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PLANETARY SPIN‐RATE AND EVOLVING CORES

As human beings, we are constantly urged not to judge others by ourselves, and in planetary science we are urged not to overplay the terrestrial analog. Yet, reasoning by analog is one of the natural scientists’ most effective weapons. Without it, the vast edifice of knowledge of geological processes and of the earth’s history would lie scattered in a morass of strictly observed but uncoordinated data. Most of the “laws” of geology would go unrecognized. What makes the universe such a difficult thing to study as a unity is that we do not know of any other examples for purposes of comparison. But on the level of stars and planets, a great deal can be achieved by arranging individuals in empirical sequences. Ordering is on the basis of least differences within any given frame of reference. Complete disorder of ideal entropy is denied by the very existence of each celestial body. By the same token, if each body were organized randomly, without rules, each would require a unique, ad hoc explanation. And empirically, we observe that nature abhors the ad hoc. It is to be deplored that many studies of our celestial neighbors appear to be undertaken on the assumption that they are unique and independent units, totally isolated, so to speak, in laboratory test-tubes. Without further apologia, therefore, it is proposed to consider the Earth and its planetary neighbors, particularly Mars, in order to extract certain attributes that may emerge as common principles.