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Thesis Format

Integrated Article

Degree

Master of Science

Program

Geology

Collaborative Specialization

Planetary Science and Exploration

Supervisor

Osinski, Gordon R.

2nd Supervisor

Flemming, Roberta L.

Co-Supervisor

3rd Supervisor

Pontefract, Alexandra

Affiliation

Georgetown University

Co-Supervisor

Abstract

Impact craters represent excellent astrobiological targets for planetary exploration missions to Mars. The impact of an asteroid or comet into a crystalline, H2O-bearing target may result in development of a hydrothermal system and increase the habitability of the substrate for the colonization of endolithic microorganisms. Given that Mars’ surface is covered by cratered basaltic rock, this study investigated target rocks and impact breccias from Vargeão Dome and Vista Alegre impact structures that formed in basalt in the Paraná Basin of Brazil.

Utilizing petrography and micro-X-ray diffraction (μXRD), the degree of shock metamorphism in plagioclase was quantitively determined. Measuring the full-width-at-half-maximum of diffraction peaks reveals increased strain-related mosaicity. This is correlated with increased optical evidence of deformation. Density and porosity results reveal trends that support conclusions from previous work on different rock types, whereby, increased shock level appears to result in greater porosity.

Optical microscopy, electron microprobe analysis, and μXRD were used to investigate hydrothermal alteration in the Vista Alegre crater and differentiate between pre- and post-impact hydrothermalism. We confidently verify that shocked samples within the Vista Alegre impact structure are altered by impact-induced hydrothermal activity. This alteration is distinct from that occurring outside the impact structure.

Summary for Lay Audience

Impact craters present excellent targets for current and future life detection missions to Mars. Although the initial impact bombardment is often viewed as a destructive process, it may also provide a unique environment for early life to assemble and, therefore, impact craters are prime sites in the search for evidence of past life on Mars. Impact events into a water-bearing target can result in an active hydrothermal system, the extent of which depends on the size of the impact and the target rock composition. Shocked substrates favourably interact with water, presenting access to nutrients and energy. Additionally, impacts play a role in providing a thermal gradient, a fluid reservoir, and a high level of permeability within the rock, thus enhancing the habitability of a host rock and its ability to support microbial life.

Impact-induced porosity correlates strongly with the shock level of the target rock. Current classification systems of impact-shocked rocks are predominantly based on the alteration of quartz, which is excellent for terrestrial sedimentary rocks, but difficult to implement in quartz-poor basalts. Given that impact craters on Mars would have occurred primarily within basaltic rocks, Vargeão Dome and Vista Alegre basaltic impact structures were selected as terrestrial analogues corresponding to the Martian surface in order to investigate how shock metamorphism and hydrothermal alteration influence the habitability of the target substrate for microorganisms.

In this contribution we quantitively characterize the degree of shock in plagioclase grains from shocked target basalts and impact breccia samples utilizing petrography and micro-X-ray diffraction (μXRD). Density and porosity results reveal trends that support conclusions from previous work on different target rocks, whereby, increased shock state correlates with increased porosity. Additionally, shocked basalts offer a higher porosity, high surface area environment which could make viable habitats for rock-dwelling microorganisms. This study also presents the first investigation of hydrothermal alteration in the Vista Alegre impact structure. Optical microscopy, electron microprobe analysis, and μXRD were used to investigate and differentiate between pre- and post-impact hydrothermal alteration. The degree of shock metamorphism and hydrothermal alteration were used to investigate the potential for microbial colonization in shocked targets and, thus, may guide the site selection and instrumentation for future Mars sample return missions.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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