Jupiter’s icy moon Europa is a major target of astrobiology research in light of the possibility that it offers a habitable environment in the Solar System. Under its ice crust, estimated to be 10 km thick, is an ocean of liquid water of over 100 km deep. A huge source of energy deriving from gravitational interaction with Jupiter keeps this water warm.
This is what makes Europa so interesting. The US National Aeronautics and Space Administration (NASA) is planning a mission, probably for the 2030s, to study Europa’s habitability and look for evidence of biological activity in its subsurface ocean. This is a real project, and it is already in progress.
Theoretical research to evaluate the microbial habitability of Europa using data collected from analogous environments on Earth has been conducted by a group of Brazilian researchers: Thiago Altair Ferreira, Marcio Guilherme Bronzato de Avellar, Fabio Rodrigues, and Douglas Galante. An article written by these four scientists has recently been published in Scientific Reports.
The study was supported by FAPESP through the Thematic Project “The Neoproterozoic Earth System and the rise of biological complexity”, a postdoctoral scholarship for the project “Compact stars in binaries: investigating the composition of superdense matter”, and a master’s scholarship for the project “Natural radioactive environments as sources of local chemical disequilibrium and their potential prebiotic outcomes”.
“We studied the possible effects of a biologically usable energy source on Europa based on information obtained from an analogous environment on Earth,” said principal investigator Douglas Galante, a researcher at Brazil’s National Synchrotron Light Laboratory (LNLS) and the Astrobiology Research Center (NAP-Astrobio) of the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences (IAG-USP), in an interview given to Agência FAPESP.
The analogous terrestrial context was found in the Mponeng gold mine near Johannesburg, South Africa, at a depth of 2.8 km, where the bacterium Candidatus Desulforudis audaxviator has been found to survive without sunlight by means of water radiolysis, the dissociation of water molecules by ionizing radiation.
“This very deep subterranean mine has water leaking through cracks that contain radioactive uranium,” Galante said. “The uranium breaks down the water molecules to produce free radicals [H+, OH-, and others]. The free radicals attack the surrounding rocks, especially pyrite [iron disulfide, FeS2], producing sulfate. The bacteria use the sulfate to synthesize ATP [adenosine triphosphate], the nucleotide responsible for energy storage in cells. This is the first time an ecosystem has been found to survive directly on the basis of nuclear energy.”
According to Galante and colleagues, the environment colonized by bacteria in the Mponeng mine is an excellent analogue of the environment assumed to exist at the bottom of Europa’s ocean.
“We studied how the parameters found in Mponeng could be transposed to Europa so that it would have conditions suitable for hosting similar ecosystems,” Galante said.
The first and most obvious requirement is the existence of liquid water. The presence of a subsurface ocean of liquid water on Europa is due to the tidal force exerted by Jupiter’s powerful gravitational attraction.
Unlike Earth’s Moon, whose orbit is almost circular, Europa follows a highly eccentric elliptical orbit and therefore undergoes periodical geometric deformation. When it is near Jupiter, its shape is “stretched” by a strong gravitational pull, and as it moves away, it shrinks back again.
This alternation between elongation and relaxation releases huge amounts of thermal energy in Europa’s interior. While its surface temperature is that of deep space, in the range of minus 270 °C and hence close to absolute zero, its subsurface is capable of hosting an ocean of water that is not just liquid but also warm.
“Thus, in a region very far from the Sun and not reached by sunlight, there’s an environment that’s favorable to the existence of life as we know it,” Galante said. “However, it’s not enough for there to be heated liquid water. There must also be a source of chemical imbalance that can generate biologically useful energy.”
Chemical gradients – differences in concentrations of molecules, ions or electrons in distinct regions – are the basis of all the bioenergetics known on Earth, he explained. Cellular respiration, photosynthesis, production of ATP, conduction of nerve impulses and many other processes are all based on the existence of chemical gradients. These differences in concentration, which produce a flow in a certain direction, are the key that unlocks biological activity.
“Hydrothermal emanations – of molecular hydrogen [H2], hydrogen sulfide [H2S], sulfuric acid [H2SO4], methane [CH4] and so on – are important sources of chemical imbalance and potential factors of ‘biological transduction’, i.e., transformation of the imbalance into biologically useful energy,” Galante said. “These hydrothermal sources are the most plausible scenario for the origin of life on Earth.”
The model proposed by Charles Darwin (1809-82), in which an aqueous medium rich in phosphoric salts and ammonia is the scenario for the origin of life, probably did not involve a “warm little pond”, as the English naturalist imagined, but an ocean bed supplied by a hydrothermal spring.
“We set out to evaluate the possibility that something similar might be happening on Europa,” Galante said. “To do so, we needed an emanation of water from the subsurface bearing the chemical elements that could produce the required imbalance. In the current stage, we don’t have the data to tell us if this is happening on Europa. The process depends on soil chemistry, hydrothermal dynamics and other variables, which in Europa’s case are still unknown. So we looked for a more universal physical effect that was highly likely to occur. That effect was precisely the action of radioactivity.”
When the Solar System was formed, it included radionuclides produced by the supernova from the previous generation, whose explosion ejected into space the matter that was to become the Sun and all the planets that orbit around it. The various bodies of the Solar System with rocky cores contributed these radioactive materials.
“Their presence had been detected and measured on Earth, in the meteorites that come to Earth, and on Mars. So we can say with some certainty that this must have occurred on Europa as well. In our study, we worked with three radioactive elements: uranium, thorium and potassium, the most abundant in the terrestrial context. Based on the percentages found on Earth, in meteorites and on Mars, we can predict the amounts that probably exist on Europa,” Galante said.
“From these amounts, we were able to estimate the energy released, how this energy interacts with the surrounding water, and the efficiency of the water radiolysis resulting from this interaction in generating free radicals. Free radicals are the source of the chemical imbalance. In the Mponeng context, as noted, they interact with pyrite to produce sulfate, which the bacteria use to synthesize ATP.”
The study shows consistently that the existence of radioactive material in fairly realistic quantities would in and of itself be very strongly conducive to the emergence of life on Europa. Another necessary ingredient is pyrite. No one knows if there is pyrite on Europa. It is likely, since sulfur (S) and iron (Fe) are abundant throughout the Solar System, but this would be an important subject for research during a space mission to Europa.
“One of the proposals deriving from our study is that traces of pyrite should be looked for as part of any assessment of the habitability of a celestial body. This is one of the tests of our model,” Galante said.
“The ocean bed on Europa appears to offer very similar conditions to those that existed on primitive Earth during its first billion years. So studying Europa today is to some extent like looking back at our own planet in the past. In addition to the intrinsic interest of Europa’s habitability and the existence of biological activity there, the study is also a gateway to understanding the origin and evolution of life in the Universe.”
If the existence of microbial activity on Europa is confirmed, an obvious question will be whether the bacteria emerged there or migrated from other regions of the Solar System, or even farther afield. It may resemble science fiction, but this question is also asked about life on Earth. Science does not yet have an answer, since in the current stage of scientific knowledge, there is no irrefutable evidence for or against an exogenous origin for terrestrial life.
The ancient panspermia hypothesis about the propagation of life throughout the Universe – taken up with new arguments by English astronomer Fred Hoyle (1915-2001) and his former student Nalin Chandra Wickramasinghe, born in 1939 in Sri Lanka and currently director of Buckingham Center for Astrobiology at the University of Buckingham in the UK – remains an open question. It has been neither convincingly confirmed nor refuted.
“To date, no evidence of life outside Earth has been found,” Galante said. “What we’ve shown in the laboratory is that microorganisms of different kinds are highly resistant and capable of surviving space travel. One possible scenario is that microorganisms ejected from Mars by collision with a comet traveled through space and came to Earth. We know this could happen, but we don’t have any proof that it actually did happen.”
Scientists from 26 universities and research institutions in Japan are conducting the Tanpopo experiment on the International Space Station. The mission includes collecting samples of cosmic dust for later analysis in search of prebiotic compounds or even microorganisms. If these exist, even if they came to near space from Earth, the discovery would be a formidable argument in favor of the thesis that life has spread beyond the limits of Earth’s atmosphere.
The article “Microbial habitability of Europa sustained by radioactive sources” (doi:10.1038/s41598-017-18470-z) by Thiago Altair, Marcio G. B. de Avellar, Fabio Rodrigues and Douglas Galante can be read at: nature.com/articles/s41598-017-18470-z.