“All those worlds are yours… except Europa. Don’t try to land there.” This was the message that was broadcast to Earth from a Jupiter turned Sun, at the end of the 2010 novel, Odyssey Two, by Arthur C. Clarke. Because there, on that satellite, life was going to appear.
The follow-up to the highly celebrated 2001 A Space Odyssey (which was actually a product of the movie of the same name, which itself was largely based on the short story The Watcher ) was published in 1982, which is a clear indication that since then Europe was already suspected that it could be a perfect place for life.
With a surface temperature of -150º C at the equator and -223º at the poles, it is a satellite that has a completely frozen appearance. Its size is 90% of the Moon , so if we had it in our sky we would see it practically the same size as our satellite but much brighter due to its icy surface, which reflects 5.5 times more sunlight than the Moon. .
Images from space probes have shown us a world with a surface covered in dark streaks a kilometer or two wide and thousands long ; many are straight but others are curved or irregular . Although they give it a cracked appearance , we can actually compare them to brush strokes made on a white billiard ball. There are also clear ones, and in this case they are smaller than the dark ones and much more uniform. These fissures seem to reflect the effects of tectonic processes on a global scale, induced both from the outside (by gravitational tides caused by Jupiter) and by internal movements, particularly those related to some kind of heat source inside the satellite. Perhaps they are due to the rise of ice contaminated with other crustal materials that breaks the surface and spreads along the flanks of the crack. Furthermore, the orientation that these cracks show on the surface could be explained if Europa was not exactly synchronized in its rotation ; that is to say that the time it takes to make a revolution around itself is not the same as the time it takes to make a revolution around Jupiter.
We also have a mystery : a reddish-brown material that can be seen along the fractures and other areas of the surface , which scientists are unable to identify, although they venture that it may be salts and sulfur compounds mixed with the ice. In fact, in 2015 NASA scientists believe that the yellowish color of some areas is due to the presence of common salt, sodium chloride . Subjected to the same environmental conditions in Europe, researchers have discovered that table salt takes on a yellowish hue similar to that seen there. Analyzing the data from Galileo and with the new observations made by the Hubble Space Telescope, it has been possible to confirm that this suspicion has all the signs of being true. Where does that salt come from? That is the real question, although for many it is an indication that something liquid and salty is hiding under the ice cap.
Regions of darker and rougher terrain are also observed: it is speculated that they are small craters, less than 4 km in diameter. In fact, only three large craters have been identified on Europa’s surface; of these Pwyll , at 26 km, is the best known. This indicates two things: either Europa’s surface is relatively young, or craters don’t last long on the icy crust. Based on the small number of observed craters, planetary scientists have calculated that the surface of this moon cannot be more than 90 million years old (recall that cratering in the Solar System occurred essentially during the first 2 billion years). years of its existence). The passage of the Galileo probe at the end of the last century revealed the existence of strange pits and dome-shaped structures in the ice cap, perhaps due to movements caused by the heat coming from the interior of the satellite. In the same way, he discovered some regions that have been baptized by researchers as “chaotic terrain”, where blocks of embedded ice are grouped, rotated and covered with that mysterious reddish material. Scientists speculate that these blocks would be like huge icebergs moving at a desperately slow speed.
But what is really interesting about Europa is what is hidden under an icy surface that is 10 to 100 kilometers thick: an immense sea of salt water 60 to 150 km deep . This means that the ocean of this satellite, whose diameter is a quarter of that of the Earth, contains twice as much water as that of all the seas of our planet combined.
How have we been able to find out that this ocean exists under an ice layer several kilometers thick? Thanks to one of the most important measurements made by the Galileo mission , which showed a special kind of magnetic field inside Europa. This could only be due to the existence of some electrically conducting fluid below the surface . And knowing that there is ice on the surface , it is not uncommon to have a more than convincing suspicion of what may be below, salt water.
This hypothesis has been confirmed by the discovery of geysers on its surface, which were first seen in 2015. To date, seven have been found on the surface of the satellite, capable of sending their jets 200 km high. Although we do not know its composition, everyone is betting that most of it must be water from the ocean hidden behind the ice. We had observational support for this idea in November 2019, when teams at the Keck Observatory in Hawaii detected water vapor on Europa’s surface for the first time.
The curious thing is that thanks to these geysers a space probe could collect samples from that ocean without having to land on the satellite. In fact, the maneuver of collecting ejected water has already been successfully tested: the Cassini probe did it with Saturn’s moon Enceladus, which also has an ocean that ‘projects’ into space.
But the million dollar question is whether this salty sea could support any kind of life . As we well know, there are three basic prerequisites for it: liquid water, appropriate chemical elements and an energy source. Astrobiologists are reasonably convinced that two of the three hold: water and basic chemistry. What they are not so sure about is that there is a power source under the thick layer of ice. As on Earth, there could be underground volcanoes, fumaroles… in which extremophilic life forms could thrive. If Europa’s ocean exists, heating by Jupiter’s tidal forces could trigger volcanic or hydrothermal activity on the seafloor, providing nutrients that would make the ocean a suitable place for living things. But this is still pure speculation , although the discovery of the geysers is an indication that there is some hydrothermal activity at the bottom of the ocean.
The next missions to Jupiter may solve the mystery, but for now let’s stay with this curious fact: in essence, the structure of Europa is similar to that of the Earth in that it has an iron core, a rocky mantle and an ocean of salt water.