Surprise at Descartes-Cayley
In 1964 the significance of the Moon’s basin structures had only just been realised, and almost all ‘sculpture’ was attributed to the formation of the Imbrium Basin. On stratigraphically mapping the peripheral hummocky terrain, Dick Eggleton judged it to be Imbrium ejecta and named it the Fra Mauro Formation. On seeing rolling lighttoned plains just beyond, he mapped them as part of the Fra Mauro. But in 1965 Don Wilhelms drew a distinction, and mapped the latter as the Cayley Formation (a name derived from the fact that the first patch that he studied was near the 10-km Cayley Crater). Nevertheless, it was still thought to be Imbrium ejecta. As the ‘cold’ Moon theory lost favour, the Cayley was reassessed as volcanic. When the dark maria were found to be lava enriched by mafic silicates from the mantle, it became evident that the mare flows would have been of such low viscosity that they readily spread out to form vast smooth sheets. The Cayley lava was thought to be enriched with silica and so more viscous.
KeywordsLanding Site Lunar Surface Small Crater Secondary Crater Hammer Blow
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