Depth to Diameter Ratios of Recent Primary Impact Craters on Mars

Introduction: Since confirming as new impacts 19 of the 20 dark spots on Mars detected by [1], ~50 additional new impact sites have been confirmed by the High Resolution Imaging Science Experiment (HiRISE) following their discovery by the Context camera, both on the Mars Reconnaissance Orbiter. These impact events have probably occurred within the last decade or so, as indicated by the absence of associated dark spots in previous images. Several of these sites have been described elsewhere [2, 3, 4, 5]. Primary Impact Sites: New small craters are most likely to be primaries [1] because they formed at different times, in widespread locations across the planet. No new large craters have been found that could be the potential primary or primaries if these were secondaries; such a new crater would almost certainly have been detected by at least one of the eight cameras orbiting Mars on four spacecraft over the past decade. In addition, the statistical probablility of enough new large impact events within the last decade to explain these as secondaries is extremely low [2]. Statistics of small primary impact events constrain modification rates and have important implications for the broader field of planetary chronology [e.g., 8, 9]. The difficulty has been in distinguishing small primaries from the huge number of secondaries produced episodically by larger primaries. It has been suggested that most of the small craters in many regions of Mars are secondaries because nearly all of the freshest craters have depth/Diameter (d/D) ratios like those of secondaries [7]. However, we now know that small primaries are forming at a high rate, perhaps comparable to the long-term rate of secondary production. One solution to this enigma would be if small primaries on Mars are generally shallower than on the Moon, due to different surface properties, atmospheric interactions, and impact velocities. This does not seem to be the case, however, at least not for this subset of new small primaries. Depth/Diameter: The ratio d/D has long been used as a criterion for distinguishing primary from secondary craters [e.g., 6]. Now we have a data set of small craters that we know are extremely fresh primaries. Their d/D ratios can be used as a standard for studies of craters whose origin is not as clear, for example spatially random craters that could be either primaries or very distant secondaries [e.g. 7]. Other criteria used for identifying secondaries [e.g. 10,11] are not always present or reliable. Noncircularity, for example distant secondaries can be circular if ejection velocities are high enough [7, 8]. Clustering alone cannot be used to recognize secondaries. Even if it wasn’t previously known that meteroids break up in the martian atmosphere creating clusters of primary craters [13], these new impacts, 60% of which are clusters, would verify it. Optical or infrared rays are also used in identifing secondaries contained within them, but such rays are not always present or preserved [7, 8]. Method: In this work, the new craters were examined in HiRISE images, and rim-to-rim diameter and rim-to-floor depth were calculated using shadow measurement techniques of [14]. Features that were too small to be confidently measured (<4 pixels across), observations taken with solar incidence angle <45° (and thus inadequate shadows), and those with indistinct or uneven rims or shadows were excluded from the study. HiRISE’s three-band color coverage was useful for distinguishing shadows from dark material exposed in the interiors of craters. Groups of craters in clusters clearly associated with the same event were measured individually; measurements were combined to get a site average for each impact event. The resulting list is concentrated in areas of high dust cover (Fig. 1) like Tharsis and Arabia because new impacts are more easily recognized in these areas.