The Early Martian Magnetic Field

Intrinsic planetary magnetic fields are responsible for the protection of atmospheres against the solar wind. The available data of an early Martian magnetic field are restricted to measurements by Mars Global Surveyor (MGS) and the study of Martian meteorites [e.g. 39]. In 1997, MGS started with the global mapping of the Martian magnetic environment and found that the planet does not have a significant global intrinsic magnetic field [40]. However, these measurements also established the presence of significant, local, small-scale, crustal, remnant magnetisation.

This local magnetisation appears mainly in the ancient southern highlands, and is absent in the regions were large impacts occurred (like Hellas and Argyre).

Since these impact basins were formed about 4 Ga ago, it is generally argued that the Martian dynamo ceased before this time [41]. Another study used the same data and suggested that the Martian dynamo turned on less than 4 Ga ago, and turned off at an unknown time since then, with the only limitation that the present magnetic field strength must be achieved [42].

Magnetic studies of the Martian meteorite ALH 84001 revealed that about 4 Ga old carbonates contained magnetite and pyrrhotite, which carried a stable natural remnant magnetisation [43]. This result implies that Mars had established a geo-dynamo within 450 to 650 Ma after its formation with an intensity within an order of magnitude of that of the present Earth. Moreover, these results support the theory that an ancient strong dynamo ceased about 4 Ga ago. The remnant magnetism of Martian SNC meteorites, which formed 1.3 Ga to 180 Ma ago, is consistent with both theories since such field strengths could appear at the surface of Mars even at present [44].

The assumed/indicated intrinsic magnetic field, which is comparable to the terrestrial one, should have protected the atmosphere from the dense solar wind of the young Sun. On the other hand, these studies indicate that the Martian atmosphere may have been unprotected and affected by various non-thermal escape processes by the solar wind plasma during the past 3.5 Ga.

0 0

Post a comment