Abstract
On Earth, the deep subsurface biosphere of both the oceanic and the continental crust is well known for surviving harsh conditions and environments characterized by high temperatures, high pressures, extreme pHs, and the absence of sunlight. The microorganisms of the terrestrial deep biosphere have an excellent capacity for adapting to changing geochemistry, as the alteration of the crust proceeds and the conditions of their habitats slowly change. Despite an almost complete isolation from surface conditions and the surface biosphere, the deep biosphere of the crustal rocks has endured over geologic time. This indicates that the deep biosphere is a self-sufficient system, independent of the global events that occur at the surface, such as impacts, glaciations, sea level fluctuations, and climate changes. With our sustainable terrestrial subsurface biosphere in mind, the subsurface on Mars has often been suggested as the most plausible place to search for fossil Martian life, or even present Martian life. Since the Martian surface is more or less sterile, subsurface settings are the only place on Mars where life could have been sustained over geologic time. To detect a deep biosphere in the Martian basement, drilling is a requirement. However, near future Mars sample return missions are limited by the mission’s payload, which excludes heavy drilling equipment and restrict the missions to only dig the topmost meter of the Martian soil. Therefore, the sampling and analysis of Martian impact ejecta has been suggested as a way of accessing the deeper Martian subsurface without using heavy drilling equipment. Impact cratering is a natural geological process capable of excavating and exposing large amounts of rock material from great depths up to the surface. Several studies of terrestrial impact deposits show the preservation of pre-impact biosignatures, such as fossilized organisms and chemical biological markers. Therefore, if the Martian subsurface contains a record of life, it is reasonable to assume that biosignatures derived from the Martian subsurface could also be preserved in the Martian impact ejecta.
Highlights
Life is a strong force on our planet, and it has endured for billions of years despite global catastrophies, such as meteorite impacts, global glaciations, mass extinctions, and climate changes.From the fossil record, we know that microbial life has been present on Earth since at least 3.5 Giga annum (Ga) [1,2,3] and perhaps as early as 3.8 Ga [4]
We know that microbial life has been present on Earth since at least 3.5 Giga annum (Ga) [1,2,3] and perhaps as early as 3.8 Ga [4]
The study of impact ejecta on the Martian surface in future missions would enable a search for a fossil record of a sustainable Martian deep biosphere without using heavy drilling equipment
Summary
Life is a strong force on our planet, and it has endured for billions of years despite global catastrophies, such as meteorite impacts, global glaciations, mass extinctions, and climate changes. Potential subsurface settings, where water can be liquid and life can survive would be at depths of several km and out of reach for future MSRs. the current goal of the MSR missions is to reach areas unaffected by radiation and oxidation, and where organic molecules indicative of life or fossilized microorganisms might have been preserved. The study of impact ejecta on the Martian surface in future missions would enable a search for a fossil record of a sustainable Martian deep biosphere without using heavy drilling equipment. The first reason is that subsurface environments can supply a deep biosphere with the important prerequisites for sustaining life: water and accessible nutrients This has led several authors to suggest subsurface or subseafloor hydrothermal settings as the most plausible environment where life originated on the early Earth [24,25]. Thermal buffering does not occur on Mars, but on the other hand, the low Martian escape velocity results in the fact that most of the energetic ejecta escape more into space, while massive quantities of less energetic ejecta are globally distributed
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