Investigation of the Extent of the Apollinaris Patera Ash Deposits: Implications for the Origin of the Columbia Hills

Introduction: The rocks of Gusev Crater around the Spirit landing site are mainly olivine-bearing basalts [1-3], and are thought to have crystallized from lava with low viscosity [4]. These rocks are dark in color, vesicular, fine-grained, and have coatings of dust in varying degrees [5, 6]. However, the bedrock outcrops of the Columbia Hills are remarkably different from those near the landing site. These rocks are layered and the beds dip in the same direction as the topography. They have a granular texture and are rich in alteration products such as hematite and goethite, suggesting modification by an acidic aqueous fluid. The Columbia Hills are thought to be older than the surrounding basaltic plains [3], and therefore could have resulted from an eruption from Apollinaris Patera. While the floor of Gusev Crater has been extensively studied, the deposits from Apollinaris Patera have not been traced from the volcano and potentially to Gusev. This study sought to map the Apollinaris Patera ash deposits in order to determine whether or not the ash traveled far enough to be a possible source of the Columbia Hills ash in Gusev Crater, leading to greater understanding of the processes involved in the formation of the Hills. Approach: The area is too dusty to use TES data for mineralogical comparison between Apollinaris Patera and the Columbia Hills. Nighttime infrared THEMIS images [7] of Apollinaris Patera, Gusev Crater, and the surrounding plains were compiled to create a large mosaic image. This image was used to distinguish between finer-grained deposits, such as dust, that cool quickly at night and rocks, which retain heat longer. The darker units in the image are interpreted to be unconsolidated and/or fine-grained material, while the lighter areas are rock. Morphology of the area was also studied using MOLA and MOC data to examine the extent of the ash deposits. MOLA shaded relief maps were used to determine changes in elevation while albedo and morphology differences were studied in MOC images. All images were taken from the THEMIS website [7]. Geologic units were defined using all three data sets. The surface features such as roughness, craters, channels, and flow lobes were described using the MOLA and MOC data while the surface textures of the different units (unconsolidated and/or fine-grained material or rocky) were determined using the THEMIS nighttime infrared images. Results: Close examination of THEMIS visible images of the Apollinaris Patera vent reveals several different eruption events (Figure 1). The deposits from these events can be traced using a combination of THEMIS, MOC, and MOLA data.