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Deep Biosignatures on Mars and Earth

Periodic Reporting for period 1 - D BIOME (Deep Biosignatures on Mars and Earth)

Reporting period: 2017-10-01 to 2019-09-30

"This research addressed fundamental science questions about the distribution of life on Earth, the history of life on Earth, and the ongoing multi-billion dollar search for life on Mars.

It has become clear in recent decades that living organisms thrive throughout the Earth's crust to depths of several kilometres. Many subsurface ecosystems are sustained by chemical reactions between water and basaltic rock and do not require sunlight or any input of organic matter from the surface. These same conditions could have supported life on Mars long after the surface became uninhabitable. Thus, there is considerable interest in how current and future Mars missions may identify the traces of ancient subsurface life preserved in rocks now exposed on the martian surface. At the same time, the realisation that a sizeable proportion of terrestrial life resides underground motivates questions about the role of deep life in the history of our own planet. Answering these questions requires us to identify robust evidence of ancient subsurface life in rocks, and to piece this evidence together into an integrated archive: a fossil record.

Against this background, the present work was originally directed towards four main objectives:

1. To identify and describe ancient deep biosignatures (""fossils"") in both marine and terrestrial rocks on Earth, showing how their origin can be interpreted and confirmed, and extending the deep fossil record.

2. To determine experimentally whether ancient deep biosignatures in rocks on Mars could be detected by rover instruments if they are exist and are exposed on the surface, particularly using the European Space Agency’s ExoMars rover (now named Rosalind Franklin).

3. To simulate the crustal subsurface (~100–1000 m deep) experimentally and discover which environmental parameters most strongly affect the growth of microorganisms and the production of biosignatures in these conditions.

4. To model how the rock- and sediment-hosted deep biosphere has changed in size and activity over the history of the Earth, and show how an analogous biosphere could have done so if it ever existed on Mars.

The main conclusions of the work were:

1. A fossil record of the deep biosphere exists but is more difficult than previously thought to distinguish from non-biological microscopic structures (pseudofossils).

2. Calcium sulfate veins, which are widespread on Mars, have the potential to yield organic residues and other chemical (isotopic) evidence of ancient subsurface life.

3. The biomass of life on Earth was dominated by subsurface organisms for at least two billion years.

4. Sediment-dwelling microbes may have played key roles in the preservation of fossils of early macro-organisms on Earth."
"The following work was performed using a variety of field, lab, analytical and numerical techniques:

1. The fossil record of Earth's subsurface ""deep biosphere"" was surveyed and reviewed, showing it to be extensive but poorly understood.

2. New samples were collected from sites of ancient subsurface fluid flow in Italy and the UK and analysed with a variety of methods for the presence of signatures of ancient subsurface life.

3. Quantitative reasoning was used to investigate the size of the ancient deep biosphere on Earth.

4. Abiotic processes capable of mimicking fossils of subsurface-dwelling microbes were discovered and investigated.

5. Implications for the search for ancient life on Mars were drawn out using Mars-analogue sample materials and mission-analogue analytical techniques.

6. Minerals formed by microbial activity in sediment were found in association with fossils of early soft-bodied animals, helping to explain their preservation.

At the end of the fellowship period, the work had resulted in 11 articles in scientific journals (of which 1 was in review and 1 was in press); further articles based on the work are anticipated.

Results were also disseminated at international conferences and meetings: the Scottish Planetary Science Research Network meeting 2017, the European Geosciences Union meeting 2018, the European Astrobiology Network Association meeting 2019, the Palaeontological Association meeting 2018, and seminars at the Universities of Leeds, Oxford, Cambridge, and Uppsala.

Non-academic audiences were engaged through 3 articles for popular magazines (of which 2 were for children), widespread media coverage of several papers, and public talks."
This work succeeded in expanding our knowledge of the fossil record of deep life and of the uncertainties associated with it. Several of the published outputs made explicit recommendations about sampling strategies for the benefit of future Mars missions.
Mineralised and carbon-rich filaments from an ancient Mars-analogue subsurface environment: fossils?