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?
Planetary update
Ever wondered why planets rotate the way they do? All the outer ones appear to have prograde rotation, i.e. they rotate in the direction as of they were rolling along. However, Mercury and Venus are exceptions. Mercury has a very slow rotation that is explained by it being in a tidal resonance with the sun, so that is no mystery, but Venus rotates slowly, and the wrong way. Most people have viewed this in terms of the standard theory of planetary accretion, where the central body gets hit by a large number of planetesimals, or even larger bodies, from random directions, and the resultant spin is a result of preferential strikes. Earth may well have included this effect when it was struck by Theia to form the Moon. In this case the Moon's orbit also take sup angular momentum from the collision. Venus has no moon, and it spins slowly so the theory went, it was just unlucky and got hit the wrong way at the end by something big. But if that were the case, why no satellite?
There was a recent paper in Science (346: pp 632 – 635) that put a different picture on this. If the planet has an atmosphere, atmospheric temperatures oscillate from night and day, which creates large-scale mass redistribution within the atmosphere, the so-called thermal tides. The retrograde motion occurs because the hottest part of the day is a few hours after midday, due to the thermal inertia of the ground. Because of this asymmetry in atmospheric mass redistribution, the stellar gravity exerts a non-zero torque on the atmosphere, and through frictional coupling, the spin of the planet is modified. This is why Venus has retrograde spin. Atmospheric modelling then showed that the resultant torques for a planet in the Venusian position with a 1 bar atmosphere are an order of magnitude stronger than for Venus, mainly because the very thick atmosphere scatters or absorbs most of the sunlight before it reaches the surface. As a consequence, rocky planets in the habitable zone around lower mass stars may well have retrograde rotation.
During these posts, the reader may have noticed that I sometimes view computer models with scepticism. Here are two examples that illustrate why. The first is from Planet. and Space Sci. 105: 133 – 147, where two models were made of atmospheric precipitation on Mars ca 3.8 Gy BP. The valley network analysis suggests an average of 1.5 – 10.6 mm/d liquid water precipitation, whereas the atmospheric model predicts about 0.001 – 1 mm/d of snowfall, depending on CO2 partial pressure (which varied from 20 mb to 3 bar in the models) and with global mean temperatures below freezing point. The authors suggest that this shows there was a cold early Mars with episodic snow-melt as a source of the run-off. I rather fancy it shows something is left out of the analysis, i.e. there is something we do not understand, because all the evidence to date makes a persistent 3 bar atmosphere most unlikely, and even then, it only works by a near miss at the extremes. The other came from Icarus 252: 161 – 174. Here, an extensive suite of terrestrial planet formation simulations showed that the rocky planets have overlapping stochastic feeding zones. Worse, Theia, the body that formed the Moon, has to be significantly more stochastic than that of Earth, and the probability that the two would have the same isotopic composition is very small, yet the isotopic composition is essentially identical. The authors state there is no scenario for the Moon's origin consistent with its isotopic composition and a high probability event. Why not concede that the premises behind the model are wrong? And there, in my opinion, is the basic problem. Almost nobody goes back and checks initial assumptions once they have been accepted for a reasonable time. And if you do, as I have done for planetary formation, nobody cares. As it happens, each of these is properly accounted for in my Planetary Formation and Biogenesis.
There is a clear published model from Belbruno and Gott that would permit the Moon to have the same isotopic ratios as Earth, and that assumes Theia accreted at one of the two Lagrange points L4 or L5 (Astron. J. 129: 1724–1745). (Lagrange points are where gravitational effects of two major bodies more or less cancel, and a third body can be at L4 or L5 indefinitely, as long as it does not become big enough to be gravitationally significant. L4 and L5 are actually saddle points, and bodies that fall off the "saddle" have net forces that pull them back, so they carry out motion about the point. Jupiter's Trojans are examples.) So why did these other authors not cite this model as a possible way out of their problem? One possible reason is they have never heard of the model, which is almost never cited, one of the reasons being that within the standard model of stochastic accretion of ever increasingly large bodies, nothing could accrete at the Lagrange points because collisions would knock them off. So now we have a problem. The standard model would not permit the conditions by which one model would explain the observations, but the observations also effectively falsify the standard model. So, what will happen? Because there is no way to have discussions on topics such as these, other than in blogs, the whole issue will be forgotten for some length of time. Progress is held up because the modern method of disseminating information has so much information in it that linking it does not always occur.
Posted by Ian Miller on Apr 5, 2015 11:33 PM Europe/London
