NAI Director's Seminar Series Seminars

A Talk With Jim Green

We have a unique opportunity for Dr. Jim Green to discuss with us the latest activities of NASA’s Planetary Science Division and the future of astrobiology. Please join us and bring your questions!

What Can Extant Genomes Reveal About Early DNA Metabolism?

DNA serves as the molecule of modern day inheritance, ensuring continuity of life on our planet. Extant modes of DNA synthesis and repair are extremely complex, unlikely to be representative of the situation when DNA initially became the molecule of life. This discrepancy is all the more pronounced since it is likely that early genomes were not as large as those of present day organisms. This presentation will look at information that can be extracted from extant genomes on the proteins that likely co-evolved with DNA in its infancy as the molecule of life. I will, therefore, initially present an overview of the complex modern day DNA replication/repair machinery, which is conserved in all life forms on our planet. I will then present genetic information that is embedded in extant genomes and likely represents relics (ancestral proteins) of ancestral DNA replication/repair machinery. In presenting the proteins that might have co-evolved with DNA as the molecule of inheritance, I will also look at their diversity and how this diversity may be generated. In particular, I will relate the presentation to a hypothesis based on two phases of evolution outlined as follows: in the earliest collective phase of life, complexity grew rapidly due to massive horizontal gene transfer of core cellular machinery, especially the translational machinery; under these circumstances there is no notion of species. Communities varied in descent. In the modern era of individual lineages however, the information processing machineries no longer undergo horizontal gene transfer. The vertical descent of individual lineages defines species and traces out the evolutionary history of life.

What We Talk About When We Talk About Earth's Oxygenation

As the possibility of detecting the atmospheric composition of terrestrial exoplanets moves from the realm of science fiction to science we have become increasingly focused on determining what Earth would look like if analyzed remotely over its long history. Beyond just providing a record of Earth’s atmospheric composition, our goal is to determine how biological evolution has shaped surface oxygenation. A better understanding of our own planet’s atmospheric evolution will improve the framework we use to interpret exoplanetary atmospheres. To unravel the tale of Earth's oxygenation and biological evolution, many key questions need to be answered. In this talk I will focus on providing a new view of atmospheric oxygen levels through Earth's middle history, where traditional constraints on redox sensitive atmospheric components are so imprecise it is difficult to estimate Earth’s ‘detectability’. Further, I will provide new constraints on the factors leading to the climatic and biogeochemical turbulence that characterized the transition to the more modern (Phanerozoic) Earth.

Bowling With Astrobiologists: A Twisted Path Toward the Origin of DNA

At the beginning, so runs the Zen saying, one sees mountains as mountains, and rivers as rivers. In 2015, it seemed a straightforward task to document the impact of recent developments such as genomics on origin-of-life research. It was to be—and will be—the first part of a book project on the biological, scientific, and cultural history of DNA. But then, a local conference attended by many luminaries in disparate fields provided a disconcerting revelation. In three days’ time, mountains were no longer mountains, rivers no longer rivers. The origin of life turns out to be vastly complex, contentious, and colorful. There followed ten months of opportunities strange and rare: making chemical gardens and discussing jazz and dissipation at the Jet Propulsion Laboratory; being intellectually fire-hosed in Düsseldorf; talking bio-philosophy while strolling the melancholy coast of Nova Scotia; bushwhacking to remote pools of boiling acid at Yellowstone; and bowling at the White House. As the year concludes, one sees mountains once again as mountains, and rivers once again as rivers. There remains much to process, but a few principles—and perhaps a little clarity—have emerged.

SHERLOC: On the Trail of Potential Biosignatures on Mars

The next rover to explore Mars has been proposed to launch in 2020. The primary goal of the mission is to better understand the geologic and climate history of Mars including the identification of potential signs of past life on Mars. Once identified, these samples will be collected and stored by the rover for return to the earth so they can be analyzed by state-of-the art instruments in terrestrial laboratories. As part of the payload, NASA selected the Scanning Habitable Environments with Raman & Luminescence for Organics and Chemicals (SHERLOC) investigation. SHERLOC consists of a Deep UV (DUV) native fluorescence and resonance Raman spectrometer that includes a built-to-print version of the Mars Hand Lens Imager (MAHLI) instrument on the Mars Science Laboratory (MSL). It is a robotic arm-mounted instrument that utilizes a DUV laser to generate characteristic Raman and fluorescence photons from a targeted spot. The DUV laser is co-boresighted to a context imager and integrated into an autofocusing/scanning optical system that allows us to correlate spectral signatures to surface textures, morphology and visible features. An internal scanning mirror enables the generation of maps that allow for the identification of spatially resolved organic and aqueous minerology structure. SHERLOC’s science goals include the detection and classification organics and astrobiologically relevant minerals on the surface and near subsurface of Mars. It is capable of organic sensitivity of 10^-5 to 10^-6 w/w over the entire observation region of 7 mm x 7 mm. It is capable of organic sensitivity of 10^-2 to 10^-4 w/w spatially resolved at 100µm and can detect astrobiologically relevant aqueously formed mineral grains with sizes <100µm.

Characterizing Potentially Habitable Exoplanets

In the coming decades, the search for life outside our Solar System will be undertaken using astronomical observations of extrasolar terrestrial planets. To support this endeavor, the NASA Astrobiology Institute's Virtual Planetary Laboratory (VPL) team uses an interdisciplinary suite of computer models, coupled with input from observations, field data and laboratory work, to answer a single scientific question: How do we recognize whether an exoplanet can or does support life? Addressing this question drives three principal areas of research: an exploration of star, planet and planetary system interactions that affect planetary habitability, an improved understanding of how life can modify a planet’s environment on a global scale, and the identification of key signs of habitability and life to be sought in exoplanet observations. This research supports future NASA exoplanet missions by providing the scientific foundation for selection of the most promising planetary targets for detailed observation, and determining the photometric and spectroscopic measurements that will provide the most robust indication of habitability or the presence of life. In this presentation we will give an overview of VPL’s progress to date in these three areas, and highlight significant research results. These include a new framework for exoplanet target selection, methods to detect oceans on distant worlds, star-planet-planetary system interactions and their impact on planetary habitability, potential signs of life from alternative biospheres, the identification of false positives for life generated by the planetary environment, and future prospects for the characterization of potentially habitable exoplanets with JWST and beyond.

Life Underground in the Sanford Underground Research Facility

Quality samples from the marine deep subsurface biosphere are difficult to obtain, biomass there is often at very low concentrations, and slow metabolic activity thwarts many laboratory experiments. However, the host rock types are generally less diverse than their continental counterparts. The petrologic complexity of the continents is countered by the relative ease in access via mines, drill holes, and deeply-sourced springs. The Life Underground team of the NAI is investigating the deep biosphere in the paleoproterozoic iron-rich metasediments at the Sanford Underground Research Facility (SURF) in South Dakota (USA). I will present recent results in metabolic reaction energetics, microbial diversity analysis, and in situ electrode cultivation. Gibbs energy calculations of ~100 inorganic redox reactions show that per mole of electrons transferred, reactions with O2, NO3-, and MnIV are the most exergonic (-120 to -40 kJ). When normalized per kg H2O, however, the most exergonic reactions are sulfur oxidation, essentially independent of the identity of the oxidant (including CO, CO2, and FeIII). Metagenomic analysis of fluid samples led to nearly closed genomes of several novel lineages within candidate phyla, and 16S sequences of DNA extracted from fresh rock core via sterile cryo-drilling share very few taxa with fluid samples from the same mine level (~1500 m). Enrichments from 5-month-long in situ electrode incubation experiments led to the isolation at several reducing potentials of novel bacteria that are related to known Fe and Mn oxidizers and reducers.

Ironing Out Life and the Universe

Iron is unusually abundant in the universe considering its place on the Periodic Table because it represents the “end of the line” of nuclear fusion. In the interior of terrestrial planets, iron exists in reduced form as Fe(0) and Fe(II), yet, on the surface of Earth today, the oxidized form, Fe(III), is stable. Looking to the very early Earth, life originated and first proliferated in an anoxic environment where reduced iron was benign, abundant, and soluble. We hypothesize that for perhaps the first 2 billion years of life on Earth Fe(II) was a ubiquitous and generally useful cofactor for nucleic acids, with roles in folding, catalysis and processing by proteins. As Earth evolved free oxygen, and Fe(III) became stable on its surface, Fe(II) was replaced as the primary cofactor for nucleic acids by Mg(II) in parallel with known metal substitutions of metalloproteins. To test this model, we assay the ability of (i) RNA to fold and catalyze reactions in the presence of Fe(II), and (ii) nucleic acid processing enzymes to use Fe(II) in place of Mg(II) during catalysis. The results show that Fe(II) can indeed substitute for Mg(II) in RNA folding and catalysis and in protein enzymology involving nucleic acids. In fact, it appears that Fe(II) supercharges RNA. The data are consistent a model in which modern biochemical systems retain latent abilities to revert to primordial Fe(II)-based states when exposed to ancient earth conditions. If the hypothesis of a fundamental change from Fe(II) to Mg(II) for some key biological functions is correct, we can place constraints on the timing of this from the ancient rock record. Iron-rich marine sedimentary rocks, including jaspers (hematite+chert) and iron formations, contain clues in their chemical and isotopic compositions for when Fe(II) started to become limiting. One constraint is the “Great Oxidation Event” (GOE) at ~2.3 b.y. ago, but evidence now exists for oxygenation of shallow ocean water as far back as 3.2 b.y. ago, nearly one billion years before the GOE. In this talk, we will explore the changing role of iron in biology and the surface environments of the ancient Earth.

Serpentinization on Mars: Observational Evidence and Theory

The Nili Fossae region is the site of a number of proposed Landing Sites for the Mars 2020 Rover. A distinguishing feature of many of these sites is the access to large amounts of carbonate deposits (Ehlmann et al. 2008). Serpentinization has been proposed as a formation mechanism of these carbonates, including carbonated (Brown et al. 2010, Viviano, et al. 2013, McSween et al. 2014) and low temperature, near surface serpentinization (Brown et al. 2010, Ehlmann et al. 2011). The presence of talc following carbonated serpentization has been linked to Earth analogs in terrestrial greenstone belts such as the Pilbara in Western Australia, where talc bearing komatiite cumulate units of the Dresser Formation overlie the siliceous, stromatolite-bearing Strelley Pool Chert unit (Van Kranendonk and Pirajno, 2004). If a similar relationships exists on Mars, investigations of rocks stratigraphically beneath the talc-bearing units at Nili Fossae may provide the best chance to examine well preserved, siliceous organics. This hypothesis is testable at the Southern Nili Fossae proposed landing site, for example. In preparation for the the Mars 2020 landing site, we are examining the thermodynamic relationships that favor formation of serpentine and talc-carbonate and different pressures and temperatures in the crust (Barnes 2007). This is important as it will constrain the low grade metamorphism required to replicate the proposed models of serpentinisation and help us understand the regional metamophic gradient that is critical to furthering our knowledge of the ancient rocks of Nili Fossae.

Searching for Life on Mars With PIXL and the Mars 2020 Rover Mission

Finding conclusive evidence of primitive microbial life in multi-billion-year-old rocks is exceptionally difficult, as illustrated by doubt surrounding the interpretation of Earth’s earliest fossil record. Seeking evidence of ancient life on Mars is an even greater challenge – one that will be taken up by NASA’s ambitious new 2020 rover mission. 2020 builds on the success of the 2011 Curiosity rover and 2004 Mars Exploration Rovers, and is informed by tools and methods that helped resolve the mysteries of early life on Earth. PIXL (Planetary Instrument for X-ray Lithochemistry) is a microfocus X-ray fluorescence spectrometer that will be mounted on the rover arm and used to make close-up measurements of rock chemistry. Elemental chemistry measurements have played a fundamental role in previous Mars rover missions, and PIXL will build on those discoveries with higher spatial resolution chemical mapping of rock and soil targets. PIXL can measure chemistry at sub-mm resolution (~125 micron spot size), compared to 17mm for APXS on Mars Science Laboratory’s Curiosity Rover. PIXL will use a micro-context camera and optical fiducial system to accurately correlate chemical variations to textures and microstructures. This petrologic information will be important for robust assessment of past habitability, the potential for preservation of biosignatures, and seeking potential biosignatures of past life. The PIXL breadboard instrument has already been used extensively in astrobiological sample analyses on Earth, providing insights to how PIXL will help achieve the 2020 mission’s science objective to search for signs of past life on Mars.