Dark microbiome and extremely low organics in Atacama fossil delta unveil Mars life detection limits

Abstract

Identifying unequivocal signs of life on Mars is one of the most important objectives for sending missions to the red planet. Here we report Red Stone, a 163-100 My alluvial fan-fan delta that formed under arid conditions in the Atacama Desert, rich in hematite and mudstones containing clays such as vermiculite and smectites, and therefore geologically analogous to Mars. We show that Red Stone samples display an important number of microorganisms with an unusual high rate of phylogenetic indeterminacy, what we refer to as "dark microbiome", and a mix of biosignatures from extant and ancient microorganisms that can be barely detected with state-of-the-art laboratory equipment. Our analyses by testbed instruments that are on or will be sent to Mars unveil that although the mineralogy of Red Stone matches that detected by ground-based instruments on the red planet, similarly low levels of organics will be hard, if not impossible to detect in Martian rocks depending on the instrument and technique used. Our results stress the importance in returning samples to Earth for conclusively addressing whether life ever existed on Mars.

Fig. 1. Red Stone location and geological characteristics.

A Red Stone Location in the Atacama Desert (digital terrain model). B Satellite image zooming in the surrounding region (Sentinel 2020 satellite data). C Paleogeographic reconstruction of the alluvial/delta system shown in B, according to the reported sedimentary record of the Caleta Coloso-El Way formation (modified from Flint, S. Clemmey, H. & Turner, P. The Lower Cretaceous Way Group of northern Chile: an alluvial fan–fan delta complex Sediment. Geology 46, 1-22 (1986)^,). The black arrows in C show the direction of the flow of the ancient river delta, which point of origin is now under the Pacific Ocean. The red point in A, B and C shows the location of the outcrop shown in E. D Stratigraphic column of the outcrop studied shown in panel E. Red arrows show sample collection points. E Close-up view of the studied outcrop. From top to bottom: UZ upper zone, U1 Unit 1, WI wall in sensor location, U2 Unit 2, WO wall out sensor location, LZ lower zone.

Fig. 2. Red Stone site characteristics.

A Panoramic view of the inspected site. B Geological map of the Caleta Coloso-El Way formation (SERNAGEOMIN, 2003. Mapa Geológico de Chile: versión digital). JK1c (light green); transitional alluvial, fluvial and aeolic sedimentary sequences. Early Cretaceous to Late Jurassic (sandstone, limestone, lutite, conglomerate, siltstone). Ki1m (green); coastal marine sedimentary sequences. Early Cretaceous (sandstone, limestone, loam, calcarenite). M1c (light brown); alluvial fan sedimentary sequences. Miocene (sand, gravel, silt, ignimbrite). J3i (light purple); volcanic continental and marine Jurassic sequences (basalts, andesite, tuff, limestone).

Fig. 3. Close-up views of the studied outcrop.

A Top weathered loose conglomerates. B Cemented conglomerates. C, D Show sandstones (s) with fine layered mudstones (m). White arrows point to halite/gypsum veins. E One of the evaporite veins after been broken, exposing the halite/gypsum inside them and the thin layer of hematite that covers them. F Fibrous halite crust parallel to the surface of the outcrop, only observable after been exposed.

Fig. 4. Next-Generation Sequencing (NGS) characterization of Red Stone samples.

A Heatmap of major bacterial phyla determined by NGS. Numbers represent percentages of total sequences. The color intensity in each box indicates the relative percentage of each phyla. B Richness (S, bars) and Shannon diversity (H’, dots) indices for the analyzed prokaryotic communities. C Hierarchy classification of the bacterial sequences found by NGS. Gray scale colors identify higher hierarchy classifications.

Fig. 5. Lipid Biosignature analyses of Red Stone samples.

A Lipid content, including normal (i.e. straight chain) alkanes, normal fatty acids, and monounsaturated fatty acids (MUFA) identified by gas-chromatography mass spectrometry (GC-MS). Carbon numbers ranges are indicated in brackets. B Total organic carbon (TOC, % dw) and stable carbon isotopic composition (δ13C, ‰) values found. C, D and E show the samples analyzed through Raman spectroscopy, while (F) shows a positive control in which a lipid was deposited on an amorphous silica-rich substrate (panel F modified with permission from Carrizo et al.).