During its formation, Venus was in the Solar System’s habitable zone – much like Earth is now. Scientists think its surface contained liquid water, and its atmosphere was somewhat like Earth’s. Maybe there was life, too. However, as the levels of carbon dioxide kept increasing, its atmosphere became opaque, trapping most of the heat reflected by its surface, and Venus heated up to the point where its oceans boiled away. Today, life on the planet’s waterless surface is considered unlikely, except perhaps by those who’ve read a November 2014 study involving supercritical carbon dioxide, and those who believe in Hell.
— Justin Starr (@UrbanAstroNYC) April 18, 2014
Why can’t this be the case on alien worlds possessing water as well? Discoveries made since the mid-1990s – especially by the Kepler space telescope and probes in the Jovian and Saturnian systems – have unearthed a variety of worlds that could, or do, have liquid water on or below the surface. On Earth, life has been found wherever liquid water has been found, so liquid water on other planets and moons gets scientists excited about the possibility of alien life. Recent discoveries of a subsurface ocean on Europa and possibly on some other moons of Jupiter and Saturn have even prompted NASA to plan for a probe to Europa in the mid-2020s.
A study published online (paywall) in the Monthly Notices of the Royal Astronomical Society applies the brakes on that excitement to some extent. A kind of exoplanet which scientists think could host lots of liquid water—some 100-times the amount of water on Earth, in fact— are the so-called ‘waterworlds‘. They would have oceans so deep and wide that, according to the study, their effects on themselves and the planet’s climate would be incomparable to that on Earth – and altogether might not be hospitable to life the way we know liquid water can usually be.
The study’s authors write, “One important consequence is, for example, the formation of high-pressure water ice at the bottom of the ocean, which prevents the immediate contact of the planetary crust with the liquid ocean.” This in turn mutes the carbon-silicate cycle, a recycling of carbon and silicon compounds on the ocean floor that determines how much carbon dioxide is released from the oceans into the atmosphere.
The authors calculate that on an (at least) Earth-sized waterworld in the habitable zone of its star, there can be 25-100 Earth oceans for temperatures ranging from the freezing point of water to just beyond the boiling point. So a colder planet, say at 0° C, would have a smaller ocean and lesser liquid water to be able to absorb the carbon dioxide (and its absorptive capabilities can’t ‘power up’ without the carbon-silicate cycle). Yet, at lower temperatures the oceans are able to dissolve more gases, even as the pressure exerted by the gas on the ocean’s surface is higher. So a colder planet with a smaller ocean will dissolve more carbon dioxide from the atmosphere – turning the planet even cooler.
Similarly, a warmer waterworld will be able to absorb less carbon dioxide, letting the greenhouse gas accumulate in the atmosphere, heat the surface up and eventually boil the oceans away (like on Venus). In short, a waterworld whose temperatures are outside a specific range will become hotter if it’s warm and even colder if it’s cold. These runaway effects can occur pretty quickly, too.
Based on the chemical properties of water and carbon dioxide, the scientists estimate that the life-friendly temperature range is from 273 K to 400 K (0° to 127° C). And even in this range, there could be threats to life in the form of ocean acidity. On Earth, limestone that’s in contact with water dissolves and keeps the water’s acidity in check, but this may not be happening on waterworlds where large landmasses could be a rarity or relatively smaller in size.
At the same time, these pessimistic speculations are offset by some assumptions the scientists have made in their study. For example, they assume that the waterworld doesn’t have tectonic activity. Such activity on Earth involves the jigsaw of landmasses grindings against each other, sometimes subducting one below the other to push down some minerals while volcanoes in other areas spew out others—in all making for a giant geological cycle that ensures the substances needed to sustain life are constantly replenished. If a waterworld were to have tectonic activity, it would also influence the carbon-silicate cycle and keep a runaway greenhouse effect from happening.
On Earth, the warming of the oceans presents a big problem to climatologists partly because its mechanisms and consequences are not fully understood – and more so to marine creatures. And as the oceans are able to dissolve more anthropogenic carbon dioxide, they also become more acidic. Yet, the effects are relatively smaller (ignoring the presence of life for a moment) compared to that on waterworlds – comprising no above-sea-level landmasses and infinite seas 100 km deep.