Long have we talked about the requirements for a world able to harbor our kind of life. We needed liquid water for life to evolve on our planet, and we continue to need it in order to survive. And by “we” I mean not just humans, but all life -- including animals and plants. The range of orbits around a star where a planet will be able to have liquid water for at least part of its year is called the “habitable zone.” Much effort is currently being expanded to find planets in the habitable zone around other stars, and to see if these planets in fact have water. But having liquid water is by no means the only condition that needs to be met.
There was a good reason V’ger’s Ilia probe spoke of “carbon units” in Star Trek: The Motion Picture. All life on this Earth revolves around the carbon atom. As it is able to make four chemical bonds, it is able to form long and intricate chains of organic molecules, the kinds necessary to enable life. In order to form and sustain this life, carbon and other materials such as hydrogen, sulphur, potassium, oxygen, carbon-dioxide and nitrogen need to be not only available, but need to be regularly replenished and recycled in our eco-system.
Carbon atoms are produced via nuclear fusion in the cores of stars. The reason we have carbon in our Solar System is that previous generations of stars created them, then scattered them back into the clouds of gas and dust at the end of their lives. These clouds, such as the Orion Nebula, are the birthplaces of stars and solar systems. After Earth formed in the Solar nebula, outgassing from erupting volcanoes provided the beginnings of our atmosphere, at that time mostly carbon-dioxide. Early bacteria and later plant life used the carbon-dioxide during photosynthesis to produce oxygen. Herbivores consume the carbon in the plants, which they incorporate into their bodies. They in turn are eaten by carnivores. Plants, herbivores and carnivores eventually die and provide their carbon to the decomposers. Some of these might end up as fossil fuels after a very long time. So far, so good, but how do you recycle carbon and other materials in your eco-system to replenish it in the atmosphere?
For that you need the continuing existence of volcanoes. But how do you get volcanoes? The answer is plate tectonics. If we look around on Earth for the locations of volcanoes, we see that the vast majority of them are at the plate boundaries, specifically where plates converge and one plate moves underneath another one. The plate moving underneath dissolves into the mantle, generating heat and magma, which results in the formation of volcanoes on the top plate near the boundary. That’s why we have volcanoes on the West Coast of the Americas and all around the Pacific Rim, which is otherwise referred to as the “Ring of Fire.” The volcanoes erupt and release carbon dioxide and other materials back onto the surface.
Earth’s crust has just the right thickness to be able to break into good-sized plates that can ride on the upper mantle’s convection currents. Venus’ crust is too thin, the mantle’s convection currents only manage to break it up into crumbled pieces, much like crumb cake. Mars’ crust is too thick; it can’t break up into plates. So only Earth appears to have the right conditions for plate tectonics, or does it?
Earlier this year NASA and the Johns Hopkins University announced that Europa, one of Jupiter’s large moons, also features plate tectonics. Only, in this case, the plates are thick ice sheets riding on a subsurface ocean.
From a passing spacecraft, we can see Europa’s icy surface, looking somewhat cracked with mud-colored substance in the cracks.
Europa, moon of Jupiter. Image Credit: NASA.
From previous research we know that there is a liquid ocean under this about 20-mile-deep ice layer, though we do not yet know what exactly is in this ocean. There has been talk about sending a probe to drill into the ice, an idea which will likely be explored in the future. It appears that this icy surface gets renewed by liquid ascending through the cracks, freezing at the surface.
But if new material appears on the surface, older material needs to be recycled. This appears to happen, just like on Earth, by plates moving underneath other plates, dissolving in this case in the warmer layer underneath, which is warmer than the ice above. The image below illustrates what’s going on.
Europa’s layers of rock, ocean, and ice surface. Image Credit: JHU and NASA.
Europa is a rocky body, with a liquid ocean layer, above which is the ice shell. Gravitational squeezing and tidal flexing of Europa, located between massive Jupiter and the other two larger moons, results in undersea volcanoes, which warm the ocean. This in turn causes the lower ice shell to warm, and the plates of the upper ice shell to move. Just as with the converging plates on Earth, the result is the formation of volcanoes near the upper plate rim. Only in this case, these volcanoes are not spewing out lava as we’re used to on Earth, but something called cryolava. Cryovolcanoes, also called ice-volcanoes, erupt water, ammonia, and/or methane, instead of molten rock.
Europa’s upper layers. Image Credit: Noah Kroese, JHU, NASA.
Given this situation, Europa has not only liquid water, but also a mechanism to renew and recycle materials between the interior and the surface. Aside from being fascinating for geologists, it also moves Europa up in the priority list for the search for life beyond Earth. No, we haven’t found life there yet, but the potential exists.
In Star Trek we have seen several ice worlds, such as Andoria, which, like Europa, is a moon around a gas giant. Given that life evolved there, I would not be surprised to read that it might have similar conditions as Europa. As more becomes known about Europa, maybe some of the details of Andoria could be filled in.
Andoria. Image Origin: Memory Alpha.
Rura Penthe, the penal colony in Star Trek VI, was placed on an inhospitable planet on purpose, so we do not need to assume that it ever evolved life. Similarly, we don’t have enough information to draw any conclusions about the habitability of the ice world that Voyager crashed on in “Timeless.”
Rura Penthe (left) and VOY “Timeless” planet (right). Image Origin: Memory Alpha.
Delta Vega in the Vulcan system, however, did have indigenous species. The critters that chased young Kirk across the ice in Star Trek (2009) would need a suitable environment to allow their development and continued existence.
Delta Vega. Image Origin: Memory Alpha.
In conclusion, we might have to expand our idea of habitable worlds to include those that may be outside of the usual distance from their primary star to allow liquid water on the surface, but may have liquid water under the surface instead.
Here are a few links for you if you’d like to know more about this topic.
NASA Press Release (Scientists Find Evidence of ‘Diving’ Tectonic Plates on Jupiter’s Moon Europa):
Johns Hopkins University Press Release (APL Scientist Finds Evidence of ‘Diving’ Tectonic Plates on Jupiter’s Large, Icy Moon):