360 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1964 



Mars, on the other hand, is only about one-ninth the mass of the 

 Earth, and not only would the temperatures due to radioactive heating 

 at corresponding depths be rather less than in the Earth, but the 

 pressures are far too low for liquefaction yet to have occurred in its 

 central regions. So no contraction of Mars can have occurred : indeed, 

 if anything has happened as a result of internal heating, it would 

 rather have produced very slight expansion of the outer parts, possibly 

 thereby bringing about rifting of the solid surface. Whether such 

 riftings could be eroded into anything resembling terrestrial moun- 

 tains is doubtful. Although the surface is directly visible. Mars is 

 rather too distant for the question to be settled for certain at present. 

 Nevertheless it has long been believed from observations near the edge 

 of the planet's disk that there can be no irregularities of more than a 

 few thousand feet, and the absence of detectable shadows means there 

 is no direct evidence even for this amount. Photographic or other 

 kinds of survey from space probes passing close to the planet may 

 clarify the situation in the next few years. 



The same conclusion holds for both Mercury and the moon, and the 

 theory indicates that these bodies have always been solid throughout. 

 Hence no mountains of the terrestrial kind can be expected at their 

 surfaces. It is of course generally recognized that no such features 

 are to be found on the lunar surface ; all the so-called "mountains"' can 

 be associated with the remnants of the rims of large craters that have 

 been heavily eroded. 



However, special processes, perhaps chemical or radioactive, might 

 lead to the development of intense local heating in comparatively 

 small regions of the outer parts of the planets or of the moon. For 

 example, a large meteorite of high radioactive content plunging into 

 a planet might produce sufficient heating to bring about volcanic effects 

 hundreds of millions of years later. This in turn could lead to the 

 building up of volcanic mountains, but these make an almost negligible 

 contribution to the whole area of the Earth covered by mountains. 



We can conclude that if the inner planets began as molten bodies, 

 they should all possess mountams produced by thermal contraction. 

 But if they began as entirely cool bodies, only the Earth and Venus 

 can have mountains. Thus we have an absolutely clear-cut test of a 

 new hypothesis which implies a great deal about the deep interiors 

 of the planets — a realm that can be explored theoretically. And there 

 is an intriguing opportunity for space research to obtain the necessary 

 evidence by direct exploration of the surfaces of these planets. 



