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!

Piggy hide and seek

Watching the 'freshers' turn up at work this year, all bubbly and full of the promise of a new beginning, made me have a little think. Eight years ago I was one of those over-eager little souls bounding into the Physics and Astronomy Department in Sheffield. Four years ago, I was probably not too dissimilar, this time bounding into the National Oceanography Centre, Southampton. Last week my PhD and, hopefully, Post-Doc supervisor handed me a book entitled 'Advances in Forensic Taphonomy: Method, Theory, and Archaeological Perspectives'. Funny how things change!

Were you to ask me where I thought I'd be in eight years time when I started my undergrad in Physics with Astronomy at Sheffield, I would almost certainly said something along the lines of 'hopefully doing something in astronomy'. Were you to ask me where I thought I'd be in four years time when I started doing geophysics at Southampton, I'd almost certainly have replied something along the lines of 'yeah, well, I'll probably sell out to the industry and go hunting for oil'. Yet, now, here I am, writing a Post-Doc proposal to play underwater hide and seek with some dead pigs using a variety of acoustic profilers. Funny thing is, I ain't complaining!

The principle is quite simple: we have already proved that our decimetre resolution 3D seismic system, 3D Chirp, can be used to image everything from geological structure on 10s metres scale, down to individual objects some 10s centimetres in size; and a number of companies have already shown that acoustics can be used to locate dead bodies. However Police authorities around the world still primarily default to using shore walks and diver surveys when conducting missing person searches.

To a large degree, their lack of trust in these new technologjes is reasonable. Practically, these systems tend to be highly specialised, requiring a large amount of detailed knowledge to get the best out of them. Thought needs to go into which acoustic profiler to use (side scan sonar, sector scanning sonar, echoscope, or a sub-bottom profiler such as chirp or boomer), where and how to use it, how to process the acquired data, how to interpret the processed data, etc, etc. The list is almost endless. Then, to confound it all, there has been very little work done on how the acoustic response of a body varies with the level of decomposition. How long does a body have to have been deceased before they present a strong enough acoustic signature to be detected? How long before the body is so decomposed that is presents no acoustic signature? How does this vary from person to person and locality to locality? How does this vary with the frequency content of the acoustic source? Such basic, fundamental questions need solid, scientifically robust answers before the Police authorities can be expected to make extensive use of these techniques. Hopefully, we might be able to provide them with a few.

Finally, something a little more topical. I recently discovered this post by Moshe Rozali talking in very general terms about the principles of field theory, and in particular Quantum Gravity. This is a pretty meaty topic to tackle in any situation, but Peter manages it with beautiful elegance in his usual easy-to-read style. Given the recent media hype surrounding the LHC and the Higgs Boson, this post is well worth a read as it outlines some of the basic holes in our present understanding of gravity. Personally, I think it's a shame some of the 'science' journalists didn't give it a browse before they started the 'black hole to destroy the world' tirades, maybe we'd have gotten a few more interesting news reports if they had...although they would, probably, have sold fewer newspapers!