THE FIRST YEAR OF STUDYING MARS VOLCANISM WITH HIRISE

The High Resolution Imaging Science Experiment (HiRISE) is providing an unprecedented view of volcanic materials on Mars. In the well-exposed and well-preserved terrains, such as the young flood lavas in Elysium Planitia, the morphologic and topographic information provided at ~30 cm/pixel in HiRISE stereo image pairs is yielding fundamentally new insight into volcanic processes. However, in areas where the volcanic surfaces are either mantled or degraded, we find that the 6 m/pixel MRO Context (CTX) images are optimal for volcanological studies. The mantling deposit is apparently tens of meters thick on the major shield volcanoes. Also, while the HiRISE color data has proven invaluable in many studies, most Martian volcanic terrains are sufficiently dust-covered to limit the utility of these data. One counter example is that, in cross-section, the thick mantling deposit on the flanks of Olympus Mons is found to be different in color than dust-covered surfaces. This suggests that this ~60 m thick deposit may have a different composition (and genesis) than the dust that settles from the atmosphere. One possible explanation is that this is a young airfall deposit of volcanic ash. Kilometer-scale vent structures show a great deal of diversity, including examples that appear to have been primarily effusive, primarily explosive, or exhibit a combination of eruptive styles. Lava flows also show a diversity that may exceed the range of well-preserved eruptive styles seen in mafic eruptions on the Earth. In addition to many large aa-like lava flows, we have imaged platy-ridged (i.e., rubbly pahoehoe) flood lavas, and inflated pahoehoe sheet flows similar to terrestrial examples (except perhaps in erupted volume). However, there are also indications of very large turbulent floods of lava that may not have any well-preserved terrestrial counterparts. The HiRISE observations may provide insight into very large flood lava flows on Venus, the ancient Earth, the Moon, and Io. Perhaps most important for the current Mars exploration strategy is the clear evidence for recent hydrovolcanic activity in the equatorial lowlands.