nai

Executive Summary

The broad spectrum of physical and life sciences comprising the field of astrobiology is reflected in the research activities of the UCLA lead team in its fifth year of participation in the NASA Astrobiology Institute (NAI). Astrobiological research conducted by the UCLA lead team ranges from devising methods for imaging planets outside of our Solar System to studies of the evolution of life at the scale of individual genes. These seemingly disparate activities comprise a chain of informed inquiry that seeks to define the range of possible habitable environments in our Solar System and beyond and the manifestations of life that might arise in these diverse habitats.

During the fifth year of UCLA’s involvement in the NAI, progress was made on six projects dedicated to improving our understanding of planetary habitats and the evolution and diversity of life. These projects are: 1) Extrasolar planetary systems; 2) Origins of habitable environments in the solar system; 3) Celestial influences on planetary environments; 4) Characterization of Earth’s early life; 5) Geobiology and the geochemistry of early Earth; and 6) Genomic evolution and the tree of life. References listed in this report are grouped according to these project titles.

The six projects include a significant interdisciplinary component. Examples of interdisciplinary research at UCLA in year five are: collaboration between astronomers, atmospheric chemists, and cosmochemists leading to new insights into the way that planets capable of sustaining life evolve from gas and dust around stars; the bringing together of paleontologists, atmospheric chemists, and isotope geochemists to study Earth’s ancient atmosphere; application of advanced spectroscopy methods to characterize the most ancient vestiges of life on Earth; and the integration of fossil morphology and genetics to understand the evolution of complex forms of life beginning more than 550 million years before present.

Education and public outreach activities of the UCLA lead team during the 2002/2003 year included public lectures by team members in the Los Angeles area, publication of the proceedings of the Rubey Colloquium on impacts and the origin and evolution of life, continuation of the highly successful general education course entitled “Origin of the Cosmos and Life,” sponsorship of the student-run Astrobiology Society and their activities, including hosting of a public symposium held this Spring on the UCLA campus featuring well-known speakers from the scientific community. The speakers articulated their views of astrobiology in an entertaining and accessible format. In addition to these education and public outreach activities, members of the UCLA team participated in several NAI Focus Groups and were active in efforts to establish a new Astronomy Focus Group during 2002/2003.

Extrasolar Planetary Systems
Astronomy research with consequences for astrobiology is beginning to accelerate at UCLA. Research in this area includes searching for stars that might harbor planets suitable for imaging and characterization of the composition and distribution of dust related to planets or planet formation surrounding other stars. The astronomical studies of the UCLA lead team involved observing sessions at Las Campanas Observatory in La Serena, South America, the Siding Spring Observatory in New South Whales, Australia, the W.M. Keck Observatories in Hawaii, and the Lick Observatory in California. Collaborations continue with astronomers at the Carnegie Institution of Washington, among others.

Members of the UCLA team added to the list of stars that are prime targets for attempts to image directly extrasolar planets using photons (Figure 1). Only when planets are imaged directly will it be possible to measure their spectra and determine their compositions. Advances in astronomy from the ground, including adaptive optics and the infrared capabilities of the Hubble Space Telescope, improve prospects for imaging of planets with masses comparable to that of Jupiter. But such detections must be of thermal emissions from young, warm planets rather than reflected starlight that illuminates cold, older planets like Jupiter. The targets for imaging planets must therefore be young stars. Stars that are young and close to our own star offer the greatest prospects for imaging orbiting planets and need to be identified. The accomplishments of this year’s search raise the total of such “hot prospects” to more than 200 stars.

Figure 1. Map showing several recently discovered groups of newborn stars that lie near Earth (large blue ball) and are “hot prospects” for imaging extrasolar planets. In this view from the northern sky, two circles in the equatorial plane have radii of 165 and 300 light years, respectively. Red dots depict the Tucana/Horologium group and blue indicates the beta Pictoris moving group. Gray represents the Pleaides cluster.

Astronomical work continued on the detection and characterization of dust surrounding main sequence stars. In one instance, a ring of dust similar to that found around Saturn was detected around a white dwarf star, suggesting that asteroids existed there. In another case, it was observed that silicate dust (terrestrial-like planets are composed largely of silicate) is found 5 to 10 AU from the star Beta-Pic but not at larger distances, suggesting rocky bodies surrounding the star, if they exist, reside at circumstellar radii not too different from the planets of our Solar System.

Team members made progress in adapting NASA’s flying observatory mission, Stratospheric Observatory for Infrared Astronomy (SOFIA), to include instrumentation relevant to astrobiological research. The fruits of these efforts will be the ability to detect dust composed of organic compounds.

Understanding the factors that influenced the prospects for life in our Solar System is essential for assessing the likelihood for life in extrasolar planetary systems. Calculations by members of the UCLA lead team reported this year describe impacts between bolides (asteroids, comets) and Earth-like planets, suggesting that rather than blowing away nascent atmospheres, bombardment by small planetesimals may actually have contributed to the volatile reservoirs of rocky planets. This means that while impacts of asteroids or comets may be responsible for frustrating the evolution of life to complex forms, the most well-known example being the mass extinction at the end of the Cretaceous period on Earth, impacts may also be at least partly responsible for creating life-sustaining atmospheres and hydrospheres similar to those of the Earth.

Collaborations between UCLA’s cosmochemists, astronomers, and astrobiology researchers resulted in new studies of the chemical evolution of protoplanetary systems. Regardless of whether Earth’s complement of volatiles are of exogenous or indigenous origin, mechanisms for mixing dust and water ice in the protoplanetary nebula that surrounded our nascent Sun was clearly an important process leading to habitable environments in the Solar System. Investigations were begun this year into the role that water played in establishing the chemical and isotopic characteristics of primitive rock materials accessible today in the form of meteorites. Results suggest that heterogeneous reactions involving dust grain surfaces, oxygen, and water may have had a significant influence on the chemical and isotopic evolution of planet precursors in the solar nebula. Photochemistry is a significant component of this complicated chemical network that may also include organic compounds.

Celestial Influences on Planetary Environments
The long-term consequences of planetary and satellite orbits for habitability was a central theme for several theoretical studies here at UCLA in 2002/2003. These studies reveal possible connections between bolide impacts and chaos in the motions of the inner planets of the Solar System, suggesting that the extent to which life is permitted to evolve may depend on the stability of orbital systems. As if to underscore the point of these theoretical studies, this year the UCLA team reported new geological evidence for the influence of Solar System dynamics on Earth history. They found that rock layers from South Africa laid down 3.2 billion years ago contain significant quantities of asteroid material. This large amount of extraterrestrial material, representing three giant impacts that occurred over an interval of only 20 million years, testifies to a ten-fold enhancement in the flux of asteroidal material to Earth relative to the present day.

Orbital dynamics calculations were also used to predict which of the extrasolar planetary systems discovered to date, each containing at least one Jupiter-like giant planet, could also harbor terrestrial planets in stable orbits. Other calculations carried out by our team shed new light on the ways that Jupiter’s Galilean moons respond to gravitational stresses that are compatible with stable rotational states. In addition, equilibrium models of conductive and convective heat transfer published by members of our team this year show that the Galilean moons Europa, Ganymede, and Callisto may all have subsurface liquid water oceans. These studies indicate that although Europa is the most likely to contain liquid water, a warm layer may exist beneath a stagnant lid of ice on both Ganymede and Callisto due to the temperature dependence of the viscosity of water.

Characterization of Earth’s Early Life
The antiquity of life on Earth remains controversial. Doubts about the veracity of examples of extremely ancient life on Earth are a significant impediment to astrobiology (if we can’t find it here, how can we find it out there) and have stimulated the UCLA research program. Studies seeking to establish or refute evidence for life prior to ~3 billion years ago were buoyed this year by development of new techniques for characterizing what appear to be 3.4 billion year old microscopic fossil organisms. With these techniques, researchers at UCLA are examining these purported microfossils in order to apply and refine criteria for biogenicity. Among the new capabilities brought to bear on this problem this year is the ability to image fossil cellular structures at the nanometer scale of observation and spectroscopic methods that quantify the state of preservation of ancient kerogen.

Geobiology and the Geochemistry of Early Earth
Geochemical and biogeochemical cycles on ancient Earth were investigated through several fruitful collaborations between investigators from other NAI lead teams and the UCLA team. The pivotal tool for this area of research was the ion microprobe at UCLA.

Measurements of sulfur isotope ratios in sulfide inclusions in diamonds from a Kimberlite pipe from Botswana were obtained this year. The isotope ratios exhibit a non-mass dependent fractionation that is a signature of atmospheric processing. These exciting new data demonstrate that sulfur that resided in Earth’s primitive anoxic atmosphere during the Archean found its way to the depths of Earth’s mantle where it ultimately was incorporated into sulfide minerals only to be returned to the surface again by the violent explosion that formed the Kimberlite structure. The sulfur isotope ratios thus provide a long-term tracer of mass movement between the atmosphere and the lithosphere that will have profound consequences for our understanding of the evolution of the sulfur geochemical cycles of the primitive Earth. The sulfur cycle is in turn connected with the development of the biosphere.

Figure 2. Plot of Δ³³S and δ³⁴S of sulfide inclusions from individual diamonds from the Orapa kimberlite pipe (red points). The shaded region centered around Δ³³S = 0 represents the mass-dependent field. Four points are distinguished from this field and constitute evidence for S cycling between the atmosphere and the deep mantle of the Earth. Data and plot from Farquhar et al. (2002).

More sulfur isotope data were obtained that show unequivocally that the non-mass dependent isotopic signature that appears to be a marker for an O₂-free atmosphere is preserved in sulfur bearing minerals greater than 2 billion years old. These new data lay to rest the controversy about whether or not the non-mass dependent isotope effects observed in ancient rocks are artifacts of the analytical technique; the ion probe data collected at UCLA are not subject to the same analytical uncertainties associated with the vacuum line methods used previously.

UCLA began an international collaboration (in particular with workers at the Australian National University) to investigate Earth’s earliest surviving minerals during this report period. To date, the UCLA laboratory has been used to identify numerous zircon grains with ages that exceed 4 billion years. The grains, collected from Jack Hills, Western Australia, afford opportunities to investigate the age of Earth’s atmosphere and subaerial weathering on the planet. These studies will help to characterize the global habitats that gave refuge to emergent life on Earth.

Like today, primitive organisms likely affected their surroundings and vice versa on primitive Earth. A cross-team collaboration with colleagues at Penn State was begun in year 5 in which mechanisms of element exchange between the biological and inorganic chemical realms on ancient Earth were investigated. In the experiments conducted this year, conversion of olivine, a major constituent of the primitive crust of our planet, to the hydrous mineral serpentine was performed with and without microbes present. Preliminary results suggest that the microbes have a profound effect on these heterogeneous mineral-water reactions. As part of this project, our team has constructed a new, state-of-the-art laboratory for measuring the isotope ratios of Mg, Fe, and other “metals” that are useful tracers of physicochemical processes mediated by microorganisms.

Genomic Evolution and the Tree of Life
Work continued on the role that horizontal gene transfer may have played in accelerating the evolution of prokaryotic life on Earth. Analyses of various taxa demonstrate that the process of horizontal gene transfer operates between organisms occupying similar environments.

This year also saw publication of the transorientation hypothesis proposed by members of the UCLA team. This hypothesis presents a mechanistic, structural model for decoding and proofreading during protein synthesis. The mechanism described by the transorientation hypothesis is dominated by transfer ribonucleic acid (tRNA) conformational changes as well as tRNA-messenger RNA (mRNA) interactions, and points to these molecules as progenitors of protein synthesis in a pre-existing RNA world.

Explorations of the origin and evolution of eukaryotic energy-generating organelles also progressed during this fifth year of our program. The most recent sequencing and phylogenetic analyses appear to be at odds with the hydrogen hypothesis for the origin of eukaryotes which holds that eukaryotes arose from an anaerobic, hydrogen-dependent archaebacterium host and a eubacterium symbiont that produced molecular hydrogen as a waste product.

Developmental genetics during the Cambrian explosion of evolutionary change was studied with an emphasis on the development of sensory structures. A key aspect of the work in year 5 has been the acquisition of sequence data for genes that may produce identifiable morphological expression in the Cambrian fossil record.