2009 Annual Science Report
Rensselaer Polytechnic Institute Reporting | JUL 2008 – AUG 2009
Project 4: Impact History in 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 bombardment of cosmic debris onto the Earth during the Hadean likely affected the point when sustainable life could have emerged. With the Moon having been in near-Earth space since its formation at ~4.5 Ga and having a geochemical/isotopic memory of impact events, lunar samples are being used to determine the time-dependent flux. Although several hundred lunar impact glasses (~200-400 microns in size) have been individually dated by us and other investigators using the39Ar/40Ar method, the fidelity of those isotopic ages in lunar glasses is not known since Ar diffusivity in those lunar glasses is not known.
We have begun work to determine the Ar diffusivity in natural lunar glasses over a wide range of chemical compositions. Ten (10), 1-2 mm diameter, impact glass spherules from the Apollo 16 landing site have been analyzed for their major- and minor-element abundances by electron microprobe, and have also been analyzed by Raman spectroscopy to compare their atomic bond-lengths (i.e., related to the temperatures at which each was quenched; fictive temperature). With this characterization having been completed, each of these glasses will next be neutron irradiated and isotopically analyzed by a step-heating method to determine Ar diffusivity within each of these lunar glasses. That information will be used to model the range of temperature-time histories and the effect on their apparent ages.
The second method for assessing Ar diffusivity in lunar impact glasses is to synthesize glasses in the laboratory over a wide range of lunar compositions. The experimental procedures for making these synthetic lunar analogs having a simple geometry (cylindrical or spherical) have nearly been perfected during this reporting period. The experiments need to produce cylinders of bubble-free, crystal-free glass with natural lunar compositions with the appropriate oxidation state. These synthetic glasses with observed lunar compositions will be neutron irradiated and the diffusivity of the 39Ar (generated during neutron irradiation from K) will be measured by the step-heating method.
Comparison of the compositionally dependent, Ar diffusivities between the natural lunar glasses and the synthesized lunar analogs will also show whether other processes (e.g., cosmic ray exposure) on the lunar surface have detectably affected the Ar diffusivity of lunar glasses (i.e., retentivity of Ar).
PROJECT INVESTIGATORS:John Delano
PROJECT MEMBERS:Suzanne Baldwin
RELATED OBJECTIVES:Objective 4.3
Effects of extraterrestrial events upon the biosphere