High-Resolution Imaging of Small Volcanic Vents on Mars by MRO HiRISE

Volcanic centers on Mars are now known to include numerous small pyroclastic and effusive constructs in addition to the principal large volcanoes recognized during Mariner and Viking missions. We have targeted many of these with the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE) camera now in orbit at Mars. Together with the more regional perspectives of the MRO Context Camera (CTX) and Odyssey Thermal Emission Imaging System (THEMIS) camera data, several classes of small volcanic features are recognized: (1) small steep-sided cones associated with summits of larger edifices and rootless cones associated with explosive substrate devolatilization in other areas of youthful flood lavas and channels; (2) small rootless shield volcanoes fed by lava tubes and terminal breakouts on the lower flanks of large volcanoes; and (3) isolated cones, cratered cones, and cratered domes in lowland plains.

Steepening of slopes occurs in the summit of some plains small shield volcanoes. Absence of angular blocks, thin layering, and irregular collapse of outcrops around the summit crater imply relatively variable (non-massive) lithology, either small narrow flows or small-scale variations in pyroclastic accumulation. Summit edifice development is evidence for secular changes in eruption rates, corresponding lava flow dimensions, and/or relative volatile/magma ratios. Similar late stage trends are observed in some terrestrial fissure eruptions in which diminishing eruption rates are out-paced by de-volatilization of the magmas and late stages of eruption become more fragmental.

Elsewhere in older plains, cratered cones in linear arrays and in more random concentrations remain difficult to assign likely origins. Some are consistent with volcanic origins. Others are probable residual landscape features, while cratered domes of the northern lowlands have been interpreted as possible mud volcanoes.