2013 Annual Science Report
Arizona State University Reporting | SEP 2012 – AUG 2013
The “Follow the Elements” NAI Team at ASU carries out research, education and outreach activities centered on the chemical elements of life. Our activities are motivated by a simple observation: that life-as-we-know-it uses a non-random selection of the chemical elements. This observation prompts many questions:
* What are the rules that govern the selection of these “bioessential” elements? * How might these elements differ in extreme environments on Earth or beyond? * How common are the bioessential elements in the extraterrestrial environments that might harbor life? * How are the distributions of these elements in the cosmos shaped by astrophysical processes?
The answers to these questions will shape the future exploration for life on other worlds. We seek to answer these questions through laboratory, field and computational research, and use them as the basis for much of our education and outreach. To this end, the project is organized around three major research themes: The Stoichiometry of Life; The Habitability of Water-Rich Environments; and Astrophysical Controls on the Elements of Life. We pursue each theme by way of a number of focused research tasks.
During Year 5 of our project, the Astrobiology Program at Arizona State University brought to fruition many of the major research tasks described in our CAN5 proposal, as well as associated projects begun during this time. These efforts resulted in ~ 50 publications that appeared in print during the reporting period, including several publications in high-profile journals. Collectively, this ongoing research advanced our goal of integrating life science, geoscience, planetary science and astrophysics to understand how the distribution of chemical elements shapes the distribution of life in space and time, and to guide the search for life beyond Earth. This work is comprehensively described in the main report. Key outcomes are highlighted here.
1. The Stoichiometry of Life
This theme includes several tasks aimed at elucidating relationships between the availability of bioessential elements and prokaryotic ecosystems. These tasks involved: experimental studies in the laboratory (Task 1); field studies and associated laboratory analyses at (Tasks 2a, b, and c); evolutionary studies in the geologic and genomic records (Tasks 3a and 3b, respectively); and a coupled modeling-experimental project to understand the connections between biogeochemical cycles of carbon, oxygen and bioessential nutrient elements in prokaryote-dominated oceans (Task 4).
Notable milestones and outcomes in Year 5:
* In Task 1, we completed analytical work for a study of metabolomics of the cyanobacterium Synechocystis sp. PCC6803 under different nutrient conditions in the laboratory, carried out in Year 4 in collaboration with Autonomous University of Barcelona. A host of elemental, biological macromolecule, metabolomic, and transcriptomic changes were observed, including unexpected increases in metal:C ratios under phosphorous limitation. Interpretations are in progress.
* Task 2a encompasses many basic research projects arising from fieldwork at Yellowstone National Park. Highlighted findings include: documentation of active methane cycling in an acidic hot spring; demonstration, in collaboration with the MIT team, that compound-specific C isotopes in lipids from chemoautotrophic communities track C sources; elucidation of the pH-dependence of the balance between microbial and abiotic iron cycling; improved understanding of energy supplies across the “photosynthetic fringe”; and evidence that C:N:P ratios are similar in photosynthetic and chemoautotrophic communities, while Mg and Zn enrichments varied.
* Task 2b centers on fieldwork in Cuatro Cienegas. Fieldwork wrapped up in Year 5, with whole-pond fertilization experiment to complement and compare to mesocosm experiments. Comparative analyses of similarities and differences are ongoing. Fieldwork was also completed in a study that revealed the effects of nutrient limitation on the growth of oncoid stromatolites.
* Task 2c saw processing of DNA samples extracted from floating pumice produced in the massive eruption of the Puyehue/Cordon Caulle volcano in Argentina, along with assessment of P and N uptake rates. This work promises to yield new insights into how sterile rock is colonized by microbes.
* Task 3a generated ~ 10 publications based on the geologic record of element availability, with a particular focus on a new assessment of mid-Proterozoic ocean anoxia and trace metal limitation of marine microbiota. The emerging evidence from multiple directions points to mid-Proterozoic oceans that, contrary to widespread assumptions, were not pervasively sulfide-rich, although they were less well oxygenated than today. Consistent with this view, the availability of the bioessential element Zn was found to be higher than expected according to the prior conventional wisdom, confounding theories that linked the diversification of eukaryotes to changes in ocean Zn.
* Task 3b analyses reached the final stages of data generation on the (meta)genomic diversity in the hotsprings of Yellowstone National Park, and the correlations of this diversity with variations in geochemical variables and geography.
* Task 4 saw the development of a study to assess the effects of clays at different concentrations on the formation of microbial aggregates that can promote carbon export from the ocean surface.
2. The Habitability of Water-Rich Environments
This goal of this theme is to improve our ability to infer the availability of bioessential elements in aqueous environments on Mars, in the outer Solar System, and on water-rich exoplanets. It includes tasks involving: Improving computational codes to model water-rock interactions (Task 1); application of modern geodynamics codes to ice dynamics on Europa and other icy bodies (Task 2); integration of these and other computational models with observational data to better assess the habitability Europa (Task 3), Mars (Task 4), and other small icy bodies (Task 5).
* Task 1 saw completion of a novel computational model of chemical equilibria in CO2-bearing waters. The model is suitable for application to cold icy and salty systems such as Mars and icy moons. A model of the thermodynamics of the condensation of gases to gas hydrates was developed for the outer Solar System, and a model that can be used to predict phase equilibria on Titan was published.
* Progress continues on Task 2, which saw Ph.D. student Diva Allu Peddinti receive an outstanding student paper award at the 2012 Fall AGU meeting, for her work on ice convection.
* Task 3 saw the development of novel arguments to account for the presence of sulfate minerals in non-ice material on the surface (and ocean) of Europa. Irradiaion of ices, rather than H escape was invoked.
* Mars research under Task 4 centered on development of new Raman-based approaches to biomarker detection, focusing on reducing fluorescence in rock samples, and numerical modeling of Mars basalt weathering. An important conclusion of this work is a self-consistent explanation of the distribution of secondary minerals at the Martian surface.
* Extensive output emerged from Task 5. This included: publication of a new hypothesis that the surface mineralogy of Ceres is a consequence of ancient bombardment processes rather than placid ocean chemistry; development of new models of the internal structures of Kuiper Belt Objects (KBO) that predict subsurface water oceans are common on these bodies as well as Ceres and Pluto; first observations in a campaign to loo for observational evidence of undifferentiated crust on KBOs; and research and advocacy revolving around a mission to return samples from Enceladus’ plumes to Earth.
3. Astrophysical Controls on the Elements of Life
This theme aims to elucidate the influence of supernovae and other nucleosynthetic processes on the formation of solar systems, their composition, and evolution, with an emphasis on factors crucial to the formation of Earth-like planets. The theme includes tasks involving: High-precision isotopic studies of meteorites to quantify the timescales of the injection of supernova-derived materials (Task 1); computational modeling of the physical and chemical evolution of massive stars (Task 2); quantification of the injection of supernova ejecta to star-forming molecular clouds (Task 3) and protoplanetary disks (Task 4); modeling the chemical evolution of star-forming regions (Task 5); determining what elements might be used as observational proxies in stellar spectra for elements and isotopes not amenable to direct observation (Task 6); and incorporate element abundance data in the “HabCat” of nearby stars that could support life (Task 7).
* Task 1 work was completed in Year 4 and is now headed for publication.
* In Task 2, important consequences for stellar evolution and hence habitable zone evolution arising from element variability were quantified for a range of parameters. The stellar O/Fe ratio was found to be particularly important, modifying the habitable zone lifetime by billions of years.
* Year 5 progress on Task 3 consisted of publication of the critical finding that supernova gas can cool quickly enough to penetrate into the molecular cloud. This means that stars can be chemically contaminated with supernova material just as they are forming, with implications for our Solar System’s history.
* Task 4 was largely completed in prior years, when it was found that the probability of injection of supernova material into a proto-planetary disk is very low. However, research into the effects of photo-evaporation on disk evolution advanced under this task.
* In Task 5, a simple relationship was derived that describes the evolution of the low-metallicity fractionation of star-forming molecular clouds in stellar clusters. This insight will advance efforts to understand the compositional evolution of the earliest stars and the transition from early metal-poor stellar populations to the typical populations today that are able to host Earth-like planets.
* Task 6 was largely completed in Year 4, but spurred a statistical analysis of element abundance data from ~ 500 stars, focusing on elements deemed by geophysicists to be the most important for determining the material properties of planets that pertain to habitability (e.g., tendency for mantle convection). No significant correlations were observed, implying that the whole range of potential diversity in abundances of these elements must be considered when assessing likely planet properties.
* In Task 7, work evolved to explore the possibility of making detailed predictions of planetary habitability by merging element abundance information gleaned from stellar spectra with geophysical considerations, focusing on C/O, Mg/Si, and Eu/Fe ratios. For example, this analysis suggests that Earth-like planets in the Tau Ceti system will have lower mantle viscosities and higher radiogenic heat production than the Earth.
Other Institute Objectives
In addition to our research tasks, the team carried out many activities supporting non-research objectives of the NAI. Many will be reported in the team’s EPO report, which is being compiled separately this year.
Through the year, the team promoted graduate training by partially supporting the stipends of approximately a dozen Ph.D. students. We also supported the activities of astrobiology postdocs Jordan Okie and Sara Walker in various ways.
Notably, Sara Walker joined the faculty of the School of Earth & Space Exploration toward the end of Year 5. Her work with the astrobiology program was a major motivation for his appointment.
The team continued its weekly “coffee seminars”, drawing a regular group of ~ 25 students and faculty for informal seminars. This seminar series is now fully organized by program students.
Finally, the team organized and sponsored two important workshops in Year 5:
a) A “workshop without walls” on the topic of “Stellar Stoichiometry”, April 11 – 12, 2013 (https://sites.google.com/a/asu.edu/stellarstoich/). This workshop included ~ 20 in-person participants at ASU and ~ 50 virtual participants in astronomy, astrobiology, and related communities, who met to discuss and address several unresolved issues related to the topic of stellar abundances and their impact on planetary systems. The workshop was largely organized by graduate students on our team.
As an outcome of the workshop, our team initiated a collaboration with five other research groups to determine why measurements of element abundances for a given star by different research groups often differ by more than the quoted observational errors. This work is expected to result in a journal article. An additional outcome will be a review article on the topic of stellar stoichiometry.
b) An in-person workshop on the topic of returning samples from Enceladus, June 14 – 15, 2013. The workshop included ~ 30 participants, who met at the DoubleTree Hotel in Monrovia, CA. Participants included scientists and engineers from ASU, NASA JPL (including the PI of the NAI Icy Worlds team), NASA Ames, the Carnegie Institute of Washington, the University of Washington, and the Japanese research institutions ISAS and JAMSTEC.
This workshop was centered on the LIFE mission concept (Tsou et al., 2012; http://www.ncbi.nlm.nih.gov/pubmed/22970863). The workshop forged strong ties among potential US-based mission team members, and began exploration of the possibility of a joint US-Japan mission model. The workshop marked the formal start of efforts to develop the LIFE concept for the next Discovery opportunity.