Astrobiology — NExSS ROCKE3D Project

Publications

For a full list of publications related to ROCKE-3D, please see the SAO/NASA Astrophysics Data System (ADS) library. The following lists those publications with NASA/GISS authors.

Finding “superhabitable” planets using the water cycle: The habitable part of a planet's land surface is important for detecting the spectral signature of vegetation. This is influenced by external parameters that control the planet's climate, atmospheric circulation, and hydrological cycle. We explore this using the ROCKE-3D GCM, focusing on water fluxes that affect where abundant complex life on land is most likely. We assess fractional habitability using an aridity index that measures the competition between the supply of water to the surface by rain and the demand for water by the atmosphere through evapotranspiration. Earth-like planets become "superhabitable" (a larger habitable surface area than Earth) as solar heating and day-length increase because their climates become more equable, reminiscent of past warm periods on Earth when complex life was abundant and widespread. Clouds increasingly obscure the surface as solar heating increases, but visibility improves for modest increases in rotation period. Thus, moderately slowly rotating rocky planets with insolation near or somewhat greater than modern Earth's appear to be promising targets for surface characterization by a future exoplanet direct imaging mission.

• Del Genio, A.D., M.J. Way, N. Kiang, I. Aleinov, M.J. Puma, and B. Cook, 2019: Climates of warm Earth-like planets III: Fractional habitability from a water cycle perspective. Astrophys. J., submitted, arXiv:1910.03479.

Seasonality and planetary habitability: High obliquity planets exhibit large seasonality, a reversed annual-mean pole-to-equator gradient of stellar heating, and novel cryospheres. A suite of 3-D GCM simulations with a dynamic ocean is performed with Earthlike atmospheres for low and high obliquity planets with various stellar fluxes, CO2 concentrations, and initial conditions to explore the propensity for high obliquity climates approaching the outer edge of the Habitable Zone to undergo global glaciation. We also simulate planets with thick CO2 or H2 atmospheres, such as those expected to develop near or beyond the outer edge of the Habitable Zone. We show that high obliquity planets are hotter than their low obliquity counterparts due to ice-albedo feedbacks for cold climates, and water vapor in warm climates, caused by the different dynamical regimes that occur between the two states. We suggest the conditions on high obliquity planets are likely to be more favorable for a robust biosphere to develop approaching the outer edge of the HZ. However, the influence of obliquity diminishes for dense atmospheres.

• Colose, C.M., A.D. Del Genio, and M.J. Way, 2019: Enhanced habitability on high obliquity bodies near the outer edge of the Habitable Zone of Sun-like stars. Astrophys. J., 884, no. 2, 138, doi::10.3847/1538-4357/ab4131.

A framework for comparing rocky exoplanet GCMs: Upcoming telescopes may soon be able to characterize, through transmission spectroscopy, the atmospheres of rocky exoplanets orbiting nearby M dwarfs. One of the most promising candidates is the late M dwarf system TRAPPIST-1 which has seven known transiting planets that may be terrestrial in nature. Among these seven planets, TRAPPIST-1e seems to be the most promising candidate to have habitable surface conditions, receiving ∼66% of the Earth's incident radiation, and thus needing only modest greenhouse gas inventories to raise surface temperatures to allow surface liquid water to exist. GCMs offer the most detailed way to simulate planetary atmospheres, but intrinsic differences between GCMs can lead to different climates and thus observability of gas and/or cloud features. We present a protocol to inter-compare planetary GCMs, using four test cases for TRAPPIST-1e: two land planets with pure N2 and CO2 atospheres, respectively, and two aquaplanets with a modern Earth and a CO2-rich composition. Currently there are 4 participating models (LMDG, ROCKE-3D, ExoCAM, UM), however this protocol is intended to let other teams participate as well.

• Fauchez, T., M. Turbet, E.T. Wolf, I. Boutle, M.J. Way, A.D. Del Genio, N.J. Mayne, K. Tsigaridis, R.K. Kopparapu, J. Yang, F. Forget, A. Mandell, and S.D. Domagal Goldman, 2019: TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI). Motivations and protocol. Geosci. Model Dev., submitted, doi:10.5194/gmd-2019-166.

The inner Solar System's habitability through time: Earth, Mars, and Venus, irradiated by an evolving Sun, have had fascinating but diverging histories of habitability. Although only Earth’s surface is considered to be habitable today, all three planets might have simultaneously been habitable early in their histories. We consider how physical processes that have operated similarly or differently on these planets determined the creation and evolution of their atmospheres and surfaces over time.

• Del Genio, A.D., D. Brain, L. Noack, and L. Schaefer, 2019: The inner Solar System's habitability through time, submitted for inclusion in Planetary Astrobiology, University of Arizona Press.

A transient secondary atmosphere early in the Moon’s history: The lunar history of water deposition, loss, and transport after accretion is an important consideration for a possible a human lunar outpost. Recent work has shown that a secondary primordial atmosphere of up to 10 mb could have been emplaced ∼3.5 billion years ago. We model this atmosphere using ROCKE-3D to test its viability and the possibility of transporting water outgassed from the maria to the permanently shadowed regions at the poles. We find that atmospheres as thin as 1 mb are viable and that water can indeed be transported to the poles via eddies and a Hadley circulation. A secondary lunar atmosphere would inform our understanding of space weathering in the early solar system. In addition, it should be possible to find clues to the secondary atmosphere's existence in future lunar samples.

• Aleinov, I., M.J. Way, C. Harman, K. Tsigaridis, E.T. Wolf, and G. Gronoff, 2019: Modeling a transient secondary paleolunar atmosphere: 3-D simulations and analysis. Geophys. Res. Lett., 46, no. 10, 5107-5116, doi:10.1029/2019GL082494.

A limited habitable zone for complex life: Enhanced CO2 is required for a planet’s surface to remain above the freezing point of water and thus habitable for planets that receive much less stellar heating than modern Earth. However, most complex aerobic life on Earth is precluded by CO2 concentrations of just a small fraction of a bar. At the same time, many habitable zones reside in proximity to M dwarfs, which are more numerous than Sun-like G dwarfs but are predicted to promote greater abundances of CO in the atmospheres of orbiting planets, a highly toxic gas for complex aerobic organisms with circulatory systems. We find that for CO2 tolerances of 0.001-1 bar, the habitable zone for complex life is only ∼20-50% as wide as the conventional habitable zone for a Sun-like star and that CO concentrations may limit complex life throughout the entire habitable zone of the coolest M dwarfs. These results cast new light on the likely distribution of complex life in the universe and have important practical ramifications for the search for exoplanet biosignatures and technosignatures

• Schwieterman, E.W., C.T. Reinhard, S.L. Olson, C.E. Harman, and T.W. Lyons, 2019: A limited habitable zone for complex life. Astrophys. J., 878, no. 1, 19, doi:10.3847/1538-4357/ab1d52.

Estimating albedos and surface temperatures of habitable planets: The potential habitability of known exoplanets is often categorized by assuming a Bond albedo of either 0.3, similar to Earth, or 0. As an indicator of habitability, this leaves much to be desired, because albedo on other planets can be very different, and because surface temperature exceeds equilibrium temperature due to the atmospheric greenhouse effect. We use an ensemble of ROCKE-3D simulations to show that for a range of habitable planets, much of the variability of Bond albedo and even surface temperature can be predicted with useful accuracy from incident stellar flux and stellar temperature, two currently known external parameters for every confirmed exoplanet.

• Del Genio, A.D., N.Y. Kiang, M.J. Way, D.S. Amundsen, L.E. Sohl, Y. Fujii, M. Chandler, I.A. Aleinov, C.M. Colose, S.D. Guzewich, and M. Kelley, 2019: Albedos, equilibrium temperatures, and surface temperatures of habitable planets. Astrophys. J., 884, no. 1, 75, doi:10.3847/1538-4357/ab3be8.

Fractional habitability and silicate weathering dependence on stellar heating and planet rotation: We use ROCKE-3D to study the effects of rotation rates slower than Earth’s and increased stellar heating on the fractional habitability and silicate weathering rate of an Earth-like world. We find a moderate increase in the fraction of the planet that can sustain liquid water at temperatures conducive to life with a 10-20% increase in stellar heating and a maximum at sidereal day-lengths 8-32 times that of modern Earth. By tracking precipitation and run-off we show that at rotation periods near 4 days, silicate weathering is maximized, particularly at higher stellar heating. Because of weathering's integral role in the long-term carbonate-silicate cycle, climate stability may be strongly affected by the anticipated rotational evolution of temperate terrestrial-type worlds, and should be considered a major factor in their habitability.

• Jansen, T., C. Scharf, M.J. Way, and A.D. Del Genio, 2019: Climates of warm Earth-like planets II: Rotational 'Goldilocks' zones for fractional habitability and silicate weathering. Astrophys. J., 875, no. 2, 79, doi:10.3847/1538-4357/ab113d.

The Silurian Hypothesis: If an industrial civilization had existed on Earth many millions of years prior to our own era, what traces would it have left and would they be detectable today? We summarize the likely geological fingerprint of the Anthropocene, and demonstrate that while clear, it will not differ greatly in many respects from other known events in the geological record. We then propose tests that could plausibly distinguish an industrial cause from an otherwise naturally occurring climate event.

• Schmidt, G.A., and A. Frank, 2019: The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record? Intl. J. Astrobiol., 18, 142-150, doi:10.1017/S1473550418000095.

Tidal dissipation in a possible ancient Venus ocean: The solar tidal dissipation and torque on the planet from a possible ancient Venusian ocean is simulated using a numerical tidal model with varying ocean depth and rotation periods. The results show that tidal dissipation could have varied by more than 5 orders of magnitude, depending on rotation period and ocean depth. The associated tidal torque is about 2 orders of magnitude below that of the present Venusian atmospheric torque, and could change the Venusian day length by up to 72 days per million years depending on rotation rate. Consequently, an ocean tide on ancient Venus could have had significant effects on the history of de-spinning of Venus. This also has implications for the rotation periods of similarly close-in exoplanets and the location of the inner edge of the liquid water habitable zone.

• Green, J.A.M., M.J. Way, and R. Barnes, 2019: Consequences of tidal dissipation in a putative Venusian ocean. Astrophys. J. Lett., 876, no. 2, L22, doi:10.3847/2041-8213/ab133b.

Rethinking CO antibiosignatures: Some gases have been proposed as counter-indicators to the presence of life on an exoplanet (i.e., antibiosignatures). The most common example is CO, 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. We demonstrate that reducing biospheres can maintain CO levels of ∼100 ppmv even at low H2 fluxes. Additionally, we show that photochemistry around M dwarf stars is particularly favorable for the buildup of CO. 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.

Schwieterman, E.W., C.T. Reinhard, S.L. Olson, K. Ozaki, C.E. Harman, P.K. Hong, and T.W. Lyons, 2019: Rethinking CO antibiosignatures in the search for life beyond the solar system. Astrophys. J., 874, 9, doi:10.3847/1538-4357/ab05e1.

The role of topography and other boundary conditions in planetary climate: We describe how to ingest 3D topographic data from NASA's Venus Magellan Spacecraft radar observations into ROCKE-3D. We also explain how boundary condition choices such as ocean/lake coverage/depth, rotation rate, atmospheric constituents and other factors influence surface conditions in paleo-Venus simulations. Studies such as these should also be considered when examining liquid water habitability in similar exoplanet experiments.

• Way, M., and J. Wang, 2019: Venus topography and boundary conditions in 3D general circulation modeling. In Planetary Cartography and GIS. H. Hargital, Ed., Lecture Notes in Geoinformation and Cartography. Springer International, pp. 325-335, doi:10.1007/978-3-319-62849-3_19.

Possible climates of Proxima Centauri b: The first exoplanet discovered orbiting the nearest star to Earth, Proxima Centauri, may be small enough to be a rocky planet and at a distance from its star that gives it a chance to be habitable. To inform future observations, we simulated possible climates of Proxima Centauri b with ROCKE-3D. We find that the presence of a dynamic ocean that moves excess heat to colder regions permits a wide range of climates with an least some liquid water on the surface, even on the nightside of planets that are never illuminated by the star.

• Del Genio, A.D., M.J. Way, D.S. Amundsen, I. Aleinov, M. Kelley, N.Y. Kiang, and T.L. Clune, 2019: Habitable climate scenarios for Proxima Centauri b with a dynamic ocean. Astrobiology, 19, no. 1, 99-125, doi:10.1089/ast.2017.1760.

Simulations and observational signatures of runaway greenhouse exoplanets: The detection bias of methods used to find most exoplanets favors planets close to the host star where the stellar heating may create a runaway greenhouse state. One such exoplanet, Kepler-1649b, receives a similar flux from its star as modern Venus does from the Sun, and so was categorized as a possible “exoVenus.” We use ROCKE-3D to show that various Kepler-1649b atmospheres move toward a runaway greenhouse with rapidly escalating surface temperatures. Transmission spectra for this planet relevant to the James Webb Space Telescope Near-Infrared Spectrograph demonstrate the detectability of key atmospheric signatures of a runaway greenhouse state and demonstrate the future prospects of characterizing potential Venus analogs.

• Kane, S.R., A.Y. Ceja, M.J. Way, and E.V. Quintana, 2018: Climate modeling of a potential ExoVenus. Astrophys. J., 869, no. 1, 46, doi:10.3847/1538-4357/aaec68.

Effects of rotation and stellar heating on planetary climates: We present a large ensemble of ROCKE-3D simulations of an Earth-like world as a function of increasing insolation and rotation period. The simulations include two types of oceans: one without ocean heat transport (OHT) between grid cells, as is common in the exoplanet literature, and the other a fully coupled dynamic ocean. The dynamical regime transitions that occur as day length increases induce climate feedbacks producing cooler temperatures, first via the reduction of water vapor with increasing rotation period despite decreasing shortwave cooling by clouds, and then via decreasing water vapor and increasing shortwave cloud cooling, except at the highest insolations. Simulations without OHT are more sensitive to insolation changes for fast rotations, while slowly rotating planets are relatively insensitive to ocean choice. Uncertainties in cloud physics preclude a precise determination of habitability but do not affect robust aspects of exoplanet climate sensitivity. The data sets from this study are open source and publicly available.

• Way, M.J., A.D. Del Genio, I. Aleinov, T.L. Clune, M. Kelley, and N.Y. Kiang, 2018: Climates of warm Earth-like planets I: 3-D model simulations. Astrophys. J. Supp. Series, 239, no. 2, 24, doi:10.3847/1538-4365/aae9e1.

Biosignature false positives: In our search for life — whether within the earliest part of Earth's geologic record, on planets within our solar system such Mars, or especially for extrasolar planets — we must infer the presence of life from its impact on the local or global environment. These "biosignatures," often identified from the known influence of terrestrial organisms on the Earth's atmosphere and surface, could be misdiagnosed when we apply them to alien worlds. The so-called false positives may occur when another process or suite of processes masks or mimics a biosignature. Here, we examine several leading biosignatures, then introduce potential false positives for these signals, and finally discuss methods to discriminate between the two using current and future detection technologies. We conclude that it is the astrobiology community's responsibility to thoroughly exhaust all possibilities before we resort to "life" as an explanation.

• Harman, C.E., and S. Domagal-Goldman, 2018: Biosignature false positives. In Handbook of Exoplanets. H.J. Deeg and J.A. Belmonte, Eds. Springer International, doi:10.1007/978-3-319-30648-3_71-1.

Lightning as a limiter of oxygen false positives: In the last several years, a number of authors have suggested that molecular oxygen (O2) or ozone (O3) generated by abiotic processes may accumulate to detectable concentrations in a habitable terrestrial planet's atmosphere, producing so-called 'false positives' for life. But various models have occasionally disagreed with each other. We show that photochemical false positives derive either from inconsistencies in the treatment of atmospheric and global redox balance or from the treatment (or lack thereof) of lightning. For habitable terrestrial planets with even trace amounts of atmospheric N2, NO produced by lightning catalyzes the recombination of CO and O derived from CO2 photolysis and should eliminate all reported false positives. O2 thus remains a useful biosignature gas for Earth-like extrasolar planets, provided that the planet resides within the conventional liquid water habitable zone and did not experience distinctly non-Earth-like, irrecoverable water loss.

• Harman, C.E., R. Felton, R. Hu, S. Domagal-Goldman, A. Segura, F. Tian, and J.F. Kasting, 2018: Abiotic O2 levels on planets around F, G, K, and M stars: Effects of lightning-produced catalysts in eliminating oxygen false positives. Astrophys. J., 866, no. 1, 56, doi:10.3847/1538-4357/aadd9b.

Fractionation of sulfur isotopes in the Archean atmosphere: The anomalous abundances of sulfur isotopes in ancient sediments provide the strongest evidence for an anoxic (lacking oxygen) atmosphere prior to ∼2.45 Ga, but the mechanism for producing this "mass-independent" fractionation pattern remains in question. The prevailing hypothesis has been that it is created by differences in the UV photolysis rates of different SO2 molecules that differ only in their isotopes. We investigate here a recently proposed additional source of fractionation during gas-phase formation of elemental sulfur (S4 and S8) “chains”. The simulated fractionations produced by chain formation do not directly match fractionations from the rock record. The mismatch might be explained if the isotopic signals leaving the atmosphere were significantly modulated by life, by uncertainties in the rates of reactions of both major and minor isotopic sulfur species, or by the relatively large potential range of atmospheric parameters.

• Harman, C.E., A.A. Pavlov, D. Babikov, and J.F. Kasting, 2018: Chain formation as a mechanism for mass-independent fractionation of sulfur isotopes in the Archean atmosphere. Earth Planet. Sci. Lett., 496, 238-247, doi:10.1016/j.epsl.2018.05.041.

Exoplanet biosignatures: The rapid rate of discoveries of exoplanets has expanded the scope of the science possible for the remote detection of life beyond Earth. A NExSS workshop engaged the international scientific community across diverse scientific disciplines, to assess the state of the science and technology in the search for life on exoplanets, and to identify paths for progress. The workshop produced and introduction and five review papers that provide: 1) a review of known and proposed biosignatures; 2) an in-depth review of O2 as a biosignature, rigorously examining false positives and negatives for evidence of life; 3) a Bayesian framework to organize current understanding to quantify confidence in biosignature assessments; 4) an extension of that framework in anticipation of increasing planetary data and novel concepts of biosignatures, and 5) a review of the upcoming telescope capabilities to characterize exoplanets and their environments.

• NASA release: Will We Know Life When We See It? NASA-led Group Takes Stock of the Science

• Kiang, N.Y., S. Domagal-Goldman, M.N. Parenteau, D.C. Catling, Y. Fujii, V.S. Meadows, E.W. Schwieterman, and S.I. Walker, 2018: Exoplanet biosignatures: At the dawn of a new era of planetary observations. Astrobiology, 18, 619-629, doi:10.1089/ast.2018.1862.

• Schwieterman, W., N.Y. Kiang, M.N. Parenteau, C.E. Harman, S. DasSarma, T.M. Fisher, G.N. Arney, H.E. Hartnett, C.T. Reinhard, S.L. Olson, V.S. Meadows, C.S. Cockell, S.I. Walker, J.L. Grenfell, S. Hegde, S. Rugheimer, R. Hu, and T.W. Lyons, 2018: Exoplanet biosignatures: A review of remotely detectable signs of life. Astrobiology, 18, no. 6, 663-708, doi:10.1089/ast.2017.1729.

• Catling, D.C., J. Krissansen-Totton, N.Y. Kiang, D. Crisp, T.D. Robinson, S. DasSarma, A. Rushby, A.D. Del Genio, W. Bains, and S. Domagal-Goldman, 2018: Exoplanet biosignatures: A framework for their assessment. Astrobiology, 19, no. 6, 709-738, doi:10.1089/ast.2017.1737.

• Fujii, Y., D. Angerhausen, R. Deitrick, S. Domagal-Goldman, J.L. Grenfell, Y. Hori, S.R. Kane, E. Palle, H. Rauer, N. Siegler, K. Stapelfeldt, and K.B. Stevenson, 2018: Exoplanet biosignatures: Observational prospects. Astrobiology, 18, no. 6, 739-778, doi:10.1089/ast.2017.1733.

• Walker, S.I., W. Bains, L. Cronin, S. DasSarma, S. Danielache, S. Domagal-Goldman, B. Kacar, N.Y. Kiang, A. Lenardic, C.T. Reinhard, W. Moore, E.W. Schwieterman, E.L. Shkolnik, and H.B. Smith, 2018: Exoplanet biosignatures: Future directions. Astrobiology, 18, no. 6, 779-824.

Re-thinking the moist greenhouse limit for the inner edge of the habitable zone: The traditional view of the inner edge of the habitable zone has been that once a planet receives a stellar flux slightly greater than Earth does, convective storms inject large amounts of water vapor into the stratosphere, eventually leading to loss of the oceans. ROCKE-3D simulations show that this transition is caused by a circulation driven by stellar heating rather than by convection and is gradual rather than sudden, implying that planets can remain habitable much closer to their stars than previously thought.

• NASA release: New NASA Study Improves Search for Habitable Worlds.

• Fujii, Y., A.D. Del Genio, and D.S. Amundsen, 2017: NIR-driven moist upper atmospheres of synchronously rotating temperate terrestrial exoplanets. Astrophys. J., 848, no. 2, 100, doi:10.3847/1538-4357/aa8955.

Release of the first version of ROCKE-3D: The first generation of the ROCKE-3D GCM, called Planet_1.0, has been created and is being used for current research projects. The code is available at the Simplex software repository, and a paper describing the model has been published.

• Way, M.J., I. Aleinov, D.S. Amundsen, M.A. Chandler, T. Clune, A.D. Del Genio, Y. Fujii, M. Kelley, N.Y. Kiang, L. Sohl, and K. Tsigaridis, 2017: Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics 1.0: A general circulation model for simulating the climates of rocky planets. Astrophys. J. Supp. Series, 231, no. 1, 12, doi:10.3847/1538-4365/aa7a06.

Climate effects of the evolution of eccentricity of an Earthlike planet due to a nearby giant planet: An orbital evolution model was used to predict long-term changes in the orbital eccentricity of a neighboring Earthlike planet, and ROCKE-3D was used to predict the resulting swings in climate over almost 7000 years. The presence of a dynamic ocean that stores heat and transports it from warm to cold places keeps the climate stable despite the large variations in incident sunlight between periapsis and apoapsis.

• Georgakarakos, N., I. Dobbs-Dixon, and M.J. Way, 2016: Long term evolution of planetary systems with a terrestrial planet and a giant planet. Mon. Not. Roy. Astron. Soc., 461, no. 2, 1512-1528, doi:10.1093/mnras/stw1378.

• Way, M.J., and N. Georgakarakos, 2017: Effects of variable eccentricity on the climate of an Earth-like world. Astrophys. J. Lett., 835, no. 1, L1, doi:10.3847/2041-8213/835/1/L1.

Ancient Venus may have been habitable for several billion years: ROCKE-3D simulations assuming an early Venus shallow ocean consistent with the observed D/H ratio and a land-ocean pattern determined by Magellan topography data suggest that if Venus cooled down after its formation and retained some of its initial water inventory, its climate may have become stable and remained that way between 2.9 and 0.7 Gya due to the reflective convectively generated clouds that can shield the dayside of a planet if it rotates slowly enough.

• NASA release: NASA Climate Modeling Suggests Venus May Have Been Habitable.

• Way, M.J., A.D. Del Genio, N.Y. Kiang, L.E. Sohl, D.H. Grinspoon, I. Aleinov, M. Kelley, and T. Clune, 2016: Was Venus the first habitable world of our solar system? Geophys. Res. Lett., 43, no. 16, 8376-8383, doi:10.1002/2016GL069790.

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