HR: 10:20h
AN: GP42A-01    [Abstracts]
TI: Crustal Magnetization of Mars in the Noachian Controlled by Lithology or Cooling rate in a Reversing Dynamo?
AU: * Rochette, P
EM: rochette@cerege.fr
AF: University of Aix-Marseille 3, CEREGE BP80 Cedex 4, Aix en Provence, 13545 France
AB: The presence or lack of crustal magnetization on Mars has been attributed initially to the cooling of the crust during or after the activity of a dynamo in the core of Mars. In that interpretation the Noachian crust should be strongly magnetized over its entire surface. Later was evidenced the lack of magnetization in the Noachian crust above large impact craters, as well as around these craters, due to demagnetization during impact, occurring at a time after dynamo shut-down. However, a rather large surface of Noachian crust is non-or weakly magnetized, without possibility for impact demagnetization. This has been discussed recently by Hood et al. [Icarus, vol.177 p.144-173, 2005]. They proposed a lithological origin (hydrothermalism) for the presence/lack of magnetization in the non-impacted Noachian crust, corresponding to the presence/lack of large amount of magnetic minerals in the crust. The purpose of the present contribution is to put forward an alternative hypothesis, taking into account that the crustal magnetization was acquired by cooling in a reversing dynamo, as demonstrated by alternating positive and negative magnetizations observed in the magnetized Noachian crust. The proposed model assumes constant amount of magnetic minerals, but variable reduction of total magnetization over the magnetic thickness, due to variable record of successive polarities of the magnetizing field. Large crustal cooling rates (e.g. over 90 K/Myr for polarity chrons duration of 10 Myr) would correspond to strongly magnetized zone, and slow cooling rates to poorly magnetized zone. Based on this model I derive possible explanations for the proposed confinement of magnetized zones within the intertropical band. The coincidence noted by Hood et al [2005] between near-equatorial valley networks and strongly magnetized, i.e. rapidly cooled crust, could be due to a positive feedback between river incision and crustal uplift, as described in terrestrial mountain ranges.
DE: 1535 Reversals: process, timescale, magnetostratigraphy
DE: 1595 Planetary magnetism: all frequencies and wavelengths
DE: 5440 Magnetic fields and magnetism
DE: 6225 Mars
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005