James R. Zimbelman

Polygenic eruptions on Alba Patera, Mars

A combination of photogeologic mapping, analysis of Viking Orbiter thermal inertia data, and numerical modelling of eruption conditions has permitted us to construct a new model for the evolution of the martian volcano Alba Patera. Numerous digitate channel networks on the flanks of the volcano are interpreted to be carved by sapping due to the release of non-juvenile water from unconsolidated flank deposits. Using the thermal inertia measurements, we estimate the particle size of these deposits to be 3–10 µm, which, together with theoretical modelling of the disperison of explosively derived volcanic materials, leads us to conclude that the flank deposits on Alba Patera are low-relief pyroclastic flows. The recognition of numerous late-stage summit and sub-terminal lava flows thus makes Alba Patera a unique martian volcano that is transitional between the older pyroclastic-dominated highland paterae and the more recent effusive central-vent volcanoes such as the Tharsis Montes.

Rheology of the 1983 Royal Gardens basalt flows, Kilauea Volcano, Hawaii

Ten carefully surveyed topographic profiles across a 1983 Royal Gardens basalt flow from the East Rift of the Kilauea Volcano were matched to digitally derived preflow profiles to construct accurate flow cross sections. Geometric parameters measured on these sections were then used to compute yield strengths and viscosities by means of several rheologic models. Calculated yield strengths (1.5–50 × 103 Pa) and viscosities (0.2–8.2 × 106 Pas) are comparable to earlier field estimates and slightly higher than laboratory determined values for aa basalt. Both yield strength and viscosity increased systematically downstream. The maximum observed temperature drop of 30 °C is insufficient to account for the 30-fold increase in yield strength, but could explain the three-fold order-of-magnitude increase in viscosity. The yield-strength increase downstream is more likely due to increasing crystallization and brecciation with time. For any cross section, calculations of rheologic parameters based on flow-margin depths generally gave lower values than those based on the dimensions of levees. This relationship may be attributed to the earlier formation and less complex evolution of the margins. The various equations gave more consistent results for upstream profiles, suggesting that calculations for remotely observed flows should avoid measurements near flow termini.

Spatial resolution and the geologic interpretation of martian morphology: Implications for subsurface volatiles

Abstract Viking orbiter images of Mars provide global coverage of the major geologic and tectonic provinces on the planet. However, only a fraction of the orbiter images are useful for assessing the processes responsible for the generation or modification of surface landforms. An example from Acheron Fossae illustrates that aeolian dunes visible at 9 m/pixel resolution are indistinguishable at 57 m/pixel resolution. Dunes in Acheron Fossae valleys lend support to an aeolian interpretation for lineated valley fill at this location; this result provides an alternative hypothesis for the proposed ice-rich interpretation of this terrain. Consequently, it seems unlikely that images with a spatial resolution >50 m/pixel can provide strong constraints on the relative importance of various geologic processes that have acted upon the surface. A survey of the best Viking Orbiter images with

Surface properties of the Pettit wind streak on Mars: Implications for sediment transport

Abstract Pettit crater (12°N, 174°W) is associated with a complex wind streak consisting of both Type I (bright) streak and Type II (dark) streak materials. Broadband albedo measurements of the streak indicate a minimum of .20 and a maximum of .32, with .27 on the surrounding plains. The darkest materials have the highest thermal inertias, which can be related to an effective grain diameter typical of medium sand. Most of the dark streak materials have thermal inertias representative of fine sand, the material most easily set in motion by the wind on Mars. The bright portions of the streak have thermal inertias higher than that of the surrounding plains; the bright streak values are representative of very fine sand. Differences between temperatures measured at 11 and 20 μm indicate there is no difference in the areal abundance of unresolved high thermal inertia materials (e.g., blocks, bedrock) between the Pettit streak and the surrounding plains. The dark streak material is consistent with models involving sand saltation and transport but the bright streak thermal inertia is not consistent with a volumetrically significant enhancement of unconsolidated dust in the streak.

The Geology of Mars: Volcanic features of New Mexico analogous to volcanic features on Mars

The arid climate, extensional rift setting, range in type and age of volcanic eruptions, and generally widespread and geologically youthful volcanism in New Mexico contribute to an environment rich in geologic processes and landforms analogous to many of those on Mars. Young (<5 Ma) volcanoes and associated volcanic rocks are more widely distributed throughout the state than in many other volcanic localities on the North American continent. All of the principal volcanic landforms occur including long lava flows, viscous domes, calderas, composite volcanoes, monogenetic scoria cones, small shield volcanoes, and numerous hydromagmatic vents. The morphologies, volcanic emplacement processes, and dissected structures, and the arid environment, result in many volcanic landforms analogous to those on Mars. These features provide some clues to the details of geologic processes responsible for their Martian counterparts that are uncommon in areas where volcanism is less abundant and where the environments are less arid. The largest young caldera (Valles Caldera), largest young lava flows (McCartys and Carrizozo), abundance of Quaternary volcanic fields, volatile-rich magmatism, including non-juvenile (maars) and juvenile types (Shiprock-Narbona Pass), spring deposits, and one of the great modern rift valleys on Earth (Rio Grande rift) occur in an arid setting where annual precipitation is between 8 and 15 inches (20-40 cm) per year. Combined with arid dissection and eolian in-fill, these contribute to a landscape that mimics the appearance of many volcanic terrains on Mars. In addition to surficial geologic processes, the interaction between subsurface ground water