Is science sometimes in danger of getting tunnel vision? Recently published ebook author, Ian Miller, looks at other possible theories arising from data that we think we understand. Can looking problems in a different light give scientists a different perspective?

What Is The Purpose Of A Review?

In my opinion, it is not to list data and work done (although that is valuable), it is not to make people comfortable, but rather it is to make statements that summarize knowledge. Thus stating that the force due to gravity varies inversely proportional to the square of the distance between bodies summarizes all data on that topic. Of course, not everything is so well-studied, but that does not mean that we can avoid some obvious errors. One review (R. Brasser, Space Sci. Rev. DOI 10.1007/s11214-012-9904-2), which aims to explain the small size of Mars, came to my mind . The standard theory involves a distribution (without turning points) of planetesimals formed by a mechanism that is not understood then colliding gravitationally to form Mars-sized bodies called embryos, which then collide to form planets. This mechanism leads to various scenarios, depending on assumed initial conditions, but whenever you get four rocky planets, Mercury and Mars are always bigger. So why are they so small?
 
The review shows that if the rocky planets formed from an annulus of planetesimals from between 0.7 -1-A.U. (1 A.U. is the Earth-Sun distance), then you get what we see, with the exception of Mercury. Why this annulus? One proposition is that Jupiter and Saturn migrated in, then migrated back out again, and while doing so, cleared out a lot of planetesimals. (These have to be moved to permit the movement of the giants while conserving angular momentum and energy.) Therefore it was argued that the small size of Mars supported this theory. Further, it is argued that Mars must be a simple embryo, and hence formed early. Support for this comes from isotope measurements, specifically the ratio between 182W and 182Hf, which fix the time of differentiation. (182W dissolves readily in iron, 182Hf prefers to stay in silicates, and, of course, decays to tungsten, so the levels indicate when the iron separated from the silicates.)
 
The review then argues planetary water was brought in the embryos. Venus is drier because its embryos formed in hotter regions, but if so, Mars should be much wetter per unit mass than Earth. There are three reasons for this: Mars formed in a cooler region, Earth has a large iron core and from a chemical perspective, iron is unlikely to bring water with it, and finally, the enormous heat generated in embryo collisions should drive off a significant of water. (The collision of Theia with Earth formed an essentially anhydrous Moon, and, according to modeling, a similar mass of silicates at about ten thousand degrees C, much of which was lost to space.)
 
For me, there was a glaring problem; having made a prediction, the review overlooked the fact that it was just plain wrong. All the evidence is that while Mars definitely had large amounts of water flowing, but the total water is only a few per cent of that of Earth, per unit mass.
 
So, what have we got? A theory that invokes a very specific migration of two planets to explain the small size of another, while ignoring the small size of the remaining one, and which makes one only prediction, and that prediction is not met. Things that are uncomfortable are ignored. Chemists wouldn't do that, of course, would they? Watch for a future post.
Posted by Ian Miller on Jul 14, 2012 5:06 AM Europe/London

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