Tuesday, 30 September 2008

We like da Moon

As I wittered about a few days ago, I'm quite keen on the concept of Big History. Something I meant to talk about in that post, but kinda ran out of room, was an idea that has been rattling around my head for some time now. It goes something like this...

The Moon, our big white faced friend in the sky, is believed to have been formed by a collision between the proto-Earth and a Mars-sized body some 4.45 billion years ago. The evidence for this particular theory for the lunar origin, first proposed by William Hartmann and Ronald Davis in the journal Icarus during April 1975, has been steadily growing over past decades. Where as almost all other planetary satellites are thought to be either captured asteroids (e.g., Mars' moons Phobos and Deimos) or co-formed accumulations of material during the planet's formation (e.g., the Galilean satellites of Jupiter), the anomalously large size ratio and angular momentum of the Earth-Moon system make either of these scenarios unlikely. Just to add weight to this arguement, the Moon also appears to have a small (< 5% total mass) iron core, which one would expect to be much larger in both former cases.The third theory popular prior to Hartmann and Davis' writing, which involves the moon breaking away from a rapidly spinning Earth, also falls down with this latter piece of evidence as one would expect no (or almost no) iron core (let alone mismatches in the amount of angular momentum!).

I suppose, the long and short of what I'm trying to say is, basically, that it has been circling our little blue globe as long as life has been wandering round, eating, fighting, and having sex.

In his talk at IGC, Walter Alvarez postulated that having such a large moon had a massive impact in the development of complex life on Earth. This, he said, is likely to be caused by the lunar tides making the transition from aquatic to terrestrial life easier. In both of these statements, he is correct. Having a large Moon has almost certainly had a massive effect on the development of life on our planet, while the presence of a tidal zone (which would have been considerably larger when life was forming in the Archaen, since the Moon would have been much closer) one can conceptually imagine would smooth the transition into land dwelling life forms.

However, I disagree that this is the most lasting influence of the Moon-Earth system on the development of life. One could argue this from the philosophical view-point that Alvarez is implying that all truly complex life will be terrestrial in origin. To put it politely, this it utter codswallop! Marine life and the marine ecosystem is incredibly complex, and just because they don't pour concrete over everything doesn't stop them from being highly evolved. Taking the more scientific approach, the largest implication for having such an Earth-Moon system is, I think, one which has been loosely discussed for some time, namely the stabilizing effect of the Moon on the Earth's obliquity (angle of the Earth's rotation pole to it's orbital plane). Presently, the complex interaction between the Earth's orbital and rotational momenta, with the gravitational effects of the Moon, Sun, and Gas Giants, holds this stably at c. 23 degrees (plus/minus c. 1.25 degrees). Without the stabilizing effect of the Moon, it has been shown that the highly variable effect of the Gas Giants (caused by their changing position relative to the Earth during the orbital cycle) could cause this to vary by up to 80 degrees!

The small fluctuations in the Earth's obliquity are known to be one of the driving forces behind the Milankovic cycles, which are thought by climate scientists to be the dominant cause the glacial cycle. Conceptually, introducing larger amplitude and shorter period variations in the obliquity would dramatically change the Earth's climate history; which is generally built on long periods of warmer climate and low ice levels, separating somewhat shorter periods of colder climate with large ice levels. Shortening these calm time periods between ice ages is likely to have a large effect on the evolution of complex life. Although one could argue that it is unlikey to stop the development of complex life completely, shorter calm periods would certainly have slowed evolution down a notch or two.

To my knowledge no one has tried running any kind of altered obliquity simulation. Maybe on of these days I'll get round to it!


Chuck said...

Can we blame the moon for Earth's unusually thin crust, and by extension, plate tectonics?

Mark said...

Interesting question. A lot of very clever people have spent their entire lives banging their heads together trying to figure out why the Earth alone has plate tectonics. New Scientist recently ran an article on Earth's greatest mysteries, ranking plate tectonics second (if my memory is correct).

The answer to why we've got plate tectonics seems to be a hideously complex thing, resulting from the interaction between: planet size; gravitational effect of nearby objects; constitution of the proto-planetary nebula (which therefore means distance from Sun); and probably a bunch of other stuff I can't think of.

I know some people have suggested that the lunar forming impact resulted in our having a planetary surface consisting of only 30% continental crust. True, it would have probably limited the amount of continental crust the Earth could form, but then why this limited amount would have all bunched up together is a bit of a mystery. I do, however, think that the tidal interaction between the Moon and the Earth might have had some effect, though how dominant this is compared to geothermal cooling is questionable?

Certainly food for thought!