Microstructural characteristics of deformed areas including shock-related mechanical twinning, continuous crystal lattice bending, and high frequency of fractures, seen in the martian meteorite Lafayette. Image credit: Daly et al., 2019

A new study reports evidence of shock-induced aqueous alteration recorded in a group of martian meteorites, the nakhlites. The nakhlites are igneous rocks, rich in the mineral augite, that crystallized approximately 1.4 to 1.3 billion years ago. The nakhlites also contain significant amounts of material deposited by liquid water on Mars: carbonate mineral, clay minerals, and gel; they formed long after crystallization, at approximately 630 million years ago.

Daly and his colleagues studied the crystal orientations of the augite mineral grains in the nakhlites using electron backscatter diffraction analysis. They found that the nakhlites contain localized regions where the augite was broken (brecciated), plastically deformed, and mechanically twinned by the shock of an asteroid impact. Fine-grained material in the shocked areas was preferentially altered to the clays and carbonates, which suggests that the alteration was related to the asteroid impact. To explain this, Daly proposes that an asteroid impact onto Mars approximately 630 million years ago simultaneously deformed the nakhlite parent rocks and generated liquid water by the melting of permafrost. The meltwater then permeated the nakhlites along the shock-broken zones and preferentially deposited the carbonate and clay minerals there. The nakhlite source location must then have two spatially coincident craters, one that formed approximately 630 million years ago, and another that formed approximately 11 million years ago that ejected the nakhlites from Mars and sent them toward the Earth. READ MORE