SPATIAL ANALYSES FOR SMALL VENTS SOUTH OF PAVONIS MONS

Small shield and fissure vents appear to be common throughout the Tharsis province on Mars, and are associated with each of the Tharsis Montes. The nature of these small vents and their deposits can provide insight into eruptive styles prevalent during what may have been the last stages of active volcanism on Mars. More importantly, information on the spatial distribution of small vents can be compared to other areas of volcanism to ascertain whether or not significant similarities or differences exist. Any differences may be due to changes in eruptive style as volcanic activity migrated from one volcanic center to the next (Arsia to Pavonis to Ascraeus), or possibly a reflection of the lava distribution systems (influenced by internal plumbing and crustal structure) of the three volcanic centers. In this study, 88 small vents have been systematically identified for a sample area (270 km x 480 km) south of Pavonis Mons. Approximately 45% of the sample area is covered by two adjacent High Resolution Stereo Camera image strips, and ~ 65 % of the remaining sample area is covered by a Thermal Emission Imaging System visible image mosaic. Analysis is based on the distance from each feature to its nearest neighbor (NN) to determine whether the spatial distribution is random and, if so, the character of randomness. The objective is to obtain insight into the underlying formation processes and the possibility of interactions among the neighboring features. The distribution of NN distances for small vents is very well fit by the classic Poison Nearest Neighbor (PNN) distribution, with a mean value of 14.1 km ± 2 km. Because the distribution is well-described by the PNN, we infer that the spacing of small vents at Pavonis is consistent with a randomly distributed Poisson process. Thus, small vents are equally likely to occur at any location within the sample area, and the existence of any vent does not influence the location of any other vent. Analyses of skewness and kurtosis of the NN distribution also fall within the acceptable ranges for a Poisson process, further supporting the basic inference that vents at Pavonis are spatially random in a Poisson sense. This inference is consistent with the hypothesis that development of each small eruptive vent is fed by independent shallow magma chambers and not driven by a single, centralized plumbing system.