Extraterrestrial tube worms
A thought exercise in astrobiology
Roughly since the invention of the telescope and the paradigm shift to a heliocentric model of the Solar System, life outside Earth has been a subject of theories and debates. Current advanced technologies have allowed us to explore space farther and with more detail than ever before, even beyond our own solar system. In 2017 seven Earth-sized planets were discovered orbiting a dwarf star outside our solar system (Gillon et al., 2017). Three of these planets fall in the so-called Goldilocks zone, the habitable zone of a solar system. In a paper by Borucki et al. (2013) the habitable zone is characterised as “the annulus around a star where a rocky planet with a CO2-H2O-N2 atmosphere and sufficiently large water content (such as on Earth) can host liquid water on its solid surface.” For a long time it was believed that while the chance of life outside Earth within the known universe is statistically high, to find life within the Solar System is less likely. This has to do with Earth being the only planet within our solar system situated in the Goldilocks zone. Nevertheless, finding life outside the habitable zone is not out of the realm of possibility.
In order to understand what kind of life can survive beyond Earth one can look at the life forms that live in extreme conditions here on Earth. One of these kind of ‘extremophiles’ are vestimentiferan tube worms that inhabit the hydrothermal vents deep underwater in parts of the mid-ocean ridge like the East Pacific Rise (Hunt et al., 2004) and the Galápagos Rift (Corliss et al., 1979). There are three requisites for all life as we know it: a source of energy (Pace, 2001), a source of carbon (Miller & Orgel, 1974), and water (Feinberg & Shapiro, 1980). Most life on Earth is powered by the Sun which is why the Goldilocks zone is not too close, nor too far, from the star of our solar system. For many years the understanding was that life needs energy from the Sun. However, living deep in the ocean, beyond the reach of solar radiation, these tube worms are completely independent from the energy provided by the Sun (Hunt et al., 2004). Corliss et al. (1979) explain this anomaly by the energy provided by the Earth itself. Instead of photosynthesis tube worms develop a symbiosis with bacteria that carry out chemosynthesis (Adams et al., 2012). These bacteria use non-organic chemical reactions which occur due to volcanic activity in the mid-ocean ridge. There are tens of thousands magma chambers underneath the mid-ocean ridge; as the magma meets with seawater, hydrothermal vents emitting sulfide minerals are created. The iron-sulfur world hypothesis (Wächtershauser, 1988) suggests that life on Earth started from these sulfide-laden hydrothermal vents, also known as black smokers. The energy sources of microbial life are hijacked by multicellular life forms, thus adapting to life without solar energy. The life forms independent of solar energy inhabiting the mid-ocean ridge can be seen as a window to what kind of life could survive outside Earth, and what kind of environments within our solar system provide a habitat for it.
In the past decade, data provided by NASA’s Cassini spacecraft has indicated that one of Saturn’s moons, the icy Enceladus, might be habitable for microbial life. At first, Enceladus was not unique in terms of what is known about its properties and its capability to host life. In 2015, research based on the Cassini spacecraft data confirmed that Enceladus has a global ocean beneath its surface of ice. The ice crust of one of Jupiter’s moons, Europa, also hides a layer of water (Chang, 2015). The smoothness of the surface and the plumes of vapour escaping the cracks of Europa’s surface provide strong evidence for this global ocean. Additionally, some evidence has been found for the existence of water on two of Jupiter’s other moons, Ganymede and Callisto. The possibility of any of these moons hosting water beneath their surfaces is interesting when looking for life beyond Earth, and for some time NASA’s priority in the hunt for extraterrestrial life has been to find water. Life as we know it needs a liquid as solvent and water has been found to be the liquid able to dissolve the most substances. The second thing to look for is a rocky floor for the water, one which Europa’s global ocean might have. The red colouring of Europa’s surface has led scientists to speculate that the colouring is due to salt in the ocean which in turn would indicate that the water is in contact with rock. These findings made Europa the most important subject for exploring habitability and seeking signs of life. That is, until more was learned about Enceladus.
Recent data suggests Enceladus has volcanic activity on the ocean floor: vapour is escaping from the cracks of Enceladus’s ice surface like from Europa, and as Cassini flew through one of these plumes it was confirmed to be full of hydrogen. In an interview (Franz, 2017) by Public Radio International a space physicist Scott Bolton said Enceladus is not sufficiently big to have a gravitational field which could trap the amount of hydrogen that has been observed. This means it is probable that Enceladus’s subsurface ocean is reacting with rock in the same kind of process as hydrothermal vents do on Earth. This hydrothermal activity produces the so-called black and white smokers underwater which modify the seawater chemically so that it is rich with minerals and produce hydrogen. These kind of hot fluid vents are what the tube worms live off of on Earth among other life forms. Also, it has been suggested that Mars has inactive hydrothermal vents on its crust which might have fossilised evidence of life (Paine, 2001). Finding fossilised evidence on Mars could be technically easier since there have been successful Mars landings, whereas landing on Enceladus has not been attempted yet and getting to its ocean would be tricky, as the lander would need an extremely powerful drill to make its way through the thick ice crust.
The deep-sea life on Earth evidences that the Sun is not the only source of energy, thus introducing the possibility of extraterrestrial life outside the Goldilocks zone. A habitat for microbial life could in theory also be a habitat for cellular life. As mentioned, the iron-sulfur world hypothesis suggests that life on Earth originates from hydrothermal vents in the borders of tectonic plates with volcanic activity. If neither Enceladus nor Europa host life at the moment, in theory it could still be possible for life forms similar to those in Earth’s oceans to evolve. However, by appreciating how little is known about the life in the depths of our oceans here on Earth, extraterrestrial tube worms do not seem such a far-fetched concept.
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