Donald D. Bogard

Radiometric chronology of the Moon and Mars

“How old is the Earth?” was a topic of scientific inquiry in the late nineteenth century. In the twentieth century the question became “How old are the Earth and other objects in the Solar System?” Related questions are: “How old is the Solar System?”; “How has the Earth changed over geologic time?”; and “How have the planetary bodies in the Solar System changed over time?” Also, “What has made Earth unique in our Solar System?” With the aid of spacecraft-acquired data and samples, and through the study of lunar and Martian meteorites, we are beginning to answer some of these questions as they relate to the Moon and Mars.

Issues in dating young rocks from another planet: Martian shergottites

Abstract The 40Ar/39Ar ages of a group of Martian meteorites called shergottites are systematically older by about 25% or more than ages obtained using Sm–Nd and other radiometric dating methods. The older 40Ar/39Ar ages indicate the presence of 40Ar not derived in situ from the radiogenic decay of 40K. The ‘excess’ argon can be associated with several different components, including the Martian atmosphere and mantle. We discuss the sources of Ar in shergottites, and the methods used to separate and identify them.