Abigail C. Allwood

Stromatolite reef from the Early Archaean era of Australia

The 3,430-million-year-old Strelley Pool Chert (SPC) (Pilbara Craton, Australia) is a sedimentary rock formation containing laminated structures of probable biological origin (stromatolites). Determining the biogenicity of such ancient fossils is the subject of ongoing debate. However, many obstacles to interpretation of the fossils are overcome in the SPC because of the broad extent, excellent preservation and morphological variety of its stromatolitic outcrops—which provide comprehensive palaeontological information on a scale exceeding other rocks of such age. Here we present a multi-kilometre-scale palaeontological and palaeoenvironmental study of the SPC, in which we identify seven stromatolite morphotypes—many previously undiscovered—in different parts of a peritidal carbonate platform. We undertake the first morphotype-specific analysis of the structures within their palaeoenvironment and refute contemporary abiogenic hypotheses for their formation. Finally, we argue that the diversity, complexity and environmental associations of the stromatolites describe patterns that—in similar settings throughout Earths history—reflect the presence of organisms.

Controls on development and diversity of Early Archean stromatolites

The ≈3,450-million-year-old Strelley Pool Formation in Western Australia contains a reef-like assembly of laminated sedimentary accretion structures (stromatolites) that have macroscale characteristics suggestive of biological influence. However, direct microscale evidence of biology—namely, organic microbial remains or biosedimentary fabrics—has to date eluded discovery in the extensively-recrystallized rocks. Recently-identified outcrops with relatively good textural preservation record microscale evidence of primary sedimentary processes, including some that indicate probable microbial mat formation. Furthermore, we find relict fabrics and organic layers that covary with stromatolite morphology, linking morphologic diversity to changes in sedimentation, seafloor mineral precipitation, and inferred microbial mat development. Thus, the most direct and compelling signatures of life in the Strelley Pool Formation are those observed at the microscopic scale. By examining spatiotemporal changes in microscale characteristics it is possible not only to recognize the presence of probable microbial mats during stromatolite development, but also to infer aspects of the biological inputs to stromatolite morphogenesis. The persistence of an inferred biological signal through changing environmental circumstances and stromatolite types indicates that benthic microbial populations adapted to shifting environmental conditions in early oceans.

Sulfur isotopes of organic matter preserved in 3.45-billion-year-old stromatolites reveal microbial metabolism

The 3.45-billion-year-old Strelley Pool Formation of Western Australia preserves stromatolites that are considered among the oldest evidence for life on Earth. In places of exceptional preservation, these stromatolites contain laminae rich in organic carbon, interpreted as the fossil remains of ancient microbial mats. To better understand the biogeochemistry of these rocks, we performed microscale in situ sulfur isotope measurements of the preserved organic sulfur, including both Δ33S and . This approach allows us to tie physiological inference from isotope ratios directly to fossil biomass, providing a means to understand sulfur metabolism that is complimentary to, and independent from, inorganic proxies (e.g., pyrite). Δ33S values of the kerogen reveal mass-anomalous fractionations expected of the Archean sulfur cycle, whereas values show large fractionations at very small spatial scales, including values below -15‰. We interpret these isotopic patterns as recording the process of sulfurization of organic matter by H2S in heterogeneous mat pore-waters influenced by respiratory S metabolism. Positive Δ33S anomalies suggest that disproportionation of elemental sulfur would have been a prominent microbial process in these communities.

Planning for Mars Returned Sample Science: Final Report of the MSR End-to-End International Science Analysis Group (E2E-iSAG)

Returning samples from Mars to Earth for scientific analysis has been, and continues to be, among the highest priority objectives of planetary science. Partly for this reason, the 2011 Planetary Science Decadal Survey placed high priority on a proposed 2018 rover mission that would conduct careful in situ science and use that scientific information to select and cache samples that could be returned to Earth by a potential future mission. In order to ensure that the potential contributions of the 2018 rover to the proposed MSR Campaign are properly planned, this study was undertaken to consider the science of the MSR Campaign concept from end to end. This white paper is the principal output of the Mars Sample Return (MSR) End-to-End International Science Analysis Group (E2E-iSAG): a group chartered by MEPAG (Mars Exploration Program Analysis Group).

Morphological Biosignatures in Gypsum: Diverse Formation Processes of Messinian (∼6.0 Ma) Gypsum Stromatolites

The ∼5.3-6.0 million-year-old evaporitic gypsum deposits of Cyprus and Crete contain a variety of stromatolites that formed during the Messinian salinity crisis. We recognize four stromatolite morphotypes, including domical, conical, columnar, and flat-laminated structures. Observations of morphological and textural variations among the different morphotypes reveal significant diversity and complexity in the nature of interactions between microorganisms, gypsum deposition, and gypsum crystal growth. Nonbiological processes (detrital gypsum deposition, in situ crust precipitation, syntaxial crystal growth, subsurface crystal growth, and recrystallization) interacted with inferred microbial processes (including localized growth of biofilms, trapping and binding of grains in mats, nucleation of gypsum on cells) to produce distinct morphological-textural assemblages. Evidence for biological origins is clear in some stromatolite morphotypes and can come from the presence of microfossils, the spatial distribution of organic matter, and stromatolite morphology. In one stromatolite morphotype, the presence of the stromatolite, or the biota associated with it, may have determined the morphology of gypsum crystals. In some stromatolite morphotypes, definitive evidence of a microbial influence is not as clear. There are broad similarities between the Messinian gypsum stromatolites and carbonate stromatolites elsewhere in the geologic record, such as the formation of precipitated and granular layers; the development of domed, columnar, and conical morphotypes; the potential for microbes to influence mineral precipitation; and the recrystallization of deposits during burial. However, in detail the array of microbial-sedimentary-diagenetic process interactions is quite distinct in gypsiferous systems due to differences in the way gypsum typically forms and evolves in the paleoenvironment compared to carbonate. Unique aspects of the taphonomy of gypsum compared to carbonate chemical sediments, generally speaking, include the following: the potential for growth of individual crystals to determine the shape of a stromatolite (and possibly vice versa), a more diverse set of outcomes relating to preservation versus destruction of textures through crystal growth and recrystallization, and a greater likelihood of preserving microfossils through encapsulation in large crystals. These insights gained from the study of terrestrial gypsum sedimentary rocks provide valuable guidance for the search for clues to past life in sulfate chemical sediments on Mars.

Experimental results of rover-based coring and caching

Experimental results are presented for experiments performed using a prototype rover-based sample coring and caching system. The system consists of a rotary percussive coring tool on a five degree-of-freedom manipulator arm mounted on a FIDO-class rover and a sample caching subsystem mounted on the rover. Coring and caching experiments were performed in a laboratory setting and in a field test at Mono Lake, California. Rock abrasion experiments using an abrading bit on the coring tool were also performed. The experiments indicate that the sample acquisition and caching architecture is viable for use in a 2018 timeframe Mars caching mission and that rock abrasion using an abrading bit may be feasible in place of a dedicated rock abrasion tool.1 2

Texture-specific elemental analysis of rocks and soils with PIXL: The Planetary Instrument for X-ray Lithochemistry on Mars 2020

PIXL (Planetary Instrument for X-ray Lithochemistry) is a micro-focus X-ray fluorescence instrument for examining fine scale chemical variations in rocks and soils on planetary surfaces. Selected for flight on the science payload for the proposed Mars 2020 rover, PIXL can measure elemental chemistry of tiny features observed in rocks, such as individual sand grains, veinlets, cements, concretions and crystals, using a 100 μm-diameter, high-flux X-ray beam that can be scanned across target surfaces.

Conference summary: life detection in extraterrestrial samples.

In February 2012, a conference was convened at the Scripps Institution of Oceanography in La Jolla, California, on the subject of life detection in extraterrestrial samples (program and abstracts available at http://www.lpi.usra.edu/meetings/ lifedetection2012). The aim of the conference was to explore the kinds of tools, methods, and approaches necessary for detecting evidence of life in extraterrestrial samples, including those that arrive on Earth by natural processes and those that are deliberately returned by engineered missions. Samples that might be returned from Mars by a future mission were a primary topic of interest. Presentations and discussions at the conference drew upon diverse fields of research, including meteorite studies, modern and ancient terrestrial analog studies, studies of samples returned by past lunar and comet sample return missions, studies of modern traces of life on Earth, and studies of the facilities needed to conduct this kind of research. The conference program was organized with extensive discussion sessions. This report summarizes the results of the conference. The topic of life detection was examined from two different but partially overlapping perspectives: the ‘‘science perspective’’ arising from the desire to know whether life ever arose on Mars and the ‘‘planetary protection perspective’’ arising from the need to protect our own planet from contamination by any potentially harmful living extraterrestrial organisms that may be contained in returned samples. The former relates to detection of any kind of evidence of either ancient or present-day life, whereas the latter is concerned with evidence of present-day viable organisms. A review of the topic of life detection is timely given the scope of recent advances in life-detection studies on Earth, the publication of the National Research Council’s Planetary Science Decadal Survey (which identified seeking the signs of life via Mars sample return (MSR) as its highest priority in the flagship class of missions; National Research Council, 2011), as well as the strategic emphasis within both NASA and ESA on life detection. One of the primary approaches to life detection is via the study of extraterrestrial samples, although other astrobiological approaches also exist. In the case of a potential MSR campaign, significant forward planning is required to ensure best possible practices are implemented throughout the campaign (iMARS Working Group, 2008; MEPAG E2E-iSAG, 2012): from the design and operation of a sample collection rover to containment and preservation of samples in transit, and appropriate handling and analysis of the samples after they have returned to Earth. The array of planned or possible life-detection strategies and measurements has implications for virtually every aspect of a sample return campaign. Thus, it is critical to understand these strategies and measurements well in advance to avoid compromising the fundamental scientific objectives and planetary protection requirements of an MSR campaign. Much of the discussion summarized below assumed MSR would be a robotic endeavor. However, the mission may ultimately involve humans rather than robots. In that case, some aspects of laboratory analyses and sample handling may need to be reassessed. The conference was also an introduction to a subsequent planetary protection workshop dealing specifically with the planetary protection test protocol.

Geology: Evidence of life in Earth's oldest rocks

When did life first arise on Earth? Analysis of ancient rocks in Greenland that contain structures interpreted as bacterial in origin suggest that Earth might have been an abode for life much earlier than previously thought. See Letter p.535 Stromatolites are sedimentary formations created by the layered growth of microorganisms in shallow marine settings. Fossil stromatolites constitute some of the earliest evidence for life on Earth. Allen Nutman et al. describe metamorphosed stromatolites deposited around 3,700 million years ago in what is now Greenland. This is more than 200 million years older than the previous record-holders for earliest-known fossils, so these stromatolites rank as the Earths earliest fossils by some margin. Although there is indirect evidence from isotope geochemistry that the pedigree of life on Earth is even older, this report is likely to be controversial.

The NASA Mars 2020 Rover Mission and the Search for Extraterrestrial Life

Abstract The NASA Mars 2020 rover mission will explore an astrobiologically relevant martian site to investigate regional geology, evaluate past habitability, seek signs of ancient life, and assemble a returnable cache of samples. The spacecraft is based on successful heritage design of the Mars Science Laboratory Curiosity rover, but includes a new scientific payload and other advanced capabilities. The Mars 2020 science payload features the first two Raman spectrometers on Mars, the first microfocus X-ray fluorescence instrument, the first ground-penetrating radar, an infrared spectrometer, an upgraded microscopic and stereo context cameras and weather station, and a demonstration unit for oxygen production on Mars. The instrument suite combines visible and multispectral imaging with coordinated measurements of chemistry and mineralogy, from the submillimeter to the regional scale. Using the data acquired by the science instruments as a guide, the team will collect core samples of rock and regolith selected to represent the geologic diversity of the landing site and maximize the potential for future Earth-based analyses to answer fundamental questions in astrobiology and planetary science. These samples will be drilled, hermetically sealed, and cached on the martian surface for possible retrieval and return to Earth by future missions. The Mars 2020 spacecraft is designed and built according to an unprecedented set of biological, organic, and inorganic cleanliness requirements to maximize the scientific value of this sample suite. Here, we present the scientific vision for the Mars 2020 mission, provide an overview of the analytic capabilities of the science payload, and discuss how Mars 2020 seeks to further our understanding of habitability, biosignatures, and possibility of life beyond Earth.