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The crustal magnetic field of Mars differs markedly from Earth’s as the Martian features are an order of magnitude stronger and are not distributed over the whole globe as on Earth, but are concentrated in a band in the southern highlands. Unless the Martian dynamo gave a surface field more than an order of magnitude stronger than the current geodynamo gives on Earth, which seems unlikely, some special process, which has not affected the terrestrial crust, operated on Mars to give the observed distribution of anomalies and to magnetize its ancient crust so efficiently. We suggest that water reacting with ancient Martian atmospheric carbon dioxide could give rise to fluids that dissolve igneous rocks in the crust and precipitate iron-rich carbonates, as observed in Martian meteorites. In the southern highlands, thermal decomposition of such iron-rich carbonates during metamorphism could give rise to plentiful single-domain magnetite and generate a potent source for the Martian crustal field. The lack of anomalies in the northern plains may result from higher water–rock ratios that prevented the formation or decomposition of iron-rich carbonates or the survival of single-domain magnetite. |
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