Identifying Evidence for Explosive Volcanism on Mars Through Geomorphologic and Thermophysical Observations

Accurately identifying the products of explosive volcanism on Mars is critical for unraveling the evolution of the martian crust and interior. Recent work using high-resolution datasets suggest explosive volcanic processes may have dominated over effusive activity in early martian history. However, distinguishing the products of explosive volcanism from non-volcanic sediments remains challenging since both are similar in thermophysical and geomorphologic datasets. The objective of this study is to identify geomorphologic and thermophysical characteristics of possible explosive volcanic deposits on Apollinaris Mons, one of the best-known candidates for explosive volcanism on Mars, using visible and thermal infrared imaging datasets. These geomorphic and thermophysical characteristics are compared to terrestrial volcanic analogs to more accurately identify evidence for explosive volcanism on Mars. Deposits that best exemplify explosive volcanism on Apollinaris Mons are then applied to Arabia Terra, an area of Mars controversially proposed to contain explosive volcanic calderas. The primary indicators that Apollinaris Mons is volcanic are the summit caldera and surrounding edifice, which resemble caldera complexes on Earth. Thermal inertia values throughout the Apollinaris Mons region are low (~90 J·m-2·K-1·s-1/2), consistent with fine-particulate and weakly-indurated material at the surface. In addition, the flank is dissected by numerous drainages, further suggesting a composition of weakly-consolidated material. Outcrop-scale findings include planar bedding, cross-bedding, and breccia deposits within exposed crater walls along the debris fan and within the caldera. Breccia deposits are also present along a valley wall on the southwestern flank, where boulders grade from coarser to finer with distance from the caldera wall. Planar bedding could be consistent with fall deposits, or discrete lateral flow events. Cross-bedding is consistent with traction transport in dilute mass flows, such as dilute pyroclastic density currents. However, such cross-bedding could also be formed due to truncation of earlier deposits by later erosive flows. Breccias also form during mass flows, such as concentrated pyroclastic density currents, debris avalanches or debris flows. Gradation of blocks down slope is further evidence for lateral transport since heavier blocks are dropped out of the flow at more proximal regions due to decreasing carrying capacity with distance from source. The combined evidence of a caldera with a prominent edifice, the regional low thermal inertia, and presence of extensive friable deposits with evidence of lateral transport away from the caldera supports an explosive volcanic history for Apollinaris Mons. This study revealed no evidence for past effusive activity (high strength materials), although current data limitations prevent access to the entire volcanic history. Evidence for explosive volcanism on Apollinaris Mons is applied to three proposed calderas in Arabia Terra, named Eden, Ismenia, and Siloe Paterae, to test a previously published and controversial hypothesis that they are volcanic in origin. Like Apollinaris Mons, thermal inertia values of the proposed Arabia Terra calderas are consistently low. However, unlike Apollinaris Mons, the proposed calderas are at the same level as the surrounding topography, and lack a positive edifice. Concentric fracturing surrounding some of the proposed calderas, while consistent with ring-fractures at volcanic calderas, is not unique to volcanic collapse. As such, while there is evidence that the Arabia Terra depressions experienced collapse, the mechanism for collapse cannot be tied directly to volcanism. Bedding found within the proposed caldera walls and plateaus resembles those found on Apollinaris Mons in that it is friable in nature and of a similar scale. However, since the bedding was not found within an obvious edifice, it cannot be directly related to the proposed caldera or mechanism for collapse. No primary volcanic features, such as high strength materials (i.e., lava flows) are found within the proposed caldera floors. Rectilinear fracturing and chaotic terrain are identified in patches along the floor of the proposed calderas; however, neither of these morphologies are consistent with features found within Apollinaris Mons or terrestrial calderas. The proposed calderas more closely resemble the dewatering style collapse features present within the Circum-Chryse region of Mars. Dissected, polygonal-shaped plateau structures within the proposed caldera interiors are consistent with large, fractured fault blocks common to dewatering collapse features, but not terrestrial calderas. The morphologic similarities between Circum-Chryse and Arabia Terra would suggest the proposed calderas are instead impact craters that experienced subsequent collapse following the removal of subsurface water. This research demonstrates that geomorphologic and thermophysical observations of large-scale and fine-scale features can be combined to determine volcanic and non-volcanic origins of structures on Mars. However, without sufficient geologic context such as a volcanic edifice, distinguishing volcaniclastic materials from other sedimentary deposits on Mars remains a challenge. It is still possible that the bedded, friable deposits exposed within Arabia Terra have a volcanic origin; however, the source has yet to be identified. In conclusion, careful and detailed geologic mapping, when compared with terrestrial analogs, can lend insight into the geologic history of Mars