Physics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.211c&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 211
Physics
Scientific paper
Discussion and controversy have featured in the interpretation of magnetometer data from martian fly-by and orbiter missions regarding the existence of a martian magnetic field (Dolginov 1987, Russell 1987). Any evidence of an ancient magnetic field on Mars is clearly of interest in this context, and the SNC meteorites (if of martian origin) offer the possibility of detecting such a field. The presence of a primary, stable remanent magnetization (NRM) in terrestrial and extraterrestrial rocks indicates the presence of a significant ambient magnetic field during some event or events in the rocks' history. Such events include heating and subsequent cooling, metamorphism, and impact shock. The SNC meteorites that have been studied possess an NRM, the characteristics of which generally suggest the existence of a weak but significant magnetic field on the parent body at some time in the past. Attempts to determine the strength of the field have only been partially successful. However, although a rather wide range of values has been obtained (~0.3-3 microteslas), they are significantly larger than the maximum values for the present field given by Dolginov (1987) and Russell (1987) of 0.1 microtesla and 0.01 microtesla respectively. Two basic magnetizing processes may be proposed. A "conventional" process would be thermoremanent magnetization (TRM), acquired when the material cooled from a high temperature (>~800 degrees C) in an ambient field. An alternative possibility is magnetization by shock in either a steady ambient magnetic field or a transient magnetic field generated during an impact event. Such transient fields have not been shown to exist on a planetary scale, and shock magnetization or TRM in a global magnetic field is favored as the source of the NRM. Since SNC meteorites are likely to have been subjected to moderate or severe shock during ejection from the martian surface, shock magnetization cannot be ruled out. Shergotty and ALHA 77005 are believed to have been severely shocked about 180 Ma ago and later collisional events are suggested by exposure ages (McSween (1985). Nakhla shows no evidence of sufficient shock to impart shock magnetization, and TRM is favored for this sample. Thermoremanent magnetization would certainly have occurred during cooling and crystallization, if a field was present, at ~1.3 Ga ago. The interplanetary field is unlikely to have been as strong as indicated above (at least two orders of magnitude stronger than its present value) during the relevant timespan, and does not appear to be a viable magnetizing field. It is tentatively concluded that there was a significant global magnetic field on Mars at some time between 1.3 Ga ago and the present. Further work on existing SNCs, and hopefully on future falls and finds, is desirable before present indications, with their important implications for the martian core and dynamo field generation within it, can be confirmed. References: Dolginov, Sh.Sh. (1987) Earth, Moon and Planets 37, 17-52. Russell, C.T. (1987) In Geomagnetism 2 (ed. J.A. Jacob), pp. 457-523. Academic Press, London. McSween, H.Y. (1985) Rev. Geophys. 23, 391-416.
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