Papers by Victoria Meadows
Astrobiology, Jan 4, 2018
In the coming years and decades, advanced space- and ground-based observatories will allow an unp... more In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly ...
Planetary Environmental Signatures for Habitability and Life
Springer eBooks, Aug 28, 2008
In the vast blackness of space, our home planet is a single sparkling oasis of life. Whether the ... more In the vast blackness of space, our home planet is a single sparkling oasis of life. Whether the Universe harbors other worlds that can support even simple life is a question that has been pondered, yet remained unanswered, for over two thousand years. Motivated by the discoveries over the past decade of hundreds of extrasolar planets, NASA and ESA have
The Astrophysical Journal, May 15, 2014
The Lunar CRater Observation and Sensing Satellite (LCROSS) observed the distant Earth on three o... more The Lunar CRater Observation and Sensing Satellite (LCROSS) observed the distant Earth on three occasions in 2009. These data span a range of phase angles, including a rare crescent phase view. For each epoch, the satellite acquired near-infrared and mid-infrared full-disk images, and partial-disk spectra at 0.26−0.65 µm (~500) and 1.17−2.48 µm (~50). Spectra show strong absorption features due to water vapor and ozone, which is a biosignature gas. We perform a significant recalibration of the UV-visible spectra and provide the first comparison of high-resolution visible Earth spectra to the NASA Astrobiology Institute's Virtual Planetary Laboratory three-dimensional spectral Earth model. We find good agreement with the observations, reproducing the absolute brightness and dynamic range at all wavelengths for all observation epochs, thus validating the model to within the ~10% data calibration uncertainty. Data-model comparisons reveal a strong ocean glint signature in the crescent phase dataset, which is well matched by our model predictions throughout the observed wavelength range. This provides the first observational test of a technique that could be used to determine exoplanet habitability from disk-integrated observations at visible and near-infrared wavelengths, where the glint signal is strongest. We examine the detection of the ozone 255 nm Hartley and 400−700 nm Chappuis bands. While the Hartley band is the strongest ozone feature in Earth's spectrum, false positives for its detection could exist. Finally, we discuss the implications of these findings for future exoplanet characterization missions.
Astrobiology, Sep 1, 2010
Main sequence M stars pose an interesting problem for astrobiology: their abundance in our galaxy... more Main sequence M stars pose an interesting problem for astrobiology: their abundance in our galaxy makes them likely targets in the hunt for habitable planets, but their strong chromospheric activity produces high energy radiation and charged particles that may be detrimental to life. We studied the impact of the 1985 April 12 flare from the M dwarf, AD Leonis (AD Leo), simulating the effects from both UV radiation and protons on the atmospheric chemistry of a hypothetical, Earth-like planet located within its habitable zone. Based on observations of solar proton events and the Neupert effect we estimated a proton flux associated with the flare of 5.9×10 8 protons cm-2 sr-1 s-1 for particles with energies >10MeV. Then we calculated the abundance of nitrogen oxides produced by the flare by scaling the production of these compounds during a large solar proton event called the "Carrington event". The simulations were performed using a 1-D photochemical model coupled to a 1-D radiative/convective model. Our results indicate that the ultraviolet radiation emitted during the flare does not produce a significant change in the ozone column depth of the planet. When the action of protons is included, the ozone depletion reached a maximum of 94% two years after the flare for a planet with no magnetic field. At the peak of the flare, the calculated UV fluxes that reach the surface, in the wavelength ranges that are damaging for life, exceed those received on Earth during less than 100 s. Flares may therefore not present a direct hazard for life on the surface of an orbiting habitable planet. Given that AD Leo is one of the most magnetically-active M dwarfs known, this conclusion should apply to planets around other M dwarfs with lower levels of chromospheric activity.
We show that photoevaporation of small gaseous exoplanets ("mini-Neptunes") in the habitable zone... more We show that photoevaporation of small gaseous exoplanets ("mini-Neptunes") in the habitable zones of M dwarfs can remove several Earth masses of hydrogen and helium from these planets and transform them into potentially habitable worlds. We couple X-ray/extreme ultraviolet (XUV)-driven escape, thermal evolution, tidal evolution and orbital migration to explore the types of systems that may harbor such "habitable evaporated cores" (HECs). We find that HECs are most likely to form from planets with ∼ 1 M ⊕ solid cores with up to about 50% H/He by mass, though whether or not a given mini-Neptune forms a HEC is highly dependent on the early XUV evolution of the host star. As terrestrial planet formation around M dwarfs by accumulation of local material is likely to form planets that are small and dry, evaporation of small migrating mini-Neptunes could be one of the dominant formation mechanisms for volatile-rich Earths around these stars.
The Astrophysical Journal, Feb 8, 2017
Hazes are common in known planet atmospheres, and geochemical evidence suggests early Earth occas... more Hazes are common in known planet atmospheres, and geochemical evidence suggests early Earth occasionally supported an organic haze with significant environmental and spectral consequences. The UV spectrum of the parent star drives organic haze formation through methane photochemistry. We use a 1D photochemical-climate model to examine production of fractal organic haze on Archean Earthanalogs in the habitable zonesof several stellar types: the modern and early Sun, AD Leo (M3.5V), GJ 876 (M4V), Eridani (K2V), and σ Boötis (F2V). For Archean-like atmospheres, planets orbiting stars with the highest UV fluxes do not form haze due to the formation of photochemical oxygen radicals that destroy haze precursors. Organic hazes impact planetary habitability via UV shielding and surface cooling, but this cooling is minimized around M dwarfs whose energy is emitted at wavelengths where organic hazes are relatively transparent. We generate spectra to test the detectability of haze. For 10 transits of a planet orbiting GJ 876 observed by the James Webb Space Telescope, haze makes gaseous absorption features at wavelengths < 2.5 µm 2-10σ shallower compared to a haze-free planet, and methane and carbon dioxide are detectable at >5σ. A haze absorption feature can be detected at 5σ near 6.3 µm, but higher signal-to-noise is needed to distinguish haze from adjacent absorbers. For direct imaging of a planet at 10 parsecs using a coronagraphic 10-meter class ultraviolet-visible-near infrared telescope, a UV-blue haze absorption feature would be strongly detectable at >12σ in 200 hours.
Bulletin of the AAS, 2021
The rapid advance of exoplanet discovery, planetary systems science, and telescope technology wil... more The rapid advance of exoplanet discovery, planetary systems science, and telescope technology will soon allow scientists to search for life beyond our Solar System through direct observation of extrasolar planets. This endeavor will occur alongside searches for habitable environments and signs of life within our Solar System. These searchers will require separate observational techniques, and exoplanets pose an additional challenge of having relatively limited data on any individual world. However, these searches are thematically related and will inform each other. This white paper will explore those synergies, and encourage an exchange of "lessons learned" between the Solar System and exoplanet communities, in particular focusing on quantitative frameworks for interpretation of biosignatures. Enhanced communication across the NASA Science Mission Directorate (in particular between the Astrophysics and Planetary Science Divisions) will prevent stove-piping of data and evaluation protocols, and will enhance the science return and bolster the theoretical and empirical foundation to interpret life detection mission results. Key recommendations: • Hold workshops to educate and socialize exoplanet and planetary communities to their different lineages of biosignature research and life detection approaches. • Increase communication and coordination between Astrophysics and Planetary Science Divisions and ROSES R&A funding opportunities, especially as related to funding exoplanet research within Planetary programs. • Host more cross-divisional meetings between the different analysis groups (AGs): Mars Exploration Program Analysis Group (MEPAG), Outer Planets Assessment Group (OPAG), Small Bodies Assessment Group (SBAG), and Exoplanet Exploration Program Analysis Group (ExoPAG). • Establish "exoplanet participating scientist" or "planetary participating scientist" positions in Solar System and exoplanet life detection missions, respectively (Arney et al., 2020; Marley et al., 2020). • Amplify and reinforce recommendations in other exoplanet-Solar System synergy white papers: ○ Please refer to the Arney et al. (2020) white paper "Exoplanets in our Backyard" for a report on the scientific and programmatic findings from the recent joint exoplanet-planetary workshop, which shows how NASA and the science community can encourage and nurture research at the intersection of the Solar System and exoplanet fields. ○ Please refer to the Marley et al. (2020) white paper "Enabling Effective Exoplanet/Planetary Collaborative Science" for specific structural and policy recommendations. ○ Please refer to the Schmidt et al. (2020) white paper on "Enabling Progress Towards Life Detection on NASA Missions: A White Paper from the Network for Life Detection" and the Hoehler et al. (2020) white paper "Groundwork for Life Detection," which advocate that community-level efforts be pursued to establish a standardized, evaluative framework that supports apples-to-apples assessment of diverse biosignatures' utility in addressing life detection objectives.
The Astrophysical Journal Letters, 2021
The observation of a 266.94 GHz feature in the Venus spectrum has been attributed to phosphine (P... more The observation of a 266.94 GHz feature in the Venus spectrum has been attributed to phosphine (PH3) in the Venus clouds, suggesting unexpected geological, chemical, or even biological processes. Since both PH3 and sulfur dioxide (SO2) are spectrally active near 266.94 GHz, the contribution to this line from SO2 must be determined before it can be attributed, in whole or part, to PH3. An undetected SO2 reference line, interpreted as an unexpectedly low SO2 abundance, suggested that the 266.94 GHz feature could be attributed primarily to PH3. However, the low SO2 and the inference that PH3 was in the cloud deck posed an apparent contradiction. Here we use a radiative transfer model to analyze the PH3 discovery, and explore the detectability of different vertical distributions of PH3 and SO2. We find that the 266.94 GHz line does not originate in the clouds, but above 80 km in the Venus mesosphere. This level of line formation is inconsistent with chemical modeling that assumes genera...
Geochimica et Cosmochimica Acta, 2014
Minor sulfur isotope anomalies indicate the absence of O 2 from the Archean atmosphere. A rich da... more Minor sulfur isotope anomalies indicate the absence of O 2 from the Archean atmosphere. A rich dataset showing large variations in magnitude and sign of D 33 S and D 36 S, preserved in both sulfates and sulfides, suggests that further constraints on Archean atmospheric chemistry are possible. We review previous quantitative constraints on atmospheric D 33 S production, and suggest that a new approach is needed. We added sulfur species containing 33 S and 34 S to a 1-D photochemical model and describe the numerical methodology needed to ensure accurate prediction of the magnitude and sign of D 33 S produced by and deposited from the Archean atmosphere. This methodology can test multiple MIF-S formation mechanisms subject to a variety of proposed atmospheric compositions, yielding D 33 S predictions that can be compared to the rock record. We systematically test SO 2 isotopologue absorption effects in SO 2 photolysis (Danielache et al., 2008), one of the primary proposed mechanisms for D 33 S formation. We find that differential absorption through the Danielache et al. (2008) cross sections is capable of altering predicted D 33 S as a function of multiple atmospheric variables, including trace O 2 concentration, total sulfur flux, CO 2 content, and the presence of hydrocarbons, but find a limited role for OCS and H 2 S. Under all realistic conditions, the Danielache et al. (2008) cross sections yield D 33 S predictions at odds with the geologic record, implying that additional pathways for sulfur MIF formation exist and/or the cross sections have significant errors. The methodology presented here will allow for quantitative constraints on the Archean atmosphere beyond the absence of O 2 , as soon as additional experimental measurements of MIF-S producing processes become available.
False Positives for Life: Atmospheric Ozone and Oxygen on Lifeless Rocky Exoplents
Oxygen (O2) and Ozone (O3) are two of the more commonly-cited biosignature gases for future life-... more Oxygen (O2) and Ozone (O3) are two of the more commonly-cited biosignature gases for future life-detection and planet characterization missions. In this presentation, we discuss the possibility for abiotic processes to produce these gases and examine the chemical and stellar contexts for these processes. Specifying these contexts and their observables will lead to a discussion on how false positives can be discriminated from true positives, and what the implications are for the capabilities of future exoplanet characterization missions.
Astrobiology, 2017
Oxygenic photosynthesis is Earth's dominant metabolism, having evolved to harvest the largest exp... more Oxygenic photosynthesis is Earth's dominant metabolism, having evolved to harvest the largest expected energy source at the surface of most terrestrial habitable zone planets. Using CO 2 and H 2 O-molecules that are expected to be abundant and widespread on habitable terrestrial planets-oxygenic photosynthesis is plausible as a significant planetary process with a global impact. Photosynthetic O 2 has long been considered particularly robust as a sign of life on a habitable exoplanet, due to the lack of known ''false positives''-geological or photochemical processes that could also produce large quantities of stable O 2. O 2 has other advantages as a biosignature, including its high abundance and uniform distribution throughout the atmospheric column and its distinct, strong absorption in the visible and near-infrared. However, recent modeling work has shown that false positives for abundant oxygen or ozone could be produced by abiotic mechanisms, including photochemistry and atmospheric escape. Environmental factors for abiotic O 2 have been identified and will improve our ability to choose optimal targets and measurements to guard against false positives. Most of these false-positive mechanisms are dependent on properties of the host star and are often strongest for planets orbiting M dwarfs. In particular, selecting planets found within the conservative habitable zone and those orbiting host stars more massive than 0.4 M 1 (M3V and earlier) may help avoid planets with abundant abiotic O 2 generated by water loss. Searching for O 4 or CO in the planetary spectrum, or the lack of H 2 O or CH 4 , could help discriminate between abiotic and biological sources of O 2 or O 3. In advance of the next generation of telescopes, thorough evaluation of potential biosignatures-including likely environmental context and factors that could produce false positives-ultimately works to increase our confidence in life detection.
Astrobiology, Feb 1, 2018
Proxima Centauri b provides an unprecedented opportunity to understand the evolution and nature o... more Proxima Centauri b provides an unprecedented opportunity to understand the evolution and nature of terrestrial planets orbiting M dwarfs. Although Proxima Cen b orbits within its star's habitable zone, multiple plausible evolutionary paths could have generated different environments that may or may not be habitable. Here, we use 1-D coupled climate-photochemical models to generate self-consistent atmospheres for several evolutionary scenarios, including high-O, high-CO, and more Earth-like atmospheres, with both oxic and anoxic compositions. We show that these modeled environments can be habitable or uninhabitable at Proxima Cen b's position in the habitable zone. We use radiative transfer models to generate synthetic spectra and thermal phase curves for these simulated environments, and use instrument models to explore our ability to discriminate between possible planetary states. These results are applicable not only to Proxima Cen b but to other terrestrial planets orbiti...
The Astrophysical Journal, 2013
Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary... more Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing radial velocity and transit exoplanet surveys and proposed future space missions. Most current estimates of the boundaries of the HZ are based on 1-D, cloud-free, climate model calculations by . However, this model used band models which were based on older HITRAN and HITEMP line-by-line databases. The inner edge of the HZ in model was determined by loss of water, and the outer edge was determined by the maximum greenhouse provided by a CO 2 atmosphere. A conservative estimate for the width of the HZ from this model in our Solar system is 0.95-1.67 AU.
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Papers by Victoria Meadows