Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p43d..02r&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P43D-02
Mathematics
Logic
5420 Impact Phenomena, Cratering (6022, 8136), 5430 Interiors (8147), 5440 Magnetic Fields And Magnetism, 6225 Mars, 8130 Heat Generation And Transport
Scientific paper
Mars currently has no global dynamo-driven magnetic field, but strong crustal fields [1] indicate that a global field existed in the past. Surface observations [2,3] indicate that 15 giant impacts (leaving basins >1000 km in diameter) occurred over a period of 100 Ma at about the same time the global magnetic field disappeared. Furthermore, the youngest basins in the sequence (e.g. Hellas and Argyre) are all demagnetized, suggesting that a dynamo operated early, but stopped in the mid-Noachian [4]. Giant impacts can deliver significant amounts of heat to the interior [5], reducing the rate of core cooling. Recent dynamo simulations suggest that if the Martian dynamo was subcritical, a decrease in core-mantle boundary (CMB) heat flow of as little as 1% would be sufficient to shut down the global magnetic field, but that an increase in heat flow of ~25% above the subcritical level is needed in order to restart the dynamo [6]. Thus, if the dynamo fails once, it may not return even if the original core cooling rate is restored. Here we investigate a possible link between the giant impacts during the early to mid-Noachian and the contemporaneous cessation of the Martian dynamo. We determined the heating of the Martian mantle due to impactors [5] that formed the 20 largest exposed and buried impact basins with diameters greater than 1000 km [2]. At model times corresponding to the Hartmann-Neukum ages of the impact basins [3] we introduced this impact heating into 3D spherical convection models [7] of the Martian mantle and examined the effect on the CMB heat flow. We find that impact heating associated with the larger basins (D > 2000 km) can cause the global heat flow at the CMB to decrease significantly (>10%) and even briefly go negative for the largest basins (D > 3000 km). We suggest that such a reduction in core heat flow may have led to the cessation of the Martian dynamo. Utopia is the largest and the oldest demagnetized impact basins. We suggest that impact heating associated with the Utopia impact could permanently shut down the Martian magnetic field if the dynamo was subcritical at that time. The loss of the global magnetic field would have exposed the atmosphere to stripping and sputtering by the solar wind, resulting in atmospheric loss over time. Such loss may be responsible for the global shift in climate from wet (Phyllosian era) to drier, more acidic conditions (Theikian era) which has been inferred from mineralogical results from the OMEGA experiment [8], to have occurred < 200 Ma after the dynamo died. The atmospheric loss and climate change may have serious consequences for planetary habitability during the mid-Noachian. [1] Acuña, M.H. et al. (2001), JGR, 106, 23,403-23,418. [2] Frey, H. V. (2008), GRL 35, L13023. [3] Lillis, R. J. et al. (2008), GRL 35, L14203. [4] Arkani-Hamed, J. and D. Boutin (2004), JGR, 109, E03011. [5] Watters, W. A. et al. (2008), JGR 113, in press. [6] Kuang, W. et al. (2008) GRL 35, L14204. [7] Zhong, S. et al. (2000), JGR 105, 11,063-11,082. [8] Bibring, J. F. et al. (2006), Science 312, 400-404.
Lillis Robert J.
Manga Michael
Roberts James Hirsch
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