--struck the proto-Earth.
Unfortunately, researchers have had trouble getting the giant-impact model to work without the contrivances that scuttled earlier theories. "Putting enough material into orbit to form the moon seemed to require a rather narrow set of impact conditions," says Robin M. Canup of the Southwest Research Institute in Boulder, Colo. But a new study by her and Erik Asphaug of the University of California at Santa Cruz may have broken the logjam.
Although the giant-impact model became dominant in the mid-1980s, fleshing it out has been a gradual process. Simulations have attempted to reconcile the angular momentum with three other basic facts: Earth's mass, the moon's mass and the moon's iron content. These four quantities depend on three basic attributes of the collision: the impactor's mass, the proto-Earth's mass and the impact angle.
Four facts and three parameters is a recipe for contradiction. To explain the moon's low iron content, you need to avoid a grazing collision (corresponding to a large impact angle), lest too much of the impactor's iron spill into orbit. Then, to explain the angular momentum, you need to compensate for the smallish angle with a hefty impactor. Then, to explain the moon's mass, you need to adjust the proto-Earth's mass. In the end, you might find that the total mass is incorrect.
In 1997 Alastair G. W. Cameron, one of the fathers of the giant-impact theory, now at the University of Arizona, arrived at a total mass that was a third too low. He suggested that subsequent asteroid impacts made up the difference. But few liked the idea, as the asteroids would have added extra iron.
Canup and Asphaug argue that the fault lies not in the stars but in our simulations. The calculations rely on a technique known as smoothed-particle hydrodynamics, which subdivides the bodies and applies the laws of physics to each piece. Early runs tracked 3,000 pieces--leaving the iron core of the moon to be represented by just a single piece. Even the slightest computational imprecision could vastly overstate the iron content, in which case the computer compensated by reducing the impact angle. The result was a bias toward heavy impactors and light proto-Earths. Because Canup and Asphaug use 30,000 particles, they get by with a much smaller impactor. Everything--mass, iron, momentum--clicks into place.
Considering all the twists and turns in lunar science, nobody claims that the models are complete just yet. Cameron says Canup and Asphaug's model doesn't track events for a long enough time, and moon modeler Shigeru Ida of the Tokyo Institute of Technology says that further increases in resolution could cause more upheaval. Still, it may not be long before you'll need a different cocktail-party question.
Well, isn't that convienient? They just needed to simulate 30,000 particles, rather than 3,000. This -- for a process that actually involved trillions of particles.
Computer models of the "giant-impact hypothesis" for the origin of the moon are similar to computer models of global warming -- they can be tweaked to yield whatever answer you desire.
Didn't you read the article? "Tweaking the model" is exactly what they did! ....
The calculations rely on a technique known as smoothed-particle hydrodynamics, which subdivides the bodies and applies the laws of physics to each piece. Early runs tracked 3,000 pieces--leaving the iron core of the moon to be represented by just a single piece. Even the slightest computational imprecision could vastly overstate the iron content, in which case the computer compensated by reducing the impact angle. The result was a bias toward heavy impactors and light proto-Earths. Because Canup and Asphaug use 30,000 particles, they get by with a much smaller impactor. Everything--mass, iron, momentum--clicks into place.
They increased the number of tracked particles by a factor of 10 and re-ran the orignal model. I call that a "tweak." The only "dilemma" they had was that one over-simplified model didn't (mis)fit the facts as well as another over-simplified model. None of this tells us anything fundamental about lunar origin -- it's just a way to make (an approximate) moon, not THE way the moon was made.
Yeah, I read your article. You said, "Computer models ... can be tweaked to yield whatever answer you desire."
You just don't know what you are talking about.
I would have them not misrepresent computer model results as a "solution" to the problem of lunar origin. They're not and shouldn't be touted as such.
A cryptomoon is what a cryptomoron calls a moon.
When the Days Were Shorter
Alaska Science Forum (Article #742) | November 11, 1985 | Larry Gedney
Posted on 10/04/2004 10:31:59 AM PDT by SunkenCiv
http://www.freerepublic.com/focus/chat/1234919/posts
Note: this topic is from . Thanks callisto.
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The final scientific answer. Until the next one comes along.
What is your experience in finite element modeling?
Lunar Capture keyword:
I was waiting for Ken Ham to tell me how the Moon formed.
V.A. Firsoff (Valdemar Axel Firsoff, as it turns out), Strange World of the Moon , published 1959, ten years before the manned landings started, and even before the first robotic landers, is interesting in that it shows the prevailing ideas about what would be found on the Moon (it was already believed during the 19th century, and more relevantly, by the 1920s and 1930s in Germany, that humans would visit the Moon). In a chapter "The Earth's Fair Child or a Foundling?" discusses the concept of the birth of the Moon via an overspin (doesn't use that word) condition on the Earth, which appears to be his view....the Moon clearly could not have been the satellite of the Earth then, for a total period of about 2,000 million years... Spurr points out that the face of the Moon shows two systems of great surface fractures, or faults, lying about 30 degrees from the two poles and trending from west-south-west to east-north-east. This is explained by him as a result of the halting of the Moon's rotation... Curiously, the face of the Earth, too, shows a similar structure, with the same general trend -- the Highland Boundary Fault... The poles of the Earth would also seem to have shifted place on at least three occasions, in the Cambrian, Permian, and (lastly) Quaternary Periods, brining ice and cold to previously warm lands... some mighty force made the crust of the Earth slip (the rotational stability of the axis of a mass as large as the Earth is enormous) and the position of the poles wobbled... there exists on the Moon a triple grid of surface fractures... perpendicular to each other within each grid, the grids being of different ages... Cambrian, Perm-Carboniferous, and Tertiary.Fascinating idea, based though it is on outmoded ideas about impact (i.e., Firsoff's view that there was no role for impact). He's basically given us a snapshot of the problems inherent with a fission origin (either by overspin or by impact), not least of which is that the fission origin also requires in orbit formation of the lunar sphere and capture by the Earth, while showing that capture is possible. He appears to envisage three encounters between the formed Moon and the Earth, resulting in temporary capture twice leading to the eventual outright capture.
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