In around five billion years, the Sun will turn into a red giant, and it will be doomsday for Earth. But something interesting will happen in the outskirts of the Solar System: the temperature will become warm enough for icy planets and satellites like Pluto, Europa and Titan to become temperate. They will spend some time in the “habitable zone”, with temperatures suitable for liquid oceans. These periods of time will be short compared to the time scale of life on Earth, but will still be counted in millions of years.
In a previous post, we followed the fate of Pluto. Let’s now turn to Titan, the giant moon of Saturn. Titan today is shrouded in a thick nitrogen atmosphere, with methane lakes on the surface and temperatures around 90 K (minus 180 degrees Celsius). As the Sun turn into a red giant, Titan will spend some time in the habitable zone. Since water is the main constituent of the interior of the planet, a water-vapour atmosphere will form and water oceans might be stable at the surface.
Whereas Pluto will only remain in the habitable zone for a few million years under the fast-changing Sun of the helium-flash stage, Titan will stay in the right temperature range under a stable red giant Sun for more than a hundred million years.
The far future of Titan
Using the Dartmouth models for the evolution of the Sun, I calculated the changes in the temperature of Titan near the end of the life of the Sun. During the stable, helium-burning stage of the life of the Sun as a red giant, Titan will be close to the cold edge of the habitable zone (like Mars today).
Two experts have published a model of Titan under a red giant Sun (Lorenz & Lunine 1997, Geophys. Res Lett. 24, 2905-8, Titan under a red giant sun: a new kind of “habitable” moon). Their main finding is that the “anti-freeze” action of ammonia dissolved in water oceans, combined with the powerful greenhouse effect of water vapour, should allow oceans to remain liquid on the future Titan, even near the cold edge of the habitable zone.
Could life evolve again from scratch in the late oceans of Titan? One hundred million years is comparable to the time scale of the apparition of life on Earth, to the best of our knowledge. So there is no show-stopper than we know of, although Titan’s oceans would be lacking one elements that is thought to have played a key role in the emergence of life on Earth, a volcanic bedrock at the bottom of the oceans.
The future of Jupiter’s icy moons
What about the icy satellites of Jupiter: Ganymede, Callisto and Europa? They will be close to the warm edge of the habitable zone around a the red-giant Sun. Their water surfaces will melt, bringing to light the underground oceans that we know are present today under their icy crust. Jupiter will be surrounded by three magnificent ocean planets (its innermost large satellite, Io, is entirely rocky, having lost its water long ago because of the intense volcanism caused by the tidal influence of Jupiter).
But with so much water vapour in the atmosphere, the greenhouse effect will be enormous, and might well turn the icy moons into “steam planets”, with dense water vapour atmospheres gradually turning into hot high-pressure water, without any actual ocean surface. The steam atmospheres of Ganymede, Callisto and Europa might therefore be too hot for life as we know it.
Disclaimer: there are other factors to consider for a full picture. For instance, the Sun will loose part of its envelope during the red giant stage. This will widen the orbit of Saturn and Jupiter, and submit their moons to very large solar winds. Titan and the other moon will also have been blasted by the bright “helium flash” Sun for a few million years before the red giant stage, which might modify their surface and atmospheric composition.
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