When lava meets ice: Explosive eruptions in the late Amazonian in Tharsis, Mars

Insight into the past geological evolution of Mars is limited by our ability to view the Martian subsurface. Therefore, our understanding of geological evolution relies primarily on remotely sensed observations, which mainly constrain the latest stages of the geological processes responsible for shaping the observed landforms. However, in specific cases, certain surficial landforms can reveal aspects of the geological history of particular regions. On Earth, when lava encounters (near)surficial ice deposits or water, it triggers explosive phreatomagmatic activity, forming rootless cones that serve as evidence of lava-water interaction. Such landforms indicate that waterlogged or ice deposits were present at the time of the volcanic activity. Although volcanism has played a dominant role in shaping the Tharsis surface, and despite the presence of cold-based tropical glaciers on the flanks of its major volcanoes, there is little evidence of lava-water interactions. To address this, through detailed analysis of Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) surface imagery, coupled with stereo-pair–derived topographic data, we report the presence of rootless volcanic cones located south and southeast of Ascraeus Mons. Directly atop the individual lava flows dated to younger than 215 Ma, we identified >2,000 conical edifices that form a morphologically homogenous population with an average basal width of 96 ± 31 m (1 standard deviation; SD; n = 249) and a crater width of 43 ± 18 m (1 SD; n = 207). Digital elevation models (DEMs) indicate that these edifices have an average height of 3.8 ± 2.0 m (1 SD; n = 178). Their morphological parameters and structural relationship with the hosting lava flows closely resemble both terrestrial and Martian rootless constructs. Furthermore, their exclusive superposition on individual lava flows indicates that their formation was strictly controlled by, and limited to, lava flow emplacement. This, in turn, enables a more accurate spatiotemporal reconstruction of ice distribution at the time of volcanic activity, providing insight not only into the geological evolution of this particular region but also into the obliquity state of Mars during that period. Moreover, the presence of spectrally-identified monohydrated sulfates suggests past hydrothermal circulation driven by lava-water interactions. Consequently, we propose that these young, small landforms, interpreted as rootless cones, provide valuable constraints for reconstructing the Martian paleoclimate by delineating former ice-rich zones. They should also be considered high-priority targets in future life-detection missions, as they satisfy key habitability criteria.

This project was conducted within the framework of the MARIVEL project, funded by the National Science Centre of Poland (grant no. 2024/53/B/ST10/00488).