2013 Annual Science Report
Rensselaer Polytechnic Institute Reporting | SEP 2012 – AUG 2013
Project 3: Impact History of the Earth-Moon System
The influx of interplanetary debris onto the early Earth represents a major hazard to the emergence of life. Large crater-forming bodies must have been common in the early solar system, as craters are seen on all ancient solid surfaces from Mercury to the moons of the outer planets. Impact craters are few in number on the Earth today only because geologic activity and erosion gradually erase them. The Earth’s nearest neighbor, the Moon, lacks an atmosphere and significant tectonic activity, and therefore retains a record of past impacts. The goal of our research is to reconstruct the bombardment history of the Moon, and by proxy the Earth, to establish when the flux of sterilizing impacts declined sufficiently for the Earth to became habitable.
The goal of this project is to analyze the chemical compositions of lunar impact glasses, which were formed during impacts on the Moon, in order to understand the lunar impact flux in the Earth-Moon system over time. During this past year, 24 lunar impact glasses from the Apollo 14, 16, and 17 landing sites were age dated (40Ar/39Ar) at the geochronology facility at the University of Arizona and another 62 were selected for irradiation in the near future.
Careful analyses of the composition, age, shape, and size of hundreds of lunar impact glasses (including Culler et al. 2000; Levine et al. 2005; Delano et al. 2007; Zellner et al. 2009a, 2009b; Hui 2011; Norman et al. 2011; and Norman et al. 2012) lead us (Zellner & Delano, in review) to believe that not all lunar impact glasses are good candidates for age dating and thus for helping to elucidate the impact flux in the Earth-Moon system. For example, we propose that the likelihood of 40Ar/39Ar ages of lunar impact glasses (independent of shape) recording the actual age of the impact melting event, instead of a minimum value (i.e., apparent age) caused by diffusive-loss of radiogenic 40Ar, increases with (i) size of the glass sample, as noted by Gombosi et al. (in review), and (ii) the fraction of non-bridging oxygens (i.e., chemical composition; XNBO). Since lunar impact glasses with anorthosite-norite-troctolite (ANT) highlands compositions have low XNBO values, only the largest pieces of those ANT glasses would be capable of recording 40Ar/39Ar ages either during, or prior to, the terminal lunar cataclysm (≥ 3900 Ma). Additionally, while it has been observed that the impact-produced glass spheres from four Apollo landing sites show a pronounced enhancement of 40Ar/39Ar ages <1000 My, we suggest that this is due to the limited lifespans (~100 My half-life) of impact-produced glass spheres prior to being broken into angular shards and not to an increase in the recent impact rate.
Lunar impact glasses from the Apollo 15 site also continue to be evaluated. Stefan Blachut (undergraduate student at Albion College) selected several Apollo 15 lunar impact glasses for 40Ar/39Ar dating and then spent the summer analyzing the compositions of hundreds of lunar impact glasses (i.e., glasses found in eight regolith samples from five different Apollo landing sites) in order to determine which glasses may have been formed in the same impact event, regardless of collection site. Further analyses of those data continue.
Oral and poster conference presentations of this work occurred at the annual meeting of the Michigan Space Grant Consortium (2012) and at the annual Lunar and Planetary Science Conference (2013).
PROJECT INVESTIGATORS:John Delano
PROJECT MEMBERS:Nicolle Zellner
RELATED OBJECTIVES:Objective 4.3
Effects of extraterrestrial events upon the biosphere