Tidally heated exomoons

No exomoons are discovered so far, but as instruments are developing, it is probable that these observations will begin soon. For this reason it is important to have basic knowledge about exomoons by the first detections, that will help in selecting further observation targets. In the selection habitability is an important aspect, since life can evolve on a celestial body with suitable inner characteristics and outer environmental effects. In the case of moons such effects are the radiation of the central star, the distance from the star, reflected radiation and thermal emission from the planet, the orbital elements of the moon, the atmosphere, the composition, material quality and tidal heating.

Tidal heating is an inner energy source caused by gravitational forces of the nearby body (the planet in this case). Tidal forces arise inside the moon that cause friction, which leads to a warming effect. In the Solar System there are several examples of moons that have significant warming caused by tidal heating. For example Io, moon of Jupiter, which is the most volcanically active body in the Solar System. Or Europa, that has a huge ocean below the ice sheet. The possibility of life inside this ocean is intensively studied. On such exomoons that are too far from their central star and hence their surface temperature is low, it is possible that the warmth of tidal heating allows the emergence of life.

We applied a viscoelastic model for exomoons for the first time to describe the tidal heating and the melting of the material inside the body (Dobos & Turner, 2015). This model is more realistic than the widely used fixed Q model that ignore the temperature dependency of the parameters which describe internal heat dissipation of tidal deformations. Using the viscoelastic model, we define the circumplanetary Tidal Temperate Zone (TTZ) which is the region where the tidal-heat-induced surface temperature of the satellite is between 0 and 100°C. We have found that the viscoelastic model predicts 2.8 times more exomoons in the TTZ than the fixed Q model with orbital periods between 0.1 and 3.5 days 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.

Dobos V., Turner E. L., 2015, Viscoelastic Models of Tidally Heated Exomoons, The Astrophysical Journal 804, 41 (online)

Forgan D., Dobos V., 2016, Exomoon climate models with the carbonate-silicate cycle and viscoelastic tidal heating, Monthly Notices of the Royal Astronomical Society (online)

Dobos V., Heller R., Turner E. L., 2017, The effect of multiple heat sources on exomoon habitable zones, Astronomy and Astrophysics 601, A91 (online)