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Open Access Publications from the University of California

Scholarly Works (33 results)

  • Article
  • Peer Reviewed
UC Riverside Previously Published Works (2018)
Thousands of planets orbiting stars other than the Sun (exoplanets) have been discovered, and the pace of discoveries will only accelerate as new observing missions are deployed. As the state of the field moves from exoplanet detection to characterization we are inching ever closer to indepth determination of the properties of rocky planets within the so-called 'habitable zones' of their host stars. The question now becomes-how would we recognize the signs of habitability and life on a distant exoplanet? We must begin with the only known example of a habitable and inhabited world-our own. But Earth affords more than one glimpse into conditions on a lifebearing world. Throughout geologic time the prevailing atmospheric and chemical state of our planet has undergone titanic shifts including from a hazy, orange, oxygen-free, methane-choked global habitat to the oxygen-rich pale blue dot we now take for granted. Here, we discuss ongoing efforts by astrobiologists and astronomers to catalogue the potential signatures of habitability and life we may find elsewhere in the universe by using Earth-both present and past-as a laboratory for the possible signatures of inhabited exoplanets.
    Cover page: Signs of life on a global scale: Earth as a laboratory for exoplanet biosignaturesCreative Commons 'BY-NC-ND' version 4.0 license
    • Thesis
    • Peer Reviewed
    UC Riverside Electronic Theses and Dissertations (2012)

    The evolution of oxygenic photosynthesis has dramatically reshaped the chemistry of the surface Earth, and the presence of significant quantities of O2 in the atmosphere and ocean now drives the fundamental dynamics of nearly all quantitatively significant biogeochemocal cycles (C, S, P, N, Fe). Whether by direct consumption through the metabolic demands of large, complex organisms, or through the recycling of essential substrates within microbial ecosystems, biologically produced O2 provides nearly all of the compounds used in metabolic electron transfer on a global scale. Although it is widely accepted that the partial pressure of O2 in Earth's atmosphere has increased through time (with attendant, although somewhat complex, changes in ocean ventilation), there is still much debate surrounding the timing of the emergence of oxygenic photosynthesis and little is known about the detailed tempo and mode with which this metabolic innovation came to shape early Earth surface chemistry.

    This dissertation explores the early oxygenation of Earth's atmosphere and the relationship between atmospheric oxygen levels and ocean ventilation from a variety of perspectives. First, empirical data based on an integrated suite of paleoredox proxies is used to suggest that biological oxygen production emerged and began exerting significant effects on Earth surface chemistry at least 100 million years prior to the initial accumulation of large quantities of O2 in the atmosphere (often referred to as the "Great Oxidation Event", approximately 2.4 billion years ago). The implications of this time lag between metabolic innovation and large-scale biogeochemical reorganization are explored through a series of quantitative models, focusing on the thermodynamics and kinetics of mineral reactions under various Earth surface conditions, regional oceanographic modeling of surface ocean O2 cycling, and a global sulfur isotope mass balance model that explicitly incorporates rare sulfur isotope systematics (33S, (36S) and the dynamics of sedimentary recycling on long timescales. Finally, the dynamics of ocean ventilation following the initial accumulation of oxygen in the atmosphere are explored by combining a large trace metal database with a spatially explicit mass balance model that exploits the differing redox behavior and surface cycling of molybdenum (Mo) and chromium (Cr).

      Cover page: Exploring the Texture of Ocean-Atmosphere Redox Evolution on the Early Earth
      • Thesis
      • Peer Reviewed
      UC Riverside Electronic Theses and Dissertations (2018)

      The rise of oxygen in Earth's atmosphere from negligible levels on the earliest Earth to the high levels that we enjoy today is the consequence of oxygenic photosynthesis and it is the most profound impact that life has had on Earth's surface environment. Yet details of this transition remain poorly constrained. This dissertation examines the cause-and-effect relationships between Earth's biosphere, atmosphere, and climate system with a particular focus on oxygen and its many impacts. Topics include: (1) the relationship between oxygen production and oxygen accumulation on early Earth, (2) the mechanisms that regulate oxygen levels in Earth's atmosphere, (3) the role of oxygen in early animal ecosystems, and (4) the interplay between oxygen and trace greenhouse gases. Understanding the relationships between life and its environment is essential to understanding not only the requirements and emergence of complex life on Earth but on other planets as well---and for ultimately recognizing the chemical fingerprints of life in the atmospheres of other inhabited worlds.

        Cover page: Earth's Oxygenation: Causes, Consequences, and Implications for Exoplanet Life Detection
        • Article
        UC Riverside Previously Published Works (2018)
        The strongest remotely detectable signature of life on our planet today is the photosynthetically produced oxygen (O2) in our atmosphere. However, recent studies of Earth's geochemical proxy record suggest that for all but the last ~500 million years, atmospheric O2 would have been undetectable to a remote observer--and thus a potential false negative for life. During an extended period in Earth's middle history (2.0 - 0.7 billion years ago, Ga), O2 was likely present but in low concentrations, with pO2 estimates of ~0.1 - 1% of present day levels. Although O2 has a weak spectral impact at these low abundances, O3 in photochemical equilibrium with that O2 would produce notable spectral features in the UV Hartley-Huggins band (~0.25 um), with a weaker impact in the mid-IR band near 9.7 um. Thus, taking Earth history as an informative example, there likely exists a category of exoplanets for which conventional biosignatures can only be identified in the UV. In this paper, we emphasize the importance of UV capabilities in the design of future space-based direct imaging telescopes such as HabEx or LUVOIR to detect O3 on planets with intermediate oxygenation states. We also discuss strategies for mitigating against 'false positives'--that is, O3 produced by abiotic processes. More generally, this specific example highlights the broad implications of studying Earth history as a window into understanding potential exoplanet biosignatures.
          Cover page: The Importance of UV Capabilities for Identifying Inhabited Exoplanets with Next Generation Space Telescopes
          • Article
          • Peer Reviewed
          UC Riverside Previously Published Works (2017)
          Ocean-atmosphere chemistry on Earth has undergone dramatic evolutionary changes throughout its long history, with potentially significant ramifications for the emergence and long-term stability of atmospheric biosignatures. Though a great deal of work has centered on refining our understanding of false positives for remote life detection, much less attention has been paid to the possibility of false negatives, that is, cryptic biospheres that are widespread and active on a planet's surface but are ultimately undetectable or difficult to detect in the composition of a planet's atmosphere. Here, we summarize recent developments from geochemical proxy records and Earth system models that provide insight into the long-term evolution of the most readily detectable potential biosignature gases on Earth-oxygen (O2), ozone (O3), and methane (CH4). We suggest that the canonical O2-CH4 disequilibrium biosignature would perhaps have been challenging to detect remotely during Earth's ∼4.5-billion-year history and that in general atmospheric O2/O3 levels have been a poor proxy for the presence of Earth's biosphere for all but the last ∼500 million years. We further suggest that detecting atmospheric CH4 would have been problematic for most of the last ∼2.5 billion years of Earth's history. More broadly, we stress that internal oceanic recycling of biosignature gases will often render surface biospheres on ocean-bearing silicate worlds cryptic, with the implication that the planets most conducive to the development and maintenance of a pervasive biosphere will often be challenging to characterize via conventional atmospheric biosignatures. Key Words: Biosignatures-Oxygen-Methane-Ozone-Exoplanets-Planetary habitability. Astrobiology 17, 287-297.
            Cover page: False Negatives for Remote Life Detection on Ocean-Bearing Planets: Lessons from the Early Earth.Creative Commons 'BY-NC' version 4.0 license
            • Article
            • Peer Reviewed
            UC Riverside Previously Published Works (2024)
            Abstract: JWST has created a new era of terrestrial exoplanet atmospheric characterization, and with it, the possibility to detect potential biosignature gases like CH4. Our interpretation of exoplanet atmospheric spectra, and the veracity of these interpretations, will be limited by our understanding of atmospheric processes and the accuracy of input modeling data. Molecular cross sections are essential inputs to these models. The photochemistry of temperate planets depends on photolysis reactions whose rates are governed by the dissociation cross sections of key molecules. H2O is one such molecule; the photolysis of H2O produces OH, a highly reactive and efficient sink for atmospheric trace gases. We investigate the photochemical effects of improved H2O cross sections on anoxic terrestrial planets as a function of host star spectral type and CH4 surface flux. Our results show that updated H2O cross sections, extended to wavelengths >200 nm, substantially impact the predicted abundances of trace gases destroyed by OH. The differences for anoxic terrestrial planets orbiting Sun-like host stars are greatest, showing changes of up to 3 orders of magnitude in surface CO levels, and over an order of magnitude in surface CH4 levels. These differences lead to observable changes in simulated planetary spectra, especially important in the context of future direct-imaging missions. In contrast, the atmospheres of planets orbiting M-dwarf stars are substantially less affected. Our results demonstrate a pressing need for refined dissociation cross-section data for H2O, where uncertainties remain, and other key molecules, especially at mid-UV wavelengths >200 nm.
              Cover page: The Impact of Extended H2O Cross Sections on Temperate Anoxic Planet Atmospheres: Implications for Spectral Characterization of Habitable WorldsCreative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UC Riverside Previously Published Works (2019)
              Abstract The habitable zone (HZ) is commonly defined as the range of distances from a host star within which liquid water, a key requirement for life, may exist on a planet’s surface. Substantially more CO2 than present in Earth’s modern atmosphere is required to maintain clement temperatures for most of the HZ, with several bars required at the outer edge. However, most complex aerobic life on Earth is limited by CO2 concentrations of just fractions of a bar. At the same time, most exoplanets in the traditional HZ reside in proximity to M dwarfs, which are more numerous than Sun-like G dwarfs but are predicted to promote greater abundances of gases that can be toxic in the atmospheres of orbiting planets, such as carbon monoxide (CO). Here we show that the HZ for complex aerobic life is likely limited relative to that for microbial life. We use a 1D radiative-convective climate and photochemical models to circumscribe a Habitable Zone for Complex Life (HZCL) based on known toxicity limits for a range of organisms as a proof of concept. We find that for CO2 tolerances of 0.01, 0.1, and 1 bar, the HZCL is only 21%, 32%, and 50% as wide as the conventional HZ for a Sun-like star, and that CO concentrations may limit some complex life throughout the entire HZ of the coolest M dwarfs. These results cast new light on the likely distribution of complex life in the universe and have important ramifications for the search for exoplanet biosignatures and technosignatures.
                Cover page: A Limited Habitable Zone for Complex Life
                • Article
                • Peer Reviewed
                UC Riverside Previously Published Works (2017)
                Ocean-atmosphere chemistry on Earth has undergone dramatic evolutionary changes throughout its long history, with potentially significant ramifications for the emergence and long-term stability of atmospheric biosignatures. Though a great deal of work has centered on refining our understanding of false positives for remote life detection, much less attention has been paid to the possibility of false negatives, that is, cryptic biospheres that are widespread and active on a planet's surface but are ultimately undetectable or difficult to detect in the composition of a planet's atmosphere. Here, we summarize recent developments from geochemical proxy records and Earth system models that provide insight into the long-term evolution of the most readily detectable potential biosignature gases on Earth-oxygen (O2), ozone (O3), and methane (CH4). We suggest that the canonical O2-CH4 disequilibrium biosignature would perhaps have been challenging to detect remotely during Earth's ∼4.5-billion-year history and that in general atmospheric O2/O3 levels have been a poor proxy for the presence of Earth's biosphere for all but the last ∼500 million years. We further suggest that detecting atmospheric CH4 would have been problematic for most of the last ∼2.5 billion years of Earth's history. More broadly, we stress that internal oceanic recycling of biosignature gases will often render surface biospheres on ocean-bearing silicate worlds cryptic, with the implication that the planets most conducive to the development and maintenance of a pervasive biosphere will often be challenging to characterize via conventional atmospheric biosignatures. Key Words: Biosignatures-Oxygen-Methane-Ozone-Exoplanets-Planetary habitability. Astrobiology 17, 287-297.
                  Cover page: False Negatives for Remote Life Detection on Ocean-Bearing Planets: Lessons from the Early Earth
                  • Peer Reviewed
                  UC Riverside Previously Published Works (2018)
                  Our present-day atmosphere is often used as an analog for potentially habitable exoplanets, but Earth's atmosphere has changed dramatically throughout its 4.5 billion year history. For example, molecular oxygen is abundant in the atmosphere today but was absent on the early Earth. Meanwhile, the physical and chemical evolution of Earth's atmosphere has also resulted in major swings in surface temperature, at times resulting in extreme glaciation or warm greenhouse climates. Despite this dynamic and occasionally dramatic history, the Earth has been persistently habitable--and, in fact, inhabited--for roughly 4 billion years. Understanding Earth's momentous changes and its enduring habitability is essential as a guide to the diversity of habitable planetary environments that may exist beyond our solar system and for ultimately recognizing spectroscopic fingerprints of life elsewhere in the Universe. Here, we review long-term trends in the composition of Earth's atmosphere as it relates to both planetary habitability and inhabitation. We focus on gases that may serve as habitability markers (CO2, N2) or biosignatures (CH4, O2), especially as related to the redox evolution of the atmosphere and the coupled evolution of Earth's climate system. We emphasize that in the search for Earth-like planets we must be mindful that the example provided by the modern atmosphere merely represents a single snapshot of Earth's long-term evolution. In exploring the many former states of our own planet, we emphasize Earth's atmospheric evolution during the Archean, Proterozoic, and Phanerozoic eons, but we conclude with a brief discussion of potential atmospheric trajectories into the distant future, many millions to billions of years from now. All of these 'Alternative Earth' scenarios provide insight to the potential diversity of Earth-like, habitable, and inhabited worlds.
                    Cover page: Earth: Atmospheric Evolution of a Habitable Planet
                    • Article
                    • Peer Reviewed
                    UC Riverside Previously Published Works (2020)
                    For most of Earth's history, the ocean's interior was pervasively anoxic and showed occasional shifts in ocean redox chemistry between iron-buffered and sulfide-buffered states. These redox transitions are most often explained by large changes in external inputs, such as a strongly altered delivery of iron and sulfate to the ocean, or major shifts in marine productivity. Here, we propose that redox shifts can also arise from small perturbations that are amplified by nonlinear positive feedbacks within the internal iron and sulfur cycling of the ocean. Combining observational evidence with biogeochemical modeling, we show that both sedimentary and aquatic systems display intrinsic iron-sulfur bistability, which is tightly linked to the formation of reduced iron-sulfide minerals. The possibility of tipping points in the redox state of sediments and oceans, which allow large and nonreversible geochemical shifts to arise from relatively small changes in organic carbon input, has important implications for the interpretation of the geological rock record and the causes and consequences of major evolutionary transitions in the history of Earth's biosphere.
                      Cover page: Bistability in the redox chemistry of sediments and oceans
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