Exomoon Climate Models with the Carbonate-Silicate Cycle and Viscoelastic Tidal Heating

Exomoon Climate Models with the Carbonate-Silicate Cycle and Viscoelastic Tidal Heating

Authors:

Forgan et al

Abstract:

We presence of tidal heating, eclipses and planetary illumination in the exomoon energy budget combine to produce both circumstellar and circumplanetary habitable regions. Analytical calculations suggest that the circumplanetary habitable region is defined only by an inner edge (with its outer limits determined by orbital stability). Subsequent calculations using 1D latitudinal climate models indicated that the combined effect of eclipses and ice-albedo feedback can produce an outer edge to the circumplanetary habitable zone. But is this outer edge real, or an artefact of the climate model's relative simplicity?

We present an upgraded 1D climate model of Earth-like exomoon climates, containing the carbonate-silicate cycle and viscoelastic tidal heating. We conduct parameter surveys of both the circumstellar and circumplanetary habitable zones, and we find that the outer circumplanetary habitable edge remains provided the moon's orbit is not inclined relative to that of the planet. Adding the carbonate-silicate cycle pushes the circumplanetary habitable zone outward, by allowing increases in atmospheric partial pressure of carbon dioxide to boost the greenhouse effect. Viscoelastic tidal heating widens the habitable zone compared to standard, fixed-Q models. Weakening the tidal heating effect due to melting allows moons to be habitable at higher eccentricity, and pushes the inner circumstellar and circumplanetary habitable zone boundary inward.

Exomoons in the Habitable Zone may Need to be at Least .5 Earth Mass to Retain an Significant Atmosphere

Origin and Stability of Exomoon Atmospheres - Implications for Habitability

Authors:

Lammer et al

Abstract:

We study the origin and escape of catastrophically outgassed volatiles (H2O, CO2) from exomoons with Earth-like densities and masses of 0.1M⊕, 0.5M⊕ and 1M⊕ orbiting an extra-solar gas giant inside the habitable zone of a young active solar-like star. We apply a radiation absorption and hydrodynamic upper atmosphere model to the three studied exomoon cases. We model the escape of hydrogen and dragged dissociation products O and C during the activity saturation phase of the young host star. Because the soft X-ray and EUV radiation of the young host star may be up to ∼100 times higher compared to today's solar value during the first 100 Myr after the system's origin, an exomoon with a mass less than 0.25M⊕ located in the HZ may not be able to keep an atmosphere because of its low gravity. Depending on the spectral type and XUV activity evolution of the host star, exomoons with masses between ∼0.25−0.5M⊕ may evolve to Mars-like habitats. More massive bodies with masses greater than 0.5M⊕, however, may evolve to habitats that are a mixture of Mars-like and Earth-analogue habitats, so that life may originate and evolve at the exomoon's surface.

What Does it Take to Form a Mars Mass Icy ExoMoon Around Super Jovians?

Conditions for water ice lines and Mars-mass exomoons around accreting super-Jovian planets at 1 - 20 AU from Sun-like stars

Authors:

Heller et al

Abstract:

Exomoon detections might be feasible with NASA's Kepler or ESA's upcoming PLATO mission or the ground-based E-ELT. To use observational resources most efficiently we need to know where the largest, most easily detected moons can form. We explore the possibility of large exomoons by following the movement of water (H2O) ice lines in the accretion disks around young super-Jovian planets. We want to know how different heating sources in those disks affect the H2O ice lines. We simulate 2D rotationally symmetric accretion disks in hydrostatic equilibrium around super-Jovian exoplanets. The energy terms in our semi-analytical model -- (1) viscous heating, (2) planetary illumination, (3) accretional heating, and (4) stellar illumination -- are fed by precomputed planet evolution tracks. We consider planets accreting 1 to 12 Jupiter masses at distances between 1 and 20 AU to a Sun-like star. Accretion disks around Jupiter-mass planets closer than ~4.5 AU to Sun-like stars do not feature H2O ice lines, but the most massive super-Jovians can form icy satellites as close as ~3 AU to Sun-like stars. Super-Jovian planets forming beyond ~5 AU can host Mars-mass moons. We study a broad range of disk parameters for planets at 5.2 AU and find that the H2O ice lines are universally between ~15 and 30 Jupiter radii when the last generation of moons is forming. If the abundant population of super-Jovian planets at ~1 AU formed in situ, then they should lack giant icy moons because their disks did not host H2O ice in the final stages of accretion. In the more likely case that these planets migrated to their current locations from beyond a few AU, they might be orbited by large, H2O-rich moons. In this case, Mars-mass ocean moons might be common in the stellar habitable zones. Future exomoon searches can provide powerful constraints on the formation and migration history of giant exoplanets.

Habitability of Possible Exomoons in Observed F-star ExoPlanetary Systems From the Impact of Ultraviolet Light

UV Habitability of Possible Exomoons in Observed F-star Planetary Systems

Authors:

Sato et al

Abstract:

In the present study we explore the astrobiological significance of F-type stars of spectral type between F5 V and F9.5 V, which possess Jupiter-type planets within or close to their climatological habitable zones. These planets, or at least a subset of them, may also possess rocky exomoons, which potentially offer habitable environments. Our work considers eight selected systems. The Jupiter-type planets in these systems are in notably different orbits with eccentricities ranging from 0.08 to 0.72. Particularly, we consider the stellar UV environments provided by the photospheric stellar radiation in regard to the circumstellar habitability of the system. According to previous studies, DNA is taken as a proxy for carbon-based macromolecules following the paradigm that extraterrestrial biology might be based on hydrocarbons. Thus, the DNA action spectrum is utilized to represent the impact of the stellar UV radiation. Atmospheric attenuation is taken into account based on parameterized attenuation functions. We found that the damage inflicted on DNA is notably different for the range of systems studied, and also varies according to the orbit of the Jupiter-type planet, especially in systems of high ellipticity. For some systems large values of damage are attained compared to an Earth-type planet at Earth-like positions in the solar system. A highly protective exomoon atmosphere would be required in most systems to foster habitable environments, notwithstanding extremophiles or systems based on nonstandard exobiology, which are beyond the scope of the present study.

A Better Model for Exomoon Tidal Heating

Viscoelastic Models of Tidally Heated Exomoons

Authors:

Dobos et al

Abstract:

Tidal heating of exomoons may play a key role in their habitability, since the elevated temperature can melt the ice on the body even without significant solar radiation. The possibility of life is intensely studied on Solar System moons such as Europa or Enceladus, where the surface ice layer covers tidally heated water ocean. Tidal forces may be even stronger in extrasolar systems, depending on the properties of the moon and its orbit. For studying the tidally heated surface temperature of exomoons, we used a viscoelastic model for the first time. This model is more realistic than the widely used, so-called fixed Q models, because it takes into account the temperature dependency of the tidal heat flux, and the melting of the inner material. With the use of this model we introduced the circumplanetary Tidal Temperate Zone (TTZ), that strongly depends on the orbital period of the moon, and less on its radius. We compared the results with the fixed Q model and investigated the statistical volume of the TTZ using both models. We have found that the viscoelastic model predicts 2.8 times more exomoons in the TTZ with orbital periods between 0.1 and 3.5 days than the fixed Q model for plausible distributions of physical and orbital parameters. The viscoelastic model gives more promising results in terms of habitability, because the inner melting of the body moderates the surface temperature, acting like a thermostat.

Attempting to Understand the Potential for Habitability of ExoMoons

The Effect of Planetary Illumination on Climate Modelling of Earthlike Exomoons

Authors:

Forgan et al

Abstract:

From analytical studies of tidal heating, eclipses and planetary illumination, it is clear that the exomoon habitable zone (EHZ) - the set of moon and host planet orbits that permit liquid water on an Earthlike moon's surface - is a manifold of higher dimension than the planetary HZ.

This paper outlines 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 temperature on an Earthlike moon orbiting a Jupiterlike planet, by adding planetary illumination.

We investigate the EHZ in four dimensions, running two separate suites of simulations. The first investigates the EHZ by varying the planet's orbit, keeping the moon's orbit fixed, to compare the EHZ with planetary habitable zones. In general, planetary illumination pushes EHZs slightly further away from the star.

Secondly, we fix the planet's orbit and vary the moon's orbit, to investigate the circumplanetary inner habitable edge. We demonstrate that an outer edge can exist due to eclipses (rather than merely orbital stability), but this edge may be pushed outwards when the effect of the carbonate-silicate cycle is taken into account.