Louise M. Prockter, Robert T. Based on our terrestrial view, the primary ingredients for life are water, organic compounds, and chemical energy. Europa may have all three: water of the ocean, organic compounds that have been delivered to the satellite, and chemical energy from radiolysis and possibly chemosynthesis. The evidence for liquid water within Europa is strong, as discussed earlier, and Europa's sub-ice ocean may have a greater volume than that of all Earth's surface water. Cometary and asteroidal impactors have rained onto the surfaces of the Galilean satellites throughout solar system history.
Just as Ganymede and Callisto have been darkened by impactor material, similar material must have been delivered to Europa, where its young and bright surface implies that much of this material is now incorporated into the ice shell and ocean. Metabolic reactions within living cells depend upon chemical reactions between oxidants and reductants.
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For animals, this depends on taking in oxygen, which is combined with sugars to produce CO 2 and water. For plants, CO 2 is combined with water to form sugars and oxygen. The key is that chemical disequilibrium must exist, which organisms then exploit to create the energy needed for life.
Whether Europa has sufficient chemical energy to support life is the most significant unknown in understanding Europa's potential for life. Irradiation of surface ice can form molecules of oxygen and hydrogen, with most of the hydrogen floating away but much of the oxygen and other oxidants remaining behind, like a condensed out atmosphere frozen into the uppermost centimeters of ice.
If these oxidants can be delivered to the ice shell and ocean, they maybe able to power the chemical reactions necessary for life. Some of these oxidants will be churned into the upper meter of ice by small impacts. Geological processes such as chaos formation may be able to deliver near-surface materials to the ocean, but the means of surface-ocean communication remain poorly understood. Some oxygen and hydrogen is also produced within the ice shell and ocean by radioactive decay of potassium, but this alone could not provide much energy for life.
If Europa's rocky mantle is tidally heated, then hydrothermal systems could exist on Europa's ocean floor. On Earth, hot chemical-laden water pours into the oceans, delivering organic materials and reductants into the water. If hydrothermal systems exist at the bottom of Europa's ocean, and if oxidants are delivered from the ice shell above, then the necessary chemical disequilibrium that could be used by life exists.
Another important consideration is whether Europa's interior environment is stable enough through time, such that if life ever developed it would still exist today.
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Europa's ocean may have persisted for aeons thanks to internal radioactive heating and the warming resulting from Jupiter's gravitational tug. However, the internal heating induced by the Laplace resonance is not necessarily ancient, and as discussed earlier the intensity of tidal heating may have varied perhaps cyclically through time.
It is an open question whether chemical energy sources for life exist within Europa and have been sufficiently stable to support life through time. Even if life does not exist within Europa today, it may have existed in the past. Most ecosystems on earth ultimately rely on photosynthesis, with the energy source being solar. In marked contrast, deep-sea hydrothermal ecosystems are based predominantly on chemosynthesis , with the energy source being geothermal.
Many of the chemosynthetic microbes are fueled by hydrogen sulfide, which is present at low-temperature vents in concentrations up to several hundred micromoles per liter and at high-temperature vents in concentrations up to milimoles per liter. The tubeworms have no mouth, no digestive system, and no anus; in short, no opening to the external environment.
Hydrogen sulfide diffuses across cell membranes and is transported via the hemoglobin-containing circulatory system to the trophosome, where it is utilized by the associated symbionts. Mussels Bathymodiolus thermophilus Figures 7 and 8 and vesicomyid clams Calyptogena magnifica Figure 9 , common along both the Galapagos Rift and East Pacific Rise EPR , represent two of the other dominant members of the vent megafauna that house chemosynthetic symbionts.
In the case of each of these bivalves, the symbionts are associated with the gills and both species have modified feeding apparatuses relative to those of shallow-water related species likely a result of their predominant reliance on the associated symbionts for nutrition. Closely related mussels and clams within the families Mytilidae and Vesicomyidae are common constituents of the fauna associated with vents along mid-oceanic ridge and back-arc spreading centers as well as at many cold-water hydrocarbon seeps throughout the world's oceans.
The top two-thirds of the edifice is covered with vestimentiferan tubeworms, both Riftia pachyptila larger organisms and Tevnia jerichonana smaller organisms , as well as numerous brachyuran crabs Bythograea thermydron and zoarcid fish Thermarces andersoni. Figure 5. Higher magnification of the side of Tubeworm Pillar depicted in Figure 4 , showing tubeworms Riftia pachyptila and Tevnia jerichonana and scavenging brachyuran crabs Bythograea thermydron. Figure 6. Close-up image of a cluster of Tevnia jerichonana , together with a brachyuran crab Bythograea thermydron and a zoarcid fish Thermarces andersoni.
Figure 7. A dense population of mussels Bathymodiolus thermophilus inhabiting a low-temperature hydrothermal vent field along the East Pacific Rise. Associated fauna in the field of view include tubeworms Riftia pachyptila , brachyuran crabs Bythograea thermydron , zoarcid fish Thermarces andersoni , and a galatheid crab Munidopsis subsquamosa lower left. Figure 8. Close-up of mussels Bathymodiolus thermophilus attached to the tubes of the tubeworm Riftia pachyptila. Limpets Lepetodrilus elevatus are seen attached to the external surfaces of both the mussel shells and tubeworm tubes.
Figure 9. Walter H. All organisms and therefore their ecosystems require energy to function.
What Are Chemosynthetic Bacteria?
For most higher plants and algae, that energy source is solar, through the process of photosynthesis. Energy from chemosynthesis , and particularly from volcanic vents along mid-ocean ridges, is quite interesting, but globally is very small as compared to photosynthesis. For many animals and bacteria, the energy source, through food webs, is based directly in higher plants and algae.
However, for some very large water ecosystems e. It is well known that small, planktonic algae, protozoa, and bacteria which can be considered particulates are fed on by a wide variety of larger filter feeders, and provide the base of open-water food webs. These particulates are not, by any means, the end point. They continue to be an energy source in mid-water detrital food webs.
Even in the dissolved or extremely small particulate state, these organic materials can be absorbed by bacteria, some animals, and algae, they can aggregate to form larger particulates again, or they can be adsorbed onto larger particles and organic films that have developed on surfaces Figure 6. In addition, ecosystems are rarely closed in the sense that the energy supply is only directly from solar sources. Almost invariably there is an input from an adjacent ecosystem of living or dead organisms and organic particulate materials that are derived from dead organisms, including fecal materials.
This happens rapidly, causing a non-equilibrium condition called quenching— where the ratio of gases at a high temperature gets frozen in the same concentration upon transferring into a low-temperature environment.
So if you quench, then you may have a low-temperature mixture still having a high concentration of carbon monoxide. The quenching is important because it creates a chemical potential. The chemical potential drives the reactions between gases and metals at hydrothermal vents. In other words, certain reactions create byproducts that then are used to speed up other reactions, leading to longer, more complex strings of organic molecules.
After that, as the theory goes, life evolved higher levels of complexity and ventured beyond vent communities. The rest, as they say, is history. And his is only one of several competing origin-of-life theories , which generally fall into two camps. Metabolism-first theories like the Iron-Sulfur World Theory start with simple molecules that build increasing levels of complexity. Replicator-first theories, on the other hand, suggest that simple organic molecules occur naturally and are able to self-replicate right from the start.
Chemist Stanley Miller is in the replicator-first camp. But his findings did lay the cornerstone for origin-of-life research and sparked many curious minds to test all sorts of combinations of conditions that could have been around on the early Earth.
Still, 60 years later, none of these efforts have been able to replicate life from scratch in a lab. Advocates of this theory say there was a time when RNA alone handled all maintenance activities of a cell, acting as both genetic material and a catalyst for metabolism reactions. The bacteria use reduced sulfur as an energy source for the fixation of carbon dioxide.