Starting conditions for hydrothermal systems underneath Martian craters: Hydrocode modeling

Abstract

Mars is the fi rst place to look for any sign of present or past extraterrestrial life. Its surface shows many features indicative of the presence of surface and subsurface water, while impact cratering and volcanism have provided temporary and local surface heat sources throughout Mars' geologic history. In particular, impact-generated hydrothermal systems could have been some of the most favorable sites for the origin of life on Mars.We present preliminary results of hydrocode simulations of impacts on Mars aimed at constraining the initial conditions for modeling the onset and evolution of a hydrothermal system on the red planet. Simulations of the early stages of impact cratering can identify the role of target composition and inhomogeneities in characterizing shock melting due to asteroidal and cometary impacts on the Martian surface. We fi nd that cometary impacts produce signifi cantly larger melt volumes than asteroidal impacts, thus affecting the thickness of the resulting melt sheet and its cooling history. The description of mixed targets is the main challenge of this study: unfortunately at this time, the lack of specifi c experimental data does not allow us to identify what is the best modeling approach (mixed cells versus mixed material equation of state) for mixed material targets. Modeling crater collapse is a necessary step to determine the fi nal thermal state of the target underneath. We fi nd that the combination of shock and plastic heating and the structural uplift of initially deeper strata create a water-bearing zone at depths where water is in the liquid stability fi eld. In the central uplift, the high temperatures cause water to evaporate (steam-driven circulation). The simulations show that for a mid-sized crater (rim diameter around 30 km) the hydrothermal circulation is probably restricted to a "column" contained well within the fi nal crater.

Impact craters in the northern hemisphere of Mars: Layered ejecta and central pit characteristics

“…There is currently no obvious correlation of the three central pit types with diameter, contrary to previous studies (Barlow and Bradley 1990). Hydrocode models of impacts into soil-ice mixed targets shows that a vapor zone is produced near the center of most transient craters (Pierazzo et al 2005). Sudden release of this vapor could produce the central pit morphology, but why some pits are on the crater floor while others are raised is not currently understood.…”

“…The lack of central pit craters on volatile-poor bodies such as the Moon and Mercury argues against cometary impacts alone being the primary cause of central pit formation. Recent numerical modeling of impacts into ice-soil mixed targets shows that a region under the center of the transient crater cavity reaches high enough temperatures for ice to vaporize (Pierazzo et al 2005). Both asteroidal and cometary impacts produce this high-temperature region, although the temperatures are higher for cometary impacts due largely to their higher impact velocities.…”

Effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on Mars

“…Thus, it is suggested that impact-melt rocks and glasses are likely to be much more common on Mars than previously thought. This is supported by the results of numerical simulations and experiments, which suggest that substantial amounts of melt should be produced from impacts into ground-ice-rich targets on Mars (Pierazzo et al 2005;Stewart and Ahrens 2005).…”

Modeling the formation of the K–Pg boundary layer

“…Part of the rationale for using a wet porosity is to 298 investigate whether it leads to a significant increase in ejection velocity, as suggested by Kieffer 299 and Simonds, 1980 (Artemieva, 2007). Specific strength properties of porous rocks were not 300 taken into account, however special EOS, assuming a total thermodynamic equilibrium between 301 water and calcite, has been constructed similar to a basalt-water mixture (Pierazzo et al, 2005). 302…”