Mathematics – Logic
Scientific paper
Dec 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt........33c&link_type=abstract
Thesis (PhD). UNIVERSITY OF VIRGINIA, Source DAI-B 60/06, p. 2746, Dec 1999, 161 pages.
Mathematics
Logic
Scientific paper
With the advent of Clementine data it is now possible to determine lithology and extent of areally small geologic units on the Moon, but age-dating these materials is difficult. A model is presented for estimating their age by analyzing the morphometry of degraded craters 1-3 km in diameter. Photoclinometry was used to extract the topography of fresh craters from monoscopic Clementine images. A two-dimensional computer model simulating linear diffusional creep was applied to fresh craters 3 km in diameter. The resulting profiles were then compared to photoclinometric profiles of degraded craters of known ages for calibration. Application of the resulting model to degraded craters in mare deposits of the central Apollo basin (-36.5° lat., 208.0° long.) indicates that this unit was emplaced during the early Imbrian period ( ~ 3.85 Gya). By calculating the amount of material eroded from each of the degraded craters observed in this unit, the average erosion rate is estimated to be 2.0 +/- 0.1 × 10-7 mm/yr on the Moon since the Imbrian. On Mars, degraded craters are indicative of an early climate that was much different than the present. Photoclinometric analyses of degraded crater morphometry reveal the complex stages of crater modification. A simple algebraic model based on the observed morphometric relations suggests that craters were enlarged by 7 to 10% during modification. By calculating the volume of material eroded during crater degradation and valley network formation, the erosion rates on early Mars were estimated to be on the order of ~ 0.0003 to 0.004 mm/yr, similar to those determined for modern day Canada or Alaska. Two-dimensional simulations of some possible degradational processes suggest that linear diffusional creep often combined with fluvial erosion and deposition produce equivalent degrees of modification through the range of crater diameters investigated in this study (10 to 50 km). These results imply that geologic processes related to precipitation dominated the early martian environment. Our working hypothesis is that this precipitation was due to the presence of a primordial atmosphere which condensed and collapsed (i.e., precipitated) into the martian regolith; a process which ceased during the late Hesperian/early Amazonian periods (3.5 to 1.8 Gya).
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