Those living on Earth today, perhaps, are destined to find out the answer to one of the most ancient questions of interest to mankind: are we alone in the universe?
As soon as an all-terrain robot latching onto the underwater side of an ice floe in one of the lakes in Alaska receives a signal from NASA's Jet Propulsion Laboratory in Pasadena, California, a searchlight flashes on it. "It worked!" - exclaims engineer John Leicty, huddled in a tent on the ice. Probably, this event cannot be called a big step in technology, but as the first step on the path of exploring a distant satellite of another planet, it will do.
More than seven thousand kilometers south in Mexico, geomicrobiologist Penelope Boston wanders knee-deep in water through the impenetrable darkness of a cave. Like other scientists in her group, Boston pulled on a powerful respirator and dragged a can of air so as not to be poisoned by hydrogen sulfide and carbon monoxide, which seep into the grottoes, and the underground stream washing her boots carries sulfuric acid. Suddenly, a Boston flashlight beam illuminates an elongated drop of thick, translucent liquid that oozes from the porous limestone wall of the cave. "Isn't it lovely?" She exclaims.
Perhaps, in a frozen Arctic lake and a tropical cave filled with toxic fumes, it will be possible to find clues that will help answer one of the most intractable and ancient questions on Earth: is there life on Mars? (Well, or at least somewhere outside our planet?) The life of other worlds, whether in our solar system or near other stars, may well lurk under the ice that covers entire oceans, as on Europa, the moon of Jupiter, or in tightly sealed and gas-filled caves, of which there are probably many on Mars. If you learn to identify and identify life forms that thrive in similar conditions on Earth, it will be easier to find something similar outside of it.
It’s hard to say at what point the search for life among the stars turned from science fiction to science, but one of the key events was the meeting of scientists in November 1961. It was organized by Frank Drake, a young radio astronomer, keen on the idea of finding radio waves of alien origin.
"Back then," recalls Drake, now 84, "the search for extraterrestrial intelligence [in Search for Extraterrestrial Intelligence - SETI] was kind of a taboo." However, with the support of the director of his laboratory, Frank brought together several astronomers, chemists, biologists and engineers to discuss the issues that astrobiology - the science of extraterrestrial life - is dealing with today.
Drake wanted his colleagues to tell him how wise it would be to devote significant radio telescope time to listening to radio transmissions from aliens, and what method of searching for extraterrestrial life might be the most promising. He was also interested in how many civilizations our galaxy, the Milky Way, can have, and before the guests arrived, Frank wrote an equation on the blackboard.
Promotional video:
This now famous Drake equation determines the number of civilizations that we can detect, based on the rate of formation of stars in the Milky Way, multiplied by the fraction of stars with planets, then by the average number of planets with suitable conditions for life in one star system (planets must be the size about the size of the Earth and be in the habitable zone of its star), then to the share of the planets where life could arise, and to the share of those of them where the mind could appear, and, finally, to the share of those where the intelligent life forms are able to achieve such a level of development to send recognizable radio signals, and for the average time during which such civilizations continue to send them or even exist.
If such societies are inclined to destroy themselves in nuclear war only a few decades after the invention of the radio, then their number will probably be very small at any given time.
The equation is great, except for one inconsistency. No one had even a vague idea of what all these fractions and numbers were equal to, except for the very first variable, the rate of formation of stars similar to the sun. Everything else was pure guesswork. Of course, if scientists searching for life in space were able to detect an extraterrestrial radio signal, all these assumptions would lose their meaning. But, in the absence of such, specialists in all the variables of the Drake equation had to find their exact values - to find out how often solar-type stars have planets. Well, or reveal the secret of the origin of life on Earth …
A third of a century passed before even approximate values could be substituted into the equation. In 1995, Michel Mayor and Didier Kelo of the University of Geneva discovered the first planet in another solar-class star system. This planet - 51 Pegasi b, 50 light-years distant from us, is a huge gaseous ball about half the size of Jupiter; its orbit is so close to the star that the year on it lasts only four days, and the temperature at the surface exceeds a thousand degrees Celsius.
No one even thought that life could arise in such hellish conditions. But the discovery of even a single exoplanet was already a huge success. Early the next year, a group led by Jeffrey Marcy, then at the University of San Francisco and now at Berkeley, found a second exoplanet, and then a third, and the dam burst. Today astronomers know almost two thousand of the most different exoplanets - both larger than Jupiter and smaller than Earth; several thousand more (most were discovered with the ultra-sensitive Kepler space telescope) are waiting for the discovery to be confirmed.
None of the distant planets is an exact copy of the Earth, but scientists have no doubt that this will be found in the near future. Based on data from several larger planets, astronomers have estimated that more than a fifth of solar-type stars have habitable, Earth-like planets. There is a statistical probability that the closest of them is located 12 light-years from us - by cosmic standards, on the next street.
This is encouraging. However, in recent years, inhabited world hunters have realized that it is not at all necessary to limit their searches to stars similar to the Sun. “When I was in school,” recalls David Charbonneau, an astronomer at Harvard, “we were told that the Earth revolves around the most ordinary, average star. But this is not so. In fact, 70 to 80 percent of the stars in the Milky Way are small, relatively cool, faint, reddish bodies - red and brown dwarfs.
If a terrestrial planet revolved around such a dwarf at the correct distance (closer to the star than the Earth, so as not to freeze over), the conditions for the emergence and development of life could develop on it. Moreover, a planet does not need to be the size of Earth to be habitable. "If you're interested in my opinion," says Dimitar Sasselov, another Harvard astronomer, "then any mass between one and five Earths is ideal." It seems that the variety of habitable stellar systems is much richer than Frank Drake and his conference participants could have assumed in 1961.
And that's not all: it turns out that the temperature difference and the variety of chemical environments in which extremophile organisms (literally, “lovers of extreme conditions”) can thrive are also wider than one could have imagined half a century ago. In the 1970s, oceanographers, including National Geographic Society-sponsored Robert Ballard, discovered super-hot springs on the ocean floor - black smokers, near which there are rich bacterial communities.
Microbes that feed on hydrogen sulfide and other chemical compounds, in turn, serve as food for more complex organisms. In addition, scientists have found life forms that thrive in geysers on land, in icy lakes hidden under a layer of Antarctic ice hundreds of meters thick, in conditions of high acidity, alkalinity or radioactivity, in salt crystals and even in rock microcracks deep in the bowels of the Earth. … “On our planet, these are inhabitants of narrow niches,” says Lisa Kaltenegger, who works part-time at Harvard and the Max Planck Astronomical Institute in Heidelberg, Germany. "However, it is easy to imagine that on other planets, they can prevail."
The only factor, without which, according to biologists, life as we know it cannot exist, is liquid water - a powerful solvent capable of delivering nutrients to all parts of the body. As for our solar system, after the expedition of the interplanetary station Mariner 9 to Mars in 1971, we know that once upon a time streams of water flowed along the surface of the red planet. Perhaps life also existed there, at least microorganisms - and it is possible that one of them could survive in a liquid medium under the planet's surface.
On the relatively young ice surface of Europa, Jupiter's moon, cracks are visible, indicating that the ocean is rippling under the ice. At a distance of about 800 million kilometers from the Sun, the water should freeze, but in Europa, under the influence of Jupiter and several of its other satellites, tidal phenomena constantly occur, which is why heat is released, and the water under the ice layer remains liquid. In theory, life can exist there too.
In 2005, NASA's Cassini interplanetary spacecraft discovered water geysers on the surface of Enceladus, another moon of Jupiter; research conducted by Cassini in April this year confirmed the presence of underground water sources on this moon. However, scientists do not yet know how much water is hidden in the ice sheet of Enceladus, nor how long the water remains in a liquid state to serve as the cradle of life. Titan, the largest moon of Saturn, has rivers and lakes, and it rains. But this is not water, but liquid hydrocarbons like methane and ethane. Perhaps there is life there, but it is very difficult to imagine what it is.
Mars is much more like Earth and much closer to it than all these distant satellites. And from each new descent vehicle, we expect news of the discovery of life there. And now NASA's Curiosity rover is exploring Gale Crater, where a huge lake was located billions of years ago, conditions in which, judging by the chemical composition of the sediments, were favorable for the existence of microbes.
Of course, a cave in Mexico is not Mars, and a lake in northern Alaska is not Europe. But it was the search for extraterrestrial life that led NASA astrobiologist Kevin Hand and members of his team, including John Lakety, to Lake Sukok in Alaska. And it is for this that Penelope Boston and her colleagues repeatedly climb into the poisonous Cueva de Villa Luz cave in the vicinity of the Mexican city of Tapihulapa.
Astrobiologist Kevin Hand prepares to launch a robot under the ice of Lake Sukok in Alaska.
And there, and there, scientists are testing new technologies for finding life in conditions that are at least partially similar to those in which space probes may find themselves. In particular, they look for "traces of life" - geological or chemical signs that indicate its presence, now or in the past.
Take a Mexican cave, for example. Orbiters have obtained information that there are cavities on Mars. What if microorganisms survived there, after the planet lost its atmosphere and water on the surface about three billion years ago? The inhabitants of the Martian caves would have to find a source of energy other than sunlight - just like the drop of slime that delighted Boston. Scientists refer to these unattractive streaks as snotites by analogy with stalactites. [In Russian, this term could sound like "snotty". - Approx. translator.] There are thousands of them in the cave, from a centimeter to half a meter long, and they look unattractive. In fact, this is a biofilm - a community of microbes that form a viscous, viscous bubble.
“The microorganisms that create snotites are chemotrophs,” Boston explains. "They oxidize hydrogen sulfide, the only source of energy available to them, and release this mucus." Snotites are just one of the local communities of microorganisms. Boston, a researcher at the New Mexico Institute of Mining and Technology and the National Caves and Karst Research Institute, says: “There are about a dozen such communities in the cave. Each has a very distinctive appearance. Each is built into a different nutritional system. " One of these communities is especially interesting: it does not form drops or bubbles, but covers the walls of the cave with patterns of spots and lines, similar to hieroglyphs.
Astrobiologists called these patterns bioverms, from the word "vermicule" - an ornament made of curls. It turns out that such patterns "draw" not only microorganisms living in the vaults of caves. “Traces like these appear in a wide variety of places where nutrition is scarce,” says Keith Schubert, an engineer and imaging systems specialist at Baylor University who traveled to Cueva de Villa Luz to set up cameras for long-term monitoring in the cave. … - The roots of grass and trees also create bioverms in arid regions; the same happens during the formation of desert soils under the influence of bacterial communities, as well as lichens."
Today, the traces of life that astrobiologists are looking for are mainly gases, such as oxygen, that living organisms on Earth give off. However, oxygen communities can be just one of the many forms of life. “For me,” says Penelope Boston, “bioverms are interesting because, despite their different scale and nature of manifestation, these patterns are very similar everywhere.”
Boston and Schubert believe that the emergence of bioverms, conditioned by simple rules of development and struggle for resources, can serve as an indicator of life characteristic of the entire Universe. Moreover, bioverms persist even after the death of the microbial communities themselves. "If the rover finds something like this in the vaults of a Martian cave," Schubert said, "it’s immediately clear where to focus."
Shivering scientists and engineers work at Lake Sukok with a similar purpose. One of the surveyed areas of the lake is located next to a camp of three small tents, which they dubbed "NASAville", another - with a single tent - is located about a kilometer away. Since the bubbles of methane released at the bottom of the lake disturb the water, polynyas are formed on it, and in order to get from one camp to another by snowmobile, you have to take a circuitous route - otherwise you will not fall through the ice for long.
It was thanks to methane that in 2009 scientists first drew attention to Sukok and other nearby lakes in Alaska. This gas is released by methane-forming bacteria, decomposing organic matter, and thus serves as one of the signs of life that astrobiologists can detect. However, methane is released, for example, during volcanic eruptions, formed naturally in the atmosphere of giant planets such as Jupiter, as well as in the atmosphere of Saturn's moon Titan. Therefore, it is important for scientists to distinguish methane from biological sources from methane from non-biological sources. If the subject of research is ice-covered Europe, like Kevin Hand's, then Lake Sukok is far from the worst place to prepare.
Hand, holder of the National Geographic Grant for Young Explorers, favors Europe over Mars for one reason. “Suppose,” he says, “we go to Mars and find living organisms under its surface, and they have DNA, like on Earth. This could mean that DNA is a universal molecule of life, and this is very likely. But it could also mean that life on Earth and on Mars has a common origin."
It is known for certain that rock fragments knocked out of the surface of Mars by asteroid impacts reached the Earth and fell in the form of meteorites. Probably, and fragments of terrestrial rocks reached Mars. If there were living microorganisms inside these space wanderers that could survive the journey, they would give birth to life on the planet where they "landed". "If it turns out that Martian life is based on DNA," says Hand, "then it will be difficult for us to determine whether it arose independently of Earth." Here Europe is located much further from us. If life is found there, it will indicate its independent origin - even with DNA.
Europe undoubtedly has conditions for life: plenty of water, and there may be hot springs on the ocean floor that may supply micronutrients. Comets sometimes fall on Europe, which contain organic matter, which also contributes to the development of life. Therefore, the idea of an expedition to this moon of Jupiter seems very attractive.
Beneath the cracked ice sheet of Europa, which we see in this image from the Galileo spacecraft, lies an ocean where all the conditions necessary for life can be found.
Unfortunately, the launch of the spacecraft, which the US National Research Council estimated at $ 4.7 billion, was deemed, while scientifically justified, too expensive. A team at the Jet Propulsion Laboratory, led by Robert Pappalardo, went back to the blueprints and developed a new project: the Europa Clipper would orbit Jupiter rather than Europe, which would use less fuel and save money; at the same time, it will approach Europe 45 times so that scientists can see its surface and determine the chemical composition of the atmosphere, and indirectly - of the ocean.
Pappalardo said the new project will cost less than $ 2 billion. "If this idea is approved," he says, "we could launch in the early or mid-2020s." The Atlas V launch vehicle will help make it to Europe in six years, and if the new launch system that NASA is currently developing is involved, it will take just 2.7 years.
At NASA's Jet Propulsion Laboratory, scientists are examining a probe similar to what will soon be able to penetrate the ice of Jupiter's moon Europa.
Probably, Clipper will not be able to find life on Europa, but it will collect data to justify the next expedition, already a descent vehicle, which will take ice samples and study its chemical composition, as the rovers did. In addition, Clipper will identify the best landing sites. The next step after the lander - to send a probe to Europa to study the ocean - can be much more difficult: everything will depend on the thickness of the ice cover. Scientists also offer a fallback: to explore the lake, which may be near the surface of the ice. "When our submersible is finally born," says Hand, "it will be Homo sapiens compared to the Australopithecus we are testing in Alaska."
The device, which will be tested on Lake Sukok, crawls along the underside of a 30-centimeter ice floe, snuggling against it, and its sensors measure temperature, salinity and acidity levels and other water parameters. However, he is not looking for living organisms directly - this is the task of scientists working on the other side of the lake. One of them is John Priscu of the University of Montana, who last year discovered living bacteria in Lake Willians, located 800 meters below the West Antarctica ice sheet. Together with geobiologist Alison Murray of the Institute for Desert Research in Reno, Nevada, Prisu is figuring out what cold water conditions must be like to support life, and who lives there.
As useful as the study of extremophiles is for understanding the nature of life outside our planet, it provides only earthly clues to unravel extraterrestrial mysteries. However, soon we will have other ways to find the missing variables of the Drake equation: NASA has planned for 2017 the start of the telescope - TESS (Transiting Exoplanet Survey Satellite, or a satellite for studying passing exoplanets, that is, those that pass against the background of the disk of their star). TESS will not only search for planets near the stars closest to us, but also identify traces of gases in their atmosphere, indicating the presence of life. Although the old man Hubble allowed the discovery of clouds on the super-earth - GJ 1214b.
However, the fascination with the search for traces of life and extremophiles implies that on all planets the molecules of living things contain carbon, and water serves as a solvent. This is perfectly acceptable since carbon and water are abundant throughout our galaxy. Plus, we just don't know what signs to look for non-carbon life. “If we proceed from such premises in our search, we may find nothing at all,” says Dimitar Sasselov. "You need to imagine at least some of the possible alternatives and understand what else you need to pay attention to when studying the alien atmosphere." Imagine, for example, instead of the carbon cycle prevailing on Earth, the sulfur cycle …
Among these semi-fantastic projects, the idea with which astrobiology began half a century ago is completely lost. Frank Drake, although officially retired, continues to search for extraterrestrial signals - a search that, if he succeeds, will overshadow everything else. Despite the fact that funding for SETI has almost stopped, Drake is full of enthusiasm for a new project - to search for flashes of light emitted by extraterrestrial civilizations instead of radio signals. "We need to try all the options," he says, "because we have no idea what and how aliens are actually doing."
National Geographic July 2014