(ORDO NEWS) — Which moons of the solar system have liquid water and how do we know about its existence? How do these oceans survive in the cold of space? Is it possible to detect an extraterrestrial sea in a matter of minutes, as American scientists recently promised?
The geysers of Enceladus as depicted by an artist
Some moons of the giant planets hide real oceans under the ice shells. This is at least Saturn’s moon Enceladus and Jupiter’s moon Europa.
“We are one hundred percent sure that the two satellites of the giant planets have a subglacial ocean, since we have seen water ejections. This is Enceladus – seen repeatedly – and Europa – twice seen with the Hubble telescope.
There is no doubt about the existence of oceans under the ice of these two satellites, Enceladus and Europa,” comments Vladimir Surdin, senior researcher at the P. K. Sternberg State Astronomical Institute and associate professor at Moscow State University named after M. V. Lomonosov.
On Europa and Enceladus, liquid water coexists with organic matter, so it’s hard to find a better place for an extraterrestrial biosphere in the entire solar system.
Unless other icy moons, for example, Titan, literally flooded with complex organic matter. But Titan and several other satellites are still only “on suspicion”: there is not enough data to understand whether they sleep there under the ice of the sea.
Recently, employees of several US research centers have developed a new method for searching for extraterrestrial oceans.
The researchers claim that the interplanetary probe will be enough for this 12 minutes. The authors were aiming at Triton, a distant and mysterious satellite of Neptune. But their brainchild is also suitable for studying the much more accessible moons of Jupiter and Saturn.
On the face cold, liquid inside
Water is one of the most abundant substances in the universe, and the solar system is no exception. Many satellites of the giant planets are largely composed of water.
That’s just not from liquid, but from frozen. The temperature on the surface of, say, Jupiter’s moon Ganymede does not rise above -110 degrees Celsius. The Saturnian system is even colder. What to do, the giant planets are too far from the Sun.
But there is another source of heat, from which the heart of an impregnable block of ice can literally melt. Well, not the heart (core), but the water mantle. This “heater” is tidal forces.
Tidal forces arise from the fact that one hemisphere of the icy moon is attracted to the planet (or neighboring satellite) more strongly than the other. Why? Yes, simply because the hemisphere facing the attracting object is a little closer to it than the opposite.
This difference in attraction deforms the celestial body: a tidal hump is formed. It looks like a water hump on the surface of the oceans, caused by the attraction of the moon (it is what causes the tides). And where there is deformation, there is friction. Where there is friction, there is heat.
True, this heat is barely enough to keep the water from freezing. According to the calculations of planetologists, its temperature is slightly below zero, but dissolved mineral salts, and possibly other natural antifreezes, come to the rescue here.
Geysers of Enceladus (photo from the Cassini)
Is life gushing?
The hardest thing to doubt is the existence of liquid water on Saturn’s moon Enceladus. Plumes of huge geysers regularly rise above the south pole of this celestial body. True, you can’t boil an egg for breakfast in this boiling water.
Liquid water that breaks through to the surface boils not because of the high temperature, but because of the low pressure. The pressure of the vapor ejects the jet into space, where, of course, it immediately solidifies into grains of ice.
Almost all of them fall back on Enceladus in a strange hail. But some pieces of ice remain in space and replenish one of the rings of Saturn (ring E).
Impressive photos of the activity of Enceladus were taken by the Cassini spacecraft, which explored the Saturn system from 2004 to 2017. The probe not only photographed geysers at a distance, but also flew into the very plumes, determining their composition.
“It turned out that in the composition of the water flying out of the geysers of Enceladus, there are minerals, not exactly characteristic, but hinting that there are black smokers at the bottom of this ocean.
That is, groundwater outlets rich in dissolved minerals. And this is close to the oases at the bottom of our oceans, where life prefers to develop,” says Surdin.
Cassini found in the emissions of Enceladus not only minerals, but also complex organic matter. The most remarkable molecules had a mass of more than 200 atomic units. This is more than glucose or caffeine.
Unfortunately, the Cassini instruments were not very well adapted to the accurate analysis of organics and were not at all intended for the search for life. No one expected that such equipment would be needed for an orbital probe.
It may be that Enceladus’ ejecta are teeming with extraterrestrial bacteria, but we will not know about this until we launch a new probe into the realm of geysers.
However, scientists are incredible bores and will always find a way to doubt the dizzying prospects. According to some models , reservoirs of liquid water on Enceladus are not permanently present, but periodically appear for a short time on a geological scale.
If so, then there is no need to talk about any life there. However, other researchers consider the subglacial ocean of Enceladus to be eternal.
Natural color image of Europe
Live in Europe
It looks like geysers are also rising above Jupiter’s moon Europa. They were repeatedly recorded by Hubble: the first publication was published in 2014, the second – in 2016, and a year later the results of new observations were published.
True, all these data were obtained at the very limit of the telescope’s capabilities. So there were also skeptics who dispute the conclusion about the existence of geysers.
But there is other evidence as well. In 1997, the Galileo probe passed only 200 kilometers above the surface of Europa. At this point, he crossed the magnetic anomaly . Geysers create similar anomalies above the surface of Enceladus when cosmic radiation ionizes the erupted matter.
And in 2016, the Keck telescope spotted a huge release of water vapor above the surface of Europa. The observations were made in infrared, and the vapor leaves a clear spectral trace in this range.
The fact that the ocean splashes under the ice shell of Europa is evidenced by the shell itself. Other icy moons are pitted with meteorite craters, but this one is as smooth as a billiard ball.
Perhaps NATO forces approached the defense of Europe so responsibly that they shoot down meteoroids flying towards it (well, the command got confused about which Europe it is, with whom it does not happen)? But if it’s no joke, there can be only one explanation for the absence of craters: the ice is regularly updated.
According to some models , the ice on Europa moves like the oceanic crust on Earth: in local “subduction zones” ice sinks into the mantle and melts, while “mid-ocean ridges” spewing new portions of ice into the light of day.
By the way, organic matter was found in the breaks of the “European” ice crust. It is possible that she got there from the subglacial ocean. Of course, from organic matter to life, there may not even be seven miles of forest, but seven parsecs of space. But no one can stop hoping for the best.
In 2030, the NASA Europa Clipper probe (not yet launched) should arrive in Europe. And in a year, the Clipper will be accompanied by the European device JUICE, aimed at studying Europe (how can Europe not study Europe?), Ganymede and Callisto. Both probes are orbital, without landers. But observation from orbit will also help to study the mysterious icy moon well.
Another contender for the presence of a water mantle is Saturn’s moon Titan. The Cassini apparatus once measured its gravitational field.
It turned out that the gravity of Saturn creates an obscenely high tidal hump on Titan: about ten meters. This is ten times more than one would expect from a completely solid moon. And since Titan is more than half water ice, it is natural to assume that its internal liquid is also water, and nothing else.
Titan is interesting in that it is downright a chemical plant for the production of organics. Its atmosphere (and it is the only satellite to have a dense atmosphere) is about two percent methane. Seas and lakes of liquid methane and ethane splash on the surface of Titan.
And in the upper layers of the atmosphere, ultraviolet rays create complex organic matter (up to benzene and cyanoacetylene), which falls to the surface and lies there in drifts of organic snow. If there is a subglacial ocean at this Butlerov factory, is it possible to think of a better place for the origin of life?
There is another way to detect the subglacial ocean. Salt water is a conductor. Therefore, the movement of the satellite in the magnetic field of the planet induces an electric current in the water mantle. This current creates the satellite’s own magnetic field, which can be detected.
Magnetic measurements confirm the existence of subglacial oceans on Enceladus and Europa. Moreover, they suggest that two more satellites of Jupiter, Ganymede and Callisto, have such oceans.
But making such measurements is not easy. First, the satellite’s magnetic field is much weaker than the background magnetic field of the giant planet. Secondly, its atmosphere (more precisely, the ionosphere) also contributes to the magnetism of the moon, even if it is very rarefied.
After all, the ionosphere consists of charged particles, and their movement is a current that generates a magnetic field. All this is superimposed by the inevitable instrumental noise, the short duration of measurements, the limitations of the probe trajectory in space, and other difficulties.
Neptune’s moon Triton
On the edge of a trident
A team of scientists from the United States defied all these troubles. Researchers have taken on the most difficult task possible: searching for the subglacial ocean on Neptune’s moon Triton during the Trident mission .
So far, the only spacecraft that has entered the Neptune system is Voyager 2. During a rendezvous with Triton in 1989, he photographed what appeared to be active geysers. But a brief visit by a probe flying by gave too little data to speak with confidence about the subglacial ocean.
The Trident mission was conceived to study Triton. She, too, was thought of as a flyby: we know too little about the Neptune system to try to put probes there into a stable orbit. So the device would have only 12 minutes to measure the satellite’s magnetic field.
The authors of the new study argue that this is enough. They modeled 13,000 variations of the magnetic field, which could be generated by the ocean, the ionosphere, or both.
By comparing observational data with these models, you can choose the most appropriate one. Of course, the selection procedure involves complex calculations, but we will not bore the reader with mathematical details.
True, the Trident project in the most literal sense did not take off. The mission was just one of the contenders for funding under NASA’s Discovery program. In February 2020, this concept reached the final of the competition along with three others.
And, although Bolivar of financing could endure two, four were definitely beyond his power. In June 2021, the Trident and the mission to Jupiter’s moon Io were left out, and the lucky tickets were drawn by the Venus probes: DAVINCI+ and VERITAS.
The choice of experts is understandable. The flight to Venus takes only about six months. In addition, the device can go into orbit around this planet and study it for many years.
Compared to Mars, it has been studied rather poorly, so a lot of new results are guaranteed. This is much more tempting than 13 years of flight for the sake of a few minutes of rendezvous with Neptune’s satellite.
But by doing so, humanity missed the chance to explore the “geyser” region of Triton, discovered by Voyager 2.
The illumination of this satellite by the Sun changes according to the annual cycle of Neptune, and it takes 165 Earth years. If a probe (Trident or whatever) does not appear over Triton before 2040, the region of geyser activity will go into shadow for the next hundred years.
At the same time, the window of launches to Neptune does not open often: it requires the correct location of Jupiter. The next opportunity will be in October 2025 and October 2026.
If humanity misses it (and it looks like it intends to miss it), the next launch window will have to wait until 2039. Another 13 years will take a flight to Neptune, so no one will have time to get there by 2040.
However, the authors claim that the method they developed for finding the oceans is suitable for any icy moon. And there are enough of them even closer to Neptune.
Judging by the projects Europa Clipper and JUICE, in the coming decades, everyone’s attention will be riveted on the Jupiter system. Then Saturn’s turn may come again: the launch of the ambitious Dragonfly mission is scheduled for 2027.
This “Dragonfly” should fly nothing less than in the atmosphere of Titan. Projects for flights to Uranus are also being discussed. By the way, the landforms on all five large moons of this planet (Miranda, Ariel, Umbriel, Titania and Oberon) also make it possible to suspect periodic ice renewal.
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