Is the planet earth a real extraterrestrial
Astrobiology - Is There Life in Space?
The electronic signal from a NASA laboratory in California reaches a robotic vehicle in Alaska. The rover hangs in a lake on the underside of a 30 centimeter thick layer of ice. The vehicle's headlight comes on. “It worked!” Shouts John Leichty, a young engineer who works at the Jet Propulsion Lab (JPL) in Pasadena, California. When he's not huddled in a tent on the Alaska ice. His success message may be the first step in exploring a distant moon.
7,000 kilometers further south, in Mexico, the microbiologist Penelope Boston wades 15 meters underground in a pitch-black cave through muddy water. She carries a breathing apparatus and a compressed air bottle, because toxic hydrogen sulfide and carbon monoxide gases often pass through the cave. Suddenly the light of her headlamp falls on the thread of a viscous, semi-transparent liquid that hangs from the craggy limestone ceiling. "Isn't it beautiful?" She exclaims enthusiastically.
Both places - the frozen lake in the Arctic and the poisonous tropical cave - could provide clues for solving one of the oldest riddles of mankind: Is there life outside of the earth?
In other worlds, whether in our solar system or in the orbits of distant stars, living beings may have to assert themselves in ice-covered oceans. For example on Jupiter's moon Europa. Or in gas-filled caves, as one suspects under the surface of Mars. Finding life forms on earth that thrive under similarly extreme conditions would give hope to be able to discover them on other celestial bodies as well.
It is not possible to say exactly when the search for life in space went from science fiction to real science. An international astronomy conference in November 1961 was undoubtedly of great significance. It was organized by Frank Drake, a young radio astronomer. Even then, he was eagerly listening for possible radio signals from extraterrestrials.
For the majority of his colleagues, the search for extraterrestrial intelligence, or “Seti” for short, was “more or less taboo,” remembers Drake, who is 84 today. But with the blessing of his institute director, he invited a handful of astronomers, chemists, biologists and engineers. Together they discussed astrobiology, as this branch of research is called today: the science of life outside the earth. Drake wanted to know: How useful is it anyway to spend expensive radio telescope uptime listening for extraterrestrial radio signals?
At the very beginning he wrote an equation for this on the board.
N = R * × ƒp × ne × ƒl × ƒi × ƒc × L
TED video with scientist Penelope Boston: "There might just be life on Mars":
His scribble became world famous as the "Drake Formula". The solution (N) says the probability of being able to contact extraterrestrial intelligences. The starting factor is the frequency with which sun-like stars are formed in the Milky Way (R *).
This number is multiplied by the proportion of stars that have a planetary system (& # x192; p) and this result by the number of planets in a life-friendly zone (ne) - i.e. by the number of planets that are roughly as large as the earth and orbiting its star at a distance that makes life possible. The new value is multiplied by the proportion of planets on which life really develops (& # x192; l) and that in turn by the proportion of planets on which intelligence develops (& # x192; i). In the penultimate step, the intermediate result is multiplied by the proportion of planets on which a technology has emerged that enables the transmission of radio signals that we could discover (& # x192; c).
Now one more factor is missing: the lifespan of such a radio-capable civilization (L). Because there are many dangers that threaten life on a planet - from catastrophic volcanic eruptions to asteroid impacts and nuclear war. It could be that we just missed the time window in which extraterrestrial intelligences were sending radio signals into space.
The equation was perfectly plausible. There was only one catch: Nobody knew how big the numbers in each part of the formula were. Only the very first variable was known: the frequency with which sun-like stars form. Everything else was speculation. Now the experts in the various fields were asked to fill in the positions of the Drake equation with justifiable numbers - for example about the proportion of sun-like stars with a planetary system and the proportion of such planets on which life could have originated.
For a generation of researchers, even rough estimates could not be entered into the equation. The first planet to orbit a sun-like star outside of our solar system was discovered in 1995: "51 Pegasi b" is around 50 light years away from Earth, a giant gas ball half the size of Jupiter. Because of its narrow orbit, its "year" only lasts four days, where it is over 1000 degrees.
Nobody believed in living in such hellish conditions. But the discovery of this planet was the breakthrough. After a second and a third extrasolar planet had been detected shortly afterwards, the floodgates were open. Today astronomers know almost 2,000 exoplanets. The smallest are smaller than Earth, the largest larger than Jupiter. There are clues for thousands more, but they have yet to be confirmed.
None of these planets are exactly like Earth, but astronomers are confident that they will find one sooner or later. According to the latest estimates, one in five Sun-like stars could be orbited by planets that have life-friendly conditions.
This is good news for astrobiologists. In addition, in recent years it has become clear to the planet hunters that there is no reason to limit the search to stars that resemble our sun. "When I was at school we learned that the earth orbits an average star," says Harvard University astronomer David Charbonneau. "But that's not true at all." In fact, 80 percent of the stars in the Milky Way are so-called M-dwarfs: small, cool, faintly glowing, reddish celestial bodies. If an earth-like planet orbits an M dwarf at the correct distance - it would have to be smaller than the distance between the earth and the sun, otherwise it would be too cold - life could arise there just as easily as on an earth-like planet of a star that resembles our sun .
A planet doesn't even have to be about the same size as the earth to be able to produce life. "Anything between one and five - maybe even ten - earth masses is an option," says Harvard astronomer Dimitar Sasselov. In short: The number of stars with possibly life-friendly planets is probably much larger than Frank Drake had rather cautiously estimated in 1961.
And that's not all: Extremophiles can thrive in a much wider range of temperatures and chemical environments than the researchers envisioned at Drake's meeting. Marine researchers, including Robert Ballard, sponsored by National Geographic, discovered the “black smokers” 50 years ago. These are chimneys on the sea floor from which mineral-rich hot water escapes: the basis of life for a rich ecosystem of bacteria. The microbes feed on hydrogen sulfide and other compounds dissolved in the water and in turn serve as food for larger animals.
Other organisms thrive in hot springs, in icy lakes under the Antarctic ice cap, in extremely acidic, alkaline or saline environments, with high levels of radioactivity or in microscopic rock cracks more than a thousand meters below the earth's surface. "Here on earth these are small ecological niches," says Lisa Kaltenegger from the Max Planck Institute for Astronomy in Heidelberg, "but that could be the normal situation on another planet."
According to biologists, only one thing is essential for life as we know it: water in liquid form, which can transport nutrients within an organism to wherever they are needed.
Water used to flow on Mars. We have known that since 1971, when the “Mariner 9” space probe mapped the Red Planet. So there could have been life there. It is even conceivable that traces of life will be found under the surface of Mars, where there may still be liquid water or ice. Cracks in the ice-covered surface of Jupiter's moon Europa are an indication that there is an ocean of liquid water under the ice. Because Europe is about 800 million kilometers from the sun - more than three times as far as the earth - the water there should actually be permanently frozen. But the moon is constantly deforming due to the pulling and pushing of the tides caused by Jupiter and its other moons. This creates heat that keeps the water under the ice coat liquid. So in theory there could be life there.
NG-Video: Kevin Hand - Exploring Alien Oceans:
According to optimistic estimates, every fifth sun-like star could be orbited by planets with life-friendly conditions.
An underground water reservoir has also been confirmed on Saturn's moon Enceladus since spring. It is still unclear how much water there is and whether it has been liquid long enough to allow the development of life. On the surface of Saturn's largest moon, Titan, there are even rivers, lakes and rain - but not from water, but from liquid hydrocarbons such as methane and ethane. We can only speculate about what kind of life could exist in it - especially since we still do not know how life arises in principle and what is necessary for it.
After all, Mars is closer to us in several ways than these distant moons. The “Curiosity” robotic vehicle is currently exploring the “Gale” crater. Billions of years ago there was a lake there. We know from new studies on Earth that the chemical environment was favorable for microorganisms.
Of course, a cave in Mexico - in which Penelope Boston enjoys viscous threads of slime - is not Mars. And a lake in northern Alaska - where John Leichty is testing a remote-controlled probe under the ice - is not Jupiter's moon Europa. But at both locations, the scientists are testing new methods of finding life under conditions that are at least remotely similar to what space probes might find. Your focus is on "biosignatures". These are visible or chemically detectable indications that point to life - to earlier or existing ones.
The cave in Mexico could be a model for Mars. Space probes have discovered that there are caves on the Red Planet as well. Microorganisms could have survived in them when the planet lost its atmosphere and surface water three billion years ago. Such Martians would not get their energy from sunlight. But maybe from the dripping mud that Boston is so fond of. Scientists refer to such unsightly drops as "snottites", based on the well-known stalagmites and stalactites. There are thousands of them in the cave, some a few millimeters long, others more than half a meter long. The slime is teeming with microbes. "They are chemotrophic," explains Boston, "they get their energy from the conversion of hydrogen sulfide."
The snottites are just one of many forms of the microbial communities that live here. Boston has already found around a dozen in the cave. «Every shape is different. And each one taps into different nutrient systems. "
One is particularly noticeable. It doesn't form drops or clumps of slime, it creates patterns on the cave walls: dots, lines and even networks of lines that look almost like writing. Astrobiologists call such traces “biovermiculations” - worm-shaped patterns - or bioverms for short.
“They come in different sizes, mostly in places where some resource is scarce,” says engineer Keith Schubert of Baylor University in Texas. He is a specialist in imaging systems and installs cameras in the Cueva de Villa Luz for long-term monitoring of the interior of the cave. Similar bioverms are found in crusts that cover the ground in deserts, he says. They contain communities of bacteria, mosses and lichens. So far it is only a hypothesis, but perhaps such bioverms are a kind of biosignature that leaves primitive life elsewhere as well. Because the patterns may be based on simple laws governing growth and competition for resources. Oxygen is always involved on earth. On other planets, however, organisms could leave such signatures through the conversion of other energy-supplying substances. "We saw Bioverms in all sorts of sizes and in very different environments," says Boston. "In their nature, however, the patterns are always very similar."
Such structures are preserved in caves even after the microorganisms causing them have long since died. If a Mars robot were to discover something like this on a cave wall, says Schubert, "we would know where to take a closer look".
At the upper end of North America, researchers at Lake Sukok in Alaska have set similar goals. Your focus here is on the methane that rises from the lake floor. This gaseous hydrocarbon is produced by microorganisms which experts group together in the group of methanogens.
They decompose organic material, creating another biosignature that astrobiologists could look for on alien worlds.
However, methane can also come from volcanoes and other non-biological sources. It is also constantly forming in the atmosphere of giant planets such as Jupiter and also on Saturn's moon Titan. It is therefore crucial that the scientists can distinguish biologically produced methane from the non-biologically produced gas. If you're interested in Jupiter's ice-covered moon Europa, like astrobiologist Kevin Hand of the Jet Propulsion Laboratory in California, ice-covered, methane-rich Lake Sukok is not a bad place to practice.
Hand, who is also supported by National Geographic, has good reasons for favoring Europe rather than Mars in his research. “Let's assume,” he says, “we find living things on Mars. And suppose it is based, like on Earth, on the DNA molecule in which all of our vital functions are coded. That could mean that DNA is a universal life molecule. That would be possible."
But it could also mean that life on Earth and Mars have a common origin. We know that boulders that were knocked out of Mars by asteroids ended up on Earth. The other way around it probably happened the same way. If microorganisms were trapped in such chunks, they could have survived the flight. And wherever they fell, they acted as life-giving germs.
"If it turns out that life on Mars is also based on DNA," says Hand, "we would have to think about whether the planets have fertilized each other." Jupiter's moon Europa, on the other hand, is much further away. If life were found there, that would be an indication of an independent origin - even if it was based on DNA there too.
The basic building blocks are apparently available on Europe. There is an abundance of liquid water, and on the ocean floor, as on Earth, there could be “black smokers” who provide nutrients. Asteroids regularly hit the surface of Europe and deposit organic compounds that could also serve as building blocks for living beings. Electrically charged particles from Jupiter's radiation belt split the oxygen from the hydrogen in the ice. This creates a spectrum of different molecules with the help of which living things could process the chemical nutrients from the chimneys.
Frank Drake is still hoping for signals from extraterrestrial intelligences: "Who knows how they seek contact?"
The great unknown is: How do such compounds get through Europe's ice mantle into the water below? After all, the ice is 15 to 25 kilometers thick. The recordings of various space probes have shown, however, that it is riddled with cracks. By analyzing telescope images it has been known since last year that salts from Europe's ocean find their way to the surface, probably through such crevices. Images from the “Hubble” space telescope at the end of 2013 also showed fountains of liquid water rising from the South Pole of Europe.The ice is therefore not impenetrable.
So what could be more natural than sending a probe into orbit to find out more? State research commissions in the USA rated such a project as promising, but at costs of 4.7 billion dollars as too expensive. But the scientists at JPL did not give up. They sat down under the direction of astrophysicist Robert Pappalardo and designed the project from scratch. Their result: the “Europa Clipper” probe should not orbit Jupiter's moon, but Jupiter itself. That would save a lot of fuel and money. The probe would have to fly past Europe 45 times to collect enough data about the atmosphere, the surface and the ocean below the ice.
Pappalardo says the project would cost less than two billion dollars. "We envision the start in the middle of the next decade." With an “Atlas V” rocket, the flight to Jupiter's moon Europa would take about six years. “But it is also possible,” he says, “that we can start with the new Space Launch System SLS that NASA is currently developing. That would mean we would be there in 2.7 years. "
The clipper itself would probably not discover any life on Europe, but would provide arguments for landing on the moon later with a probe - and immediately locating the best landing sites. How this probe will then penetrate the thick ice mantle in order to get into the ocean below, no one can say today. Kevin Hand's researchers are already testing control and measurement methods at Lake Sukok in Alaska.
The sensors of the remote-controlled probe measure the temperature, salinity, acidity and other properties of the water under the ice. However, the device does not search for life directly; other scientists in Hand's team are responsible for this. One of them is John Priscu from Montana. Last year he succeeded in extracting live bacteria from Lake Williams. The lake is 800 meters below the Antarctic ice cap. Priscu investigates how extremely cold environments have to be in order to still allow life. And of course what kinds of living things actually exist there.
Research into extremophiles, however, only provides “terrestrial” indications of possible properties of extraterrestrial life. Other projects will provide more useful data for adding missing values to the Drake equation. NASA has just approved the “Transiting Exoplanet Survey Satellite” (“Tess”) space telescope. From 2017 on, it will look for planets and biosignatures in our neighboring stars. The search is made easier by the “James Webb” space telescope, which is due to start in 2018. In 2024, “Plato” will join the search, the space telescope that is being built under the direction of the German Aerospace Center (DLR).
Some researchers even allow themselves mental excursions into science fiction. Why, you ask, do we assume that life on other planets, like ours, evolved on the basis of carbon and water? They are everywhere in the Milky Way. But we don't even know what the biosignatures of life could look like that is not based on carbon compounds.
"But if we limit our search in such a way, we could fail," warns Harvard astronomer Sasselov. "We should at least think about some possibilities of what their biosignatures might look like in the atmosphere." While most researchers focus on Earth-like planets, Sasselov's group is also looking for completely different forms of biology on planets with completely different conditions. For example, for life that is not based on carbon but on sulfur compounds.
Frank Drake, with whom astrobiology began more than half a century ago, is still involved. Although officially retired, he continues to search for signals from aliens. It annoys him that the financing for "Seti" has largely been stopped. To this end, he is pursuing a new project with great interest: Instead of listening to radio waves from extraterrestrial civilizations, they want to find them through the light that the systems of a high-tech society radiate into space. "We should take every conceivable approach," says Drake: "Who knows how extraterrestrials are currently seeking contact."
(NG, issue 7/2014, page (s) 34 to 53)
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