How Did the Moon Really Form?
The great thing about science is that, unlike religions, it has real, hard evidence. In fact, it's all about evidence. It doesn't matter what clever hypotheses scientists come up with, until they are supported by evidence, they remain just hypotheses. And even when evidence is found it is never assumed to have proved the hypothesis beyond doubt, placing it in some realm of certainty never to be questioned again.
The only certainty in science is that there are no certainties.
Take for example the hypothesis that was devised to explain why Earth has an axis of rotation which is tilted in relation to those of other planets and in relation to the surface of the Sun, and also why Earth has such a large moon relative to its size, compared to what we normally see for other planets like Mars or Jupiter.
The hypothesis that explained this - the so-called
'giant impact hypothesis' - was that the Earth and Moon were formed early in the life of the Solar System from the remains of an impact between the earlier, smaller Earth and another even smaller, Mars-sized planet, which has been named 'Theia', that had drifted out of orbit. The impact tilted the axis of rotation of Earth. Some of the debris from this impact, mostly from Theia which broke up on impact, but some of it from Earth, fell back to Earth and became merged into it, while the remainder formed an accretion disk around the now larger Earth which coalesced into the Moon. The Moon formed by this process would have been hot, driving off the lighter elements and water which, with the low lunar gravity, would have been lost into space, leaving the arid landscape with no atmosphere that the Moon has today.
However, this hypothesis, although explaining what we can observe in terms of Earth's axis of rotation and large Moon, not only suffered from a lack of hard evidence but the evidence available appeared to contradict it. As
Daniel Clery explains in an
article in Science this week:
But one bit of evidence just doesn’t fit: the composition of moon rocks. Researchers have found that rocks from different parts of the solar system (brought to Earth as meteorites) have subtle differences in their composition. Oxygen, for example, comes in different varieties, called isotopes. Oxygen-16 (O-16) is the most common type, followed by oxygen-17 (O-17)—which has one extra neutron in its nucleus—and oxygen-18, with two extra neutrons. Meteorites from different parts of the solar system have different proportions of these isotopes. So a rock from Mars would have a markedly different ratio of O-17 compared with O-16 than, say, a piece of an asteroid or a rock from Earth. These ratios are so reliable that researchers use them to identify where meteorites come from.
Here’s the puzzle: The giant impact hypothesis predicts that the moon should be made of about 70% to 90% material from the impactor, so its isotope ratios should be different from Earth’s. But ever since researchers got hold of Apollo moon rocks for analysis, they have failed to find any significant difference in isotope ratios on Earth and the moon. Studies of the isotopes of oxygen, titanium, calcium, silicon, and tungsten have all drawn a blank.
It was getting to the stage where some people were suggesting that the collision had not taken place.
Dr Daniel Herwartz
University of Cologne, Germany
So, the question for science was, is this apparently contradictory evidence sufficient to destroy the hypothesis or does it's explanatory power for other observed phenomena still make it a viable hypothesis?
For the hypothesis was the fact that both Earth's axis of rotation and the size of the Moon are not really in doubt while the ratios of the different isotopes of oxygen and other elements could have another explanation - it
is only an assumption that Theia should have had very different ratios, we don't have any way of knowing for sure what they would have been.
Against the hypothesis was the argument that these difference should have been detected by now so it's beginning to look suspiciously like a prediction made by the hypothesis is being falsified.
The problem was, as I have pointed out above, without knowing exactly what that prediction is in the absence of any information about Theia's oxygen isotope ratios, we can't even say with any degree of confidence that it
has been falsified.
It is a relief that a [disparity in ratios] has been found, since the total absence of difference between Earth and moon would be hard to explain.
David Stevenson, planetary scientist
California Institute of Technology, Pasadena, USA
(e-mail to Science quoted in the above article.
Now, however, a team of scientists led by
Daniel Herwartz of the University of Cologne, Germany, seem to solved the mystery. The problem may have been that the Moon rock samples previously used were meteorites collected from Earth and so had been subjected to the effects of weathering. This may have skewed the results. When they compared samples brought back by Apollo Missions 11, 12 and 16 with samples from Earth's mantle, they found that the Moon has an O-16 to O-17 ratio 12 parts per million higher than those of Earth rock. The results suggest the Moon may be composed of about equal proportions of Earth and Theia, so strongly supporting the big impact hypothesis.
ABSTRACT
The Moon was probably formed by a catastrophic collision of the proto-Earth with a planetesimal named Theia. Most numerical models of this collision imply a higher portion of Theia in the Moon than in Earth. Because of the isotope heterogeneity among solar system bodies, the isotopic composition of Earth and the Moon should thus be distinct. So far, however, all attempts to identify the isotopic component of Theia in lunar rocks have failed. Our triple oxygen isotope data reveal a 12 ± 3 parts per million difference in Δ17O between Earth and the Moon, which supports the giant impact hypothesis of Moon formation. We also show that enstatite chondrites and Earth have different Δ17O values, and we speculate on an enstatite chondrite–like composition of Theia. The observed small compositional difference could alternatively be explained by a carbonaceous chondrite–dominated late veneer.
Now that a difference has been found, many will work to confirm or deny it and do battle over what it means... The possible significance of enstatite chondrites is interesting, but at present we are stuck with speculating about the bodies that went into making Earth, since they are no longer around.
David Stevenson, op. cit.
So, does that settle the issue once and for all? Can science now claim to be certain that the Earth/Moon system was created by the 'big impact' in the early life of the solar system? Of course not. It is always possible that this team's findings might be shown to be wrong, or some unexpected evidence might be found which would cause the entire thing to be revised and discarded. The team themselves point out that Earth may have been bombarded by material with a different oxygen isotope ratio after the impact.
Imagine religious dogma being subjected to this sort of constant review and revision with nothing sacred at all in terms of firm conclusions. What little scraps of evidence they have, no matter how tenuous, like the
Turin Shroud, are carefully guarded against too much independent scientific scrutiny and when they are, as is
the case with the Turin Shroud where three independant teams all concluded that the shroud is made of linen made from flax which grew in the 14th-century, the results are dismissed or ignored.
Religion has developed a whole edifice of beliefs and dogmas based on nothing more substantial than myths and fantasies and a dogmatic assumption that, for no other reason than wishful thinking and clerical necessity, these myths and fantasies
must be true. None of this would be needed if they had any real evidence either for the existence of gods, or the nature of those gods. They then have to defend those dogmas against all arguments by developing apologetics invariably based on circular reasoning and the presupposition that the dogmas are right in the first place.
The art of a good apologist is to convince believers that they were right all along and few of them require much convincing, so refuted fallacies, false claims, circular logic, parochial ignorance and cultural arrogance are all brought into play, like a snake-oil salesman playing to a credulous and anxious audience.
The level of intellectual honesty and moral integrity needed to be a theologian or a professional religious apologists would quickly result in a research scientist being shown the door of any respectable research institute foolish enough to have employed him or her in the first place. It's a level of personal integrity more suited to the role of door-to-door salesman, second-hand car dealer, political spin doctor or real-estate agent.
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