70th Annual Meeting of the Meteoritical Society: Abstracts A63 5008 CRATER RETENTION AGES OF PLANETARY SURFACE PROCESSES: NEW PROGRESS

Introduction: Impact crater densities (craters/km2) have allowed successful estimate of planetary surface ages and survival times of topography on modified surfaces [1–3]. Malin et al. recently reported a breakthrough observation: formation rate of small (~20 m) Martian craters [4]. If correct, this will allow even more direct age measurement. History of Technique: The technique was introduced in the 1960s when Canadian impact craters were used to predict the ~3.6 Ga age of lunar lava plains [5], and an “early intense bombardment” prior to 3.6 Ga [6], confirmed by Apollo. In the 1970s, cratering rates were extended to Mars by scaling arguments [1]. This led to age estimates of a few hundred Ma for Mars’ broad lava plains [1], supported in the 1980s by ages of Martian basaltic meteorites [7, 8]. My 2005 “isochron” plot [2] shows expected crater densities on Martian surfaces of different age, in the absence of erosion. The new Malin et al. data [4] agree with the isochrons within a factor 3 [9]. This opens the door to direct Martian age measurements without scaling arguments. It also negates most recent critiques of the technique [10]. Example: Debris Aprons, Ice Flow, and Climate/Obliquity Cycles: A striking application illustrates the power of the method. Martian features attributed to ice deposition include debris aprons, mantles, and possible glaciers [2, 11, 12]. Measurements of ages of decameter-scale structure on these surfaces repeatedly give ages in the range ~3–50 Ma, measured either by my isochrons, Neukum’s isochrons, or the new Malin et al. measurement [2–4, 9]. These ages are in the range of the last few highobliquity episodes, and support suggestions that Mars is undergoing ~10 Ma cycles of climate change in which ice deposition occurs [2, 12–14]. Conclusions: The technique offers great promise for further work on Mars and extension to other planetary bodies. References: [1] Hartmann W. 1973. Journal of Geophysical Research 78:4096–4116. [2] Hartmann W. 2005. Icarus 174:294–320. [3] Hartmann W. and Neukum G. 2001. Space Science Reviews 96:165–194. [4] Malin M. et al. 2006. Science 314:1573–1557. [5] Hartmann W. 1965. Icarus 4:157–165. [6] Hartmann W. 1966. Icarus 5:406–418. [7] Hartmann W. et al. 1981. In Report of Basaltic Volcanism Study Project. Elmsford: Pergamon. [8] Nyquist L. et al. 2001. Space Science Reviews 96:105–164. [9] Hartmann W. Icarus. Forthcoming. [10] McEwen A. et al. 2005. Icarus 176:351–381. [11] Arfstrom J. and Hartman W. 2005. Icarus 174:321–335. [12] Head J. et al. 2003. Icarus 426:797–801. [13] Mustard J. et al. 2001. Nature 412: 411–413. [14] Costard F. et al. 2001. Science 295: 110–113. 5104 LIGHT NOBLE GAS COMPOSITION OF DIFFERENT SOLAR WIND REGIMES: RESULTS FROM GENESIS V. S. Heber1, H. Baur1, D. S. Burnett2, D. B. Reisenfeld3, R. Wieler1, and R. C. Wiens4. 1Isotope Geology and Mineral Resources, NW C, ETH, 8092 Zurich, Switzerland. E-mail: [email protected]. 2CalTech, JPL, Pasadena, CA 91109, USA. 3Physics and Astronomy, University of Montana, Missoula, MT 59812, USA. 4LANL, Space & Atmospheric Science, Los Alamos, NM, 87544, USA.