1 2 It has long been suggested that hydrothermal systems might have provided habitats for the origin 3 and evolution of early life on Earth, and possibly other planets such as Mars. In this contribution 4 we show that most impact events that result in the formation of complex impact craters (i.e., >2-5 4 and >5-10 km diameter on Earth and Mars, respectively) are potentially capable of generating 6 a hydrothermal system. Consideration of the impact cratering record on Earth suggests that the 7 presence of an impact crater lake is critical for determining the longevity and size of the 8 hydrothermal system. We show that there are six main locations within and around impact 9 craters on Earth where impact-generated hydrothermal deposits can form: 1) crater-fill impact 10 melt rocks and melt-bearing breccias; 2) interior of central uplifts; 3) outer margin of central 11 uplifts; 4) impact ejecta deposits; 5) crater rim region; and 6) post-impact crater lake sediments. 12We suggest that these six locations are applicable to Mars as well. Evidence for impact-13 generated hydrothermal alteration ranges from discrete vugs and veins to pervasive alteration 14 depending on the setting and nature of the system. A variety of hydrothermal minerals have been 15 documented in terrestrial impact structures and these can be grouped into three broad categories: 16(1) hydrothermally-altered target-rock assemblages; (2) primary hydrothermal minerals 17 precipitated from solutions; and (3) secondary assemblages formed by the alteration of primary 18 hydrothermal minerals. Target lithology and the origin of the hydrothermal fluids strongly 19 influences the hydrothermal mineral assemblages formed in these post-impact hydrothermal 20systems. There is a growing body of evidence for impact-generated hydrothermal activity on 21 Mars; although further detailed studies using high-resolution imagery and multispectral 22 information are required. Such studies have only been done in detail for a handful of Martian 23 4 craters. The best example so far is from Toro Crater (Marzo et al., 2010). We also present new 1 evidence for impact-generated hydrothermal deposits within an unnamed ~32-km diameter crater 2 ~ 350 km away from Toro and within the larger Holden Crater. Synthesizing observations of 3 impact craters on Earth and Mars, we suggest that if there was life on Mars early in its history, 4 then hydrothermal deposits associated with impact craters may provide the best, and most 5 numerous, opportunities for finding preserved evidence for life on Mars. Moreover, 6hydrothermally altered and precipitated rocks can provide nutrients and habitats for life long 7 after hydrothermal activity has ceased. 8 5 1

Large impacts provide a mechanism for resurfacin g planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peak s transition to peak ri ngs. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Ex pedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust

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The nature of the groundmass of surficial suevite from the Ries impact structure, Germany, and constraints on its origin Abstract-Surficial suevites from the Ries impact structure have been investigated in the field and using optical and analytical scanning electron microscopy. The groundmass of these suevites comprises calcite, clay minerals, impact melt glass, crystallites (plagioclase, garnet, and pyroxene), francolite, and Ba-phillipsite. The latter zeolite is a secondary phase. Abundant textures have been observed: intricate flow textures between the various groundmass phases, globules of each phase in the other phases, spheroids of pyrrhotite in calcite, the "budding-off" of clay globules into silicate glass and/or calcite, euhedral overgrowths of francolite on apatite clasts, and quench-textured crystallites in the groundmass. Groundmass-forming calcite displays higher FeO, MnO, and SiO 2 contents than limestone target material. The composition of suevite "clay minerals" is highly variable and not always consistent with montmorillonite. Three types of glasses are distinguished in the groundmass. Type 1 glasses are SiO 2 -rich and are clearly derived from sandstones in the sedimentary cover, while the protoliths of the other two glass types remains unclear. Analytical data and micro-textures indicate that the calcite, silicate glass, francolite, and clay minerals of the groundmass of the Ries suevites represent a series of impact-generated melts that were molten at the time of, and after, deposition. On cooling, plagioclase, pyroxene, and garnet crystallized from the groundmass. These results are at variance with the current, traditional descriptive definition of suevite. Given that Ries is the original type occurrence of "suevite," some modification to the traditional definition may be in order. As the results of this study are most consistent with the groundmass of Ries surficial suevites representing a mix of several types of impact-generated melts, we suggest that a possible origin for these suevites is as some form of impact melt flow(s) that emanated from different regions of the evolving crater.

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Abstract-The Haughton impact structure has been the focus of systematic, multi-disciplinary field and laboratory research activities over the past several years. Regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to ∼1880 m. New 40 Ar-39 Ar dates place the impact event at ∼39 Ma, in the late Eocene. Haughton has an apparent crater diameter of ∼23 km, with an estimated rim (final crater) diameter of ∼16 km. The structure lacks a central topographic peak or peak ring, which is unusual for craters of this size. Geological mapping and sampling reveals that a series of different impactites are present at Haughton. The volumetrically dominant crater-fill impact melt breccias contain a calciteanhydrite-silicate glass groundmass, all of which have been shown to represent impact-generated melt phases. These impactites are, therefore, stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets. The crater-fill impactites provided a heat source that drove a post-impact hydrothermal system. During this time, Haughton would have represented a transient, warm, wet microbial oasis. A subsequent episode of erosion, during which time substantial amounts of impactites were removed, was followed by the deposition of intra-crater lacustrine sediments of the Haughton Formation during the Miocene. Present-day intracrater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the High Arctic. Modern modification of the landscape is dominated by seasonal regional glacial and niveal melting, and local periglacial processes. The impact processing of target materials improved the opportunities for colonization and has provided several present-day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain.

Highly expanded Cretaceous–Paleogene (K-Pg) boundary section from the Chicxulub peak ring, recovered by International Ocean Discovery Program (IODP)–International Continental Scientific Drilling Program (ICDP) Expedition 364, provides an unprecedented window into the immediate aftermath of the impact. Site M0077 includes ∼130 m of impact melt rock and suevite deposited the first day of the Cenozoic covered by <1 m of micrite-rich carbonate deposited over subsequent weeks to years. We present an interpreted series of events based on analyses of these drill cores. Within minutes of the impact, centrally uplifted basement rock collapsed outward to form a peak ring capped in melt rock. Within tens of minutes, the peak ring was covered in ∼40 m of brecciated impact melt rock and coarse-grained suevite, including clasts possibly generated by melt–water interactions during ocean resurge. Within an hour, resurge crested the peak ring, depositing a 10-m-thick layer of suevite with increased particle roundness and sorting. Within hours, the full resurge deposit formed through settling and seiches, resulting in an 80-m-thick fining-upward, sorted suevite in the flooded crater. Within a day, the reflected rim-wave tsunami reached the crater, depositing a cross-bedded sand-to-fine gravel layer enriched in polycyclic aromatic hydrocarbons overlain by charcoal fragments. Generation of a deep crater open to the ocean allowed rapid flooding and sediment accumulation rates among the highest known in the geologic record. The high-resolution section provides insight into the impact environmental effects, including charcoal as evidence for impact-induced wildfires and a paucity of sulfur-rich evaporites from the target supporting rapid global cooling and darkness as extinction mechanisms.