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Impacts in the Earth-Moon System

Prof. Nicolle Zellner, Albion College
Tuesday, March 29, 2011, 3:45pm, Science Building 110
Jointly sponsored with Geology

The Moon provides the most clear and complete history of impact events in the inner Solar System since its formation about 4.5 billion years (Ga) ago.  Since the Moon and Earth are close together in space, if properly interpreted, the Moon’s impact record can be used to gain insights into how the Earth has been influenced by impacting events over billions of years.  The timing of impacts on the Moon, however, is not well understood (see Figure 1).  In fact, the National Academy reported that interpreting this impact flux should be one of the top science priorities for NASA’s return to the Moon. 

My research focuses on obtaining geochemical and chronological data on lunar impact glasses, which are pieces of melted regolith (lunar soil) created by energetic impacting events on the Moon.  These impact glasses possess the composition of the target material and can be dated by the 40Ar/39Ar (argon) method in order to determine their age of formation.  Understanding the ages of impact glasses, along with their compositions, allows for a full interpretation of the impact history of the Moon, and from them, we can begin to piece together information about the rate of impact events and their effects on Earth.

Graph of history of moon impacts

Figure 1. Schematic diagram illustrating the variation among the models of the early impact cratering history on the Moon. While it is generally agreed that there was a decrease in the impact rate after 3.8 Ga, it is still unsettled whether a) there was an anomalously high flux ~3.9 Ga, b) if there was a high flux ~3.9 Ga, how long it lasted, and c) whether the impact rate between ~4.4 and ~4.0 was relatively high or low.  Peaks like that shown at 3.9 Ga have also been suggested to occur several times between 4.4 and 4.0 Ga. Modified from Kring (2003).


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