Dynamic Landscapes at High Latitudes on Mars: Constraints from Populations

Introduction: Populations of impact craters are widely used in planetary science to study surface ages and the nature of resurfacing. Apparent surface age depends on scale; at smaller scales natural surfaces tend to be younger. High resolution imaging of Mars has revealed that at the scales of decameters and smaller, surprisingly many different terrains are devoid of small craters and hence are extremely young [1]. HiRISE images [2] with their high resolution and dynamic range allow identification of smaller craters and hence quantitative studies of younger crater population. Very young crater populations are thought to be unaffected by distal secondary craters, because they are formed after the most recent secondary-forming impact. All surface above ~60° latitude in both hemispheres is very young at the small scales. Except polar layered deposits, dunes, residual ices and rare very steep slopes, all surfaces at these latitudes are covered by continuous polygonal patterns [3-5], regardless of geological settings. Here I present a progress report in my study of small crater populations on these terrains. Production function: To apply quantitative methods to crater populations, one needs to know the production function, that is the size-frequency distribution (SFD) of newly formed craters. For small craters it has not been known: the Neukum-Hartmann production function (NPF) [6] is established for crater diameters D > 10 m. To obtain it for smaller diameters, I studied [7] a population of small craters on proximal ejecta of Zunil crater. This population is very young and there is a good chance that it is not affected by secondary craters; crater morphology indicates that obliteration of craters is minor, and the population is close to pure accumulation regime; there is a good coverage with 4 full-resolution HiRISE images. I registered locations and diameters of all craters I could identify in the images, about 5700 craters in the 290 km 2 study area. Some craters in the population form clusters, which occurs due to break-up of projectiles in the atmosphere [8]. For age estimations, each cluster should be considered as a single impact event. I ran a "cluster-ing" algorithm, which identified clusters and replaces them with "effective" craters. The "clustering" is not 100% reliable; some large clusters may be erroneously identified as a few separate clusters; some craters may happen be too close to each other occasionally. However , estimations showed that the effect of these errors can be neglected.