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M.A. Bullock, D.H. Grinspoon (SwRI), J.W. Head, III (Brown University)
It is possible that geochemical tracers of past climate change currently exist at the Venus surface. For example, Johnson and Fegley [2000; 2003] found in laboratory experiments that tremolite is metastable, decomposing on timescales of billions of years. Because of the large range of temperatures from lowlands to highlands on Venus (~100 K), measurements of differential tremolite abundance can constrain the time a particular lithographic unit formed, the average surface temperature since its emplacement, or both.
Alternatively, past episodes of high temperatures may have left their imprint on differential abundances of other minerals, such as other amphiboles, halogen-bearing minerals, or even carbonates. Clearly, this scheme uses a highly idealized description of how a particular reaction can proceed differentially on a planet’s surface, and how in situ measurements at different altitudes can yield age and/or time-averaged surface temperature. Most likely, rocks at different altitudes will have had different histories, even if their lithologies look similar. The variation of atmospheric water vapor with time and altitude will affect tremolite stability, as will possibly other trace species and total atmospheric pressure. Nevertheless, determining ages and possibly averaged or maximum surface temperatures over some time period is of utmost for understanding the history of Venus. The development and testing of this scheme, including its predictive possibilities and its weaknesses and limitations, will be discussed.
This work was supported by NASA Planetary Atmospheres Grant NASG511039, and NASA Planetary Geology and Geophysics Grant NAG510330.
Johnson, N.M., and B. Fegley, Icarus, 146, 301-306, 2000.
Johnson, N.M., and B. Fegley, Icarus, 165, 340-348, 2003.
The author(s) of this abstract have provided an email address for comments about the abstract: bullock@boulder.swri.edu
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.