Post contributed by Dr. Sandro Rossato, Department of Geosciences, University of Padova, Padova, Italy
Terrestrial maar-diatremes are small volcanoes (see this previous post for a general description) which have craters whose floor lies below the pre-eruptive surface and are surrounded by a tuff ejecta ring 2-5 km wide (Figure 1) that depends on the size of the maar itself and on the depth of the explosion (Lorenz, 2003). Maar-diatremes constitute highly valuable sites for in situ investigations on planetary bodies, because they expose rocks at the surface from a great range of crustal depths and are sites which could preferentially preserve biomarkers.
The explosive eruptions that produce Maar-diatremes can penetrate down to at least 2.2 km (Valentine, 2012), meaning that material deriving from different depths in the crust can be analyzed and sampled quite easily by a rover at the surface, having been exposed and fragmented. Secondly, but more importantly in the search for signs of life, many maar-diatreme structures on Earth host, or have hosted long-lived lakes (White & Ross, 2011), due to their interaction with watery environments and to the elevation of the crater floor, often below the groundwater table. Post-eruptive lacustrine sediments are commonly found on Earth inside the crater, in some cases hosting invaluable fossil records (e.g. the Messel pit UNESCO fossil site, Germany; Figure 2).
On Mars examples of hydrovolcanism have been reported (e.g. Keszthelyi et al., 2010) but only in rare cases they have been ascribed to a maar-diatreme structure (e.g. in the Nephentes/Amenthes region, on the southern margin of Utopia Planitia; Figure 3). Maar-diatreme structures usually derive from multiple blasts that may alter the morphology of the landform, making its identification more demanding.
On the contrary, those recently found in the Simud Vallis area (southern margin of Chryse Planitia; Figure 4) are morphologically very similar to terrestrial analogs, presenting rounded to sub-rounded shapes and intact tuff rings with low inclination. By means of comparisons with laboratory experiments (Figure 5), the Simud Vallis maar-diatremes are expected to have originated from one, or at maximum two, blasts which occurred at the optimal depth for maximizing the size of the craters and the distinctiveness of the morphologies.
Brož, P., Hauber, E. (2013). Hydrovolcanic tuff rings and cones as indicators for phreatomagmatic explosive eruptions on Mars. Journal of Geophysical Research E: Planets, 118 (8), pp. 1656-1675.
Graettinger, A.H., Valentine, G.A., Sonder, I., Ross, P.-S., White, J.D.L., Taddeucci, J. (2014). Maar-diatreme geometry and deposits: Subsurface blast experiments with variable explosion depth. Geochemistry, Geophysics, Geosystems, 15 (3), pp. 740-764.
Lorenz, V. (2003). Maar-Diatreme Volcanoes, their Formation, and their Setting in Hard-rock or Soft-rock Environments. Geolines, 15, pp. 72-83.
Keszthelyi, L.P., Jaeger, W.L., Dundas, C.M., Martínez-Alonso, S., McEwen, A.S., Milazzo, M.P. (2010). Hydrovolcanic features on Mars: Preliminary observations from the first Mars year of HiRISE imaging. Icarus 205, pp. 211-229.
Pajola, M., Rossato, S., Baratti, E., Mangili, C., Mancarella, F., McBride, K., Coradini, M. (2016). The Simud-Tiu Valles hydrologic system: A multidisciplinary study of a possible site for future Mars on-site exploration. Icarus, 268, pp. 355-381.
Schulz, R., Buness, H., Gabriel, G., Pucher, R., Rolf, C., Wiederhold, H., Wonik, T. (2005). Detailed investigation of preserved maar structures by combined geophysical surveys. Bulletin of Volcanology, 68 (2), pp. 95-106.
Valentine, G.A. (2012). Shallow plumbing systems for small-volume basaltic volcanoes, 2: Evidence from crustal xenoliths at scoria cones and maars. Journal of Volcanology and Geothermal Research, 223-224, pp. 47-63.
White, J.D.L., Ross, P.S. (2011). Maar-diatreme volcanoes: A review. Journal of Volcanology and Geothermal Research, 201 (1-4), pp. 1-29.