Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmgp32a0189h&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #GP32A-0189
Physics
1540 Rock And Mineral Magnetism, 5109 Magnetic And Electrical Properties, 6030 Magnetic Fields And Magnetism, 6200 Planetology: Solar System Objects
Scientific paper
To date, comparisons of the global distribution of Martian crustal field anomalies with surface geology have revealed several basic characteristics (e.g., Acuña et al., Science, v. 284, p. 790, 1999). First, the strongest anomalies occur over the older southern highland terrane and are relatively weak over the younger, resurfaced northern lowlands. Second, a zone of especially strong anomalies occurs in the southern hemisphere over Noachian terrane centered approximately on the 180o longitude meridian. Third, anomalies are especially weak in the near vicinities of the large southern hemisphere impact basins, Hellas and Argyre. The simplest interpretation of the first characteristic is that a core dynamo was operative at least during the Noachian epoch and possibly afterward. The third characteristic does not necessarily require that the dynamo ceased operation before these basin-forming impacts. It is possible that highly susceptible sources were not produced as a consequence of these impacts. If so, no strong anomalies would be present even if a core dynamo were still operative. Similarly, the relative absence of anomalies over the northern lowlands does not necessarily require that the dynamo ceased prior to the resurfacing event. Because the Martian crust is very inhomogeneously magnetized, excavation of the Hellas and Argyre basins ("holes in the crust") would not produce strong anomalies if a core dynamo previously magnetized the crust. The most probable explanation for the present near absence of anomalies near these basins is that impact shock effects demagnetized the previously inhomogeneously magnetized crust. Experimental evidence shows that shock pressures of ~ 10 kbar or less can remove magnetic remanence while smaller pressures of a few kbar can reduce it. Magnetic susceptibility can also be reduced by shock effects. Using pressure attenuation curves and approximate scaling laws, it can be estimated that an impact that would produce a basin on Mars would produce shock pressures of 10 kbar out to several basin radii. Hence, the preferred concentration of anomalies over the southern highlands near 180o may reflect mainly the reduction of anomalies elsewhere in the southern highlands by the Hellas and Argyre impacts.
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