From hot to cold? – Hydrothermal activities as a source for icy-debris flows on Dryas Mons, Terra Sirenum, Mars

Abstract

During an extensive grid-mapping campaign on Mars' southern hemisphere, we have detected so-called sheet flow deposits, which are defined by distinctive lobate fronts, thin and mostly planar layers, and faint sub-parallel surface lineations. They originate from the high plains of Dryas Mons, a large massif of impact and tectonic origin, and follow the topographic gradient down into the adjacent basins. Their sources often coincide with steep and multi-layered outcrops. This work addresses the formation of these deposits, and if they are related to the unique tectonic and endogenic environment of Dryas Mons. We applied photogeological mapping, age determinations by crater counts, as well as topographic and heat flow measurements in order to reconstruct the evolution of these landforms. The calculated ages (mid to late Amazonian), their location in the mid-latitudes, as well as some specific morphologies like lobate flow fronts are typical for standard viscous-flow features on Mars. In contrast to such viscous-flow features, the sheet flow deposits occur isolated in the Dryas Mons region. Aside from that, major other landforms, typical for viscous-flow features are lacking, like sublimation pits, brain terrain or arcuate deformations. Considering these similarities and differences, we suggest that both landforms viscous-flow features and the sheet flow deposits of Dryas Mons were formed by the involvement of volatiles; however, at varying amounts and by different emplacement processes. As the formation of the sheet flow deposits is located in an area with one of the highest heat flow values planet-wide, we calculated if the heat flow could be a potential trigger for the release of outcropping volatile-rich layers. However, our calculations have shown that the heat flow is still insufficient to enable near-surface melting. Instead, we suggest that the volatiles originate from deep layers affected by magmatic and/or intrusive activities in a tectonically active environment. These conditions led to melting and mobilisation of the volatiles by advective hydrothermal processes. Thus, the volatiles migrated upward along hypothesized deep-seated fault systems, formed by Noachian/Hesperian impacts and tectonic activities, until they reached conductive layers outcropping at Dryas Mons, enabling them to drain. After their release to the cold Amazonian atmosphere, this mixture of volatiles and solids partially froze, resulting in a comparatively high viscosity, and hence, a slow and laminar (non-turbulent) movement. This flow behaviour might have led to the formation of the lineations at their surface along shear zones. Hence, we suggest that the sheet flow deposits may have originally formed as slowly moving icy debris flows during the mid- to late Amazonian.