Knobby terrain on ancient volcanoes as an indication of dominant early explosive volcanism on Mars

Determining if the mechanically weak materials of the upper crust are products of early explosive volcanism or generated by modification of extensive effusive lava flows is important for understanding the geologic and thermal history of Mars. We examined 75 recently identified preserved ancient volcanoes, whose eruption styles are representative of early volcanism. We describe a unique knobby terrain that is associated with 17 of these volcanoes. This newly identified terrain is characterized by a combination of high-resolution images and thermophysical and thermal infrared hyperspectral data. The morphology characteristics of these knobby terrains are similar to terrestrial eroded ignimbrites, and the thermophysical properties indicate that they are composed of unconsolidated fine-grained materials. The spectral analysis indicates that they experienced some aqueous alteration. We explain the knobby terrain associated with these ancient volcanoes as the products of an early explosive volcanic phase in Martian history, followed by subsequent modification of these deposits.