| More than a thousand exoplanets have been identified to date (Schneider 2014); however, moons orbiting exoplanets have not been discovered, yet. Nevertheless, it is probable that exomoons will be detected in the near future if they are not much more rare in exoplanetary systems than they are in the Solar System (Heller & Barnes 2013, Peters & Turner 2013, Kaltenegger 2010). For this reason it is important to begin to develop basic theoretical models of exomoons in advance of the first detections. Habitability is a particularly important aspect, since life might well develop on a non-planetary body if it has suitable characteristics and environmentals. Tidal forces produced by the planet which the moon orbits can induce friction inside the satellite that will have a warming effect (Peale et al. 1979). In the Solar System there are several examples for moons that have significant heating due to tidal forces. On exomoons that are too far from their central star to have habitable surface temperatures due to radiative heating, it is possible that warmth area of the surface produced by tidal heating could allow the emergence of life (Scharf 2006). This investigation focuses on tidal heating, studying the surface temperature of hypothetical exomoons for different orbital and interior parameters of the body. The aim of our research is to describe the conditions that allow the existence of water reservoirs in the liquid phase state on or near the surface. We modified and extended the public code of Heller & Barnes (2013) for the purposes of this investigation.
References
Heller, R., Barnes, R. 2013 Exomoon Habitability Constrained by Illumination and Tidal Heating, Astrobiology 13, 18
Kaltenegger, L. 2010 Characterizing habitable exomoons, The Astrophysical Journal Letters 712, 125
Peale, S. J., Cassen, P., Reynolds, R. T. 1979 Melting of Io by Tidal Dissipation, Science 203, 892
Peters, M. A., Turner, E. L. 2013 On the direct imaging of tidally heated exomoons, The Astrophysical Journal 769, 98
Scharf, C. A. 2006 The Potential for Tidally Heated Icy and Temperate Moons around Exoplanets, The Astrophysical Journal 648, 1196
Schneider, J. 2014 The Extrasolar Planets Encyclopaedia, http://exoplanet.eu/ |
| Earth Through Time as an Exoplanet: Lessons for Exoplanet Astrobiology |
| Dr.
Shawn Domagal-Goldman (NASA Goddard Space Flight Center) |
| The Archean Earth represents the most alien biosphere for which we have data. Oxygenic photosynthesis was not the dominant primary production metabolism at the surface, as it is on modern-day Earth. Due to this, the atmospheric composition, climate, and ocean chemistry of the planet were all dramatically different than they are on today's planet, even though life was present at the time. These dramatic differences are instructive on biology in a planetary context. Furthermore, they provide an example of a "working inhabited planet” that would have different biosignatures, climates, and spectral features. We can thus use the lessons from the rock record to inform us about the possibilities for and improve our ability to search for life. When we do that, we discover that by looking strictly for the "traditional" biosignatures from methane, oxygen, and ozone, we may conclude dead planets to be alive and living planets to be dead. In some cases, we may not even be looking for life on the right planets. In this talk, we will discuss these issues and their implications for future space-based observatories designed to search for life beyond the solar system.
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| The Effect of Planetary Illumination on Climate Modelling of Earthlike Exomoons |
| Dr.
Duncan Forgan (Institute for Astronomy, University of Edinburgh) |
| Compared to an Earthlike planet, there are several extra sources of energy available to an Earthlike exomoon, including tidal heating and radiative flux received directly from the host planet, either from thermal blackbody radiation or stellar radiation reflected from the planet's upper atmosphere. Regular eclipses of the star by the planet provides an extra effective sink of energy.
There have been many studies applying analytical calculations as to how these three factors affect the potential habitability of moons, and the morphology of the exomoon habitable zone (EHZ), which is clearly a manifold of higher dimension than the planetary HZ. I will discuss the first attempt to produce climate models of exomoons which possess all the above sources and sinks of energy.
We expand on our previous 1D latitudinal energy balance models (LEBMs) which follow the evolution of the latitudinal temperature distribution on an Earthlike moon orbiting a Jupiterlike planet, which in turn orbits a Sunlike star. Originally, this model incorporated the physical processes of stellar insolation, atmospheric cooling, heat diffusion across latitudes, stellar eclipses and tidal heating. We now add planetary illumination to assess its contribution to the morphology of the EHZ.
We investigate a four-dimensional EHZ, composed of the planet's semimajor axis and eccentricity, and the moon's semimajor axis and eccentricity. To do this, we run two separate suites of simulations. The first suite investigates the EHZ in terms of the planet's orbital parameters, keeping the moon's orbit fixed. This allows us to compare the EHZ with standard planetary habitable zones, which are pushed away from the star by planetary illumination (albeit not very far). Secondly, we investigate the EHZ in terms of the moon's orbital parameters, keeping the planet's orbit fixed. This allows us to investigate the recently-coined ``circumplanetary habitable edge''. This is generally considered to be only an inner edge, but we demonstrate that an outer circumplanetary habitable edge also exists, well within the orbital stability limit. |
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| Rocky Planetary Debris Around Young WDs |
| Prof.
Boris Gaensicke (University of Warwick) |
| The vast majority of all known planet host stars, including the Sun,
will eventually evolve into red giants and finally end their lives as
white dwarfs: extremely dense Earth-sized stellar embers. Only
close-in planets will be devoured during the red-giant phase. In the
solar system, Mars, the asteroid belt, and all the giant planets will
escape evaporation, and the same is true for many of the known
exo-planets.
It is hence certain that a significant fraction of the known white
dwarfs were once host stars to planets, and it is very likely that
many of them still have remnants of planetary systems.
The detection of metals in the atmospheres of white dwarfs is the
unmistakable signpost of such evolved planetary systems. The strong
surface gravity of white dwarfs causes metals to sink out of the
atmosphere on time-scales much shorter than their cooling ages,
leading unavoidably to pristine H/He atmospheres. Therefore any metals
detected in the atmosphere of a white dwarf imply recent or ongoing
accretion of planetary debris. In fact, planetary debris is also
detected as circumstellar dust and gas around a number of white
dwarfs. These debris disks are formed from the tidal disruption of
asteroids or Kuiper belt-like objects, stirred up by left-over
planets, and are subsequently accreted onto the white dwarf,
imprinting their abundance pattern into its atmosphere.
Determining the photospheric abundances of debris-polluted white
dwarfs is hence entirely analogue to the use of meteorites, "rocks
that fell from the sky", for measuring the abundances of planetary
material in the solar system.
I will briefly review this new field of exo-planet science, and then
focus on the results of a large, unbiased COS snapshot survey of
relatively young (~20-100Myr) white dwarfs that we carried out in
Cycle 18/19.
* At least 30% of all white dwarfs in our sample are accreting
planetary debris, and that fraction may be as high as 50%.
* In most cases where debris pollution is detected, the low C/Si ratio
demonstrates that the planetary material is of rocky nature.
* None of the 9 systems where we measure the C/O ratio shows evidence
for carbon-dominated chemistry, implying that "carbon planets" are
not common.
* In the most polluted white dwarfs, we measure the debris abundances
of up to 11 elements, enabling a detailed comparison between the
chemistry of exo-planetary material with that of solar system
meteorites. We find that the exo-planetary debris shares many
characteristics of solar-system material, i.e. a wide spread in the
relative abundances of Mg, Fe, Si, and O, a constant Al/Ca ratio,
and evidence for differentiation in the form of Fe over-abundances
All of the above is suggestive that thermal and collisional
processing of planetary material in those systems might have been
similar to that in the solar system.
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| Exoplanet Demographics with Microlensing Surveys |
| Bernard Gaudi (The Ohio State University) |
| Because of its unique sensitivity to low-mass, long-period, and
free-floating planets, microlensing is an essential complement to our
arsenal of planet detection methods. I motivate microlensing surveys
for exoplanets, and in particular describe how they can be used to
test models for planet formation, as well as inform our understanding
of the frequency and potential habitability of low-mass planets
located in the habitable zones of their host stars. I review results
from current microlensing surveys, and then discuss expectations for
next-generation experiments. I explain why a space-based mission is
necessary to realize the full potential of microlensing. When
combined with the results from complementary surveys such as Kepler, a
space-based microlensing survey will yield a nearly complete picture
of the demographics of planetary systems throughout the Galaxy. |
|
| The Habitability of the Milky Way Galaxy |
| Mr.
Michael Gowanlock (University of Hawaii ) |
| The Galactic Habitable Zone is defined as the region(s) of the Galaxy that may support complex life. Studies of the habitability of the Milky Way are becoming increasingly important with the growing number of extrasolar planet detections, and the multitude of conditions that life is found to thrive on the Earth. Through the evolution of the Galaxy, the distribution of stars and the planets that they host vary throughout space and time. Combining the information of the frequency of extrasolar planets, and the prospects for life in a range of environments within our evolving galaxy, we are able to make initial estimates of the habitability of the Milky Way. Some of the prerequisites for complex life include having enough metallicity, or building blocks for planet formation, enough time for biological evolution and low exposure to transient radiation events, such as supernovae. Our previous work suggests that the inner disk of the Milky Way may contain the greatest number density of habitable planets at the present day at a galactocentric distance of R>2.5 kpc, despite the higher supernovae rate in the region in comparison to the Sun’s location at 8 kpc. I will discuss our previous work, and present an overview of dangers to habitable planets beyond supernovae in different galactic environments.
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| Venus and Mars as Failed Biospheres |
| Dr.
David Grinspoon (Library of Congress) |
| What kinds of planets can support life? A widely held belief is that to support life, a planet should have stable bodies of liquid surface water. This assumption has in turn led to the conventional notion of a habitable zone (HZ) as a range of distances from a star where water can exist on the surface of a solid planet for biologically relevant timescales.
As our understanding of terrestrial planet evolution has increased, the importance of water abundance as a substance controlling many evolutionary factors has become increasingly clear. This is true of biological evolution, as the presence of liquid water is widely regarded as the key to the possibility of finding “life as we know it” on other worlds. It is also true of geological and climatic evolution. Water is among the most important climatically active atmospheric gasses on the terrestrial planets. It is also a controlling variable for tectonic style and geologic processes, as well as a mediator of surface-atmosphere chemical reactions.
Of the three local terrestrial planets, two have lost their oceans either to a subsurface cryosphere or to space, and one has had liquid oceans for most of its history. It is likely that planetary desiccation in one form or another is common among extrasolar terrestrial planets near the edges of their habitable zones. Thus, understanding the sources and sinks for surface water and characterizing the longevity of oceans and the magnitude of loss mechanisms on terrestrial planets of differing size, composition and proximity to stars of various stellar types, as well as the range of physical parameters which facilitates plate tectonics, is key to defining stellar habitable zones.
The global biosphere of Earth has greatly altered many physical properties of the planet, and it is unclear to what extent the long-term habitability of Earth is the result of its inhabitation. Only comparative planetology, eventually including comparison with other inhabited planets, will answer this question. If extrasolar planet discovery thus far is any guide, then the variety of terrestrial planets is likely to be large and surprising. Making sense of this diversity with such a small baseline of local terrestrial planets seems like a daunting task. However, in the first billion years of solar system evolution, Venus, Mars and Earth were all very different from their current states. These differences would be observable at interstellar distances. To the extent that we can understand the likely past and future states of local terrestrial planets, we can expand our knowledge base to more than the three examples provided by the current states of these planets.
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| Assessing the Suitability of Nearby Red Dwarf Stars as Hosts to Habitable Life-Bearing Planets |
| Prof.
Edward Guinan (Villanova University) |
| As part of our NSF/NASA sponsored ``Living with a Red Dwarf Star'' program, we are carrying out a comprehensive study of red dwarf stars across the electromagnetic spectrum (X-ray-IR) to assess their suitability as hosts of habitable planets. These cool, dim, long-lived, low mass stars comprise >75% of the stars in our Galaxy. Moreover an increasing number of (potentially habitable) large Earth-size planets are being found hosted by red dwarfs. With intrinsically low luminosities (L < 0.02 Lsun), the habitable zones (HZs) of hosted planets are close to their host stars (typically 0.05 AU < HZ < 0.4 AU). However, red dwarf stars have strong magnetic-dynamo generated magnetic fields and resulting coronal and chromospheric X-ray to UV (XUV) emissions, as well as strong flares. These XUV emission greatly decrease with increasing age and slower rotation.
Our study indicates red dwarf HZ planets without strong (protective) magnetic fields are especially susceptible to atmospheric erosion & loss by the host star's XUV radiation and frequent flares. We have also estimated the ages of planet-hosting stars using our Age-Rotation-Activity relations. Frequent flares of young red dwarf stars and tidal-locking of close-in planets could challenge the development of life. But tidal locking of these planets could have some advantages for the development of life. The long lifetimes of the red dwarfs ( >100 Ga) could be favorable for the development of complex (possibly even intelligent) life for the many old red dwarfs in the solar neighborhood - such as GJ 581 and HD 85512 - both are old and host HZ Earth-size planets.
We gratefully acknowledge the support from NSF-Grant AST-10-09903, Chandra Grants GO1-12124X & GO2-13020X and HST Grant GO-10920. |
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| How Life and Rocks Have Co-Evolved |
| Prof.
Robert Hazen (Carnegie Institution) |
| The near-surface environment of terrestrial planets and moons evolves as a consequence of selective physical, chemical, and biological processes—an evolution that is preserved in the mineralogical record. Mineral evolution begins with approximately 12 different refractory minerals that form in the cooling envelopes of exploding stars. Subsequent aqueous and thermal alteration of planetessimals results in the approximately 250 minerals now found in unweathered lunar and meteorite samples. Following Earth’s accretion and differentiation, mineral evolution resulted from a sequence of geochemical and petrologic processes, which led to perhaps 1500 mineral species. According to some origin-of-life scenarios, a planet must progress through at least some of these stages of chemical processing as a prerequisite for life. Once life emerged, mineralogy and biology co-evolved and dramatically increased Earth’s mineral diversity to >4000 species.
Sequential stages of a planet’s near-surface evolution arise from three primary mechanisms: (1) the progressive separation and concentration of the elements from their original relatively uniform distribution in the presolar nebula; (2) the increase in range of intensive variables such as pressure, temperature, and volatile activities; and (3) the generation of far-from-equilibrium conditions by living systems. Remote observations of the mineralogy of other terrestrial bodies may thus provide evidence for biological influences beyond Earth.
Recent studies of mineral diversification through time reveal striking correlations with major geochemical, tectonic, and biological events, including large-changes in ocean chemistry, the supercontinent cycle, the increase of atmospheric oxygen, and the rise of the terrestrial biosphere.
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| Specialization of Bacillus in the Geochemically Challenged Environment of Death Valley |
| Ms.
Sarah Kopac (Wesleyan University) |
| Death Valley is the hottest, driest place in North America, a desert with soils containing toxic elements such as boron and lead. While most organisms are unable to survive under these conditions, a diverse community of bacteria survives here. What has enabled bacteria to adapt and thrive in a plethora of extreme and stressful environments where other organisms are unable to grow? The unique environmental adaptations that distinguish ecologically distinct bacterial groups (ecotypes) remain a mystery, in contrast to many animal species (perhaps most notably Darwin’s ecologically distinct finch species). We resolve the ecological factors associated with recently diverged ecotypes of the soil bacteria Bacillus subtilis and Bacillus licheniformis, isolated from the dry, geochemically challenging soils of Death Valley, CA.
To investigate speciation associated with challenging environmental parameters, we sampled soil transects along a 400m stretch that parallels a decrease in salinity adjacent to a salt flat; transects also encompass gradients in soil B, Cu, Fe, NO3, and P, all of which were quantified in our soil samples. We demarcated strains using Ecotype Simulation, a sequence-based algorithm. Each ecotype’s habitat associations were determined with respect to salinity, B, Cu, Fe, NO3, and P. In addition, our sample strains were tested for tolerance of copper, boron and salinity (all known to inhibit growth at high concentrations) by comparing their growth over a 20 hour period.
Ecotypes differed in their habitat associations with salinity, boron, copper, iron, and other ecological factors; these environmental dimensions are likely causing speciation of B. subtilis-licheniformis ecotypes at our sample site. Strains also differed in tolerance of boron and copper, providing evidence that our sequence-based demarcations reflect real differences in metabolism. By better understanding the relationship between bacterial speciation and the environment, we can begin to predict the habitability of unexplored extreme and extra-Earth environments.
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| DiskDetective.org: Finding Homes for Exoplanets Through Citizen Science |
| Marc Kuchner (NASA GSFC) |
| The Disk Detective project is scouring the data archive from the WISE
all-sky survey to find new debris disks and protoplanetary disks that other programs have missed.
In its first month, volunteers on this new Zooniverse citizen science website
have performed more than 400,000 classifications of WISE sources.
Using the power of crowdsourcing, we stand to increase the pool of known
debris disks by ~375 and triple the solid angle in clusters of young stars
examined with WISE, finding new targets for JWST and exoplanet imaging.
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| Moon Radius Limits for a Habitable Zone Kepler Transiting Planet Candidate |
| Dr.
Karen Lewis (ELSI) |
| In addition to planets being potentially habitable bodies, moons, both inside and beyond the habitable zones of their host star may also be suitable sites for life. One promising method to detect such habitable moons is the through the transit technique, in particular using the high quality, long baseline Kepler dataset. Planets in the habitable zone of Sun-like stars tend to have long orbital periods and thus exhibit few transits within the 3.5 year Kepler mission. In addition, candidate planets are more likely to be confirmed if they are in multiple systems where planetary perturbations may make moon detection through transit timing very challenging. As a result we focus on the direct detection moon technique first described by Sartoretti and Schneider (1999), which involves searching and fitting the extra dip due to a moon in each transit light curve directly. To test this method in the presence of realistic photometric noise, we developed a Kepler light curve simulator that generates noisy light curves corresponding to physically consistent planet-moon systems. Using this program we calculate sets of unique light curve realisations for a Kepler candidate (KOI3681.01) in the habitable zone of a Sun-like star, for a grid of physically realistic moon radii and semi-major axes, and process them using our detection code. This allows us to robustly place constraints on potentially habitable terrestrial moons thus demonstrating the power of this approach. |
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| The Potential Habitability of Dwarf Planet Ceres |
| Dr.
Jian-Yang Li (Planetary Science Institute) |
| As the largest object in the main asteroid belt of the Solar System, Ceres is 940 km in diameter and accounts for ~1/3 of the total mass of the asteroid belt. The recent unequivocal discovery of water vapor associated with localized sources on Ceres by Herschel Space Telescope confirmed enrichment in volatiles in Ceres as suggested by its low density and previous theoretical models. Hence water must have played a significant role in the evolution of Ceres and even affected its current state. Indeed, the shape and size of Ceres measured from Hubble Space Telescope pointed to a differentiated interior with a ~60 km thick outer ice shell. Although no definitive agreement on the compositions of Ceres' surface has been reached, the pervasive signature of hydrated minerals over the surface of Ceres and the albedo and spectral homogeneity across its surface suggest that processes involving liquid-phase activity at the global scale may have occurred in the past. Moreover, the Herschel Space Telescope observations directly pointed us to an active world.
The current evolution models of Ceres indicate that liquid water was present following an early differentiation and drove hydrothermal activity for a few tens of My since its formation. Silicate leaching could lead to the concentration of soluble species in the ocean that could play a role in decreasing the freezing temperature of that layer. The likely accretion of low-eutectic species such as ammonia hydrates could have promoted the long-term preservation of a deep liquid layer at the base of an icy shell over extended periods of time (possibly until present).
Therefore, models and observations emphasize the importance of Ceres as a potentially habitable object. The Dawn spacecraft is scheduled to arrive at Ceres in March 2015 to perform detailed geological, spectroscopic, compositional, and gravity mapping. In the mean time, we have begun an observing campaign using ground- and space-based facilities that cover wavelengths from UV to sub-mm, aiming to fully characterize the nature of water and hydration features detected at Ceres, and to facilitate theoretical studies. We expect that our knowledge about the history and current status of water on Ceres will be significantly advanced in the coming years, possibly putting Ceres into the category of potentially habitable planets that is by far one of the most accessible to human beings.
Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.
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| Habitable Moons and Planets Around Post-Main Sequence Stars |
| Dr.
Ralph Lorenz (JHU APL) |
| Habitability is ephemeral, and arises against the backdrop of stellar evolution. Atmospheric modulation of incoming and outgoing radiative fluxes can restrict or extend the insolation domain in which habitable conditions can persist, and feedbacks (notably, silicate weathering of CO2) may fortuitously adapt that modulation to counteract evolving luminosity. But eventually the star will win. What happens then depends on the histories of stellar luminosity, and of stellar mass loss. While the enhancement of luminosity may render the outer solar system habitable in a classic radiative/convective equilibrium sense, a scenario studied in most detail in connection with Saturn's moon Titan, the enhanced solar wind associated with the latter may strip atmospheres unprotected by magnetic fields. The question of post-main sequence habitability is therefore not a simple one. |
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| Exo-Planetary Phoenix: Rebirth of Planetary Systems Beyond the Main Sequence |
| Massimo Marengo (Iowa State University) |
| Mounting evidence suggests that planetary systems may be a common feature of stars that have evolved beyond the main sequence. Warm debris disks around white dwarfs and "pulsar" planets orbiting a neutron star are a strong indication that planetary systems may, at least in same cases, survive the dramatic phenomena leading to stellar death. A close look at these late evolutionary stages, however, suggests that these systems may be more than mere survivors of doomed pre-existing exo-planetary systems. The circumstellar environment of post-main sequence stars bears surprising similarities to the conditions leading to pre-main sequence planetary formation: a metal-rich environment often characterized by the presence of circumstellar or circumbinary disks. Are these conditions conducive to the birth of a second-generation planetary system, like a phoenix rising from the ashes of ancient worlds? In this talk we will discuss how the physical conditions in the winds of dusty giant stars may be favorable for renewed planetary formation, with particular emphasis on the effects of enhanced metallicity, binarity and the timescales available for the formation of a new generation of planets. |
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| Early Earth(s) Across Time and Space |
| Prof.
Stephen Mojzsis (University of Colorado) |
| The geochemical and cosmochemical record of our solar system is the baseline for exploring the question: “when could life appear on a world similar to our own?” Data arising from direct analysis of the oldest terrestrial rocks and minerals from the first 500 Myr of Earth history – termed the Hadean Eon – inform us about the timing for the establishment of a habitable silicate world. Liquid water is the key medium for life. The origin of water, and its interaction with the crust as revealed in the geologic record, guides our exploration for a cosmochemically Earth-like planets.
From the time of primary planetary accretion to the start of the continuous rock record on Earth at ca. 3850 million years ago, our planet experienced a waning bolide flux that partially or entirely wiped out surface rocks, vaporized oceans, and created transient serpentinizing atmospheres. Arguably, “Early Earths” across the galaxy may start off as ice planets due to feeble insolation from their young stars, occasionally punctuated by steam atmospheres generated by cataclysmic impacts. Alternatively, early global environments conducive to life spanned from a benign surface zone to deep into crustal rocks and sediments. In some scenarios, nascent biospheres benefit from the exogenous delivery of essential bio-elements via leftovers of accretion, and the subsequent establishment of planetary-scale hydrothermal systems.
If what is now known about the early dynamical regime of the Earth serves as any measure of the potential habitability of worlds across space and time, several key boundary conditions emerge. These are: (i) availability and long-term stability of liquid water; (ii) presence of energy resources; (iii) accessibility of organic raw materials; (iv) adequate inventory of radioisotopes to drive internal heating; (v) gross compositional parameters such as mantle/core mass ratio, and (vi) P-T conditions at or near the surface suitable for sustaining biological activity. Life could emerge in or on a suitable planet with Earth characteristics, within ca. 150 Myr after its formation.
Our understanding of the thermal histories and chemical transformations of the crusts of early Earth, Moon, Mars and asteroids have accumulated to the point where it is now feasible to deduce the habitable potential of the early solar system and to place some upward constraints on the timing of life’s appearance. The natural lifetime of a biosphere is strongly dependent not simply on its proximity to its star, but on the age and composition of the host planet.
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| Life in the Slow, Dark, Salty, Cold and Oxygen-Depleted Lane – Insights on Habitability from Lake Vida |
| Prof.
Alison Murray (Desert Research Institute) |
| Ice-entrained Lake Vida brine has provided an accessible natural habitat to study life in the slow lane – where cellular growth is limited, but not extinguished. We measured in situ stable isotopic signatures of N2O, SO42-, H2, conducted experiments utilizing stable isotope geochemical tracers to detect microbial transformations and employed radioisotopically-labeled amino acid precursors to detect cellular macromolecule biosynthesis. The results indicated a dominance of abiotic processes in the brine – yet support metabolically active life through detection of nominal rates of protein biosynthesis. At the same time, the brine has posed a challenge to our understanding of ecosystem energetics. Data collected thus far suggests that the brine is isolated from surfical processes and receives no new mass or energy from above. Calculations have estimated carbon remineralization rates, which indicate that resources should be depleted to the level of small molecules perhaps supporting a methanogenic ecosystem given the amount of time since encapsulation at the temperatures recorded – yet the brine is resource-rich harboring abundant bacteria and large molecules, in addition to a complex mixture of both reduced and oxidized compounds. This has motivated explorations into alternative sources of energy such as hydrogen – which was detected at levels ~ 10 micromolar - that could be generated by brine-rock interactions and supply endogenous energy to this closed ecosystem. This cold, salty, anoxic and organically rich brine, provides insight into a new category of habitable earth ecosystems that may also give us food for thought when considering habitability of giant planet icy worlds or of icy exoplanets. However, the methods we use, and the framework of scientific inquiry applied, are limited by perception and familiarity of rates of change that are important in human time scales. The Vida-icy brine ecosystem provides a model for expansion of our understanding of habitability in which time scales need to be extended, and the role of intermingling abiotic and biotic processes need to be considered. |
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| From Magma Oceans to Plates: A Habitability Transition in the Earth's Early History |
| Prof.
Peter Olson (Earth & Planetary Sciences) |
| Scenarios for Earth's early history link the development and maintenance of habitable surface conditions to the transition from a dynamical regime dominated by near-surface magma oceans to one dominated by mobile plate tectonics.
Plate tectonics is considered necessary for maintaining a vigorous rock cycle that buffers atmospheric CO2 with negative feedbacks involving balances between crustal uplift, erosion, weathering, and volcanism. Heat transfer through the mantle by plate tectonics also promotes dynamo activity in the core, thereby generating a long-lived magnetic field to inhibit gas escape from the atmosphere and provide a radiation shield for the near-surface environment. In this talk I review various hypotheses about the thermal evolution of the Earth, focusing on the implications for early Earth history of the transition from magma oceans and magmatic heat transport to plate tectonics and mantle heat transport by solid-state flow. In addition to summarizing some of the observational constraints and the physical controls on the timing of this transition, I review current ideas on the bifurcation from mobile plates to single plate (stagnant lid) forms of mantle convection, showing how this bifurcation can lead to a lack of habitability in single-plate terrestrial planets such as Venus.
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| Formation and Volatile Content of Terrestrial Exoplanets |
| Sean Raymond (Laboratoire d'Astrophysique de Bordeaux) |
| Earth contains only 0.1% water by mass. Yet Earth formed in a part of the protoplanetary disk that was probably completely dry. Water and other volatiles were delivered from colder regions to the growing terrestrial planets. Recent thinking suggests that Jupiter's large-scale migration played a prominent role in delivering water to Earth (the "Grand Tack" model).
A large fraction of Sun-like stars host planets somewhat larger than Earth (so-called "super-Earths"). Planets as small as Earth have been discovered in the habitable zones of their host stars. To estimate the volatile contents of these planets we first need to understand how they formed. Current thinking suggests that they either accreted in-situ or, more likely, migrated inward from farther out. The planets' volatile contents should vary substantially in the different models.
When systems contain gas giants, they have a strong effect on water delivery. Most of the observed systems with giant exoplanets probably only harbor small, dry terrestrial planets on eccentric and inclined orbits.
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| Extremophiles: Making a Living in Galactic Circumstances |
| Dr.
Frank Robb (University of Maryland) |
| "The other thing which is important to note is that, apart from carbon monoxide, which is a very abundant gas in space, carbon in its allotrope forms -- diamond, fullerene, and graphite -- is only present in very small quantities." Quote by Pascale Ehrenfreund, 2006.
Earth provides an exceptionally constant and friendly milieu, in cosmic terms. The exoplanets that have been discovered to date are unlikely to support life as we know it. In order to consider the adaptive strategies that might allow life to extend into some environments elsewhere in the Universe, we consider the terrestrial extremes, from pH 0-13, from -6C to 122C, from 0.1- 75 mPa and saturating salts or distilled water, using a variety of lifestyles.
Metabolic adaptation is equally important as many life forms from low-luminosity habitats, which may be common in galactic distribution, may be relying on chemosynthesis. The geothermal sites can inform us of the different metabolic strategies that support life.
(methanogenesis, methylotrophy), CO trophy, CO biogenesis
Indeed, hydrogenogenic carboxydotrophy (CO + H2O CO2 + H2) is a geographically widespread anaerobic metabolism that appears in most hydrothermal microbial ecosystems, including those we have studied at hot springs in Uzon Caldera, Kamchatka, Russia. Carboxydotrophs have been isolated that can use CO as sole carbon and energy source in culture medium with headspace CO partial pressures ranging from ≤ 10-4 atm to ≥ 2 atm.
The question of biogenic CO is of great significance. The production of CO by sulfate reducing bacteria has been well documented by Voordouw and collaborators. Remarkably, the model sulfate reducing bacterium Desulfovibrio vulgaris Hildenborough showed a pause in sulfate reduction at the beginning of stationary phase and produced a temporary spike of 500 ppm of CO and similar levels of H2 before resuming sulfate reduction.
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| Observing How Habitable Conditions Develop (Or Not) in Protoplanetary Disks |
| Dr.
Colette Salyk (National Optical Astronomy Observatory) |
| In this talk, I will discuss how observations of protoplanetary disks can be used to study the large-scale physical processes that lead to habitable planets, and to potentially assess the occurrence of habitable conditions throughout the galaxy. I'll provide an overview of the current status of disk observations, including summarizing key observational techniques, and what we do and don't know about disk properties. Then I'll focus, in particular, on how disk observations are being used to study the processes that lead to some of the fundamental properties of planets that determine their habitability, includingn size and type (terrestrial vs. gas giant), location and chemistry.
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| Mapping Ganymede’s Time-Variable Aurora in the Search for a Subsurface Ocean |
| Joachim Saur (University of Cologne) |
| Jupiter's moon Ganymede, the largest satellite in the solar system, is the only moon known to possess two auroral ovals and a dynamo magnetic field. An important unresolved question about Ganymede is whether Ganymede harbors a subsurface water ocean under its icy crust. Next to an internal dipole magnetic field, Galileo magnetometer data can be explained by the additional presence of magnetic fields induced in a saline ocean or by magnetic quadrupole moments without induced fields (Kivelson et al., Icarus, 2002). Hence the Galileo measurements alone do not prove the existence of an ocean. Here we present a new means to address this question through HST observations of Ganymede’s auroral ovals. Due to Jupiter’s time-variable magnetic field, the locations of Ganymede’s auroral ovals are expected to rock up and down. A saline, electrically conductive water ocean will modify the time-variable field due to electromagnetic induction effects and will reduce the rocking. Our analysis of HST observations, which were particularly designed to address this question, shows that the auroral ovals only weakly rock in concert with the time-variable Jovian magnetic field. This weak rocking of the ovals is consistent with shielding of the time-variable field due to electromagnetic induction in a saline subsurface ocean on Ganymede. |
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| Energizing the Discussion of Ice-Ocean World Habitability |
| Dr.
Britney Schmidt (Georgia Institute of Technology) |
| The outer solar system boasts a wide range of worlds with oceans—moons orbiting the gas giants as well as putative ocean worlds in the Kuiper Belt. These objects span sizes from a few hundred kilometers to larger than Mercury. How do we understand these bodies as a class as well as evaluate the habitability of individual environments? Recognizing that there is more to habitability than a set of ingredients, “Follow the Energy” has become an important mantra. Earth’s biosphere is strongly coupled to its geologic activity that maintains a sort of stable chemical disequilibria that is employed by life. From this perspective, we can think of geologic activity as a planetary proxy for energy, setting up redox environments of which life can take advantage. With this as a backdrop, we will explore two of the most intriguing bodies: Europa and Enceladus.
With an icy outer shell hiding a global ocean, Europa (r=1565 km) exists in a dynamic environment, where immense tides from Jupiter potentially power an active deeper interior. Intense irradiation and impacts bathe the top of the ice shell. These processes are sources of energy that could sustain a biosphere. In the past few decades the debate about habitability of Europa has been focused strongly on the thickness of its ice shell. However an arguably more critical question is: how does the ice shell really work? Galileo data indicated that Europa has undergone recent resurfacing, and implied that near-surface water was likely involved. Now the detection of potential water ice plumes, subduction-like behavior as well as shallow subsurface “lakes” within the past few years implies that rapid ice shell recycling could create a conveyor belt between the ice and ocean. Mediated by processes at the ice-ocean interface, exchange between Europa's surface and subsurface could allow ocean material to one day be detected or sampled by spacecraft. At least at this level, Europa passes the energy test. But the question remains: is there enough?
Enceladus (r=250 km) is the star of the Cassini mission, shooting water ice plumes from its south pole despite its small size and relatively low tidal forcing. This surprising activity, compete with heat signatures surrounding the sources of the south polar jets, is difficult to explain. These plumes contain a wide range of compounds that include potential products of water-rock reactions. Moreover, Enceladus is two-faced—half of the body seems to have undergone immense tectonic evolution, while another region of the moon is covered with ancient craters. Recent work showed that Enceladus’ shape was consistent with a localized sea rather than a global ocean, and has now been confirmed by gravity measurements. In such a world, can geologic activity persist to set up redox conditions suitable for life? Moreover, since Enceladus can likely not sustain such activity over geologic activity, what is special about this timing and could a biosphere persist?
In this presentation, we will explore these worlds and consider how habitable environments might be produced. These considerations should form the basis for understanding habitability of ice-ocean systems both for their own intrinsic interest and as type examples for planets we may one day detect around other stars.
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| What Can Earth Paleoclimates Reveal About the Resiliency of Habitable States? An Example from the Neoproterozoic Snowball Earth |
| Dr.
Linda Sohl (Columbia University) |
| The Neoproterozoic “Snowball Earth” glaciations (~750-635 Ma) have been a special focus for outer habitable zone investigations, owing in large part to a captivating and controversial hypothesis suggesting that Earth may have only narrowly escaped a runaway icehouse state on multiple occasions (a.k.a. “the hard snowball”; Hoffman and Schrag 2001). A review of climate simulations exploring snowball inception (Godderis et al. 2011) reveals that a broad range of models (EBMs, EMICs and AGCMs) tend to yield hard snowball solutions, whereas models with greater 3-D dynamic response capabilities (AOGCMs) typically do not, unless some of their climate feedback responses (e.g., wind-driven ocean circulation, cloud forcings) are disabled (Poulsen and Jacobs 2004). This finding raises the likelihood that models incorporating dynamic climate feedbacks are essential to understanding how much flexibility there may be in the definition of a planet’s habitable zone boundaries for a given point in its history.
In the first of a series of new Snowball Earth simulations, we use the NASA/GISS ModelE2 Global Climate Model – a 3-D coupled atmosphere/ocean model with dynamic sea ice response – to explore the impacts of wind-driven ocean circulation, clouds and deep ocean circulation on the sea ice front when solar luminosity and atmospheric carbon dioxide are reduced to Neoproterozoic levels (solar = 94%, CO2 = 40 ppmv). The simulation includes a realistic Neoproterozoic land mass distribution, which is concentrated at mid- to tropical latitudes. After 300 years, the sea ice front is established near 30 degrees latitude, and after 600 years it remains stable. As with earlier coupled model simulations we conclude that runaway glacial states would have been difficult to achieve during the Neoproterozoic, and would be more likely to have occurred during earlier times in Earth history when solar luminosity was less. Inclusion of dynamic climate feedback capabilities in habitable zone modeling studies is likely to result in an expansion of our view of what a “Goldilocks” state can entail. Future simulations with a modified version of the NASA/GISS GCM, ROCKE-3D, will take advantage of newly-added model capabilities that evaluate the influence of rotation rate, solar spectral variability, CO2 surface condensation and CO2 clouds on the outer edge of Earth’s habitable zone.
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| The Distribution of Plate Tectonics Planets in the Galaxy |
| Dr.
Vlada Stamenkovic (MIT) |
| Whether a rocky planet has plate tectonics or not is crucial for comprehending planetary climate and possibly habitability. The recent findings that super-Earths are common in our Galaxy and the push to find targets for future spectroscopic observations are highly motivating to determine which rocky planets support volcanism and also plate tectonics.
I present how different model assumptions (for 1D, 2D, 3D models), interior heat, initial conditions, and non-Earth-like planetary properties affect plate tectonics and furthermore discover some fundamental problems within exo-geophysics, which are too often neglected. I specifically find that:
1. Simple scaling laws are, without great modifications, not usable to study massive rocky planets. Moreover, thermal evolution models are necessary.
2. The question whether there is plate tectonics on super-Earths or not is tightly linked to how plate tectonics reacts to interior heat. Plate tectonics seems to be less likely with increasing interior heat and for massive planets with an Earth-like structure and composition.
3. Previous models disagree whether there is plate tectonics on super-Earths or not because of different model assumptions and the lack of result robustness.
4. The initial thermal conditions, the amount of iron and radiogenic heat sources in the mantle, the C/O ratio, the distribution of water between bulk mantle and surface, the planet’s core size, and whether a planet is differentiated or not impact plate tectonics as much as planetary mass does.
The presented results allow to relate the propensity of plate tectonics not only to a planet’s mass but also to its composition, structure, age, and hence partially to its formation environment, host star, and location in the Galaxy – allowing us to start embedding habitability from an interior perspective within the framework of Galactic habitability.
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| Habitable Planets: An Interior Perspective |
| Prof.
Diana Valencia (University of Toronto) |
| In the roadmap to finding a true Earth analog, we have found many super-Earths and sub-Neptune planets. Not only because they are easier to find, but also because they are very common. We can begin to explore the variety of characteristics of these low-mass planets given that we have masses and radii measurements. In the 1-10 Earth-mass range there are two overlapping populations: those that necessarily have an envelope, or sub-Neptune planets and those that are most likely rocky, or super-Earths. There is variation in composition and structure not only among these two populations but also within each population that may have an effect on habitability. Furthermore, I will focus on the rocky super-Earths to discuss two processes that are related habitability: plate tectonics and magnetic field generation. |
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| Are Post-Main Sequence Planets Doomed? |
| Prof.
Eva Villaver (Universidad Autonoma de Madrid) |
| Post-main sequence evolution directly affects the survival of planetary and sub-planetary mass bodies.
Planets orbiting evolved stars undergo orbital evolution under the influence of tides and mass-loss, can be
ejected, evaporated, and suffer multiple-body instabilities. The conditions on the planet surface
are expected to be modified as well as the result of the evolution of the star. I will discuss the new limits that
the theoretical studies allow us to set on the survival and habitability of planets as the star runs out of its hydrogen fuel. |
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| Habitable Zones Around Evolved Stars |
| Prof.
Lee Anne Willson (Iowa State University) |
| Evolutionary models give us luminosity L(t) and radius R vs. L for post-main-sequence evolution, including the first ascent red giants (RGB), the horizontal branch, and the second ascent (AGB). Evolutionary models also give a limiting L for the RGB. However, mass loss rates determine M(t) and also the maximum L and R achieved on the AGB as well as evolution of the sizes of the orbits of planets. Combining evolutionary and mass loss information allows us to determine the evolution of the habitable zone around RGB and AGB stars and to sort planets into likely survivors and those that will be destroyed by spiraling in to the star. I will review what we know of the evolutionary parameters, with particular emphasis on the role of mass loss. |
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| The Final Stages of Life |
| Don Winget (University of Texas) |
| The overwhelming majority of all stars end their lives as white dwarf stars. These stars and their environs have a deep personal significance for humanity: this is the expected fate of our own sun. Once a star becomes a white dwarf, its remaining evolution is best described as an exponential cooling. In the final throws of post-main sequence mass-loss the former stellar core becomes a white dwarf, emerging phoenix-like from amongst the ashes. Some planets may survive and others may form as a sort of second generation from the cast-off material. Life may survive or may be reborn on any planets that remain; life may also arise on newly formed planets. The prospects will depend in a significant way on the timescales of the central white dwarf star’s cooling evolution and how its radiation shapes the environment. We will discuss white dwarf evolutionary timescales with an eye towards the potential habitability of planets, both new and old. We will consider the uncertainties in these timescales from both an empirical and a theoretical perspective. We will critique the existing evidence for planets and summarize what we have learned so far through direct imaging and stellar pulsations. We will close with the very bright prospects for the future of planets and life in the final stages. |
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| Planets Around Giant Stars |
| Prof.
Alexander Wolszczan (Penn State University) |
| I will summarize the current status of searches for planets around giant and sub-giant stars. The discussion of conclusions from these searches will include topics such as the absence of planets with orbital radii < 0.6 AU, planets around lithium-polluted giants, and planet mass - stellar mass and planet frequency - stellar metallicity correlations. I will compare the currently established characteristics of planetary systems orbiting giants with those of planets around main-sequence stars. |
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