HR: 0830h
AN: G41B-0033    [PDF]
TI: Determination of Variations in Ellipsoidal Shape of Mars From Paleo-shorelines
AU: * Gratton, R
EM: gratton@chimie.univ-nantes.fr
AF: GEOTOP-UQAM-McGill, B.P. 8888 succursale centre-ville, Montreal, QC H3C 3P3 Canada
AU: Mareschal, J
EM: jcm@olympus.geotop.uqam.ca
AF: GEOTOP-UQAM-McGill, B.P. 8888 succursale centre-ville, Montreal, QC H3C 3P3 Canada
AU: Sotin, C
EM: sotin@chimie.univ-nantes.fr
AF: Laboratoire de Planetologie et Geodynamique, Universite de Nantes 2, rue de la Houssiniere B.P. 92208 cedex 3, Nantes, 44322 France
AB: Following Parker (1993), three contacts traced on Viking and MOC images have been tentatively identified by Clifford and Parker (2001) as paleo-shorelines: Meridiani, Arabia, and Deuteronilus. As Head et al. (1999) did, we checked that these contacts stay along an equipotential and we found that the deviation is too important for these contacts to be paleo-shorelines unless the reference ellipsoid changed after the formation of Meridiani, the first of these contacts. For a triaxial ellipsoid, the longitudinal variation of the potential along the equator follows the variation in radius to the center of mass. Assuming that the best-fit ellipsoid of Mars changed with time, we have found the best fitting ellipsoid along the three contacts in order to deduce the time variation in equatorial flattening. For this preliminary study, we assumed that the polar flattening stayed constant. The best-fit ellipsoid for the contact Meridiani had an equatorial flattening of 1/1100. For the contact Arabia, we obtained a different value, 1/1000 for the equatorial flattening, and we believe that there had been a regression. A second variation of the equatorial flattening to 1/2544, is needed for the contact Arabia. For the contact Deuteronilus, we also obtained two best-fitting ellipsoids. For the first one, the equatorial flattening is the same as the second Arabia, 1/2544, but there was a regression. For the second one, the equatorial flattening, is 1/5000. The present value of the equatorial flattening is 1/2544. Because we can date these contacts from the age of the surface where they are observed, we can try to relate these variations of the ellipsoid shape to past tectonic activity. The reference ellipsoid, elongated at the beginning, became more circular. A tentative interpretation is that the Meridiani and Arabia contacts formed during the Noachian and the elongated ellipsoidal shape would be related to the formation of the dichotomy. The Deuteronilus contact formed later during the Hesperian when the volcanism of Tharsis dome began (Tanaka, 1986). \\ \\ Clifford and Parker Icarus, 154, 40-79, 2001. \\ Head, J.W. et al., Science, 286, 2134-2137, 1999. \\ Parker, T.J., J. geophys. Res., 98(E6), 11061-11078, 1993. \\ Tanaka, K.L., J. geophys. Res. 91, 139-158, 1986.
DE: 1227 Planetary geodesy and gravity (5420, 5714, 6019)
DE: 5415 Erosion and weathering
DE: 5417 Gravitational fields (1227)
DE: 6225 Mars
SC: Geodesy [G]
MN: 2003 Fall Meeting