You might have heard water referred to as a ‘universal solvent’ because of the vast diversity and richness of substances it can dissolve, and most chemical reactions related to life’s metabolic processes are carried out in water. For this reason, much effort has been spent looking for traces of water on Mars with the hope that it would lead to signs of life (biosignatures). However, these efforts were mostly driven by our proximity to the red planet and our technological capabilities. The launch of NASA’s Europa Clipper mission and ESA’s JUICE mission mark a turn of the tide in the search for life beyond Earth because icy moons, such as Europa in the Jovian (Jupiter) system, contain vast liquid water oceans underneath their ice shells.

Origin of life

So far, we only know of one place where life has arisen, and that’s here on Earth. However, the ocean of Europa offers the possibility of a second abiogenesis (origin of life). Interactions between its ice shell and space, or the ocean and the rocky interior, allow for water-rock reactions that might produce the necessary ingredients to support life.

For example, hydrogen cyanide (HCN) and formamide (HCONH₂) are important precursors for the formation of nucleotides (the building blocks of DNA), and ammonium cyanide (NH₄CN) is a precursor for the formation of amino acids (the building blocks of proteins). For these reactions to take place, the molecules need to reach certain concentrations, and it is proposed that this might occur when they are frozen in the ice shell: as water freezes, molecules are excluded from the growing ice crystals, concentrating them over time in microscopic brine channels between the crystals.

Other locations on Europa might also play a role in the origin of life. Hydrothermal vents are structures proposed to be present on the Europan ocean floor that many scientists believe could be candidates for the origin of life, as has been suggested for similar environments on Earth (Figure 1).

In regions where the water (under immense pressures) seeps into the porous ocean floor, it comes into contact with the heated rock of the crust. The water-rock interactions that result from this produce chemicals of biological interest, such as methane or hydrogen, and solid precipitates that form chimneys (see Figure 1a). These structures might catalyse reactions essential for life or drive electrochemical potentials similar to the ones found in modern day cells. The chemical byproducts of hydrothermal vents have been found on other icy moons such as Enceladus, and on Earth some organisms are sustained by them.

Figure 1 Two sorts of hydrothermal vent on Earth: (a) a black smoker, and (b) a white smoker.Show descriptionTwo photographs. Figure A shows thin columns of rock growing upwards from the sea floor. These are coloured yellow, white and black. Black smoke is being emitted from the columns. Figure B shows the sea floor, with white smoke being emitted from several places.

Life’s chemical reactions

Animals, including humans, favour a metabolism (life-supporting processes) centred around oxygen: we breathe in this molecule and use it to oxidise molecules such as glucose (a type of reduction-oxidation reaction, or ‘redox’ for short). Plants are capable of photosynthesis, which means using light as a source of energy. Even though you may be familiar with these types of metabolisms, the microbial world offers an enormous diversity of alternatives which mean they can be found in environments on Earth where other organisms could not survive.

On Europa, the thick ice layer (4–20 km) prevents any light from reaching the ocean and so, if life is present, it must be sustained by the energy released by chemical reactions such as those described above. Microorganisms that do this are known as chemolithotrophs. Some are capable of ‘breathing’ inorganic sulfur compounds (Figure 2) and produce toxic molecules such as hydrogen sulfide (H₂S). Others take their energy from molecular hydrogen and produce methane (methanogens) which itself can sustain other microorganisms. Microorganisms capable of such metabolic feats are sometimes described as ‘extremophiles’ if the conditions they are found in – e.g. temperature, pH, radiation, salinity – are close to the known limits of life. However, in Europa, the ocean seems to contain regions that scientists would often associate with being habitable and even Earth-like.

Figure 2 Microorganisms capable of using sulfur compounds. Imaged under a scanning electron microscope.Show descriptionA black and white photograph of the surface of a rock. This has an irregular surface, with many pores and protrusions. This photograph has a blue colour applied to rod-like shapes, to indicate the presence of microorganisms.

Implications of discovering life

Discovering life on Europa would be one of the most profound scientific breakthroughs in human history, bringing with it deep philosophical and societal implications. It would help answer fundamental questions about our role in the universe, questions that have shaped human cultures and religions for millennia. The scientific output of life-seeking missions to Europa and other icy moons could revolutionise fields such as medicine.

The search for extraterrestrial life in the Solar System seems to stem from the deep human instinct for exploration, but this is not a task we should take lightly. Sadly, human exploration (and colonisation) of Earth has done tremendous harm not only to indigenous populations of humans but also the natural world. If we want to avoid the mistakes of the past and the risk of ecological disasters at interplanetary levels, then we must ensure that planetary protection measures are put in place and followed.