Crater Population and Resurfacing of the Martian North Polar Cap

Introduction: The Martian north polar residual cap (NRC) lies on top of the north polar layered deposits (NPLD). The NRC is about 1m thick and composed primarily of large-grained, dust-poor water ice [1]. The mass balance of the north polar cap is uncertain. Satellite imagery reveals small reversible changes in its extent on an interannual basis [2]. Images with a pixel scale of up to 0.25 m/pixel from the High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter (MRO), show brighter, or smaller-grained, (i.e. younger) ice superposing darker, or larger-grained, (i.e. older) ice. The lack of dust accumulation indicates that the material composing the NRC accumulated recently. On the other hand, the exposure of darker, larger-grained ice indicates a current state of net ablation. Small pits observed in HiRISE imagery resemble suncups and also suggest recent ablation. The NPLD are believed to preserve a record of seasonal and climatic cycling of atmospheric water and dust and could reveal important information regarding Martian geologic and climatic history. The NRC is often considered new NPLD material. Thus, understanding the NRC’s current behavior and mass-balance in relation to the current climate is an important step in reading the climatic record of the NPLD. One way to do this is to analyze the cratering record of the NRC. Previous studies concluded a NPLD resurfacing age of only ~20-100 Kyr. [3; 4]. However, due to the low resolution of available imagery, these conclusions were based on 0 or 2 craters respectively. To date, data from the Context Camera (CTX) aboard the MRO provide almost complete coverage of the NRC and NPLD with pixel scales of ~5 m/pixel (Figure 1A). To constrain the processes and rates of NRC resurfacing, we are conducting a search for craters within the CTX dataset. Using the NRC crater population data, we will use landscape evolution modeling to investigate the recent (10-100 Kyr) mass-balance history of the NRC. CTX Search: CTX products available on the Planetary Data System (PDS) are projected using ISIS (Integrated Software for Imagers and Spectrometers) [5] and the crater search is being conducted using ArcMap, a component of ESRI's ArcGIS Geographical Information System (GIS) [6]. To date, ~100 craters have been counted and measured on the NRC and NPLD combined (of which only 4 were previously known); ~70 of these craters are located on the NRC (Figure 1). HiRISE images have been acquired to follow up on 52 of the craters (several in stereo). NRC and NPLD craters range in diameter from ~10-450m (~10–212m for craters on the NRC only). Comparisons of diameters measured in HiRISE and CTX images indicate that our CTX measurements slightly overestimate diameters (Figure 2). The results of the comparison were used to systematically correct CTX-only diameter measurements.