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Executive Summary

The Ames Team pursued complementary lines of research to understand the context for habitable environments and life, the origins of life and its impact on the planetary environment, and how changing environments can affect ecosystems. These investigations address all seven goals of NASA’s current Astrobiology Roadmap, and they help to unify astrobiology and strengthen its linkages to flight missions and NASA’s new vision for exploration. Our direct involvement in several NASA missions provides context, motivation, and resource-sharing opportunities for our research, education and public outreach efforts. The Ames Team website highlights these activities (see www.amesteam.arc.nasa.gov).

We investigated the processes that influence the formation of planetary systems and the evolution of planetary atmospheres. We examined photo-evaporation effects, which can clear disks and influence the probability that habitable planets might form around a star of a given mass (Figure 1). We used our numerical model to analyze the observations of TW Hya, which is the nearest solar type star in the process of planet formation. Photo-evaporation has dispersed the gas from ~100 AU to 200 AU from the star. Gas will soon be dispersed within 100 AU, truncating gas giant planet formation and affecting any terrestrial planet formation. Our model, together with Spitzer Space Telescope observations, indicates that only a small fraction of a Jupiter mass of gas remains in a sample of stars with ages between 5 to 50 Myr. We determined those stellar masses having the longest-lived disks and therefore the highest probability of forming planets. Our models also predicted emission lines from ionized species created by UV photons from the central star. Singly ionized neon creates one of the strongest lines at 12.8 microns; the Spitzer Space Telescope has now detected this line. Such emissions provide the most sensitive means to detect gas in stellar planet-forming regions.

Figure 1. Planets form from gas and dust orbiting stars. Photoevaporation caused by the heating of the gas by X-ray and UV photons, either from an external star (a) or from the central star, (b) can remove the gas and small dust particles so quickly that it can sometimes prevent or stunt the growth of planets. Luminous high mass stars lose their gas and dust very quickly©, hindering planet formation. On the other hand, low mass stars have less gas and dust to start with. Stars with masses similar to the Sun have favored conditions for the formation of habitable planets.

We published a new condensation model that estimates the relative distributions of gas and ice in protoplanetary disks. The steepest abundance gradients occur near sublimation boundaries (“the snow line”) along the disk radius. Species abundances can differ substantially between the inner high-density planet forming regions and the lower density photospheric regions. The model predicts water vapor and ice clouds at specific disk locations. Observations of such features will help to assess whether protoplanetary disks might develop habitable environments and life. We modeled the chemical evolution of protoplanetary disks and demonstrated that only a small subset of reactions need to be modeled in order to describe the evolution of abundances of most species.

We published a study of planetary growth within binary systems where the stellar periapse distances were 5 – 10 AU. Sufficiently wide binaries leave the planet formation process largely unaffected. Binary stars with periastron qB > 10 AU have a minimal effect on terrestrial planet formation within ~ 2 AU of the primary star, whereas binary stars with qB < ~5 AU restrict terrestrial planet formation to within ~ 1 AU of the primary star. Given the observed distribution of binary orbital elements for solar-type primaries, about 40% to 50% of the binary population is sufficiently separated to allow terrestrial planet formation to take place unimpeded.
We developed a hydrodynamic simulation code that tracks the surface flow patterns on extrasolar planets under conditions of non-synchronous planetary rotation. We published results for several well-observed hot-Jupiter type planets. We will model the atmospheric dynamics of the newly discovered worlds such as Gl 581 c, which lie near the boundary of potential habitability.

We continued to develop the Systemic Console, a software package that analyzes the dynamics of planetary systems. We added functionality, including long-term stability analysis, F-test analyses of competing fits, orbital waveform sonification (for aural evaluation of stability properties), interactive system plotting, Levenberg-Marquardt fit polishing, and bootstrap-method calculation of uncertainties in the orbital elements of fits. This package is the world’s most advanced tool for extracting planetary systems from radial velocity data.

We modeled the aftermath of the Moon-forming impact on Earth. Water oceans condensed from the steam after 2 Myr but the surface would have stayed warm (~500 K) for some 10 to 100 Myrs. Thereafter a lifeless Earth would have evolved into a bitterly cold ice world. Volcanic- and impact-induced thaws episodically interrupted this cooling trend.
We approached two major milestones in the IR spectroscopy of polycyclic aromatic hydrocarbons (PAHs). We are making a database of our world-unique collection of PAH mid-IR spectra under deep space conditions. This is the spectral collection upon which the interstellar PAH model is based. The database/web user interface will be beta-tested in 2007. We published our study of unique, closed-shell, charged PAH clusters. Our paper addressing the IR properties of PAH ions in water ice analogs of interstellar and cometary ices has been accepted for publication.
We published a paper on amino acid production in cosmic ice analogs, with samples analyzed at Goddard, one of our NAI cross-team collaborations. We published our findings that doubly charged large PAH ions can be produced by low energy photons irradiating water ice. We published our study of the biogenic compounds produced by the vacuum ultraviolet photolysis of anthracene in water ice and the connection of these photoproducts with meteoritic organic compounds.

One of us is the leader of the Stardust Mission’s Organics Preliminary Examination Team and has been intimately involved with sample extraction, distribution, and analysis of the returned samples. The team discovered and published that cometary organics are much richer in oxygen and nitrogen than meteorites.

We continue to investigate the evolutionary origins of functional macromolecules using both experimental and computational approaches. We conducted the first laboratory evolution of a new non-biological enzyme derived from a partially randomized non-catalytic scaffold protein (Figure 2). This enzyme joins two fragments of RNA into a single strand. The amino acid substitutions that occurred during in vitro evolution of this well-structured protein indicate that its initial structure underwent at least local refolding. The results demonstrate that novel functions, and possibly different structures, can be obtained through a limited number of mutations in sequences of small proteins that might serve as models for ancestral macromolecules. Nature has accepted this work for publication.

Figure 2. Sequences of the starting library and selected ligases (from Seelig and Szostak, Nature, 2007, in press). Loop regions are highlighted in light blue. The cysteines highlighted in orange constitute the two pairs of CXnC (n = 2 or 5) motifs that coordinate zinc ions in the original hRXR domain. Randomized amino acids in the library are shown as x. Dashes indicate amino acids that are the same as in the starting library, whereas periods highlighted in grey symbolize deletions.

We continue to explore the evolutionary optimization of our previously evolved ATP-binding protein. Surface residue interactions play an unexpectedly strong role of in stabilizing this small protein. The results help to elucidate pathways by which primordial protein sequences attain increased degrees of functionality through the systematic accumulation of point mutations. This work led to two published papers.
To understand how primordial proteins might have transported ions across cell walls, we studied the antiamoebin channel using molecular dynamics computer simulations (Figure 3). This simple channel consists of 8 identical helices (each 16 amino acids long) and achieves efficiency comparable to that of a highly evolved voltage-gated potassium channel. We propose that channels evolved further towards high structural complexity due to evolutionary selection for precise regulation rather than for improved efficiency. Further, the amino acid sequence and function of membrane proteins depend upon the nature of membrane-forming material, indicating that channels and membranes might have co-evolved.

Figure 3. The cross-section of the antiamoebin channel spanning the phospholipids bilayer (dark green and gray) surrounded by 0.5 M aqueous solution of KCl. Potassium ions are yellow and chloride ions are blue. Note two potassium ions inside the channel.

Continuing our studies of cyanobacterial mat ecosystems, we addressed the role played by sulfate as (i) a key arbiter of the distribution and of photosynthetic energy across several possible biosignature classes (e.g., volatile vs. solid phase) and (ii) a critical influence on mechanisms of organic preservation in sulfate evaporite systems analogous to sulfate-rich evaporites on Mars. We imaged at micron resolution the lateral and vertical distributions of 34S/32S (an indicator of biological sulfur cycling and an important biosignature for interpretation of Earth’s rock record) in the surface layer of an actively photosynthesizing microbial mat. We developed a technique to link genetic identity to consumption of isotopically labeled substrates through micron-scale imagery. We will combine these imaging techniques with classical geochemical approaches to characterize extensively the oxidative sulfur cycle in microbial mats subjected to >1-year incubation under reduced sulfate levels that simulate Archean seawater. We documented the occurrence of approximately seven sulfate evaporite mineral microfacies that apparently are uniquely associated with biological mediators, based on lipid and microbial diversity assays conducted in parallel with the microfacies analysis.

As part of our studies of chemosynthetic microbial ecosystems, we published a conceptual framework to quantify and link the physical and chemical constraints on habitability through their common effect on biological energy demand (Figure 4). We also documented the occurrence of finely-laminated dolomite (high-Mg carbonate) cements in the outflow of springs that are influenced by serpentinizing host-rocks, and we are investigating potential biological features and effects.

Figure 4. From “Classical” to “Energetic” Habitability. In the classical view, habitability occurs at the conjunction of factors that support the creation, maintenance, and function of complexity (with parenthetical terms indicating the specific conditions required by life on Earth). We are working to quantify habitability by considering the balance between the biological demand for energy (as needed to support a specified complexity under a specified set of physico-chemical and resource availability conditions) and the corresponding potential for biological transduction of available energy from external sources. The goal of this work is to refine and quantify our view of habitability in order to enable prioritization of astrobiological search targets based on the perceived “degree” of present or past habitability.

We refined our numerical model to link microbial mats to global biosphere processes in order to consider the migration of live microbial cells upward as dead cells and other organic matter pools were created. We also began to model photoexcretion by mat cyanobacteria. We assumed that, if O2/CO2 abundance ratios in mat porewaters within a given layer exceed unity, photorespiration then produces glycolate, a common excretion product. Under such conditions about 12% of net photosynthesis enters the exudates pool, which sulfate-reducing bacteria and methanogens can utilize.

We assess the potential for life to move beyond its planet of origin and to survive in potentially lethal radiation environments. We conducted fieldwork in the Bolivian Andes at altitudes (~15,000 feet) where the ozone column is substantially reduced and UV radiation levels are high. We discovered previously undescribed organisms that might be highly radiation resistant. We found several microbes that are highly resistant to desiccation and UV radiation. We visited new field sites in the Andes, the radioactive Paralana Springs in central Australia, and Lassen Volcanic Park, California. We developed a high-throughput assay to detect both direct and indirect DNA damage, such as that caused by radiation. To prepare for ESA’s EXPOSE flight, we conducted two experiment verification tests at the ground simulation facility at DLR.
We are refining our model to assess biogeochemical cycles of South American land ecosystems in the past. We are cross-referencing past ecosystem predictions with proxy data for volcanic, orbital and solar forcings of Holocene climate. We submitted several manuscripts for publication: 1) D’Antoni H. and L. Rothschild. 2007. Measuring Ultraviolet Radiation in South America. (in revision). 2) D’Antoni, H.L., L. Rothschild, C. Schultz, S. Burgess, J.W. Skiles. 2007. Extreme Environments in the Forests of Ushuaia, Argentina. (in review). 3) Burry, Trivi, and D’Antoni. 2007. Pollen study of central Tierra del Fuego during the last three millennia (in revision). 4) Trivi, D’Antoni and Romero. 2007. History of vegetation research in Patagonia (in review). 5) Burry, D’Antoni and Frangi. 2007. History of vegetation research in Tierra del Fuego (in review).

Our education and public outreach efforts focused on our partnerships with California Academy of Sciences (CAS) and Yellowstone National Park (YNP), but we also pursued other key activities. Our chapter on thermophiles and astrobiology appeared in the 2007 edition of the Yellowstone Resources and Issues guide. Eight permanent “way side” (trail) signs that interpret microorganisms and astrobiology were installed in YNP in 2006 and have been well received by park visitors. A new exhibit on astrobiology premiered at the California Academy of Sciences; Ames team members participated in the exhibit’s design and review. We supported a Lunar Planetary Institute-organized workshop that provided teachers with astrobiology-related field and classroom experiences. We worked with TERC Corporation to create new astrobiology products. Several team members gave numerous talks in classrooms and informal public venues. The Ames team maintained its website (www.amesteam.arc.nasa.gov), which features research accomplishments and education and public outreach.

Figure 5. Yellowstone National Park Wayside Sign Exhibit.

We maintained our substantial presence in current and upcoming NASA missions. The P.I. is a strategic planning lead for science operations by the Mars Exploration Rover mission. A team Co-I is also P.I. of the CheMin XRD spectrometer for the Mars Science Laboratory mission. Another team member serves as a Co-I with NASA’s Kepler mission. One of us is a Co-I on the Stardust Mission. Several team members contribute to the SOFIA mission and others are involved with missions that are either scheduled or planned.