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Terrestrial impact structures provide the only analogs for hands-on astronaut training or robotic exercises in preparation for fieldwork on other planetary surfaces. Impact structures not only represent the dominant surface features on, inter alia, the Moon, Mars, or asteroids but are also crucial for basic geoscientific surface analysis, subsurface geological studies, and analysis of sites of possible exobiological evidence or economic resources for future colonization of other planetary bodies. We assess 11 terrestrial impact structures of varied age, type, size, and erosion level, the majority of which have already served for astronaut or geoscientist/student training purposes, for their suitability as possible impact geological training sites. This evaluation is achieved through a range of (1) practical criteria (such as access time and site infrastructure) and (2) geological criteria (such as impact geology, target geology, aspects of impact cratering, outcrop conditions, and variety). For the practical criteria, Ries, Rochechouart, and Steinheim score the highest, with a small advantage for Ries. Sudbury and Meteor Crater score similarly, yet much lower than the leaders, with Vredefort in between. Talemzane and Araguainha are just below Meteor Crater. Clearwater West, Haughton, and Mistastin are by far the least suitable ones. Regarding geological criteria, the scores vary much less. The three Northern Canada structures and Steinheim are at the end of the record, yet only 23%–39% below Ries, which comes out as the leader and is closely followed by Araguainha (only 2% below Ries). Although the Northern Canada sites compare in size and type to the younger and less eroded Ries and the Araguainha (older and more eroded) structures, the diversity of impact features and lithologies and the outcrop situation are less favorable. Considering only the geological features and lithologies factors, Rochechouart gets the highest mark, followed by Araguainha, Sudbury, Vredefort, and Ries. In view of the targeted objective, the analog testing experiment places Ries and Rochechouart in the first and second positions, respectively. Steinheim and Vredefort score almost the same in the third and fourth positions, respectively. The three Northern Canada sites score the lowest. Based on their accessibility, relative proximity to each other, and remarkable complementarity in terms of crater type and size, and in terms of impact and target features and lithologies, the combination of the three leading structures (Ries–Rochechouart–Steinheim) may represent the most appropriate target for analog training purposes, from anywhere in the world.

A possible impact crater at Mahout in Oman was recently proposed by Nasir et al. (2023 [1]). They alleged that “crater” morphology, shatter cones, shock metamorphic evidence in quartz, plagioclase, and calcite, as well as initial geochemical information and geophysical data supported this proposal. Their conclusion was that “All current analyses suggest the impact origin of the Mahout structure” (p. 20 of the article). In this Comment, their evidence is critically discussed and largely refuted. It is demonstrated that the basis to consider the Mahout structure a possible impact structure is very limited and that further detailed, state-of-the-art analyses of the mesoscopic and microscopic de-formation features are required to obtain credible evidence. Beyond the discussion of the article by Nasir et al., this Comment emphasizes the general need for careful and comprehensive analysis of the geological structures that might, a priori, suggest a possible impact origin if, ultimately, a new impact structure can be successfully confirmed.

The Rochechouart impact structure in the northwestern part of the French Massif Central (FMC) has a great diversity of impactites, including monomict impact breccias, suevite, and impact melt rocks (IMRs). The structure is strongly eroded, which allows the study of impactites of the crater fill and the transition into the crater floor. The FMC has had a multistage geological evolution from the late Neoproterozoic to the Ordovician (600–450 Ma) until the later stages of the Variscan orogeny (~300 Ma). Previous geochronological work on Rochechouart has been focused mainly on the impactites and constraining the impact age, and scarce work has been done on the FMC‐related target rocks. Here, U‐Pb isotope analysis by LA‐MC‐ICP‐MS has been conducted on zircon from two IMRs from the Recoudert and Montoume localities, and from a monzodiorite, a paragneiss, and two amphibolite samples of the basement to the impact structure. Zircon from the target rocks yielded mainly Neoproterozoic to Carboniferous ages (~924 to ~301 Ma) that can mostly be correlated to different stages of the geological evolution of the FMC. The monzodiorite also yielded a Permian age of 272 ± 12 Ma. Zircon from the IMRs, and especially from the Montoume sample, gave a comparatively higher diversity of Neoproterozoic to Jurassic ages (~552 to ~195 Ma). Provenance analysis for the zircon age populations of the impactites compared to those of the basement rocks shows overall poor correlation between the two age groups. This suggests that other target lithologies were involved in the formation of these impact melts as well. Post‐Variscan and preimpact ages (281–226 Ma) obtained for both melt rocks probably reflect a previously unconstrained event in the evolution of the regional geological history. Ages similar to the currently most widely accepted impact age of ~204–206 Ma were obtained from both IMR samples. In addition, the Montoume melt rock yielded several post‐204 Ma ages, which might reflect a to date unconstrained, about 194 Ma postimpact thermal/hydrothermal event. [ABSTRACT FROM AUTHOR]

Extensive, new outcrops along the MT‐100 state road in the northern part of the central uplift of the 40‐km diameter, 252–259 Ma old Araguainha impact structure, Central Brazil, have become available for investigation. They offer new insight into the contact relationships between the different lithologies and the genesis of different types of impact‐related rocks, as well as the current level of erosion of the structure. Three types of impact melt rock (IMR) with different field relationships and compositions can now be distinguished: (1) Type‐I of granitic composition and occurring mainly as veins and dikes, besides a few larger pods, in the central alkali granite core of the central uplift; (2) Type‐II in the form of plastically deformed clasts of mainly highly silicious compositions in polymict impact breccia; and (3) Type‐III, derived from partially melted conglomerate or sandstone precursors, and that occurs at selected sites in (meta)sedimentary strata of the basement in the immediate environs of the alkali granite core. Both polymict lithic and melt‐bearing (suevitic) impact breccias are recognized in the 110‐m thick integrated section through impact breccia directly overlying the crater floor. This crater floor is composed of (meta)‐sedimentary basement strata with granite injections and, locally, sandstones of the Devonian sedimentary Furnas Formation of the Paraná Basin. Main breccia components are (meta)‐pelites and (meta)sandstones of the basement that is currently favored to be related to the regional Paraguay Belt and to the lower sequence of the Paraná Basin sedimentary strata. Locally, breccia contains clasts of IMR Type‐II, and only very rarely are granitic fragments observed. Clasts of IMR Type‐I have never been observed in the breccia deposits. These new observations preclude significant involvement of alkali granite in the formation of the polymict breccia or in the production of shock melts. They also reveal the major role of the (meta)sedimentary precursors in the production of IMR by shock melting and provide essential information for better understanding the cratering processes involved in the formation of an impact structure in a sedimentary target, of the size of the Araguainha impact structure. [ABSTRACT FROM AUTHOR]

A possible impact crater at Mahout in Oman was recently proposed by Nasir et al. (2023 [1]). They alleged that "crater" morphology, shatter cones, shock metamorphic evidence in quartz, plagioclase, and calcite, as well as initial geochemical information and geophysical data supported this proposal. Their conclusion was that "All current analyses suggest the impact origin of the Mahout structure" (p. 20 of the article). In this Comment, their evidence is critically discussed and largely refuted. It is demonstrated that the basis to consider the Mahout structure a possible impact structure is very limited and that further detailed, state-of-the-art analyses of the mesoscopic and microscopic de-formation features are required to obtain credible evidence. Beyond the discussion of the article by Nasir et al., this Comment emphasizes the general need for careful and comprehensive analysis of the geological structures that might, a priori, suggest a possible impact origin if, ultimately, a new impact structure can be successfully confirmed. [ABSTRACT FROM AUTHOR]