How much water is necessary for life elsewhere in the solar system?

The largest and deepest body of water from all known never seen seafarers. He has no islands and shores, the wind is not it uplifting waves on the water is not running sun glare. This dark ocean you will not find on any map of the Earth - it is more than 500 million kilometers away from us, to Europe, one of the 69 known moons of Jupiter. These cosmic Galileo system, which has flown to Europe 11 times from 1995 to 2003, showed that under the icy surface of the moon is smooth immense salty ocean. Its depth should be 100 kilometers - eight times deeper into the Pacific Ocean at the maximum depth. It is two to three times more water than all the oceans of the Earth.

How much water is necessary for life elsewhere in the solar system?

We know that the universe is full of watery moons and planets. But how do we know whether they could support life?

Europe is not the only one of its kind. At least two other moons of Jupiter - Ganymede and Callisto - hide under the surface of the oceans. Titan and Mimas, a moon of Jupiter, probably, too. And there is no doubt that another moon of Saturn, Enceladus hides water under its frozen crust. Amazing and irrefutable evidence of the deep depths of Enceladus appeared in 2005, when the probe "Cassini" imprinted geysers spewing ice and water for hundreds of kilometers into space. "Cassini" even flew through the geysers in October 2015, after swimming 50 kilometers from the moon's surface to take samples of their contents.

To say that an abundance of liquid water in the outer solar system completely redefined scientists - to say nothing. Until revelations "Cassini", the Galileo and other probes the general opinion was: the satellites of Jupiter and Saturn will be similar to the moons of Mars - solid, studded with craters, barren rocks, incapable of harboring life.

How much water is necessary for life elsewhere in the solar system?

"Nobody expected that there would be subsurface oceans," says Seth Shostak, an astronomer at the SETI Institute in Mountain View, California. "Our idea of ​​the inhabited worlds has expanded, and now we expect to be able to find a life where there is no thought to look for it before. We have always thought that life should be on the planet. But now I know seven places in our solar system, where there is every reason to look for life - or at least the conditions for it. And most of them - the companions ".

With such an abundance of water we have at hand, we can say with certainty that countless planets orbiting other stars also have to be with the oceans, not to mention their satellites. Astronomers have previously identified several "water worlds" outside our solar system - planets without any land.

"It's amazing," said Christopher Gleyn scientist mission "Cassini" of the Southwestern Institute in San Antonio, Texas. "This is how to invent a new field of oceanography".

However, the existence of extraterrestrial oceans should not be so much a surprise. Hydrogen makes up 74% of the ordinary matter in the universe; oxygen - the third most common element. Put them together - get water, H 2 O. The astronomers observed the traces of water ice in the craters on the Moon and even on Mercury - the closest planet to the sun. Her many in interstellar clouds and dusty disks of nascent planetary systems; even in the atmospheres of some giant exoplanets we have found water.

"The study of exoplanets turned explosive," says Bonnie Meinke, NASA's scientist, working with the James Webb Space Telescope, which will go into space in the next year. "Over the last 20 years we have moved from a few to thousands of exoplanets. And now we know that every star in the night sky have at least one planet. I think we can assume that the majority of these planets are in a sense, and water. "

And where there is water, and can be life. "Look for water" - an old axiom astrobiologists. What makes water so indispensable? The chemical reactions that nourish life engines require liquid for dissolution and transport of molecules across the cell. Water is one of the best known solvents; it will remain liquid at a higher temperature range than any other agent. It is possible that another fluid will perform the role of water in an alien biochemistry - methane lakes, for example, that we have found on Titan. But so far no exceptions to the rule "of life need water," we did not find.

How much water is necessary for life elsewhere in the solar system?

So, the planet is completely covered with this very important matter should be the ideal haven for life? Recent studies have covered such expectations copper basin: water on such planets may be too much for life, so she came and began to flourish, given the chance. "More is not better," says Steven Desch, an astrophysicist at the University of Arizona. Desch and his colleagues conducted a computer simulation of exotic geophysical and atmospheric environments, so does the planet completely covered with this essential substance will not be the ideal haven for life? Some recent studies have cast a giant wet blanket on such expectations: many worlds can indeed be too much water for life to arise - or thrive if it started. "More is not necessarily better," - says Steven Desch, an astrophysicist at the University of Arizona. Dash and his colleagues performed computer modeling geophysical exotic and atmospheric environments that may be found in other worlds. Their goal - to create something like a field guide for future hunters exoplanets. Desch calls him "the periodic table of the planet." It will worlds types that are likely to contain the products of livelihood in the atmosphere - the oxygen or methane, for example. More importantly, these gases must be present in large enough quantities to be able to detect the telescopes of the future decades. "We have to put the study of the planets in the priority because they are the best indicators of life." Water worlds, as it turns out, may be the best place to look for life. Desh team created a computer model that resembles Earth in almost everything: size, not too cold and not too hot distance from stable star like the Sun. Then they filled the world with water in five to seven times larger than the Earth to drown all its continents. Hold down the virtual world, they have eliminated the most important process that supports life, that we, humans, generally forgotten: weathered outcrop.

In the absence of rain or running water, eroding the rock, the sea in the world created Desh team, contained very little phosphorus, an essential element for life. The sea water itself is not enough acidity to dissolve the phosphorus as effectively as fresh. "Phosphorus is very important," said Tessa Fischer, microbial ecology of the University of Arizona. "In addition to DNA and RNA, it also creates ATP transporting energy for the whole molecule biochemistry known to us. The Earth's biochemistry, as far as we know, can not function in the absence of phosphorus. "

Desh and Fisher point out that their model does not preclude the existence of life in the water world. Sea on such planets will surely contain a certain amount of phosphorus, but not enough to sustain life in a large scale and to leave an imprint in the atmosphere. "There will be an atmosphere of 30% consisting of oxygen, as on Earth," says Fisher. "Perhaps the planet is completely covered by an ocean, it is inhabited. Simple life there are so fragmented that we have it even can not be detected with the Earth. "

How much water is necessary for life elsewhere in the solar system?

There may be worlds and with so much water that life is simply impossible. Scientists estimate that the size of the planet Earth with 10% of its weight as water is absolutely lifeless. Such a planet would have the equivalent of 400 Earth's oceans; enormous pressure on the bottom of the sea it would create a dense exotic form of ice known as ice-ice and six or seven. "Water from the rock would not have cooperated, would be nothing in my life has turned out," says Desch. And no matter how strange these conditions may seem, these worlds may be more common than the rocky planets of the Earth type. Water and stone, perhaps equally common in planetary systems throughout the cosmos. In our own solar system, comets, moons, and some frozen the residents of the Kuiper belt is believed to contain the same amount of ice and rock. "The outer planets 50% of ice," says Desch. "This is normal. Abnormally only the extent to which the dry land. "

From our point of view it seems the essence of the planet Earth with the ocean - "pale blue dot" covered seas. But all these oceans spread out a thin film on the surface of the planet. By weight of earth only at 0, 025% of water. With the existing technology, astronomers could not tell whether we will exoplanets like the Earth in general any water. Astronomers use two main techniques for determining the composition ekzoplanet. First, they assess the size of the planet, observing how much light it blocks, passing in front of its star. Then they measure the vibrations of the stars, which causes the planet in its orbit, which gives us the mass of the planet. Separation of the planet mass on its volume density yields and a density of about allows astronomers count the percentage gas content of the solid matter and water of the planet.

"Think about how thin our ocean. He did not change the radius of the Earth. " Now astronomers can say that there are oceans of exoplanets, only if the water will account for about 10% of its mass. This equals 400 of Earth's oceans, huge amounts of water, crushing all living things. It turns out that the only water worlds that we can detect, using existing technologies will be suitable for life. "This is the situation at the moment," says Desch. "We have an opportunity to look for water, and even see when the water 10% of the mass of the planet, but it's too much water." Seven of these worlds revolve on Trappist-1 orbit the star 49 light-years away, named in honor of Belgian beers. All of them are the size of Earth, and even three are within a potentially habitable zone of the star, at a distance, where possible existence of water in the liquid state. Now it is the most that neither is a concern to us "possible land", but they may be too moist or bombarded with ice to their breeding grounds in the life.

Trying to determine the composition of a distant planet by a few pixels of light, find themselves in a telescope, at least will not be accurate. Given these limitations, Dash and colleagues evaluated that outer planet Trappist-1 consist of 50% of ice; inner planets consist of 10% ice and liquid water. "This is more than enough to cover the continents," says Desch. "You will get hundreds or even squeezed kilometers of ice on the ocean floor. It's a dead planet. "

How much water is necessary for life elsewhere in the solar system?

What you need to identify a "living" planet earth with a mixture of continents and seas, not too wet, not too dry? Given the range of possible worlds such as ours should be a lot. But how to find them? James Webb Space Telescope will be the king of astronomy, as soon will begin its 10-year mission in 2020; he will be able to analyze the atmosphere of giant exoplanets of Neptune type, and perhaps even find a few "super-Earths" - planets 2-10 times larger than Earth by mass. However, it will be too short-sighted to see the atmosphere of the planets, not to mention the oceans.

"It is very difficult to look at something so small - the size of Earth passing in front of a star - and see the faint gleam of the atmosphere," says Meinke. "There are plans for future telescopes that will be able to do it, and I think still see it in my lifetime. But Webb could not confirm the presence of water on Earth-like planets. " Telescopes that can visualize the oceans and land masses of another world, probably separated from us for a couple of decades. And even if the resolution is likely to be limited by a pixel or two for the entire planet. Here's what to look for one of the most important discoveries in the history of science - our first direct view of the world, similar to our own: the color of one pixel to be replaced periodically from blue to brown, as if pirouettes, alternately showing the land and the sea to our eyes.

Until that day has not come, we can find signs of the existence of life in some ekzookeanah much closer to home. And closest to us such an ocean on Enceladus, plus it has all the conditions necessary for life. When the probe "Cassini", moving at a speed of nearly 30,000 kilometers per hour, dived through the geyser "Enceladus" in October 2015, its instruments recorded hydrogen, carbon dioxide and methane, and then deep-sea hydrothermal activity is present on the satellite, as earth. "We literally tasted Enceladus ocean, flying through the plume of a geyser," says Gleyn.

The presence of hydrogen, in particular, it was an indication that the chemical reaction between the hot rocks and salt water in the sea bottom Enceladus split water into hydrogen and oxygen. Enceladus with body size usually does not have a tangible hydrogen content in general, because this element is very light and small body had to fly into space long ago. Therefore Enceladus hydrogen must somehow continually replenished, likely during reactions of water and hot rocks. Once we find the hydrogen, we can conclude that the chemical energy present her a lot, and it is the same energy that organisms in the depths of the Earth used for accommodation and food. Methanogens - type ancient bacteria which are everywhere from hydrothermal vents in the world - are combined with the carbon dioxide, hydrogen and methane and energy is released as a by-product during this reaction. Simple organisms like those inhabited by the early Earth's oceans. Even now, after billions of years after its introduction, the methanogens live independently of sunlight and take their place in the national food chain that supports the ecosystem of tube worms and giant clams.

How much water is necessary for life elsewhere in the solar system?

Can any form of life, is more complex than bacteria, occur on Enceladus, Europe or careless depths of some other lunar seas? "These subsurface oceans of life can be, but the sources of energy to sustain life is much more complex organisms, which need more food may not be available," says Shostak. "We can not say that this could not happen - there were satellites 4, 5 billion years old, so there multicellular things can be, but some tuna - is unlikely."

The only way to answer this question - visit these worlds. NASA has approved a mission to Europa Clipper, which will begin in 2024 and will reach Jupiter in 2030. The spacecraft will round Europe 45 times, it will come to an icy surface at 30 kilometers. Future missions that actually sit on Europe, Enceladus or Titan will have to look for complex amino acids and other biomolecules produced only by living beings.

With just one example - our own world - it is impossible to say whether life is quite ordinary, or incredibly cosmically rare. "It is generally believed that because the fossils or chemical evidence of life go so far into the past, life appeared fairly quickly," says Gleyn. "And people believe that if fast, so easy." Easy, difficult, or somewhere in between - it does not matter. Now we know one thing: if life needs water, water in the universe in bulk. This part of the equation for the search for life has already been solved once and for all.