Skip to comments.Earth-Shattering Theory:FINALLY, THE DETAILS FOR FORMING THE MOON WORK OUT
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
Note: this topic is from . Thanks callisto.
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.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.