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Astronomie - We may have already found signs of alien microbes on Enceladus

28.02.2018

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Microbes that produce methane may already be living on Enceladus, a moon of Saturn which is tipped to host life because it boasts a liquid water ocean beneath a crust of solid ice, and strange atmospheric plumes of water. That’s the implication of research showing that an earthbound organism which also produces methane can happily survive in conditions known to exist on Enceladus, from observations by the Cassini space probe before its mission ended last year.

 
 

Isolated from deep sea vents almost 1000 metres deep in the Okinawa Trough off Japan, Methanothermococcus okinawensis was subjected to gruelling physical and chemical conditions found on Enceladus for more than five years.

 

This microbe, called a methanogenic archaeon, survives without oxygen by combining hydrogen and carbon dioxide – both observed in Enceladus’s atmosphere – to make the energy it needs, emitting methane as a waste product. Cassini detected traces of methane in Enceladus’s plumes, and there’s a chance that some of it may have come from this kind of microbe.

 

 

 

“The conditions we mimicked in the lab are as close as possible to those inferred from Cassini on Enceladus,” says Simon Rittmann at the University of Vienna in Austria, who led the investigations.

Rittmann subjected the microbe to various combinations of gases found on Enceladus, and found that it was always able to survive when provided with the moon’s levels of hydrogen and carbon dioxide. It was still able to thrive at temperatures and pressures likely found in Enceladus’s oceans, ranging respectively from 0 to 90 degrees Celsius, and up to 50 Earth atmospheres.

Life on the seafloor

Rittmann’s team also computed how much hydrogen would be produced by a breakdown of olivine minerals – which are predicted to make up the moon’s solid core – under a range of likely geological conditions on Enceladus. They found these minerals could break down chemically to produce enough hydrogen for methanogens to thrive.

The best environment for them is likely to be the seafloor. “There, you have contact with rock and minerals, pressures of around 50 atmospheres and temperatures most likely a bit higher than 0 degrees Celsius,” says Rittmann.

“This [team] has taken the first step to showing experimentally that methanogens can indeed live in the conditions expected on Enceladus,” says Chris McKay at NASA Ames Research Center in Moffett Field, California.

Rittmann says he hopes there will be future missions to Enceladus to explore further for signs of life. He says a probe fitted with a mass spectrometer would be able to detect carbon isotope ratios unique to living organisms, as well as other potential “biomarkers” of methanogens, including lipids and hydrocarbons.

“If we find life on Enceladus, it is not likely to be very Earth-like, unless the origin of these life forms is from a common source outside the solar system, which is highly unlikely,” says Hunter Waite at the Southwest Research Institute in Texas. Proof of concept on Earth is interesting, he says, but there is no substitute for finding and studying a methanogenic organism in the unique environment of Enceladus.

Journal reference: Nature Communications, DOI: 10.1038/s41467-018-02876-y

Quelle: New Scientist

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Enceladus Could Be Teeming with Methane-Belching Microbes

New lab experiments suggest a particular microorganism could be the source of methane emanating from the oceanic depths of Saturn’s icy moon 

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This illustration shows the Cassini spacecraft diving through plumes of salty water vapor erupting from the icy crust of Enceladus, a small moon of Saturn. Cassini detected traces of methane in the plumes that could come from simple forms of alien life dwelling within the moon. Credit: NASA and JPL-Caltech

Scientists and science fiction writers alike have long wondered about what forms alien life might take on other worlds. Now researchers have strengthened the case that, at least on Saturn’s icy moon Enceladus, some alien life might closely resemble a specific type of microbe found deep in our own planet’s seas. Such alien organisms may even be living there now, and if so, could conceivably become the first discovered beyond Earth.

“We had speculated about the possibility of life outside the ‘habitable zone’ in our solar system,” says Simon Rittmann, a biochemist at the University of Vienna, referring to the limited orbital region where starlight-warmed planets can host liquid water on their surfaces. “Now we’ve found in our modeling that some of the methane produced on Enceladus could be of biological origin.” Enceladus, of course, lies far outside the habitable zone, but nonetheless boasts a deep liquid ocean beneath its icy crust.

Rittmann led a team performing a series of experiments and modeling to determine if any three methane-producing microbes could grow in the crushing depths of the ocean’s cold, briny and alkaline waters. They argue one of these so-called “methanogen” species could indeed live there, encouraging more detailed research and missions to find out for sure. They published their findings in the February 27 Nature Communications.

NASA’s Cassini spacecraft previously revealed Enceladus’s ocean by monitoring geyserlike plumes spewing from the moon’s south pole. Within the plumes it found chemical signatures of a saltwater ocean sandwiched between the moon’s icy crust and rocky core as well as tentative evidence of hydrothermal vents on that alien seafloor. It also caught whiffs of hydrogen, carbon dioxide and methane, along with signs of organic compounds such as methanol. In Earth’s oceans such a cocktail of gases would suggest the presence of hydrogen-eating, methane-belching microbes—but extrapolating that conclusion to the very different geology and chemistry of Enceladus required a rather unscientific leap of faith. More research was—and is—needed.

“Hydrogen and carbon dioxide are the only chemical sources of energy that we know are there, and who consumes that?: methanogens. This paper takes that to the next level and demonstrated it experimentally,” says Chris McKay, a planetary scientist at NASA Ames Research Center. McKay is also the lead investigator for the Enceladus Life Signatures and Habitability project, a proposed NASA space mission.

Using various mixtures of gases held at a range of temperatures and pressures in enclosed chambers called “bioreactors,” Rittmann and his co-authors cultivated three microorganisms belonging to the oldest branch of Earth’s tree of life, known as Archaea. In particular, they focused on Archaean microbes that are also methanogens, which are able to live without oxygen and produce methane from that anaerobic metabolism. The team examined the simplest types of microbes, which could be the primary producers of methane at the base of a possibly more complex ecological food chain within the moon.

They tried to simulate the conditions that could exist within and around Enceladus’s hydrothermal vents, which are thought to resemble those found at a few deep-sea sites on Earth, often near volcanically active mid-oceanic ridges. According to their tests, only one candidate, the deep-sea microbe Methanothermococcus okinawensis, could grow there—even in the presence of compounds such as ammonia and carbon monoxide, which hinder the growth of other similar organisms.

Scientists do not really know the precise conditions on Enceladus yet, of course. And in any case it is possible any life there, if it exists, is nothing like any DNA-based organism on our planet, rendering our Earth-based extrapolations moot. What’s more, these findings only show microbial life might exist in one particular subset of possible environments within the moon’s dark ocean. Rittmann’s argument is an indirect one that could explain the methane emanating from the moon, but the evidence remains muddled and inconclusive—on Earth, certain geochemical reactions between hot water and rocks beneath the seafloor can also generate significant amounts of methane without the presence of microbes.

The tantalizing possibility of life on another world orbiting our sun, even if it is in the outer solar system and hidden beneath a kilometers-thick crust of rock-hard ice, has sparked more interest from the world’s space agencies. Nascent missions to explore Enceladus further as well as other icy ocean worlds with similar conditions, especially Jupiter’s moon Europa, are already in various stages of development at NASA and the European Space Agency.

One possibility would be to send a spacecraft with a suite of powerful new instruments to orbit or fly by Enceladus, to sniff out clearer signs of habitability and even life in the plumes jetting from the moon. That approach is endorsed by Hunter Waite, a scientist at the Southwest Research Institute in San Antonio who helmed Cassini’s earlier analysis of the moon’s geysers. But to answer the question definitively, he says, one would want to deploy some kind of robotically controlled submarine in that ocean—while making sure the probe is free of any hitchhiking microbes imported from Earth.

We’re looking at 30 to 40 years for such a mission, Waite says. “But if you went and confirmed these things and saw signs of life, I think people would want to go back.”

Quelle: SCIENTIFIC AMERICAN
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