Surface textures of Mars' north polar layered deposits: A framework for interpretation and future exploration

Background: The deposition of layers within the north polar layered deposits (NPLD) is thought to be controlled by changing climate influencing the relative proportions of ice and dust accumulated in the polar regions; thus the NPLD are considered to contain the record of recent climate change on Mars and are important targets for future exploration. The physical properties and history of these layers remain relatively unknown. No detailed analysis of the morphology and texture of individual layer surfaces has been carried out until now; doing so can provide insight into the properties of the layers themselves and highlight useful measurements to make in the course of future exploration of the deposits and demonstrate what conditions such missions especially rovers or drills will face. Approach: Of the 214 highest-resolution images available of the north polar region at the time of this study, 36 contain exposures of the NPLD. We examined the morphological characteristics of the surfaces of individual layers and compared them to those of terrestrial ice sheets in order to consider what physical characteristics may cause the morphologies observed in the NPLD, and to outline what future measurements will aid in understanding the processes that formed and shaped these deposits. Results: The surfaces of the NPLD examined in the highest resolution images available display a variety of surface textures and resistances to erosion and erosion style. We classify layers according to their distinguishing characteristics; these classifications include knobby, pitted, rough, imbricated, and laminated layer textures. Several layers display evidence for deformation. Ten images between 4° and 40°E contain layer sequences that appear to include unconformities. Interpretations: Mechanisms for forming these surface textures may include variations in erosion rates and styles of layers with different physical properties such as ice to dust ratio or ice grain size. We propose a conceptual model where variations in surface characteristics can be explained through climate influence on dust and ice deposition followed by subsequent erosion of this material. Potential angular unconformities and deformation features imply that portions of the NPLD have a complex history that may include periods of significant erosion as well as some amount of flow. Conclusion: A comparison of terrestrial ice surfaces and martian layer bed surfaces provides insight into the history of polar processes. The textures of exposed individual layer surfaces may be heavily influenced by the physical properties of the underlying layer; such properties and therefore the surface textures as well may be influenced by climate variations. We propose a simple conceptual model in which layer surface texture is controlled by cyclical variation of dust content in the NPLD. We identify a number of observations that future polar surface missions should make to help distinguish among hypotheses. The diverse nature of these deposits indicate that ideally, future missions should be capable of analyzing the NPLD across multiple layers in multiple locations. Any mobile platform will need to be able to navigate rough surfaces as observed in many layers.