Evolution on TV - April 2014
by Do-While Jones

Lunacy: A Spacetime “Oddity”

President Obama was right when he said, “You can’t believe everything you see on Fox.”

People like John, who wrote the emails to us printed in this month’s newsletter, believe all the nonsense stated as fact on TV science shows. This month there is a “science” series running on the Fox network and National Geographic channel titled, Cosmos: A Spacetime Odyssey. It presents all sorts of speculation as scientific fact. For example, Neil deGrasse Tyson tells how the Moon formed, with a perfectly straight face.

The Earth took one hell of a beating the first billion years. Fragments of orbiting debris collided and coalesced until they snowballed to form our moon. The Moon is a souvenir of that violent epoch. If you stood on the surface of that long-ago Earth, the Moon would have looked 100 times brighter. It was 10 times closer back then—locked in a much more intimate gravitational embrace. As the Earth cools, seas began to form. The tides were 1,000 times higher then. Over the eons, tidal friction from the Earth pushed the Moon away. 1

If one didn’t read the professional scientific literature, one would think that scientists know how the Moon formed, and that it happened just the way Tyson said it did. The fact is, the formation of the Moon is an unsolved mystery. We know that because we really do read the professional, peer-reviewed, scientific literature. Here’s a sampling of what that literature says.

Lunar-origin studies are in flux. No current impact model stands out as more compelling than the rest. Progress in several areas is needed to rule out some theories, support others or direct us to new ones. 2

It remains troubling that all of the current impact models invoke a process after the impact to effectively erase a primary outcome of the event — either by changing the disk's composition through mixing for the canonical impact, or by changing Earth's spin rate for the high-angular-momentum narratives. 3

Our knowledge of how Earth's natural satellite [that is, the Moon] formed is increasingly being challenged by observations and computer simulations. 4

Here’s a short summary of the problem:

Since the 1980s, work on lunar origins has focused on the 'giant-impact' theory. This proposes that the collision of another planet-sized body with the forming Earth generated a disk of debris that coalesced into the Moon. Such giant collisions were common in the Solar System during the final stages of Earth's formation 4.5 billion years ago.

But we still do not understand in detail how an impact could have produced our Earth and Moon. In the past few years, computer simulations, isotope analyses of rocks and data from lunar missions have raised the possibility of new mechanisms to explain the observed characteristics of the Earth–Moon system.

The main challenge is to simultaneously account for the pair's dynamics — in particular, the total angular momentum contained in the Moon's orbit and Earth's 24-hour day — while also reconciling their many compositional similarities and few key differences. The collision of a large impactor with Earth can supply the needed angular momentum, but it also creates a disk of material derived largely from the impactor. If the infalling body had a different composition from Earth, as seems probable given that most objects in the inner Solar System do, then why is the composition of the Moon so similar to the outer portions of our planet? 5

The giant-impact hypothesis of lunar origin is celebrated for its simplicity: a late, grazing impact on the proto-Earth launches a portion of the rocky mantle into orbit and establishes the angular momentum of the Earth–Moon system. The Moon, depleted of iron and volatile elements relative to Earth, forms from this hot circumterrestrial disk of rocky mantle. Hydrodynamic simulations of giant impacts successfully produce disks of low iron content and sufficient mass to make this hypothesis plausible. The fatal issue is that simulations that lead to the present angular momentum derive most disk material from the impactor. Thus, the giant-impact model predicts that Earth and the Moon should be derived from different source material, each with distinct isotopic fingerprints, and this contradicts the geochemical (isotopic) observations. 6

As emphasized in a Royal Society meeting in September that debated the origin of the Moon, the compositional differences between Earth and the Moon that would be expected as a consequence are increasingly at odds with diverse, high-precision isotopic observations. 7

Thus, differences in oxygen, tungsten and silicon isotope ratios between target and impactor seem inevitable, and so the standard model predicts isotopic differences between Earth and the Moon that are not observed. 8

So far, the solutions proposed appeal to extra processes — such as extensive mixing of materials from both bodies or a later gravitational resonance with the Sun — the feasibility of which are unclear. 9

All scientists really know about how the Moon formed is that it happened, “somehow.” That’s not a scientific answer. But programs like Cosmos make it appear that scientists know how the Moon formed (and that life evolved)—and people like John believe it.

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Footnotes:

1 Cosmos: A Spacetime Odyssey, “Episode 1: Standing Up in the Milky Way”, March 9, 2014 (45 minutes into the program)
2 Canup, Nature, 5 December 2013, “Lunar conspiracies”, pages 27-29, http://www.nature.com/news/planetary-science-lunar-conspiracies-1.14270
3 ibid.
4 Elliott & Stewart, Nature, 5 December 2013, “Shadows cast on Moon's origin”, pages 90-91, http://www.nature.com/nature/journal/v504/n7478/full/504090a.html
5 Canup, Nature, 5 December 2013, “Lunar conspiracies”, pages 27-29, http://www.nature.com/news/planetary-science-lunar-conspiracies-1.14270
6 Elliott & Stewart, Nature, 5 December 2013, “Shadows cast on Moon's origin”, pages 90-91, http://www.nature.com/nature/journal/v504/n7478/full/504090a.html
7 ibid.
8 ibid.
9 Canup, Nature, 5 December 2013, “Lunar conspiracies”, pages 27-29, http://www.nature.com/news/planetary-science-lunar-conspiracies-1.14270