Amenthes Cones, Mars: Hydrovolcanic (tuff) Rings and Cones from Phreatomagmatic Explosive Eruptions on Mars

Introduction: Mars was volcanically active throughout most, if not all, of its history [e.g., 1-4], and volcanism played a significant role in the formation of its surface. Another factor modifying the Martian surface is water, both in liquid and frozen state, and at and beneath the surface [e.g., 5]. Interactions of magma with water and/or ice should be common on Mars, therefore. On Earth, such interactions are known to trigger hydrovolcanism [6], the natural phenomenon of magma or magmatic heat interacting with an external water source [6] producing tuff rings, tuff cones and maars as typical landforms. The existence of explosive volcanism on Mars was predicted on theoretical grounds [7], but only few direct observations are available [8-13]. Moreover, no detailed studies of the reported hydrovolcanic landforms are available (with ref. [14] as aan exception). Here we present observations of a large field of pitted cones along the dichotomy boundary in the Amenthes region (Figure 1), previously described by [15] as result of mud diapirism forming mud volcanoes. The aim of our study is to test the hypothesis of an igneous (hydro)volcanic origin of these cones. Data: We used images from several cameras, e.g., HRSC, CTX, HiRISE, for morphological analyses. Topographic information (e.g., heights and slope angles) was determined from single shots of the Mars Orbiter Laser Altimeter (MOLA) in a GIS environment, and from stereo images (HRSC) and derived gridded digital elevation models (DEM). Terrestrial data obtained from Google Earth software. Geologic setting: The study area (10°N to 20°N and 95°E to 125°E) lies close to the dichotomy boundary, between cratered highlands in the south and smoother appearing northern plains and close to the two giant impact basins, Isidis and Utopia. Recent results indicate that volcanism was common and longlived in the region, not only at the Elysium bulge [16] and the Amenthes Fossae region [17], but also in the plains of Isidis Planita [18], locations in Utopia [12], and the Nephentes region to the southeast of our study area [19]. Previous studies of the cones are sparse. To our knowledge, the only in-depth study is that of Skinner and Tanaka [15]. Based on the morphologic interpretation of an assemblage of landforms (fractured rises, isolated and coalesced depressions, mounds and the pitted cones studied here), the comparison to terrestrial analogues, and the sedimentary and tectonic setting, Skinner and Tanaka conclude that an origin as mud volcanoes best explains their formation [15]. An igneous volcanic origin is rejected by [15] because of (1) the large distance to known volcanic vents, (2) a lack of obvious structural control of dike-related eruptions, (3) the confinement to a specific latitude and elevation range, (4) the setting in a compressional tectonic regime, and (5) the pitted cones being part of a broader assemblage of landforms.