The Interaction of Terrestrial Exoplanet Oceans and Mantle

Effect of surface-mantle water exchange parameterizations on exoplanet ocean depths

Authors:

Komacek et al

Abstract:

Terrestrial exoplanets in the canonical habitable zone may have a variety of initial water fractions due to random volatile delivery by planetesimals. If the total planetary water complement is high, the entire surface may be covered in water, forming a "waterworld." On a planet with active tectonics, competing mechanisms act to regulate the abundance of water on the surface by determining the partitioning of water between interior and surface. Here we explore how the incorporation of different mechanisms for the degassing and regassing of water changes the volatile evolution of a planet. For all of the models considered, volatile cycling reaches an approximate steady-state after ∼2 Gyr. Using these steady-states, we find that if volatile cycling is either solely dependent on temperature or seafloor pressure, exoplanets require a high abundance (≳0.3% of total mass) of water to have fully inundated surfaces. However, if degassing is more dependent on seafloor pressure and regassing mainly dependent on mantle temperature, the degassing rate is relatively large at late times and a steady-state between degassing and regassing is reached with a substantial surface water fraction. If this hybrid model is physical, super-Earths with a total water fraction similar to that of the Earth can become waterworlds. As a result, further understanding of the processes that drive volatile cycling on terrestrial planets is needed to determine the water fraction at which they are likely to become waterworlds.

The Emergence and Limitations of the new Science of ExoGeophysics

EMERGING POSSIBILITIES AND INSUPERABLE LIMITATIONS OF EXOGEOPHYSICS: THE EXAMPLE OF PLATE TECTONICS

Authors:

Stamenković et al

Abstract:

To understand the evolution and the habitability of any rocky exoplanet demands detailed knowledge about its geophysical state and history—such as predicting the tectonic mode of a planet. Yet no astronomical observation can directly confirm or rule out the occurrence of plate tectonics on a given exoplanet. Moreover, the field of plate tectonics is still young—questioning whether we should study plate tectonics on exoplanets at this point in time. In this work, we determine the limitations and the emerging possibilities of exogeophysics, the science of connecting geophysics to exoplanets, on the example of plate tectonics. Assuming current uncertainties in model and planet parameters, we develop a qualitatively probabilistic and conservative framework to estimate on what kind of planets and where in the Galaxy plate tectonics might occur. This we achieve by modeling how plate yielding, the most critical condition needed for plate mobility and subduction, is affected by directly observable (planet mass, size) or indirectly, to some degree, assessable planet properties (structure and composition). Our framework not only highlights the importance of a planet's chemistry for the existence of plate tectonics and the path toward practical exogeophysics but also demonstrates how exoplanet science can actually help to better understand geophysics and the fundamentals of plate tectonics on Earth itself.

Many Magma Exoplanets may Have Surface Compositions That Change Over Time

Atmosphere-interior exchange on hot rocky exoplanets

Authors:

Kite et al

Abstract:

We provide estimates of atmospheric pressure and surface composition on short-period rocky exoplanets with dayside magma pools and silicate vapor atmospheres. Atmospheric pressure tends toward vapor-pressure equilibrium with surface magma, and magma-surface composition is set by the competing effects of fractional vaporization and surface-interior exchange. We use basic models to show how surface-interior exchange is controlled by the planet's temperature, mass, and initial composition. We assume that mantle rock undergoes bulk melting to form the magma pool, and that winds flow radially away from the substellar point. With these assumptions, we find that: (1) atmosphere-interior exchange is fast when the planet's bulk-silicate FeO concentration is low, and slow when FeO concentration is high; (2) magma pools are compositionally well-mixed for substellar temperatures ≲ 2400 K, but compositionally variegated and rapidly variable for substellar temperatures ≳ 2400 K; (3) currents within the magma pool tend to cool the top of the solid mantle ("tectonic refrigeration"); (4) contrary to earlier work, many magma planets have time-variable surface compositions.

How Much Water is TOO Much to be a Habitable World?

Water-rich planets: How habitable is a water layer deeper than on Earth?

Authors:

Noack et al

Abstract:

Water is necessary for the origin and survival of life as we know it. In the search for life-friendly worlds, water-rich planets therefore are obvious candidates and have attracted increasing attention in recent years. The surface H2O layer on such planets (containing a liquid water ocean and possibly high-pressure ice below a specific depth) could potentially be hundreds of kilometres deep depending on the water content and the evolution of the proto-atmosphere.

We study possible constraints for the habitability of deep water layers and introduce a new habitability classification relevant for water-rich planets (from Mars-size to super-Earth-size planets). A new ocean model has been developed that is coupled to a thermal evolution model of the mantle and core. Our interior structure model takes into account depth-dependent thermodynamic properties and the possible formation of high-pressure ice.

We find that heat flowing out of the silicate mantle can melt an ice layer from below (in some cases episodically), depending mainly on the thickness of the ocean-ice shell, the mass of the planet, the surface temperature and the interior parameters (e.g. radioactive mantle heat sources). The high pressure at the bottom of deep water–ice layers could also impede volcanism at the water–mantle boundary for both stagnant lid and plate tectonics silicate shells.

We conclude that water-rich planets with a deep ocean, a large planet mass, a high average density or a low surface temperature are likely less habitable than planets with an Earth-like ocean.

Geodynamics of Mars-sized Terrestrial Planets

The early geodynamic evolution of Mars-type planets

Authors:

Zhang et al

Abstract:

It is not clear whether Mars once possessed active tectonics, yet the question is critical for understanding the thermal evolution of Mars, and the origin and longevity of its early dynamo. To address these issues, we have coupled mantle flow simulations, together with parameterized core evolution models, to simulate the early evolution of Mars-like planets, and constrain the influence of early mobile-lid tectonics on core evolution. We have explored a wide parameter suite, encapsulating a range of uncertainties in initial conditions, rheological parameters, and surface strength. We present successful models that experience early mobile-lid behaviour, with a later transition into a stagnant-lid mode, which reproduce core dynamo histories similar to the magnetic history of early Mars.

Surface/atmospheric Abundances of Bio-Necessary Materials may be VERY Variable in Terrestrial Exoplanets

Tracing the Ingredients for a Habitable Earth from Interstellar Space through Planet Formation

Authors:

Bergin et al

Abstract:

We use the C/N ratio as a monitor of the delivery of key ingredients of life to nascent terrestrial worlds. Total elemental C and N contents, and their ratio, are examined for the interstellar medium, comets, chondritic meteorites and terrestrial planets; we include an updated estimate for the Bulk Silicate Earth (C/N = 49.0 +/- 9.3). Using a kinetic model of disk chemistry, and the sublimation/condensation temperatures of primitive molecules, we suggest that organic ices and macro-molecular (refractory or carbonaceous dust) organic material are the likely initial C and N carriers. Chemical reactions in the disk can produce nebular C/N ratios of ~1-12, comparable to those of comets and the low end estimated for planetesimals. An increase of the C/N ratio is traced between volatile-rich pristine bodies and larger volatile-depleted objects subjected to thermal/accretional metamorphism. The C/N ratios of the dominant materials accreted to terrestrial planets should therefore be higher than those seen in carbonaceous chondrites or comets. During planetary formation, we explore scenarios leading to further volatile loss and associated C/N variations owing to core formation and atmospheric escape. Key processes include relative enrichment of nitrogen in the atmosphere and preferential sequestration of carbon by the core. The high C/N BSE ratio therefore is best satisfied by accretion of thermally processed objects followed by large-scale atmospheric loss. These two effects must be more profound if volatile sequestration in the core is effective. The stochastic nature of these processes hints that the surface/atmospheric abundances of biosphere-essential materials will likely be variable.

Exoplanet Composition Will Greatly Impact Habitability

Connections between the bulk composition, geodynamics and habitability of Earth

Authors:

Jellinek et al

Abstract:

The bulk composition of the silicate part of Earth has long been linked to chondritic meteorites. Ordinary chondrites — the most abundant meteorite class — are thought to represent planetary building materials. However, a landmark discovery showed that the 142Nd/144Nd ratio of the accessible parts of the modern terrestrial mantle on Earth is greater than that of ordinary chondrites. If Earth was derived from these precursors, mass balance requires that a missing reservoir with 142Nd/144Nd lower than ordinary chondrites was isolated from the accessible mantle within 20 to 30 million years of accretion. This reservoir would host the equivalent of the modern continents' budget of radioactive heat-producing elements (uranium, thorium and potassium), yet has not been discovered. We argue that this reservoir could have been lost to space by ablation from early impactors. If so, Earth's radiogenic heat generation is between 18 and 45% lower than estimates based on a chondritic composition. Calculations of Earth's thermal history that incorporate such reduced radiogenic heating are consistent with a transition to the current plate tectonic mode in the past 2.5 billion years or so, a late onset of the dynamo and an evolving rate of volcanic outgassing consistent with Earth's long-term habitable climate. Reduced heat production compared with Venus and Mars could also explain aspects of the differences between the current climatic regimes of these planets and Earth.

τ Ceti's 'Habitable' Exoplanets may be Greatly Effected by Potentially Different Mineral Content

The Chemical Composition of τ Ceti and Possible Effects on Terrestrial Planets

Authors:

Pagano et al

Abstract:

τ Ceti (HD10700), a G8 dwarf with solar mass of 0.78, is a close (3.65 pc) sun-like star where 5 possibly terrestrial planet candidates (minimum masses of 2, 3.1, 3.5, 4.3, and 6.7 Earth masses) have recently been discovered. We report abundances of 23 elements using spectra from the MIKE spectrograph on Magellan. Using stellar models with the abundances determined here, we calculate the position of the classical habitable zone with time. At the current best fit age, 7.63 Gy, up to two planets (e and f) may be in the habitable zone, depending on atmospheric properties. The Mg/Si ratio of the star is found to be 1.78, which is much greater than for Earth (about 1.2). With a system that has such an excess of Mg to Si ratio it is possible that the mineralogical make-up of planets around τ Ceti could be significantly different from that of Earth, with possible oversaturation of MgO, resulting in an increase in the content of olivine and ferropericlase compared with Earth. The increase in MgO would have a drastic impact on the rheology of the mantles of the planets around τ Ceti.

Earth-Similar Geological Cycles Needed for Planetary Habitability

Sulphide oxidation and carbonate dissolution as a source of CO2 over geological timescales

Authors:

Torres et al

Abstract:

The observed stability of Earth’s climate over millions of years is thought to depend on the rate of carbon dioxide (CO2) release from the solid Earth being balanced by the rate of CO2 consumption by silicate weathering. During the Cenozoic era, spanning approximately the past 66 million years, the concurrent increases in the marine isotopic ratios of strontium, osmium and lithium suggest that extensive uplift of mountain ranges may have stimulated CO2 consumption by silicate weathering but reconstructions of sea-floor spreading do not indicate a corresponding increase in CO2 inputs from volcanic degassing. The resulting imbalance would have depleted the atmosphere of all CO2 within a few million years. As a result, reconciling Cenozoic isotopic records with the need for mass balance in the long-term carbon cycle has been a major and unresolved challenge in geochemistry and Earth history. Here we show that enhanced sulphide oxidation coupled to carbonate dissolution can provide a transient source of CO2 to Earth’s atmosphere that is relevant over geological timescales. Like drawdown by means of silicate weathering, this source is probably enhanced by tectonic uplift, and so may have contributed to the relative stability of the partial pressure of atmospheric CO2 during the Cenozoic. A variety of other hypotheses have been put forward to explain the ‘Cenozoic isotope-weathering paradox’, and the evolution of the carbon cycle probably depended on multiple processes. However, an important role for sulphide oxidation coupled to carbonate dissolution is consistent with records of radiogenic isotopes, atmospheric CO2 partial pressure and the evolution of the Cenozoic sulphur cycle, and could be accounted for by geologically reasonable changes in the global dioxygen cycle, suggesting that this CO2 source should be considered a potentially important but as yet generally unrecognized component of the long-term carbon cycle.