Siljan Impact Structure
Impact Generated Hydrothermal Deposits…Suitable Places for Life? Large impact craters on Mars are of astrobiological interest because of their associated paleolake deposits and their potential to harbor local hydrothermal deposits that could have been generated by the impact event. Earth-based impact-generated hydrothermal deposits are therefore suitable analogues for those that are likely to have formed on Mars.  Impact generatedhydrothermal deposits also provide an opportunity to study how fossil organisms or biosignatures of life might have been preserved in such rocks. To test the hypothesis that large impacts generated suitable habitats for thermophilic communities, Dr. Tomas Hode and colleagues have studied the ~370 million year old Siljan Impact Structure, which is located in south central Sweden.  The diameter of the structure is about 80 km.  It is a complex crater that displays several structural units that include a central uplift region surrounded by a ring-shaped depression.  The mineral assemblages in veins that cross-cut the structure precipitated from a low-temperature hydrothermal system that infiltrated the region along the border between the central uplift and the surrounding ring depression of the crater.  Fluid inclusion studies of the minerals indicate homogenization temperatures that range from ~75°C up to 137°C.  Given the estimated erosional unloading of about 1 km, the formation temperatures probably did not exceed 150°C in the outer part of the crater. These findings indicate that large impacts can generate hydrothermal systems that could have supported thermophilic and hyperthermophilic communities in the subsurface region beneath such crater structures. Our research of the Siljan Impact Structure now focuses on determining whether the traces of microbial life we discovered in mineral-filled veins represent an autochthonous ancient thermophilic community that lived within the impact-induced hydrothermal system.
Image from NASA WorldWind (public domain)