2.2.3 The Long Term Population of Interplanetary Micrometeoroids

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Three problems will be discussed: A) The relationship between mlcro-meteoroids and solar flare particles averaged over the recent geologic past (~1 my); B) the past record of this relationship as measured in lunar soils and lunar and meteoritic breccias; C) the determination of the time at which different extraterrestrial samples were exposed to free space. Data bearing on these points obtained from studies of special lunar rocks and from measurements on individual crystals removed from lunar cores will be presented. Progress in using ion-probe mass spectrometry to link measurements of micro-impact craters with the past properties of the solar wind will also be discussed. Comparing microcraters and solar flare tracks in individual crystals from lunar cores, we find no evidence of any extraordinary variations for a time span covering an interval of ~10⁹ yrs. Crystals 100µ to 400µ in size in mature lunar soil samples appear to have been exposed 3 to free space at the top of the lunar regolith for times from 10³ to 10⁴ yrs.