Small Fault Scarps on Mercury Detected in Low-Altitude MESSENGER Images
Tuesday, 16 December 2014
Lowering MESSENGER’s periapsis altitude has provided the opportunity to image the surface of Mercury at higher resolution than was possible during the earlier orbital phase of the mission. In images obtained at spatial scales as good as ~3 m/pixel, small thrust-fault scarps have been detected. The small scarps found thus far are less than 10 km in length and have only tens of meters of relief, much smaller than the large lobate scarps that are the dominant contractional tectonic landform on Mercury and have been attributed to cooling and contraction of the planetary interior. The small scarps on Mercury are nonetheless comparable in scale to similar small scarps documented on the Moon. High-resolution MESSENGER images also reveal that the small scarps crosscut small impact craters (<100 m in diameter). Transected small craters <100 m in diameter on the Moon are Copernican (<~800 My) in age, broadly equivalent to the Kuiperian period on Mercury. Their low relief, lack of superposed small craters, and pristine appearance support the inference that these scarps are geologically young, because small landforms on Mercury will not survive for billions of years under steady meteoroid bombardment. Lunar lobate scarps comparable in scale could be less than 50 My old on the basis of estimated rates of infilling of small, shallow graben in the back-scarp terrain. The impact flux on Mercury is estimated to be a factor of ~3 greater than on the Moon, so the rate of degradation of small landforms on Mercury is expected to be comparably greater. In addition to the detection of small, young scarps, MESSENGER is acquiring images of large lobate scarps from low altitudes. Disturbed and crosscut small impact craters (<100 m in diameter) have been found in high-resolution images on the northwest segment of Carnegie Rupes, indicating recent activity on thrust-fault segments associated with older, prominent scarps. The young, small fault scarps and small-diameter craters crosscut by both small and large scarps are evidence that the contraction of Mercury has continued to geologically recent time and is likely ongoing.