2014 年 48 巻 4 号 p. 231-243
Age is one of the most important observables in planetary science. The ages of geologic events on planets including Mars, however, are estimated with the crater chronology approach, which is largely based on radiometric age data derived from very limited places on the Moon. Obtaining age data for the inner planets, asteroids, or the satellites of the outer planets is essential for accurate understanding of the history of our solar system. Although sample-return missions can achieve this goal, they are technically very challenging and extremely expensive. Thus, in-situ geochronology measurements with one-way lander/rover missions would be very important. This paper reviews (1) primary scientific issues on planetary chronology, such as uncertainties in the crater chronology models on Mars, (2) recent developments for in-situ dating instruments based on K–Ar, Rb–Sr, or Pb–Pb dating methods, (3) in-situ K–Ar age measurements conducted by NASA's Curiosity rover, and (4) the development status of our in-situ K–Ar isochron dating method based on a combination of laser-induced breakdown spectroscopy and quadrupole mass spectrometry for measurements of potassium and argon, respectively. We also discuss perspectives for a future Japanese in-situ geochronology mission to Mars.