Although carbon monoxide (CO) is an abundant molecule and may have great importance for planetary... more Although carbon monoxide (CO) is an abundant molecule and may have great importance for planetary interiors, measurements of its properties are difficult due to its extreme volatility. We calculate the equation of state for CO over a range of temperature and density that is applicable to the conditions in planetary interiors. Previous experimental and theoretical studies cover only a limited temperature-density range. Our calculations match these early results well, but now cover the full range of relevance. The method of calculation is based on the general-purpose quotidian equation of state described by More et al. (1988), which is here used in order to generate a freely downloadable look-up table to be used by the community.
We investigate the thermal equation of state, bulk modulus, thermal expansion coefficient, and he... more We investigate the thermal equation of state, bulk modulus, thermal expansion coefficient, and heat capacity of MH-III (CH 4 filled-ice Ih), needed for the study of CH 4 transport and outgassing for the case of Titan and super-Titans. We employ density functional theory and ab initio molecular dynamics simulations in the generalized-gradient approximation with a van der Waals functional. We examine the finite temperature range of 300 K-500 K and pressures between 2 GPa-7 GPa. We find that in this P-T range MH-III is less dense than liquid water. There is uncertainty in the normalized moment of inertia (MOI) of Titan; it is estimated to be in the range of 0.33 − 0.34. If Titan's MOI is 0.34, MH-III is not stable at present in Titan's interior, yielding an easier path for the outgassing of CH 4. However, for an MOI of 0.33, MH-III is thermodynamically stable at the bottom of a ice-rock internal layer capable of storing CH 4. For rock mass fractions 0.2 upwelling melt is likely hot enough to dissociate MH-III along its path. For super-Titans considering a mixture of MH-III and ice VII, melt is always positively buoyant if the H 2 O:CH 4 mole fraction is > 5.5. Our thermal evolution model shows that MH-III may be present today in Titan's core, confined to a thin (≈ 10 km) outer shell. We find that the heat capacity of MH-III is higher than measured values for pure water-ice, larger than heat capacity often adopted for ice-rock mixtures with implications for internal heating.
Bulletin of the American Astronomical Society, May 31, 2019
The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist... more The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. The classical HZ definition makes a number of assumptions common to the Earth, including assuming that the most important greenhouse gases for habitable planets are CO2 and H2O, habitable planets orbit main-sequence stars, and that the carbonate-silicate cycle is a universal process on potentially habitable planets. Here, we discuss these and other predictions for the habitable zone and the observations that are needed to test them. We also, for the first time, argue why A-stars may be interesting HZ prospects. Instead of relying on unverified extrapolations from our Earth, we argue that future habitability studies require first principles approaches where temporal, spatial, physical, chemical, and biological systems are dynamically coupled. We also suggest that next-generation missions are only the beginning of a much more data-filled era in the not-too-distant future, when possibly hundredsthousands of HZ planets will yield the statistical data we need to go beyond just finding habitable zone planets to actually determining which ones are most likely to exhibit life.
By using a hydrodynamic atmospheric escape mechanism amit09 we show how the unusually high mass d... more By using a hydrodynamic atmospheric escape mechanism amit09 we show how the unusually high mass density of Quaoar could have been predicted (constrained), without any knowledge of a binary companion. We suggest an explanation of the recent spectroscopic observations of Orcus and Charon delsanti10,cook07. We present a simple relation between the detection of certain volatile ices and the body mass density and diameter. As a test case we implement the relations on the KBO 2003 AZ_84 and give constraints on its mass density. We also present a method of relating the latitude-dependence of hydrodynamic gas escape to the internal structure of a rapidly rotating body and apply it to Haumea.
We examine the possibility that icy super-Earth mass planets, formed over long time scales (0.1--... more We examine the possibility that icy super-Earth mass planets, formed over long time scales (0.1--1 Gyr) at large distances (∼ 200--1000 AU) from their host stars, will develop massive H-rich atmospheres. Within the interior of these planets, high pressure converts CH_4 into ethane, butane, or diamond and releases H_2. Using simplified models which capture the basic physics of the internal structure, we show that the physical properties of the atmosphere depend on the outflux of H_2 from the mantle. When this outflux is ≲ 10^10 [molec cm^-2 s^-1], the outgassed atmosphere has base pressure ≲ 1 bar. Larger outflows result in a substantial atmosphere where the base pressure may approach 10^3 - 10^4 bar. For any pressure, the mean density of these planets, 2.4--3 [g cm^-3], is much larger than the mean density of Uranus and Neptune, 1.3--1.6 [g cm^-3]. Thus, observations can distinguish between a Planet Nine with a primordial H/He-rich atmosphere accreted from the protosolar nebula and ...
Traditional definitions of the habitable zone assume that habitable planets contain a carbonate-s... more Traditional definitions of the habitable zone assume that habitable planets contain a carbonate-silicate cycle that regulates CO2 between the atmosphere, surface, and the interior. Such theories have been used to cast doubt on the habitability of ocean worlds. However, Levi et al (2017) have recently proposed a mechanism by which CO2 is mobilized between the atmosphere and the interior of an ocean world. At high enough CO2 pressures, sea ice can become enriched in CO2 clathrates and sink after a threshold density is achieved. The presence of subpolar sea ice is of great importance for habitability in ocean worlds. It may moderate the climate and is fundamental in current theories of life formation in diluted environments. Here, we model the Levi et al. mechanism and use latitudinally-dependent non-grey energy balance and single-column radiative-convective climate models and find that this mechanism may be sustained on ocean worlds that rotate at least 3 times faster than the Earth. ...
We consider super-Earth sized planets which have a water mass fraction that is large enough to fo... more We consider super-Earth sized planets which have a water mass fraction that is large enough to form an external mantle composed of high pressure water ice polymorphs and that lack a substantial H/He atmosphere. We consider such planets in their habitable zone so that their outermost condensed mantle is a global deep liquid ocean. For these ocean planets we investigate potential internal reservoirs of CO2; the amount of CO2 dissolved in the ocean for the various saturation conditions encountered, and the ocean-atmosphere exchange flux of CO2. We find that in steady state the abundance of CO2 in the atmosphere has two possible states. When the wind-driven circulation is the dominant CO2 exchange mechanism, an atmosphere of tens of bars of CO2 results, where the exact value depends on the subtropical ocean surface temperature and the deep ocean temperature. When sea-ice formation, acting on these planets as a CO2 deposition mechanism, is the dominant exchange mechanism, an atmosphere o...
Recent astronomical observations obtained with the Kepler and TESS missions and their related gro... more Recent astronomical observations obtained with the Kepler and TESS missions and their related ground-based follow-ups revealed an abundance of exoplanets with a size intermediate between Earth and Neptune (1 R ⊕ ≤ R ≤ 4 R ⊕). A low occurrence rate of planets has been identified at around twice the size of Earth (2 × R ⊕), known as the exoplanet radius gap or radius valley. We explore the geometry of this gap in the mass–radius diagram, with the help of a Mathematica plotting tool developed with the capability of manipulating exoplanet data in multidimensional parameter space, and with the help of visualized water equations of state in the temperature–density (T–ρ) graph and the entropy–pressure (s–P) graph. We show that the radius valley can be explained by a compositional difference between smaller, predominantly rocky planets (<2 × R ⊕) and larger planets (>2 × R ⊕) that exhibit greater compositional diversity including cosmic ices (water, ammonia, methane, etc.) and gaseous...
The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist... more The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. The classical HZ definition makes a number of assumptions common to the Earth, including assuming that the most important greenhouse gases for habitable planets are CO2 and H2O, habitable planets orbit main-sequence stars, and that the carbonate-silicate cycle is a universal process on potentially habitable planets. Here, we discuss these and other predictions for the habitable zone and the observations that are needed to test them. We also, for the first time, argue why A-stars may be interesting HZ prospects. Instead of relying on unverified extrapolations from our Earth, we argue that future habitability studies require first principles approaches where temporal, spatial, physical, chemical, and biological systems are dynamically coupled. We also suggest that next-generation missions are only the beginning of a much more data-filled era in the not-to...
The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist... more The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. The classical HZ definition makes a number of assumptions common to the Earth, including assuming that the most important greenhouse gases for habitable planets are CO2 and H2O, habitable planets orbit main-sequence stars, and that the carbonate-silicate cycle is a universal process on potentially habitable planets. Here, we discuss these and other predictions for the habitable zone and the observations that are needed to test them. We also, for the first time, argue why A-stars may be interesting HZ prospects. Instead of relying on unverified extrapolations from our Earth, we argue that future habitability studies require first principles approaches where temporal, spatial, physical, chemical, and biological systems are dynamically coupled. We also suggest that next-generation missions are only the beginning of a much more data-filled era in the not-to...
This is a white paper in response to the National Academy of Sciences "Exoplanet Science Str... more This is a white paper in response to the National Academy of Sciences "Exoplanet Science Strategy" call. We summarize recent advances in theoretical habitability studies and argue that such studies will remain important for guiding and interpreting observations. Interactions between 1-D and 3-D climate modelers will be necessary to resolve recent discrepancies in model results and improve habitability studies. Observational capabilities will also need improvement. Although basic observations can be performed with present capabilities, technological advances will be necessary to improve climate models to the level needed for planetary habitability studies.
We study ocean exoplanets, for which the global surface ocean is separated from the rocky interio... more We study ocean exoplanets, for which the global surface ocean is separated from the rocky interior by a high-pressure ice mantle. We describe a mechanism that can pump salts out of the ocean, resulting in oceans of very low salinity. Here we focus on the H 2 O-NaCl system, though we discuss the application of this pump to other salts as well. We find our ocean worlds to be acidic, with a pH in the range of 2 − 4. We discuss and compare between the conditions found within our studied oceans and the conditions in which polyextremophiles were discovered. This work focuses on exoplanets in the super-Earth mass range (2M ⊕), with water composing at least a few percent of their mass. Although, the principal of the desalination pump may extend beyond this mass range.
We consider super-Earth sized planets which have a water mass fraction that is large enough to fo... more We consider super-Earth sized planets which have a water mass fraction that is large enough to form an external mantle composed of high pressure water ice polymorphs and that lack a substantial H/He atmosphere. We consider such planets in their habitable zone so that their outermost condensed mantle is a global deep liquid ocean. For these ocean planets we investigate potential internal reservoirs of CO 2 ; the amount of CO 2 dissolved in the ocean for the various saturation conditions encountered, and the ocean-atmosphere exchange flux of CO 2. We find that in steady state the abundance of CO 2 in the atmosphere has two possible states. When the wind-driven circulation is the dominant CO 2 exchange mechanism, an atmosphere of tens of bars of CO 2 results, where the exact value depends on the subtropical ocean surface temperature and the deep ocean temperature. When sea-ice formation, acting on these planets as a CO 2 deposition mechanism, is the dominant exchange mechanism, an atmosphere of a few bars of CO 2 is established. The exact value depends on the subpolar surface temperature. Our results suggest the possibility of a negative feedback mechanism, unique to water planets, where a reduction in the subpolar temperature drives more CO 2 into the atmosphere to increase the greenhouse effect.
Although carbon monoxide (CO) is an abundant molecule and may have great importance for planetary... more Although carbon monoxide (CO) is an abundant molecule and may have great importance for planetary interiors, measurements of its properties are difficult due to its extreme volatility. We calculate the equation of state for CO over a range of temperature and density that is applicable to the conditions in planetary interiors. Previous experimental and theoretical studies cover only a limited temperature-density range. Our calculations match these early results well, but now cover the full range of relevance. The method of calculation is based on the general-purpose quotidian equation of state described by More et al. (1988), which is here used in order to generate a freely downloadable look-up table to be used by the community.
We investigate the thermal equation of state, bulk modulus, thermal expansion coefficient, and he... more We investigate the thermal equation of state, bulk modulus, thermal expansion coefficient, and heat capacity of MH-III (CH 4 filled-ice Ih), needed for the study of CH 4 transport and outgassing for the case of Titan and super-Titans. We employ density functional theory and ab initio molecular dynamics simulations in the generalized-gradient approximation with a van der Waals functional. We examine the finite temperature range of 300 K-500 K and pressures between 2 GPa-7 GPa. We find that in this P-T range MH-III is less dense than liquid water. There is uncertainty in the normalized moment of inertia (MOI) of Titan; it is estimated to be in the range of 0.33 − 0.34. If Titan's MOI is 0.34, MH-III is not stable at present in Titan's interior, yielding an easier path for the outgassing of CH 4. However, for an MOI of 0.33, MH-III is thermodynamically stable at the bottom of a ice-rock internal layer capable of storing CH 4. For rock mass fractions 0.2 upwelling melt is likely hot enough to dissociate MH-III along its path. For super-Titans considering a mixture of MH-III and ice VII, melt is always positively buoyant if the H 2 O:CH 4 mole fraction is > 5.5. Our thermal evolution model shows that MH-III may be present today in Titan's core, confined to a thin (≈ 10 km) outer shell. We find that the heat capacity of MH-III is higher than measured values for pure water-ice, larger than heat capacity often adopted for ice-rock mixtures with implications for internal heating.
Bulletin of the American Astronomical Society, May 31, 2019
The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist... more The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. The classical HZ definition makes a number of assumptions common to the Earth, including assuming that the most important greenhouse gases for habitable planets are CO2 and H2O, habitable planets orbit main-sequence stars, and that the carbonate-silicate cycle is a universal process on potentially habitable planets. Here, we discuss these and other predictions for the habitable zone and the observations that are needed to test them. We also, for the first time, argue why A-stars may be interesting HZ prospects. Instead of relying on unverified extrapolations from our Earth, we argue that future habitability studies require first principles approaches where temporal, spatial, physical, chemical, and biological systems are dynamically coupled. We also suggest that next-generation missions are only the beginning of a much more data-filled era in the not-too-distant future, when possibly hundredsthousands of HZ planets will yield the statistical data we need to go beyond just finding habitable zone planets to actually determining which ones are most likely to exhibit life.
By using a hydrodynamic atmospheric escape mechanism amit09 we show how the unusually high mass d... more By using a hydrodynamic atmospheric escape mechanism amit09 we show how the unusually high mass density of Quaoar could have been predicted (constrained), without any knowledge of a binary companion. We suggest an explanation of the recent spectroscopic observations of Orcus and Charon delsanti10,cook07. We present a simple relation between the detection of certain volatile ices and the body mass density and diameter. As a test case we implement the relations on the KBO 2003 AZ_84 and give constraints on its mass density. We also present a method of relating the latitude-dependence of hydrodynamic gas escape to the internal structure of a rapidly rotating body and apply it to Haumea.
We examine the possibility that icy super-Earth mass planets, formed over long time scales (0.1--... more We examine the possibility that icy super-Earth mass planets, formed over long time scales (0.1--1 Gyr) at large distances (∼ 200--1000 AU) from their host stars, will develop massive H-rich atmospheres. Within the interior of these planets, high pressure converts CH_4 into ethane, butane, or diamond and releases H_2. Using simplified models which capture the basic physics of the internal structure, we show that the physical properties of the atmosphere depend on the outflux of H_2 from the mantle. When this outflux is ≲ 10^10 [molec cm^-2 s^-1], the outgassed atmosphere has base pressure ≲ 1 bar. Larger outflows result in a substantial atmosphere where the base pressure may approach 10^3 - 10^4 bar. For any pressure, the mean density of these planets, 2.4--3 [g cm^-3], is much larger than the mean density of Uranus and Neptune, 1.3--1.6 [g cm^-3]. Thus, observations can distinguish between a Planet Nine with a primordial H/He-rich atmosphere accreted from the protosolar nebula and ...
Traditional definitions of the habitable zone assume that habitable planets contain a carbonate-s... more Traditional definitions of the habitable zone assume that habitable planets contain a carbonate-silicate cycle that regulates CO2 between the atmosphere, surface, and the interior. Such theories have been used to cast doubt on the habitability of ocean worlds. However, Levi et al (2017) have recently proposed a mechanism by which CO2 is mobilized between the atmosphere and the interior of an ocean world. At high enough CO2 pressures, sea ice can become enriched in CO2 clathrates and sink after a threshold density is achieved. The presence of subpolar sea ice is of great importance for habitability in ocean worlds. It may moderate the climate and is fundamental in current theories of life formation in diluted environments. Here, we model the Levi et al. mechanism and use latitudinally-dependent non-grey energy balance and single-column radiative-convective climate models and find that this mechanism may be sustained on ocean worlds that rotate at least 3 times faster than the Earth. ...
We consider super-Earth sized planets which have a water mass fraction that is large enough to fo... more We consider super-Earth sized planets which have a water mass fraction that is large enough to form an external mantle composed of high pressure water ice polymorphs and that lack a substantial H/He atmosphere. We consider such planets in their habitable zone so that their outermost condensed mantle is a global deep liquid ocean. For these ocean planets we investigate potential internal reservoirs of CO2; the amount of CO2 dissolved in the ocean for the various saturation conditions encountered, and the ocean-atmosphere exchange flux of CO2. We find that in steady state the abundance of CO2 in the atmosphere has two possible states. When the wind-driven circulation is the dominant CO2 exchange mechanism, an atmosphere of tens of bars of CO2 results, where the exact value depends on the subtropical ocean surface temperature and the deep ocean temperature. When sea-ice formation, acting on these planets as a CO2 deposition mechanism, is the dominant exchange mechanism, an atmosphere o...
Recent astronomical observations obtained with the Kepler and TESS missions and their related gro... more Recent astronomical observations obtained with the Kepler and TESS missions and their related ground-based follow-ups revealed an abundance of exoplanets with a size intermediate between Earth and Neptune (1 R ⊕ ≤ R ≤ 4 R ⊕). A low occurrence rate of planets has been identified at around twice the size of Earth (2 × R ⊕), known as the exoplanet radius gap or radius valley. We explore the geometry of this gap in the mass–radius diagram, with the help of a Mathematica plotting tool developed with the capability of manipulating exoplanet data in multidimensional parameter space, and with the help of visualized water equations of state in the temperature–density (T–ρ) graph and the entropy–pressure (s–P) graph. We show that the radius valley can be explained by a compositional difference between smaller, predominantly rocky planets (<2 × R ⊕) and larger planets (>2 × R ⊕) that exhibit greater compositional diversity including cosmic ices (water, ammonia, methane, etc.) and gaseous...
The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist... more The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. The classical HZ definition makes a number of assumptions common to the Earth, including assuming that the most important greenhouse gases for habitable planets are CO2 and H2O, habitable planets orbit main-sequence stars, and that the carbonate-silicate cycle is a universal process on potentially habitable planets. Here, we discuss these and other predictions for the habitable zone and the observations that are needed to test them. We also, for the first time, argue why A-stars may be interesting HZ prospects. Instead of relying on unverified extrapolations from our Earth, we argue that future habitability studies require first principles approaches where temporal, spatial, physical, chemical, and biological systems are dynamically coupled. We also suggest that next-generation missions are only the beginning of a much more data-filled era in the not-to...
The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist... more The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. The classical HZ definition makes a number of assumptions common to the Earth, including assuming that the most important greenhouse gases for habitable planets are CO2 and H2O, habitable planets orbit main-sequence stars, and that the carbonate-silicate cycle is a universal process on potentially habitable planets. Here, we discuss these and other predictions for the habitable zone and the observations that are needed to test them. We also, for the first time, argue why A-stars may be interesting HZ prospects. Instead of relying on unverified extrapolations from our Earth, we argue that future habitability studies require first principles approaches where temporal, spatial, physical, chemical, and biological systems are dynamically coupled. We also suggest that next-generation missions are only the beginning of a much more data-filled era in the not-to...
This is a white paper in response to the National Academy of Sciences "Exoplanet Science Str... more This is a white paper in response to the National Academy of Sciences "Exoplanet Science Strategy" call. We summarize recent advances in theoretical habitability studies and argue that such studies will remain important for guiding and interpreting observations. Interactions between 1-D and 3-D climate modelers will be necessary to resolve recent discrepancies in model results and improve habitability studies. Observational capabilities will also need improvement. Although basic observations can be performed with present capabilities, technological advances will be necessary to improve climate models to the level needed for planetary habitability studies.
We study ocean exoplanets, for which the global surface ocean is separated from the rocky interio... more We study ocean exoplanets, for which the global surface ocean is separated from the rocky interior by a high-pressure ice mantle. We describe a mechanism that can pump salts out of the ocean, resulting in oceans of very low salinity. Here we focus on the H 2 O-NaCl system, though we discuss the application of this pump to other salts as well. We find our ocean worlds to be acidic, with a pH in the range of 2 − 4. We discuss and compare between the conditions found within our studied oceans and the conditions in which polyextremophiles were discovered. This work focuses on exoplanets in the super-Earth mass range (2M ⊕), with water composing at least a few percent of their mass. Although, the principal of the desalination pump may extend beyond this mass range.
We consider super-Earth sized planets which have a water mass fraction that is large enough to fo... more We consider super-Earth sized planets which have a water mass fraction that is large enough to form an external mantle composed of high pressure water ice polymorphs and that lack a substantial H/He atmosphere. We consider such planets in their habitable zone so that their outermost condensed mantle is a global deep liquid ocean. For these ocean planets we investigate potential internal reservoirs of CO 2 ; the amount of CO 2 dissolved in the ocean for the various saturation conditions encountered, and the ocean-atmosphere exchange flux of CO 2. We find that in steady state the abundance of CO 2 in the atmosphere has two possible states. When the wind-driven circulation is the dominant CO 2 exchange mechanism, an atmosphere of tens of bars of CO 2 results, where the exact value depends on the subtropical ocean surface temperature and the deep ocean temperature. When sea-ice formation, acting on these planets as a CO 2 deposition mechanism, is the dominant exchange mechanism, an atmosphere of a few bars of CO 2 is established. The exact value depends on the subpolar surface temperature. Our results suggest the possibility of a negative feedback mechanism, unique to water planets, where a reduction in the subpolar temperature drives more CO 2 into the atmosphere to increase the greenhouse effect.
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