Other
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..gp32a01z&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #GP32A-01
Other
1521 Paleointensity, 1560 Time Variations--Secular And Long Term, 5440 Magnetic Fields And Magnetism
Scientific paper
Unlike Earth, Mars currently has no planetary magnetic field from an internal dynamo; however, Mars has locally strong crustal field from magnetic anomalies, predominantly in the southern highlands. It has been argued that the age of these anomalies is constrained by demagnetized areas associated with some impact basins, indicating that field that died within the first few hundred million years of the planet. This has been disputed on the basis that the impact basins might predate the dynamo, which would then have grown and died over the intervening period. A perceived difficulty with the early death of a Martian dynamo is that the basic similarity of Mars and Earth suggests that the Martian dynamo should have taken significantly longer than the allotted time to decay. We examine parameterized convection models of the Earth and Mars in order to seek firmer constraints on the lifetime of dynamos on these planets. Different but plausible assumptions concerning the nature and composition of the planetary interiors and the relationship between core heat flux and dynamo energetics were used. We find that the early demise of a Martian dynamo can by no means be excluded geodynamically. The relevant timescales are very sensitive to the partitioning of potassium into the core, which may be very different for the two planets due to the differing pressure regimes. If potassium transforms into a siderophile transition element at pressures around 30 Gpa, as hypothesized by some mineral physicists, then the Earth's core may be relatively enriched and Mars' core may be relatively depleted in potassium. This radioactive isotope can be a significant heat source. Other unknowns include the temperature of core formation, which exerts a significant control on the early heat flux of the entire planet, including the core. In contrast, a late-onset dynamo requires ``special physics'', such as a requirement for a solid inner core in order for a planet to be capable of supporting a dynamo, so that a delay in the solidification of the inner core until significantly after planetary formation would prevent an early dynamo, even if the heat flux through the early core-mantle boundary is very large and the core is vigorously convecting. In summary, for Mars we favor an early onset and early death for the core dynamo and resulting planetary magnetic field.
Frawley James J.
Ravat Dhananjay
Stegman Dave
Taylor Patrick
Zatman Stephen
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