arXiv subjects

Stephanie L. Olson

Publications and source records attributed to Stephanie L. Olson.

At least 19 recordsLinked to original sources

arXiv2026

What the Solar System Can Teach Us About Rocky Exoplanets

The number of rocky extrasolar planets being discovered continues to increase, but so too does the apparent diversity of such worlds. As we work to understand the likely bulk properties and thermal, geological, and climatological attributes we might expect of rocky exoplanets, we can look to the Solar System for guidance. Here, we review the interior, surface, and atmospheric characteristics of modern Earth, and discuss how our homeworld has changed through Solar System history. We then visit in turn Venus, Mars, Mercury and Earth's Moon, and Io, noting how these terrestrial planets are alike, how they are different, and how they have evolved through time. Finally, we consider some examples of the types of rocky worlds known or suspected to exist without direct Solar System counterparts --- but we argue that even then, for all the variety we might expect, they are still variations on a common theme. Perhaps the most important lesson the Solar System can teach us is that terrestrial bodies change through time, sometimes dramatically, and that rocky worlds of similar size and mass can have vastly different planetary outcomes.

astro-ph.EP↗

arXiv2024

Climatic Effects of Ocean Salinity on M Dwarf Exoplanets

Ocean salinity is known to dramatically affect the climates of Earth-like planets orbiting Sun-like stars, with high salinity leading to less ice and higher surface temperature. However, how ocean composition impacts climate under different conditions, such as around different types of stars or at different positions within the habitable zone, has not been investigated. We used ROCKE-3D, an ocean-atmosphere general circulation model, to simulate how planetary climate responds to ocean salinities for planets with G-star vs. M-dwarf hosts at several stellar fluxes. We find that increasing ocean salinity from 20 to 100 g/kg in our model results in non-linear ice reduction and warming on G-star planets, sometimes causing abrupt transitions to different climate states. Conversely, sea ice on M-dwarf planets responds more gradually and linearly to increasing salinity. Moreover, reductions in sea ice on M-dwarf planets are not accompanied by significant surface warming as on G-star planets. High salinity can modestly bolster the resilience of M-dwarf planets against snowball glaciation and allow these planets to retain surface liquid water further from their host star, but the effects are muted compared to G-star planets that experience snowball bifurcation and climate hysteresis due to the ice-albedo feedback.

astro-ph.EP↗

arXiv2024

The early Earth as an analogue for exoplanetary biogeochemistry

Planet Earth has evolved from an entirely anoxic planet with possibly a different tectonic regime to the oxygenated world with horizontal plate tectonics that we know today. For most of this time, Earth has been inhabited by a purely microbial biosphere albeit with seemingly increasing complexity over time. A rich record of this geobiological evolution over most of Earth's history provides insights into the remote detectability of microbial life under a variety of planetary conditions. We leverage Earth's geobiological record with the aim of a) illustrating the current state of knowledge and key knowledge gaps about the early Earth as a reference point in exoplanet science research; b) compiling biotic and abiotic mechanisms that controlled the evolution of the atmosphere over time; and c) reviewing current constraints on the detectability of Earth's early biosphere with state-of-the-art telescope technology. We highlight that life may have originated on a planet with a different tectonic regime and strong hydrothermal activity, and under these conditions, biogenic CH$_4$ gas was perhaps the most detectable atmospheric biosignature. Oxygenic photosynthesis, which is responsible for essentially all O$_2$ gas in the modern atmosphere, appears to have emerged concurrently with the establishment of modern plate tectonics and the continental crust, but O$_2$ accumulation to modern levels only occurred late in Earth's history, perhaps tied to the rise of land plants. Nutrient limitation in anoxic oceans, promoted by hydrothermal Fe = fluxes, may have limited biological productivity and O$_2$ production. N$_2$O is an alternative biosignature that was perhaps significant on the redox-stratified Proterozoic Earth. We conclude that the detectability of atmospheric biosignatures on Earth was not only dependent on biological evolution but also strongly controlled by the evolving tectonic context.

astro-ph.EP↗

arXiv2023

Earth as an Exoplanet. II. Earth's Time-variable Thermal Emission and Its Atmospheric Seasonality of Bioindicators

We assess the dependence of Earth's disk-integrated mid-infrared thermal emission spectrum on observation geometries and investigate which and how spectral features are impacted by seasonality on Earth. We compiled an exclusive dataset containing 2690 disk-integrated thermal emission spectra for four different full-disk observing geometries (North & South Pole centered and Africa & Pacific centred equatorial views) over four consecutive years. The spectra were derived from 2378 spectral channels in the wavelength range from 3.75 to 15.4 micron (nominal resolution $\approx$ 1200) and were recorded by the Atmospheric Infrared Sounder aboard the Aqua satellite. We learned that there is significant seasonal variability in Earth's thermal emission spectrum, and the strength of spectral features of bio-indicators, such as N2O, CH4, O3 and CO2 depends strongly on both season and viewing geometry. In addition, we found a strong spectral degeneracy with respect to the latter two indicating that multi-epoch measurements and time-dependent signals may be required in order to fully characterize planetary environments. Even for Earth and especially for equatorial views, the variations in flux and strength of absorption features in the disk-integrated data are small and typically $\le$ 10%. Disentangling these variations from the noise in future exoplanet observations will be a challenge. However, irrespectively of when the planet will be measured (i.e., day or night or season) the results from mid-infrared observations will remain the same to the zeroth order which is an advantage over reflected light observations.

astro-ph.EP↗

arXiv2022

Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach

Nitrous oxide (N2O) -- a product of microbial nitrogen metabolism -- is a compelling exoplanet biosignature gas with distinctive spectral features in the near- and mid-infrared, and only minor abiotic sources on Earth. Previous investigations of N2O as a biosignature have examined scenarios using Earthlike N2O mixing ratios or surface fluxes, or those inferred from Earth's geologic record. However, biological fluxes of N2O could be substantially higher, due to a lack of metal catalysts or if the last step of the denitrification metabolism that yields N2 from N2O had never evolved. Here, we use a global biogeochemical model coupled with photochemical and spectral models to systematically quantify the limits of plausible N2O abundances and spectral detectability for Earth analogs orbiting main-sequence (FGKM) stars. We examine N2O buildup over a range of oxygen conditions (1%-100% present atmospheric level) and N2O fluxes (0.01-100 teramole per year; Tmol = 10^12 mole) that are compatible with Earth's history. We find that N2O fluxes of 10 [100] Tmol yr$^{-1}$ would lead to maximum N2O abundances of ~5 [50] ppm for Earth-Sun analogs, 90 [1600] ppm for Earths around late K dwarfs, and 30 [300] ppm for an Earthlike TRAPPIST-1e. We simulate emission and transmission spectra for intermediate and maximum N2O concentrations that are relevant to current and future space-based telescopes. We calculate the detectability of N2O spectral features for high-flux scenarios for TRAPPIST-1e with JWST. We review potential false positives, including chemodenitrification and abiotic production via stellar activity, and identify key spectral and contextual discriminants to confirm or refute the biogenicity of the observed N2O.

astro-ph.EP↗

arXiv2022

The Effect of Ocean Salinity on Climate and Its Implications for Earth's Habitability

The influence of atmospheric composition on the climates of present-day and early Earth has been studied extensively, but the role of ocean composition has received less attention. We use the ROCKE-3D ocean-atmosphere general circulation model to investigate the response of Earth's present-day and Archean climate system to low vs. high ocean salinity. We find that saltier oceans yield warmer climates in large part due to changes in ocean dynamics. Increasing ocean salinity from 20 g/kg to 50 g/kg results in a 71% reduction in sea ice cover in our present-day Earth scenario. This same salinity change also halves the pCO$_2$ threshold at which Snowball glaciation occurs in our Archean scenarios. In combination with higher levels of greenhouse gases such as CO$_2$ and CH$_4$, a saltier ocean may allow for a warm Archean Earth with only seasonal ice at the poles despite receiving 20% less energy from the Sun.

astro-ph.EP↗

arXiv2022

Moderately High Obliquity Promotes Biospheric Oxygenation

Planetary obliquity is a first order control on planetary climate and seasonal contrast, which has a number of cascading consequences for life. How moderately high obliquity (obliquities greater than Earth's current obliquity up to 45$^{\circ}$) affects a planet's surface physically has been studied previously, but we lack an understanding of how marine life will respond to these conditions. We couple the ROCKE-3D general circulation model to the cGENIE 3D biogeochemical model to simulate the ocean biosphere's response to various planetary obliquities, bioessential nutrient inventories, and biospheric structure. We find that the net rate of photosynthesis increased by 35$\%$ and sea-to-air flux of biogenic oxygen doubled between the 0$^{\circ}$ and 45$^{\circ}$ obliquity scenarios, which is an equivalent response to doubling bioessential nutrients. Our results suggest that moderately high-obliquity planets have higher potential for biospheric oxygenation than their low-obliquity counterparts and that life on moderately high-obliquity habitable planets may be easier to detect with next generation telescopes. These moderately high-obliquity habitable planets may also be more conducive to the evolution of complex life.

astro-ph.EP↗

arXiv2021

Leveraging Models to Constrain the Climates of Rocky Exoplanets

In recent years, numerical models that were developed for Earth have been adapted to study exoplanetary climates to understand how the broad range of possible exoplanetary properties affects their climate state. The recent discovery and upcoming characterization of nearby rocky exoplanets opens an avenue toward understanding the processes that shape planetary climates and lead to the persistent habitability of Earth. In this review, we summarize recent advances in understanding the climate of rocky exoplanets, including their atmospheric structure, chemistry, evolution, and atmospheric and oceanic circulation. We describe current and upcoming astronomical observations that will constrain the climate of rocky exoplanets and describe how modeling tools will both inform and interpret future observations.

astro-ph.EP↗

arXiv2021

Probing the capability of future direct imaging missions to spectrally constrain the frequency of Earth-like planets

A critical question in astrobiology is whether exoEarth candidates (EECs) are Earth-like, in that they originate life that progressively oxygenates their atmospheres similarly to Earth. We propose answering this question statistically by searching for O2 and O3 on EECs with missions such as HabEx or LUVOIR. We explore the ability of these missions to constrain the fraction, fE, of EECs that are Earth-like in the event of a null detection of O2 or O3 on all observed EECs. We use the Planetary Spectrum Generator to simulate observations of EECs with O2 and O3 levels based on Earth's history. We consider four instrument designs: LUVOIR-A (15m), LUVOIR-B (8m), HabEx with a starshade (4m, "HabEx/SS"), HabEx without a starshade (4m, "HabEx/no-SS"); as well as three estimates of the occurrence rate of EECs (eta_earth): 24%, 5%, and 0.5%. In the case of a null-detection, we find that for eta_earth = 24%, LUVOIR-A, LUVOIR-B, and HabEx/SS would constrain fE to <= 0.094, <= 0.18, and <= 0.56, respectively. This also indicates that if fE is greater than these upper limits, we are likely to detect O3 on at least 1 EEC. Conversely, we find that HabEx/no-SS cannot constrain fE, due to the lack of an coronagraph ultraviolet channel. For eta_earth = 5%, only LUVOIR-A and LUVOIR-B would be able to constrain fE, to <= 0.45 and <= 0.85, respectively. For eta_earth = 0.5%, none of the missions would allow us to constrain fE, due to the low number of detectable EECs. We conclude that the ability to constrain fE is more robust to uncertainties in eta_earth for missions with larger aperture mirrors. However all missions are susceptible to an inconclusive null detection if eta_earth is sufficiently low.

astro-ph.EP↗

arXiv2020

Non-detection of O$_2$/O$_3$ informs frequency of Earth-like planets with LUVOIR but not HabEx

A critical question in the search for extraterrestrial life is whether exoEarths are Earth-like, in that they host life that progressively oxygenates their atmospheres roughly following Earth's oxygenation history. This question could be answered statistically by searching for O$_2$ and O$_3$ on exoEarths detected by HabEx or LUVOIR. The point of this paper is to compare the ability of HabEx and LUVOIR to prevent a false negative answer to this question, in which we do not detect O$_2$ or O$_3$ on any planet even if all exoEarths are Earth-like. Our approach is to assign O$_2$ and O$_3$ values drawn from Earth's history to a distribution of detectable exoEarths and determine whether O$_2$ and O$_3$ would be detectable using the Planetary Spectrum Generator. We find that if exoEarths tend to be Earth-like, we expect to detect O$_3$ with a LUVOIR-sized instrument. We also find that LUVOIR is unlikely to have a false negative scenario in the context of searching for Earth-like life on its targeted exoEarths. Because of that, if LUVOIR does not detect O$_2$ or O$_3$ on any exoEarths, we will be able to constrain the maximum number of exoEarths that could be Earth-like. In contrast, we find that even if all exoEarths are Earth-like, HabEx has up to a 22% chance of not detecting O$_2$ or O$_3$ on any of them. This is because HabEx will detect less planets and cannot reliably detect O$_2$ and O$_3$ at all potential Proterozoic levels. This is a strong argument for building a larger telescope such as LUVOIR if we want to determine whether exoEarths tend to be Earth-like.

astro-ph.EP↗

arXiv2020

Oceanic and atmospheric methane cycling in the cGENIE Earth system model

The methane cycle is a key component of the Earth system that links planetary climate, biological metabolism, and the global biogeochemical cycles of carbon, oxygen, sulfur, and hydrogen. However, currently lacking is a numerical model capable of simulating a diversity of environments in the ocean where methane can be produced and destroyed, and with the flexibility to be able to explore not only relatively recent perturbations to Earth's methane cycle but also to probe methane cycling and associated climate impacts under the reducing conditions characteristic of most of Earth history and likely widespread on other Earth-like planets. Here, we present an expansion of the ocean-atmosphere methane cycle in the intermediate-complexity Earth system model cGENIE, including parameterized atmospheric photochemistry and schemes for microbial methanogenesis, aerobic methanotrophy, and anaerobic oxidation of methane. We describe the model framework, compare model parameterizations against modern observations, and illustrate the flexibility of the model through a series of example simulations. Though we make no attempt to rigorously tune default model parameters, we find that simulated atmospheric methane levels and marine dissolved methane distributions are generally in good agreement with empirical constraints for the modern and recent Earth. Finally, we illustrate the model's utility in understanding the time-dependent behavior of the methane cycle resulting from transient carbon injection into the atmosphere, and present model ensembles that examine the effects of oceanic chemistry and the thermodynamics of microbial metabolism on steady-state atmospheric methane abundance.

physics.ao-ph↗

arXiv2020

Exogeoscience and Its Role in Characterizing Exoplanet Habitability and the Detectability of Life

The search for exoplanetary life must encompass the complex geological processes reflected in an exoplanet's atmosphere, or we risk reporting false positive and false negative detections. To do this, we must nurture the nascent discipline of "exogeoscience" to fully integrate astronomers, astrophysicists, geoscientists, oceanographers, atmospheric chemists and biologists. Increased funding for interdisciplinary research programs, supporting existing and future multidisciplinary research nodes, and developing research incubators is key to transforming true exogeoscience from an aspiration to a reality.

astro-ph.IM↗

arXiv2020

The Effect of Substellar Continent Size on Ocean Dynamics of Proxima Centauri b

The potential habitability of tidally locked planets orbiting M-dwarf stars has been widely investigated in recent work, typically with a non-dynamic ocean and without continents. On Earth, ocean dynamics are a primary means of heat and nutrient distribution. Continents are a critical source of nutrients, strongly influence ocean dynamics, and participate in climate regulation. In this work, we investigate how the size of a substellar land mass affects the oceans ability to transport heat and upwell nutrients on the tidally locked planet Proxima Centauri b using the ROCKE-3D coupled ocean-atmosphere General Circulation Model (GCM). We find that dayside ice-free ocean and nutrient delivery to the mixed layer via upwelling are maintained across all continent sizes. We also find that Proxima Centauri bs climate is more sensitive to differences among atmospheric GCMs than to the inclusion of ocean dynamics in ROCKE-3D. Finally, we find that Proxima Centauri b transitions from a lobster state where ocean heat transport distributes heat away from the substellar point to an eyeball state where heat transport is restricted and surface temperature decreases symmetrically from the substellar point when the continent size exceeds about 20 percent of the surface area. Our work suggests that both a dynamic ocean and continents are unlikely to decrease the habitability prospects of nearby tidally locked targets like Proxima Centauri b that could be investigated with future observations by the James Webb Space Telescope (JWST).

astro-ph.EP↗

arXiv2020

Oceanographic Considerations for Exoplanet Life Detection

Liquid water oceans are at the center of our search for life on exoplanets because water is a strict requirement for life as we know it. However, oceans are dynamic habitats---and some oceans may be better hosts for life than others. In Earth's ocean, circulation transports essential nutrients such as phosphate and is a first-order control on the distribution and productivity of life. Of particular importance is upward flow from the dark depths of the ocean in response to wind-driven divergence in surface layers. This `upwelling' returns essential nutrients that tend to accumulate at depth via sinking of organic particulates back to the sunlit regions where photosynthetic life thrives. Ocean dynamics are likely to impose constraints on the activity and atmospheric expression of photosynthetic life in exo-oceans as well, but we lack an understanding of how ocean dynamics may differ on other planets. We address this issue by exploring the sensitivity of ocean dynamics to a suite of planetary parameters using ROCKE-3D, a fully coupled ocean-atmosphere GCM. Our results suggest that planets that rotate slower and have higher surface pressure than Earth may be the most attractive targets for remote life detection because upwelling is enhanced under these conditions, resulting in greater nutrient supply to the surface biosphere. Seasonal deepening of the mixed layer on high obliquity planets may also enhance nutrient replenishment from depth into the surface mixed layer. Efficient nutrient recycling favors greater biological activity, more biosignature production, and thus more detectable life. More generally, our results demonstrate the importance of considering oceanographic phenomena for exoplanet life detection and motivate future interdisciplinary contributions to the emerging field of exo-oceanography.

astro-ph.EP↗

arXiv2019

No Snowball on Habitable Tidally Locked Planets with a Dynamic Ocean

Terrestrial planets orbiting within the habitable zones of M-stars are likely to become tidally locked in a 1:1 spin:orbit configuration and are prime targets for future characterization efforts. An issue of importance for the potential habitability of terrestrial planets is whether they could experience snowball events (periods of global glaciation). Previous work using an intermediate complexity atmospheric Global Climate Model (GCM) with no ocean heat transport suggested that tidally locked planets would smoothly transition to a snowball, in contrast with Earth, which has bifurcations and hysteresis in climate state associated with global glaciation. In this paper, we use a coupled ocean-atmosphere GCM (ROCKE-3D) to model tidally locked planets with no continents. We chose this configuration in order to consider a case that we expect to have high ocean heat transport. We show that including ocean heat transport does not reintroduce the snowball bifurcation. An implication of this result is that a tidally locked planet in the habitable zone is unlikely to be found in a snowball state for a geologically significant period of time.

astro-ph.EP↗

arXiv2019

A Limited Habitable Zone for Complex Life

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.

astro-ph.EP↗

arXiv2019

Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System

Some atmospheric gases have been proposed as counter indicators to the presence of life on an exoplanet if remotely detectable at sufficient abundance (i.e., antibiosignatures), informing the search for biosignatures and potentially fingerprinting uninhabited habitats. However, the quantitative extent to which putative antibiosignatures could exist in the atmospheres of inhabited planets is not well understood. The most commonly referenced potential antibiosignature is CO, because it represents a source of free energy and reduced carbon that is readily exploited by life on Earth and is thus often assumed to accumulate only in the absence of life. Yet, biospheres actively produce CO through biomass burning, photooxidation processes, and release of gases that are photochemically converted into CO in the atmosphere. We demonstrate with a 1D ecosphere-atmosphere model that reducing biospheres can maintain CO levels of ~100 ppmv even at low H2 fluxes due to the impact of hybrid photosynthetic ecosystems. Additionally, we show that photochemistry around M dwarf stars is particularly favorable for the buildup of CO, with plausible concentrations for inhabited, oxygen-rich planets extending from hundreds of ppm to several percent. Since CH4 buildup is also favored on these worlds, and because O2 and O3 are likely not detectable with the James Webb Space Telescope, the presence of high CO (>100 ppmv) may discriminate between oxygen-rich and reducing biospheres with near-future transmission observations. These results suggest that spectroscopic detection of CO can be compatible with the presence of life and that a comprehensive contextual assessment is required to validate the significance of potential antibiosignatures.

astro-ph.EP↗

arXiv2019

The remote detectability of Earth's biosphere through time and the importance of UV capability for characterizing habitable exoplanets

Thousands of planets beyond our solar system have been discovered to date, dozens of which are rocky in composition and are orbiting within the circumstellar habitable zone of their host star. The next frontier in life detection beyond our solar system will be detailed characterization of the atmospheres of potentially habitable worlds, resulting in a pressing need to develop a comprehensive understanding of the factors controlling the emergence and maintenance of atmospheric biosignatures. Understanding Earth system evolution is central to this pursuit, and a refined understanding of Earth's evolution can provide substantive insight into observational and interpretive frameworks in exoplanet science. Using this framework, we argue here that UV observations can help to effectively mitigate 'false positive' scenarios for oxygen-based biosignatures, while 'false negative' scenarios potentially represent a significant problem for biosignature surveys lacking UV capability. Moving forward, we suggest that well-resolved UV observations will be critical for near-term volume-limited surveys of habitable planets orbiting nearby Sun-like stars, and will provide the potential for biosignature detection across the most diverse spectrum of reducing, weakly oxygenated, and oxic habitable terrestrial planets.

astro-ph.EP↗