5. Planetary Magnetism
Achievement: Mars Global Surveyor discovered and mapped intense magnetization in the Martian crust. The data indicate that Mars once had a global magnetic field, driven by a dynamo that halted early in Martian history.
Significance: The early magnetic field would have helped to shield the Martian surface from the solar wind, allowing atmosphere and water to be retained. The field’s loss may have triggered the shift from the wetter Mars of geological record to the arid planet of today.
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Image: NASA, from 
The Earth’s approximately dipolar
global field. Image: NASA, Jack Connerney, Mario Acuna, Carol
LaddMars Global Surveyor collected data under Mars’s ionosphere while aerobraking, and produced the first useful information about martian magnetic properties. The planet lacks an Earth-like global magnetic field. But widespread patches of strongly magnetized crust on the martian surface indicate that a so-called dynamo field similar to Earth’s once existed there, driven by convection within a molten core. The magnetized areas are concentrated in rock from the first few hundred million years of Mars’s existence (the Noachian geological era).
Acuna, M.H. et al. (1999), “Global distribution of crustal
magnetization disovered by the Mars Global Surveyor MAG/ER experiment,” Science
284, 790 – 93.
Connerney, J. E. P., et al. (2001), “The global magnetic field
of Mars and implications for crustal evolution,” Geophys. Res. Lett., 28(21),
4015–4018.
Patches of magnetized crust on the martian surface. Geophys. Res. Lett., 28 (21), cover, 2001. ©2001 American Geophysical Union. Reproduced by permission of American Geophysical Union. |
Crustal magnetism on the martian surface. Red and blue patches indicate areas of strongest magnetization, concentrated in regions of oldest rock. Pale green background stripes are spacecraft tracks. Dark green circles are volcanoes. Open circles are impact craters. The solid line indicates the north-south dichotomy boundary. Image: PIA 02059, NASA/JPL/GFSC. |
Rock magnetization in the nineteen largest impact craters. The older 15 craters are magnetized, the younger five demagnetized. |
Impact craters superimposed on laser altimetry topographical Mars maps. Blue indicates lowlands, red highlands. Dotted lines are impacts inferred from crustal thinning data. Very large impact craters characteristically have multiple concentric rims. From Frey et al. Published 2008 by American Geophysical Union. Reproduced by permission. |
The nineteen largest impact craters on Mars (up to 2000 miles across, with some detectable only by MGS’s laser altimeter) are suggested to have been created at the end of the Noachian era, during a single brief (150 million year) window approximately four billion years ago. The rock of the fourteen craters identified as oldest is magnetized, indicating that Mars’s dynamo operated when they formed; that of the five considered youngest is demagnetized, indicating that by then the dynamo had stopped. The data suggest that the transition was geologically rapid, over only about 20 million years.
Roberts, J.H., R. J. Lillis, and M. Manga (2009), Giant impacts on early Mars and the cessation of the Martian dynamo, J. Geophys. Res. 114, E04009. Lillis, R. J., H. V. Frey, and M. Manga (2008), Rapid decrease in Martian crustal magnetization in the Noachian era: Implications for the dynamo and climate of early Mars, Geophys. Res. Lett., 35, L14203 Frey, H. (2008), Ages of very large impact basins on Mars: Implications for the late heavy bombardment in the inner solar system, Geophys. Res. Lett., 35, L13203. From Roberts et al. ©2009 American Geophysical Union. Reproduced by permission.
Without a global magnetic field, the martian atmosphere is being eroded by the solar wind. The upper atmosphere extends hundreds of kilometers into space, where it interacts directly with solar radiation that is not deflected by a magnetosphere as on Earth. Atmospheric particles are ionized by the radiation and carried off. Extrapolating the observed rates backwards four billion years suggests that solar wind erosion is responsible for Mars’s thin atmosphere (about 1% of Earth’s atmospheric pressure.)
Brain, D.A. and 26 others. A comparison of global models for the solar wind interaction with Mars, Icarus doi:10.1016/j.icarus.2009.06.030 (in press).
Lillis, R. J., H. V. Frey, and M. Manga (2008), Rapid decrease in Martian crustal magnetization in the Noachian era: Implications for the dynamo and climate of early Mars, Geophys. Res. Lett., 35, L14203 Frey, H. (2008), Ages of very large impact basins on Mars: Implications for the late heavy bombardment in the inner solar system, Geophys. Res. Lett., 35, L13203.
Image: NASA/Steve Bartlett
