Small-diameter Martian Craters: Applicability for Chronology

Introduction: Crater counting has historically been used to establish relative and absolute chronologies on planetary surfaces and to develop global stratigraphies. Typically, craters in the size range >1 km have been used. This diameter was chosen as a convenient balance between image resolution and a diameter that would indicate a formation age. Craters at larger diameters might be craters protruding from an underlying surface; smaller diameters might be affected by local resurfacing. A limitation of an approach that employs this size crater is that counts must be compiled for large areas in order to reduce the statistical uncertainties associated with comparison of counts. This results in an inability to detect small areas of variable age. The availability of high resolution MOC, THEMIS and HRSC images provides the ability to examine the population of craters in the <<1 km size range. Because the production function of craters follows a log distribution, the number of craters in the meters to hundred of meters diameter range is several orders of magnitude greater than in the km range. Thus, smaller areas can be counted, in principal, with low statistical uncertainties allowing the recognition of smaller differences in age and smaller areas of different age. [1, 2] proposed absolute chronologies for small scale craters to allow them to be used to establish not only the relative age, but the absolute age of a surface. Such chronologies are established by modifying the lunar chronology (based on returned samples) for martian condidtions (e.g., impact velocities, gravity). Applicability of Model Isochrons: The isochrons that have been proposed by Hartmann and his colleague have been used by a number of investigators. The observed size-frequency distributions have been plotted with reference to the proposed isochrons and geologic scenarios in the context of absolute time have been proposed (Figures 1 and 2). However, in many cases it is unclear if the proposed chronologies are realistic. This concern arises from the statistics of the observed size-frequency distribution. Additional concerns are raised by issues of the lifetime of individual volcanoes and the thermal evolution of Mars. In most cases, the plots show that the observed size-frequency distrubiton of impact craters lie along a slope more shallow than that of the isochrons. This results in a diameter-dependent age – that is, for the same surface, the smaller the reference crater diameter under discussion, the younger the surface age. In such cases, it is not at all clear what diameter should be used for a reference. Figure 1 illustrates this effect for a portion of the southern Elysium plains. The various distributions represent counts made for different MOC images by different people along with counts made on Viking images [3]. The distrubitons clearly lie along lower slopes than the isochrons. Thus, any surface can have any age spanning hundreds of millions to billions of year, depending upon the diameter chosen. The geologic scenario proposed for this area [3], to account for the apparent age variation, is that over time volcanic eruptions repeatedly cover the entire area, but each eruption is successively thinner such it only buries the small diameter craters.