The Relationship between Lava Fans and Tubes on Olympus Mons in the Tharis

Introduction: The acquisition of image data from the Mars Odyssey THermal EMission Imaging System (THEMIS) and the European Space Agency’s Mars Express High Resolution Stereo Camera (HRSC) have enabled new observations, and in some cases interpretations, of small volcanic features in the Tharsis region of Mars. While image resolution limitations have traditionally hampered detailed inspection of small-scale (<10 km in diameter) volcanic features on Olympus Mons, these data together provide complete coverage of the volcano at resolutions between 12 and 18 m/pixel. When co-registered with pre-existing, topography data from the Mars Orbiter Laser Altimeter (MOLA), small-scale features such as fans, tubes, ridges, and channels can be mapped, and their characteristics measured. Here we report on a study focused on lava fans on the flank of Olympus Mons. Background and Approach: Olympus Mons rises 23 km above the surrounding terrain and has a diameter of ~ 600 km [1]. Our mapping encompassed ~ 330,000 km, covering both the volcano’s main flank and areas beyond the basal scarp where Olympus Mons derived lavas flowed onto the Tharsis plains. Lava fans are delta-like features displaying lava flow morphologies that radiate down slope from an apex and are thought to represent local eruption points on the flank of Olympus Mons. Multiple hypotheses are proposed to explain the formation of lava fans such as: 1) outlets for lava tube flows [2], 2) small-scale vents occurring along dikes or rift zones [3], or 3) outpourings of lava along thrust faults [4]. Using ArcGIS (by ESRI) to examine HRSC, THEMIS, and MOLA data, we assigned a data point to the apex of all identifiable fans as originally described by Carr et al. [2] in order to provide insight into the mechanism of their formation. To test the relationship between fans and tubes, the lava tubes and ridges were traced and their relationships to fans were noted. The objective of this work is to identify fans that are associated with tubes to better enable a focused study of features that might be indicative of rift zone activity. We also conducted preliminary Nearest Neighbor analyses to the location data for Olympus Mons lava fans. This approach has been shown to provide insight into the formation of volcanic features on the Earth, Moon, and Mars [5,6,7] The statistical techniques are applied to the total population of fans, the individual fans and fan complexes, and the populations of fans that are and are not related to lava tubes. Results could help identify rift zone-related features if present. Results: Every identifiable fan is considered a localized point of eruption in this study, rootless or not, and was assigned a data point. Fans exhibit a delta-like shape formed by roughly linear boundaries trending from a single apex. The fan is often composed of outpourings of lava that can be traced to a single apex that is the topographic high-point. Small collapse pits and boulder-like features are commonly located at the apex. Fans are typically embayed by younger lava flows which probably obscure their full extent. The total population of fans is 171. Some fans exhibited adjacent and nested relationships with other fans, together forming a much larger fan complex.