Post by Nathalie Cabrol and the High Lakes Science Expedition.
Impact processes are fundamental in the creation of planets, the modification of their landscape, and for Earth, in the evolution of life. However, unlike the other planets of our solar system, Earth has not kept a large record of its impact history. Plate tectonic and erosional processes have erased most of them with time. Small impact craters, in particular, are difficult to preserve but there are still a few left, including the Monturaqui impact crater (23°56’S/68°17’W) located in the Atacama Desert in Chile.
Monturaqui’s age is estimated to be <1 Ma and since late Pleistocene (0.01Ma) erosion has been controlled mainly by mechanical weathering due to the extreme arid conditions of the area. The crater was discovered in 1962 in aerial photographs and first described by Sanchez and Cassidy . Geological, gravity, magnetic and topographic surveys were performed by Ramirez and Gardeweg  and Ugalde et al.,  and others. Monturaqui’s dimensions are 360 x 380 x 34 m [Buchwald, 1975] and it shows a central uplift of ~3 m coincident with limestone sediments [Ugalde et al., 2005]. Gibbons et al.,  described its formation in siliceous igneous rocks and an enigmatic sulfide abundance. A mathematical model shows that the impactor was a ~15 m diameter asteroid with a velocity over 17 km/s, which struck at an angle of ~ 41° and would have projected material up to 11.5 km and dust up to 23.6 km away from ground-zero. The transient crater was 226 m and the hydrothermal zone ~ 15-113 m from the nucleus of impact, generating hydrothermal processes for 3,500 ± 96 years [Echaurren et al., 2005]. An aquifer has been identified through excavation by Herczeg and Leaney  and could have interacted with the deposits and melt material from the impact. Surface aqueous processes are present and are related to rare rain events and altiplanic winter, which maintain playa formation in the crater. The playa can be seen in Image 1 as the light-tone central deposit.
By its size and morphology, Monturaqui presents many similarities with the Bonneville crater (Image 2) explored by Spirit at the beginning of its mission in Gusev Crater. Both craters are shallow (20 m for Bonneville, 34 m for Monturaqui); the size of the blocks ejected near the crater rim are similar, and both were formed in a volcanic environment, although granite has been identified in the deepest strata of Monturaqui. Since its formation Monturaqui has endured several glacial and deglaciations in the Altiplano, the latest deglaciation dating back 18,000-13,000 yr BP. These geological episodes left many gullies and ravines that eroded the ejecta. Since then, Monturaqui, like Bonneville, has mostly experienced a very arid climate. High daily winds are currently the main erosional and depositional agents, depositing and removing particles from the crater floor. Bonneville was formed while Mars was already a dry planet with little aqueous processes. Bonneville is filled with windblown ripples and unstructured sand accumulations.
Monturaqui presents an additional interesting analogy with Mars, this time with the Meridiani landing site explored by the Opportunity Rover. Petrographic studies on impactite [Bunch and Cassidy, 1972; Buchwald, 1975] show a porous texture ranging from a few mm to ~10 cm. The porous texture is given by the vesicles (30%). About 70% of the vesicles show no fill. The remainder is filled with Fe-oxides (hematite, magnetite) and limonites (goethite), or metal spherules [Ugalde et al., 2005]. What makes the site particularly interesting is that both the impactor and the target region were hematite-rich. The presence of hematite spherules in the ejecta and the study of its origin and evolution could give important information about those observed on Mars. This is the objective of the Planetary Spherules Project (PSP) which examines the main hypotheses of spherules formation for Meridiani (aqueous, volcanic, impact) through the study of terrestrial analogs, including Monturaqui.
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