In Arthur C Clarke's 2001 A Space Odyssey, the supercomputer HAL issues a last instruction to the Discovery crew ALL THESE WORLDS ARE YOURS EXCEPT EUROPA ATTEMPT NO LANDING THERE. What might we find? To prove that life exists, or did at one time, what signs should we be looking for? Possible signs of alien life might include a wide range of features. For example, cells walls or membranes made of simple carbohydrates encapsulate the most successful life-forms on our planet: bacteria. Fatty acids-chains of carbons, hydrogens and oxygens-are diagnostic of biosynthetic processes here on Earth. Certain proteins, such as DNA, can only arise from active biological processes. All of these processes and structures all have something in common, though: they use energy to renew themselves.
At their most basic, fuel cells systems that convert hydrogen and oxygen into water, and in the process, release energy.
This image shows hydrothermal-vent ecosystems vs plant photosynthesis. Hydrothermal-vent ecosystems derive their energy from chemicals in a process called "chemosynthesis." In chemosynthesis, the hydrothermal vent acts an energy source (1) instead of the sun. Both processes use carbon dioxide (2) and water to produce sugars (3). As an end product, chemosynthesis produces sulfur (4) while photosynthesis generates oxygen.
“You can use electrochemical techniques to simulate planetary seafloor systems and some reactions at the emergence of life, since many geological and biological systems function similarly to fuel cells,” said Dr. Laurie Barge, lead author and scientist with the NASA Astrobiology Institute (NAI) Icy Worlds team at JPL. “A hydrothermal vent is a ‘geochemical fuel cell’ because it can drive the transfer of electrons from hydrothermal fuels to seawater oxidants, generating an electrical current in a precipitated mineral chimney wall.”
All life-plant, animal and bacteria-utilizes energy systems based on passing around electrons. One molecule loses an electron, and other receives it. These reactions are classified as “redox”, which is short for, “oxidation-reduction.”
Where there is abundant air and fresh water, life forms hand electrons to molecules eager to accept them, like oxygen and carbon dioxide. Animal cells oxidize sugar and oxygen receives the stripped electron. Plants oxidize water into oxygen in a neat series of reactions we know as photosynthesis.
Unlike animals and plants, bacteria around hydrothermal vents are surrounded by reducers-elements that love to lose electrons-such as iron and nickel. One third of the heat from underwater volcanos and vents is channeled into bacterial communities. The communities then use the heat and those metals to survive using a process called chemosynthesis. Barge’s experiments are trying to simulate whether the geochemistry of these systems can also even encourage the emergence of life by providing enough energy to drive pre-life metabolic reactions, even in the absence of oxygen and light.
“It’s possible that other worlds, such as Europa or Enceladus, could drive similar chemical reactions between their oceans and rocky seafloors to produce electrical potentials and pH gradients,” said Barge.
When you consider what we know about tidal heating in places like Europa, the results of Barge’s experiments are electrifying. Chemosynthesis on Earth has been shown to support not only the growth of bacteria, but shrimp, tubeworms and other marine creatures. Now that Barge’s and her team’s preliminary experiments have been successful with early-Earth-abundant minerals, they can be modified to simulate the geo-electrochemical systems on other worlds that might also have oceans and a rocky seafloor: like Europa, Enceladus, or early Mars.
“If we knew the composition of Europa’s ocean, ocean crust, and the hydrothermal fluid that would result from interaction of these,” said Barge, “then we could build a fuel cell to simulate how much energy would be generated in a hydrothermal system on Europa.”
"Certain minerals could have driven geological redox reactions, later leading to a biological metabolism. We're particularly interested in electrically conductive minerals containing iron and nickel that would have been common on the early Earth."
Dr Terry Kee from the School of Chemistry at the University of Leeds, one of the co-authors of the research paper, said: "What we are trying to do is to bridge the gap between the geological processes of the early Earth and the emergence of biological life on this planet."
Previously, some scientists have proposed that living organisms may have been transported to Earth by meteorites. Yet there is more support for the theory that life emerged on Earth in places like hydrothermal vents on the ocean floor, forming from inanimate matter such as the chemical compounds found in gases and minerals.
"Before biological life, one could say the early Earth had 'geological life'. It may seem unusual to consider geology, involving inanimate rocks and minerals, as being alive. But what is life?" said Dr Kee.
"Many people have failed to come up with a satisfactory answer to this question. So what we have done instead is to look at what life does, and all life forms use the same chemical processes that occur in a fuel cell to generate their energy."
Fuel cells in cars generate electrical energy by reacting fuels and oxidants. This is an example of a 'redox reaction', as one molecule loses electrons (is oxidised) and one molecule gains electrons (is reduced).
Similarly, photosynthesis in plants involves generating electrical energy from the reduction of carbon dioxide into sugars and the oxidation of water into molecular oxygen. And respiration in cells in the human body is the oxidation of sugars into carbon dioxide and the reduction of oxygen into water, with electrical energy produced in the reaction.
Certain geological environments, such as hydrothermal vents can be considered as 'environmental fuel cells', since electrical energy can be generated from redox reactions between hydrothermal fuels and seawater oxidants, such as oxygen. Indeed, last year researchers in Japan demonstrated that electrical power can be harnessed from these vents in a deep-sea experiment in Okinawa.
In the new study, the researchers have demonstrated a proof of concept for their fuel cell model of the emergence of cell metabolism on Earth.
In the Energy Leeds Renewable Lab at the University of Leeds and NASA's Jet Propulsion Laboratory, the team replaced traditional platinum catalysts in fuel cells and electrical experiments with those composed of geological minerals.
Iron and nickel are much less reactive than platinum. However, a small but significant power output successfully demonstrated that these metals could still generate electricity in the fuel cell – and hence also act as catalysts for redox reactions within hydrothermal vents in the early Earth.
For now, the chemistry of how geological reactions driven by inanimate rocks and minerals evolved into biological metabolisms is still a black box. But with a laboratory-based model for simulating these processes, scientists have taken an important step forward to understanding the origin of life on this planet and whether a similar process could occur on other worlds.
Dr Barge said: "These experiments simulate the electrical energy produced in geological systems, so we can also use this to simulate other planetary environments with liquid water, like Jupiter's moon Europa or early Mars.
"With these techniques we could actually test whether any given hydrothermal system could produce enough energy to start life, or even, provide energetic habitats where life might still exist and could be detected by future missions."
The Daily Galaxy via University of Leeds and http://www.astrobio.net/
Image credit top of page: With thanks to Kees Veenenbos
Image Credits: Courtesy Woods Hole Oceanographic Institution and NASA Science News