Terrestrial impact sites for field training of Astronauts and future "Field Planetary Geologists"

Having knowledge in geology and more particularly in impact geology is, and will be, a necessity toward exploring, and what is more, toward exploiting the surface of nearby planetary objects (Moon, Mars and asteroids), as impact cratering is the major active geological process at work on these surfaces. Sooner or later astronauts will walk on these surfaces and will have to be able to characterize the terrain. They will do Geology. This will require specific training. If, as happens during education, a part can be "masterful", training in geology requires a minimum of experience on the ground, in order to recognize the rocks from the outcrops, in order to follow and map contacts between different lithologies, structures, in order locate oneself in space, to collect sample and to carry out various field measurements. Today, and in the future, after manned flights to the Moon will have been re-established, the Earth is and will remain the best candidate to provide this field training. Ditto for developments and testing field instruments and tools specific to geological studies on the Moon or other planetary surfaces, whether these tools are used by men or by robots. This leads to the use terrestrial analogues, especially impact sites. The purpose of this presentation is to assess how terrestrial impacts in general and Rochechouart in particular, may contribute to this objective. To this end, a large part of the presentation will be devoted to the discussion of the criteria and qualities required, and to a review of the terrestrial impacts which already participate into such efforts or which would be useful to participate (see also [2-4]).

We will retain among the criteria, the geological interest (nature, significance, diversity, quality of the rock exposures), and the practical aspects (geographical accessibility, the risks (political, health, etc.), the distance between outcrops of interest, the availability of dedicated support on site (infrastructure and skills).

Obviously, no impact crater on Earth or on another planet, can expose all the direct and indirect features associated with impacts, such as distal effects, ejecta, the various impact deposits in the crater and the wide variety of damage and manifestations observed, at the scale of the outcrop, under and on the periphery of the large impact craters (autochthonous breccias, mega-blocks, fractures, pseudotachylitic breccias, shatter cones, ultra-cataclasites, hydrothermal deposits, etc…).

Rochechouart will receive particular attention because, in addition to its geographical accessibility, it is distinguished by the diversity and richness of the impact geology exposed. Its level of erosion oscillates on either side of the crater floor, offering the geologist an unparalleled set of natural sections, both in the crater infill and into the target underneath the crater. In other terrestrial impact structures, only part of the typical impact facies is usually represented, and usually either in the crater or below it. All the facies are represented at Rochechouart, both in the crater, including the impactoclastites, and below, including the hydrothermal-type post-impact effects, which may be of economic and/or biological interest in relation to the effects of impacts on the habitability of planetary surfaces and on the emergence of Life [5]. Rochechouart allows the geologist to see and to follow the limit between the rocks filling the crater and the rocks of the target beneath, i.e., it allows to study the bottom limit of the transient cavity produced in a large, deca-kilometer impact crater. The richness of the field at Rochechouart is reinforced by the installation and availability to the geologist of the CIRIR “Impact on Shelves” facility, a large sample library that is unique in its principle and its destination [6].

Developing a field training offer for astronauts and future planetary geologists will be open to discussion.

References: [1] Phinney W. C., (2015), NASA SP-2015-626, 318 p., [2] Kring D., (2010), Nordlingen Ries Crater Workshop, LPI Contribution 1559, 7036.pdf, , [3] Mangold et al. (2022), ICF-CIRIR 2022, Kring D. A. et al., (2020), LPI Contribution No. 2576, [4] Osinski G. R., 2022, 53rd LPSC, [5] Brack A. et al. (2022), ICF-CIRIR 2022, [6] Lambert P. (2019), LMI 6^th, Brasilia, USRA 5027.pdf

Acknowledgements: Thanks to Jean Pierre Lebreton, Uwe Reimold and Frances Westall for help with the preparation of this abstract. Special thanks to the local territories (“Porte Océane du Limousin” and “Charente Limousine”) for support, and special thanks to all the members of the CIRIR consortium (list at cirir-edu.org) without whom all the research and foreseen development at Rochechouart and at other terrestrial impact sites would not be possible.