Identification and Characterization of Small-scale Polygons around the Phoenix Landing

Introduction: The Phoenix probe landed on the northern plains of Mars (68.2°N, 234.2°E) on May 25, 2008, and the landscape seen on the first images acquired on the surface confirmed what was already known since the HiRISE camera began scrutinizing the Martian surface with unprecedented detail, that the existence of polygonal terrains is a predominant feature in that area [1][2]. The polygons observed in this region have been recently reported to fall into two distinct dimensional classes [2]: small-scale polygons, 3 to 6 metres, with an average median axis of 4.6 meters; and large-scale polygons, 20 to 25 metres, with an average axis of 22.1 meters. The smaller polygons have generally darker contours thinner than what the resolution of MOC/MGS narrow-angle images was capable of illustrating, but now resolved in those of HiRISE/MRO. This work is the first step of a planned intensive study of the small-scale polygons present on these plains, using HiRISE images and extracting the same geometric and topological parameters as in previous studies [3][4], through automated segmentation [5] and characterization [6] algorithms. Identification of small-scale polygons: We began by studying the polygonal network in a region of 2,600x2,600 m centred in the location where Phoenix landed, contained on the HiRISE image PSP-0085912485. Due to some constraints on computational power available the selected region (10,400x10,400 pixels) was divided into 25 square scenes, each covering 600x600 m (2,400x2,400 pixels) with an overlap (EW and N-S) between adjacent images of 100 m [7]. We created this overlap in order to count the maximum number of polygons, since the procedure employed for neighbourhood analysis [6] only uses complete polygons, i.e., the polygons cut by the edges of the images and their neighbours are not counted to avoid bias on the measurements. The polygonal network is visually homogeneous and occupies all the area. Although the small-scale polygons are not equally discernible in every region of the image, in most of the cases the edges are clearly detected; even taking into account the cases in which the identification of polygons is uncertain, the global segmentation performance is very good. An example of this detection is presented in Figure 1. The use of this automated approach permitted us to identify some 450,000 polygons in the 25 square scenes, with about 17,500 polygons per each 600x600 m region. Figure 1. Small-scale polygon detection on HiRISE image PSP-008591-2485. Top: image detail (side is 250 m on the terrain); Bottom: corresponding segmented network (edges with 1 pixel thickness).