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Palaeomagnetism of the Vredefort meteorite crater and implications for craters on Mars

机译：Vredefort陨石坑的古磁性及其对火星陨石坑的影响

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摘要

Magnetic surveys of the martian surface have revealed significantly lower magnetic field intensities over the gigantic impact craters Hellas and Argyre than over surrounding regions(1). The reduced fields are commonly attributed to pressure demagnetization caused by shock waves generated during meteorite impact(2,3), in the absence of a significant ambient magnetic field. Lower than average magnetic field intensities are also observed above the Vredefort meteorite crater in South Africa, yet here we show that the rocks in this crater possess much higher magnetic intensities than equivalent lithologies found elsewhere on Earth. We find that palaeomagnetic directions of these strongly magnetized rocks are randomly oriented, with vector directions changing over centimetre length scales. Moreover, the magnetite grains contributing to the magnetic remanence crystallized during impact, which directly relates the randomization and intensification to the impact event. The strong and randomly oriented magnetization vectors effectively cancel out when summed over the whole crater. Seen from high altitudes, as for martian craters, the magnetic field appears much lower than that of neighbouring terranes, implying that magnetic anomalies of meteorite craters cannot be used as evidence for the absence of the planet's internally generated magnetic field at the time of impact.
机译：对火星表面的磁力调查显示，巨大的撞击坑Hels和Argyre的磁场强度明显低于周围地区（1）。减小的磁场通常归因于在没有明显的环境磁场的情况下由陨石撞击期间产生的冲击波引起的压力消磁（2,3）。在南非的Vredefort陨石坑上方，磁场强度也低于平均水平，但在此我们证明，该坑中的岩石具有比地球上其他地方的等效岩性高得多的磁场强度。我们发现这些强磁化岩石的古磁方向是随机定向的，矢量方向在厘米长度范围内变化。此外，在冲击过程中，对剩磁有贡献的磁铁矿晶粒会结晶，这直接将随机化和强化与冲击事件联系起来。当对整个弹坑求和时，强而随机取向的磁化矢量会有效抵消。从高空看，就火星陨石坑而言，磁场似乎要比邻近的陨石坑低得多，这意味着陨石陨石坑的磁异常不能用作证据表明撞击时行星内部没有产生磁场。

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