Characterizing Polar Layered Deposits at the Martian North Pole: Current Results

Introduction: Within the northern residual polar cap of Mars are dark lanes or troughs; on the walls of these exposures are layered deposits. These deposits consist of extensive lateral layers of ice and dust and are found throughout the polar cap. They were first identified in Mariner 9 images [1, 2] and later studied in detail with the Viking orbiters [e. g. 3, 4, 5, 6]. In these images, the layers appear to consist of alternating sequences of light and dark layers ~ 5 to 25 m thick [4, 5]. Recent data taken by the Mars Orbiter Camera (MOC) onboard the Mars Global Surveyor (MGS) reveal that the layers are thinner and more numerous than Viking images suggested. Layers are seen with thicknesses at the limit of resolution (~2 m) and it is possible that smaller scale layers may also exist [7]. An understanding of the formation processes of the polar layered terrain will provide insights into the processes of trough formation, lateral propagation, erosion, and deposition. It will also allow us to interpret the simple vertical record of climate change encoded in the layered deposits. Ultimately, we will gain a foundation upon which we can build a better model for the interactions of the various volatile deposits on Mars, including the current polar surface as well as the latitude-dependent layer of subsurface ice currently being mapped by Mars Odyssey [8, 9]. In order to begin this foundation of knowledge we first must characterize sections of layers and quantitatively compare the layer sequences in one trough with another. In this way, constraints may be placed on trough formation mechanisms. Polar Trough Characterization: Much work has been done to describe the layers in the northern cap, especially those in the region shown in Figure 1. The individual layers show considerable variation in thic kness and conformity. Layers range in thickness from several meters to several tens of meters [10]; some layers are observed to pinch out [7]. Additionally, layers show varying resistance to erosion. In particular, a “marker bed” 20 m thick with resistant knobs ~ 10 m wide is observed in many places [7, 11, 12] Layers also show variation in surface texture. Individual layers display surfaces with pockmarks, brick-like textures, and a pattern similar to a deformed, woven structure [10]. It is not clear if the textures are a property of the layer or are limited to the exposed surface. There is not a relation between texture and location. The variety of textures may be due to structural properties of the individual layers which caused different amounts of erosion [10]. Some textures indicate a pitted surface, which may be due to outgassing of clathrate-rich layers [12]. These layers are laterally extensive. Several images taken from a Trough A in Figure 1 show that in some regions individual layers can be traced for many hundreds of kilometers [7, 12]. Two different investigators have proposed that the marker bed can be found in multiple troughs 50 km apart [7, 11]. However, a preliminary assessment of images found that layer sequences vary greatly around the cap [10]. An additional difficulty for comparing image sequences is the seasonal surface frost deposits found throughout this area. Indeed, two sequences can look very different from each other when in reality they are taken from either side of the same image [12]. Frost can cause several layers to look like one thick layer. It can also be preferentially deposited within a segment of a dark region, revealing many layers in an area which would otherwise be thought to be one layer [12]. Figure 1c shows image M00-01754 from Trough A. Figure 1d is the DN profile taken along the dashed line in 1c. Some layer transitions are seen as sharp jumps in the DN profile; the most distinct of these are identified with letters in the two images. The region of the image above layer a clearly contains several layers. However, when examining the DN profile there are no clear jumps in DN value that indicate a layer boundary. Additionally, there is more variation in brightness within the light layer above a than our eyes can observe from the image. The DN profile reveals details about the layers that might otherwise be missed.