2014 Annual Science Report
Rensselaer Polytechnic Institute Reporting | SEP 2013 – DEC 2014
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.
This study focuses on obtaining geochemical and 40Ar/39Ar (argon) chronological data on lunar impact glasses, which are pieces of melted regolith (lunar soil) created by energetic impacting events on the Moon. The aim of the study is to understand the ages of impact glasses, along with their compositions, in order to gain a full interpretation of the impact history of the Moon.
A very important outcome of our work that is currently in review for publication in Geochimica et Cosmochimica Acta is that not all lunar impact glasses are created equal. All previous investigators (e.g., Hui 2011; Hui et al. 2010; Zellner et al., 2009a,b; Delano et al., 2007; Levine et al., 2005; Culler et al., 2000) have implicitly assumed that lunar impact glasses are highly retentive of radiogenic 40Ar during prolonged residence in the shallow lunar regolith that is subjected to diurnal temperature variations, but this is not the case. Using experimental data (Gombosi et al. 2012) and applying it to our lunar impact glass data, we have found that composition and size of the glass are important components to consider when evaluating whether or not the 40Ar/39Ar age is a true age (or just an apparent one, as the result of argon diffusion). We revisited hundreds of 40Ar/39Ar ages of lunar impact glasses and report that only a subset reflect a true formation age of the glass.
We also investigated the observation (e.g., Norman et al. 2012; Hui 2011; Hui et al. 2010; Levine et al. 2005; Culler et al. 2000) that lunar impact glass spherule ages appear to represent an increase in the recent (i.e., <500 Ma) impact flux. We conclude that lunar impact glass spheres may be short-lived. Lunar impact glasses contain high thermal stresses (Ulrich, 1974; strain exotherms) caused by rapid quenching from hyperliquidus temperatures. These thermal stresses would make impact-generated glass spheres susceptible to breaking into shards. Thus, investigators who preferentially select lunar impact glass spheres and who reported an increase in the recent (< 500 Ma) impact flux (i.e., Norman et al. 2012; Levine et al. 2005; Culler et al. 2000) are biasing their lunar impact flux curves to young ages. If we are correct in our suggestion that impact glass spheres are easily broken as a result of regolith gardening, then the high rate of occurrence of lunar impact glass spheres with ages <500 Ma, as reported in those studies, need not require a substantial increase in the impact flux during the last 500 Ma.
These are strong working hypotheses, but more data are needed. In August 2014, Zellner traveled to the Geochronology Center at the University of Arizona to use the argon mass spectrometer to obtain 40Ar/39Ar ages on ~30 impact glasses. Almost 100 glasses are still awaiting 40Ar/39Ar dating. These analyses will be concluded in 2015.
Gombosi, D. J., Baldwin, S. L., Watson, E. B., Swindle, T. D., Delano, J. W., & Roberge, W. G. (2015). Argon diffusion in Apollo 16 impact glass spherules: Implications for 40Ar/39Ar dating of lunar impact events. Geochimica et Cosmochimica Acta, 148, 251–268. doi:10.1016/j.gca.2014.09.031
PROJECT MEMBERS:Nicolle Zellner
RELATED OBJECTIVES:Objective 4.3
Effects of extraterrestrial events upon the biosphere