Venus Has Uncertain Pedigree

The five instrumented Pioneer probes that plunged into the thick Venusian atmosphere in late 1978 discovered unexpectedly large quantities of the isotope argon-36. The significance of argon-36 is that it is supposed to be primordial argon; that is, an argon isotope formed when the solar system was born. Since argon-36 is radioactive, most of the original supply of this isotope should have disintegrated and disappeared over the 4-billion-year history of the solar system. Indeed, the atmospheres of earth and Mars have much, much smaller quantities of argon-36 than Venus. Venus, therefore, may have had an origin different from that of earth and Mars -- either a much more recent birth (such that the argon-36 has not all disintegrated), or an altogether different kind of origin in which more argon-36 was created than was the case for earth and Mars.

(Anonymous; "Venus Probes Solar System Birth," New Scientist, 80:916, 1978.)