STATISTICS OF MORPHOLOGIES OF SMALL PRIMARY AND SECONDARY CRATERS ON MERCURY'S NORTHERN PLAINS

Since MESSENGER entered orbit around Mercury, the spacecraft’s narrow-angle camera has taken numerous high-resolution images of the planet’s extensive northern plains. We have studied the size-frequency and spatial density relationships of small craters, separated by morphological class, mainly in the diameter range 100 m to 10 km. These craters reveal the effects of processes affecting the plains surface at smaller scales and, potentially, during more recent times than studies of larger-scale craters and other landforms. A common trait of the region we studied (near 70º N, 320º E) is a population of less-than-pristine craters often arranged in chains or clusters, which are rare at diameters of 5 – 10 km, but nearly saturated near 1 km. We interpret these as large, near-field secondary craters, formed by ejecta from some of the largest, rather nearby primary craters. They show progressively more degraded morphologies and sharply reduced spatial densities at diameters from 1 km down to 400 m, implying that craters < 1 km diameter and topographic features less than about 100 m in vertical scale were inundated, perhaps by one or more lava flows with a cumulative depth of ~100 – 200 m. (There is abundant evidence from these and other images that the northern plains were formed by voluminous flood volcanism.) Superimposed on these subdued, undulating plains surfaces is a population of tiny craters (100 – 300 m in diameter) of rather fresh morphology. These craters are highly spatially clustered, suggesting that they must also be secondary craters; but their circular shapes indicate that they formed as far-field secondaries by ejecta impacting at higher velocities, derived from more distant primary impacts. These statistics strongly suggest that cratering and/or degradation by lava inundation was episodic. Since the clusters of tiny craters formed, later processes have modified the landscape hardly at all (≤ 10 m depth), so formation of the lava plains in this area wholly ceased before formation of the large, distant craters that peppered the surface with these clusters of small secondaries. The domination of the northern plains by secondary craters < 10 km in diameter will complicate relative dating of small geological units by measuring relative densities of small primary craters.