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Project Reports

• Understanding Past Environments on Earth and Mars

  In this task we performed research to understand the evolution of habitable environments on Earth and Mars, both of which serve as potential analogs for habitable environments on extrasolar planets. We are expanding this line of work from past reports to span the entire histories of both planets. On Earth, we have sought to understand environments and time periods spanning the origins of life to the effects of human-generated greenhouse gas emissions on modern-day climate cycles. On Mars, we focus on the ancient conditions that could have allowed liquid water to be stable at the surface; on modern Mars, we focus on the debate on the presence, amount, and variability of methane in the Martian atmosphere.

  ROADMAP OBJECTIVES: 1.1 1.2 4.1 4.2 5.1 5.2 6.1

• Biogenic Gases From Anoxygenic Photosynthesis in Microbial Mats

  This lab and field project aims to measure biogenic gas fluxes in engineered and natural microbial mats composed of anoxygenic phototrophs and anaerobic chemotrophs, such as may have existed on the early Earth prior to the advent of oxygenic photosynthesis. The goal is to characterize the biogeochemical cycling of S, H, and C in an effort to constrain the sources and sinks of gaseous biosignatures that may be relevant to the detection of life in anoxic biospheres on habitable exoplanets.

  ROADMAP OBJECTIVES: 4.1 5.2 6.1 7.2

• Charnay NAI NPP PostDoc Report

  My project focuses on the modeling of clouds and photochemical haze in the atmospheres of the early Earth and exoplanets. I use a 3D model, developed to simulate any kind of atmosphere, to study the formation, dynamics, climatic impact and observational features of clouds/haze. My first object of interest is GJ1214b, a mini-Neptune whose observations by HST revealed a cloudy/hazy atmosphere. The formation of such high and thick clouds is not understood. My second object of interest is the Archean Earth for periods with a methane-rich atmosphere leading to the formation of organic haze.

  ROADMAP OBJECTIVES: 1.2 4.1

• Eva Stüeken NPP Postdoc Report

  I study the non-marine sedimentary rock record to determine if lakes and rivers could have been important habitats for the early evolution of life on Earth. Our results suggest that the greater environmental diversity found in non-marine settings may enhance biological diversity. However, we cannot confirm previous conclusions that lakes were particularly suited for eukaryotic life. These findings may provide clues about potential biodiversity of other worlds that are characterized by smaller lake basins (e.g. early Mars) versus a global ocean (e.g. Europa).

  ROADMAP OBJECTIVES: 4.1 4.2 6.1

• Solar System Analogs for Exoplanet Observations

  The worlds of our Solar System can provide an important testing ground for ideas and techniques relevant to characterizing exoplanets. In this task, we use observations and simulations of Solar System planets to understand how astronomers and astrobiologists will recognize signs of habitability and life in future observations of rocky exoplanets. Work in this area this past year includes the first-ever direct detection of molecular nitrogen collision-induced absorption in Earth’s whole-disk spectrum, which can be used to indicate atmospheric pressure and, thus, habitability. Also in this task, VPL scientists have proposed techniques for using color to distinguish Earth-like exoplanets from other types of worlds.

  ROADMAP OBJECTIVES: 1.2 2.2 7.1 7.2

• Global Surface Biosignatures: Circular Polarization Spectra of Anoxygenic Phototrophs and Cyanobacteria

  This new project focuses on characterizing the chiral signature of biological molecules. The phenomenon of chirality is a powerful biosignature and, in principle, can be remotely observed on planetary scales using circular polarization spectroscopy. Molecules such as photosynthetic pigments are optically active and have several chiral centers, and influence the polarization of light. This can be measured using full Stokes spectropolarimetry. The goal of this interdisciplinary project is to characterize the circular polarization spectra of chiral photosynthetic pigments in anoxygenic phototrophs and cyanobacteria as global surface biosignatures.

  ROADMAP OBJECTIVES: 4.1 5.2 7.2

• The Long Wavelength Limit of Oxygenic Photosynthesis

  Oxygenic photosynthesis (OP) produces the strongest known biosignatures at the planetary scale on Earth: atmospheric oxygen and the spectral reflectance of vegetation. The pigment chlorophyll a was long considered the unique controller of both of these biosignatures, in its capability to enable water splitting to obtain electrons and thus produce oxygen as a biogenic gas, through spectral absorbance of light from the blue to 680 nm in the red. Then the discovery in 1996 of the cyanobacterium Acaryochloris marina shattered this conventional wisdom. A. marina was found to have replaced 93-97% of Chl a with Chl d, which enables it to perform oxygenic photosynthesis with much lower energy photons in the far-red/near-infrared. Since that first discovery in 1996, more far-red oxygenic phototrophs have been discovered, revealing a previously unsuspected diversity in the photosystems of oxygenic phototrophs. We seek to determine the long wavelength limit at which OP might remain viable and what factors affect the selection of that wavelength limit. This would clarify whether and how to look for OP adapted to the light from stars with a difference radiance spectrum from our Sun.

  Under this project in previous years and with other co-investigators, we spectrally quantified the thermodynamic efficiency of photon energy use in Acaryochloris marina str. MBIC11017, determined that its water-splitting wavelength is in the range 710-723 nm, and that it is more efficient than a Chl a cyanobacterium. The current focus of the project is to understand the adaptations of far-red/near-infrared (NIR) oxygenic photosynthetic organisms in general: in which environments they are competitive against chlorophyll a organisms, and what energetic shifts have been made in their photosynthetic reactions centers to enable their use of far-red/NIR photons. We are conducting field sampling and measurements to isolate new strains of far-red-utilizing oxygenic photosynthetic organisms, to quantify the spectral and temporal light regime in which they (and previously discovered strains) live in nature, and to use these light measurements to drive kinetic models of photon energy use to determine efficiency thresholds of survival.

  ROADMAP OBJECTIVES: 3.2 4.2 5.1 5.3 6.2 7.2

• Habitable Planet Formation and Orbital Dynamical Effects on Planetary Habitability

  This task explores how habitable planets form and how their orbits evolve with time. Terrestrial planet formation involves colliding rocks in a thin gaseous disk surrounding a newborn star and VPL’s modeling efforts simulate the orbital and collisional evolution of a few to millions of small bodies to determine the composition, mass, and orbital parameters of planets that ultimately reach the habitable zone. After formation, gravitational interactions with the star and planet can induce short- and long-term changes in orbital properties that can change amount of energy available for the climate and to illuminate the planetary surface. The VPL simulates these effects in known and hypothetical planetary systems in order to determine the range of variations that permit planetary habitability.

  ROADMAP OBJECTIVES: 1.1 1.2 3.1 4.3

• Global Surface Biosignatures: Reflectance Spectra of Anoxygenic Phototrophs and Cyanobacteria

  This project investigates the spectral reflectance signature of anoxygenic photosynthetic bacteria, as an alternative to the “biosignature” of the vegetation “red-edge.” The vegetation red-edge is so called due to the sharp contrast in visible light absorbance by light harvesting pigments in plant leaves versus their high reflectance in the near-infrared (NIR). This contrast occurs around 700 nm in the red/far-red. This signature is ubiquitous among plants, which all utilize chlorophyll a. However, anoxygenic phototrophs contain a diverse array primary photopigments, including bacterichlorophylls (Bchl) a, b, d, e, and g. These Bchl pigments display different absorption maxima with peaks primarily in the NIR. There is an abundance of data on plant spectral reflectance thanks to the Earth remote sensing field. However, there is a dearth of data on anoxygenic phototrophs and cyanobacteria. For this project, we measured the reflectance spectra of pure cultures as well as environmental samples of laminated microbial mats to characterize their detectable biosignature features. This work will help inform the search for life on exoplanets at a similar stage of evolution or biogeochemical state as the pre-oxic Earth.

  ROADMAP OBJECTIVES: 4.1 5.2 7.2

• Planetary Surface and Interior Models and SuperEarths

  We use computational and theoretical models to simulate the evolution of the interior and the surface of real and hypothetical planets around other stars. Our goal is to determine the characteristics that are most likely to contribute to making a planet habitable in the long run. Observations in our own Solar System show us that water and other essential materials are continuously consumed via weathering (and other processes: e.g., subduction, sediment burial) and must be replenished from the planet’s interior via volcanic activity to maintain a biosphere. The surface models we are developing will be used to predict how gases and other materials will be trapped through weathering and biological processes over time. Our interior models are designed to predict tidal effects, heat flow, and how much and what sort of materials will come to a planet’s surface through resurfacing and volcanic activity throughout its history.

  ROADMAP OBJECTIVES: 1.1 1.2 4.1 5.2 6.1

• Stellar Effects on Planetary Habitability and the Limits of the Habitable Zone

  In this task, VPL team members studied the interaction between stellar radiation (including light) and planetary atmospheres to better understand the limits of planetary habitability and the effects of stellar radiation on planetary evolution. Work this year spanned climate modeling to atmospheric escape. We showed that multiple stable states of climate could exist for water-rich worlds, including both habitable and uninhabitable states, suggesting that water-rich planets in the habitable zone are not necessarily habitable. Atmospheric escape models were used to illustrate how the pre-main sequence evolution of M-dwarf stars could strip the gaseous envelopes from mini-Neptune planets, transforming them into potentially-habitable, Earth-sized rocky bodies. We also showed that pre-main sequence evolution could lead to strong atmospheric escape of water on otherwise habitable worlds, potentially rendering them uninhabitable. We defined the first metric to rank an exoplanet’s potential to support surface liquid water based on fundamental data from transit observations. Observational work was also undertaken to characterize the frequency and characteristics of stellar flares on M dwarf stars from Kepler data, as input to future work on characterizing the effect of stellar flares on habitability.

  ROADMAP OBJECTIVES: 1.1 1.2 3.4

• Exoplanet Detection and Characterization: Observations, Techniques and Retrieval

  In this task, VPL team members use observations and theory to better understand how to detect and characterize extrasolar planets. Techniques to improve the detection of extrasolar planets, and in particular smaller, potentially Earth-like planets are developed, along with techniques to probe the physical and chemical properties of exoplanet atmospheres. These latter techniques require analysis of spectra to best understand how it might be possible to identify whether an extrasolar planet is able to support life, or already has life on it.

  ROADMAP OBJECTIVES: 1.1 2.2 7.2

• Astronomical Biosignatures, False Positives for Life, and Implications for Future Space Telescopes

  In this task, we identify novel biosignatures and also identify “false positives” for life, which are ways for non-biological processes to mimic proposed biosignatures. Of primary concern are false positives that could mimic easier to detect biosignatures like O₂, which we plan to search for with future space-based telescopes. This is a growing area of research that VPL’s past work has motivated, leading to multiple research teams across the planet following our example. Our work continues to be at the forefront of this area of work, as we have identified new non-biological mechanisms for mimicking signs of life. Further, we explained the ways in which these non-biological mechanisms could be identified, and “true positives” from biology confirmed with secondary measurements. Finally, we communicated these lessons to various teams that are studying concepts for future missions that would search for these signs of life. This connection to missions will ensure that our research is incorporated into those missions, so that they will not be “tricked” by these false positives.

  ROADMAP OBJECTIVES: 1.1 1.2 4.1 4.3 5.2 5.3 7.2

• Jon Toner NAI NPP Postdoc Report

  Aqueous salt solutions are critical for understanding the potential for liquid water to form on icy worlds and the presence of liquid water in the past. Salty solutions can form potentially habitable environments by depressing the freezing point of water down to temperatures typical of Mars’ surface or the interiors of Europa or Enceladus. We are investigating such low-temperature aqueous environments by experimentally measuring the low temperature properties of salt solutions and developing thermodynamic models to predict salt precipitation sequences during either freezing or evaporation. These models, and the experimental data we are generating, are being applied to understand the conditions under which water can form, the properties of that water, and what crystalline salts indicate about environmental conditions such as pH, temperature, pressure, and salinity.

  ROADMAP OBJECTIVES: 2.1 5.2 5.3