Europa, a moon of Jupiter, is thought to be one of the best bets for extraterrestrial life in the solar system. At first it seems crazy to think that something so far from the sun could possibly support life, yet there are numerous reasons to believe that Europa can do just that. However, to understand the nature of life on Europa, we must first review the origins and unusual circumstances of this moon of Jupiter.
Formation of Europa
The prevalent theory for the formation of the eight planets of our solar system (and any planets outside our solar system too) is called Nebular Collapse Theory. This theory states that in a protostellar nebula (a cloud of hydrogen, helium, and trace amounts of other elements that will one day form a solar system) a region of high density will begin to collapse in upon itself and, due to the conservation of angular momentum, rotate faster and faster as it collapses. While shrinking, due to a complex mixture of collisions, gravity, and gas pressure the nebula flattens itself into a disk. While the center continues collapsing and heating as predicted by the ideal gas law until it ignites into a hydrogen-fusing star, little conglomerations of silicates and metals close to the center and hydrogen compound ices farther from the center begin to form. These objects, called planetesimals, are the first stages of most things in the solar system, from tiny asteroids to the massive Jovian planets. Some of them are able to accrete mass through chemical bonding until they can hold themselves together gravitationally, and the most massive of these become the cores of planets. You can read some more about that topic here.
It turns out, while this is not how our Moon formed, the same mechanics are behind the formation of Jupiter’s major moons. Jupiter is so massive that when its planetesimal was gravitationally capturing hydrogen and helium gas, the gas cloud acted similarly to a nebula.
What do Volcanoes of Io and Oceans of Europa Have in Common?Europa is in a relatively crowded part of the solar. It is stuck in orbit around the second most massive object in the Solar System (after the sun) with a bunch of other massive moons (like Io and Ganymede) that may be considered planets or dwarf planets if they were to orbit the Sun instead of Jupiter. Europa is so close to Jupiter that there is actually a significant difference between the gravitational pull on the side of Europa facing Jupiter and the gravitational pull on the side of Europa facing away from Jupiter. This causes what is called a tidal bulge in Europa. Its sister moons also play a big role in Europa’s characteristics. This is because the three moons are in a 4:2:1 resonance which means that for every one orbit Io makes, Europa makes two, and Ganymede makes four. This forces Europa into a pretty eccentric (non-circular) orbit. An eccentric orbit means that there are times that Europa is fairly close the Jupiter and other times when it is farther away. This difference in the orbital radius causes the gravitational pull of Jupiter and the size of the tidal bulge to be variable. The periodic growth and shrinking of Europa causes heat to be generated due to friction and are thought to enable the existence of an underground ocean on this moons. These same forces called Tidal Heating cause Io (the first moon of Jupiter) to be incredibly volcanically active.
Note the first part of this video, from the Science Channel’s Into the Universe with Steven Hawking, where it describes tidal heating.
To some, even with the presence of tidal heating, the idea of a liquid water ocean so far from the sun seems implausible. However, this theory is backed up by data collected from passing Voyager and Galileo spacecraft. Perhaps the most readily observable, albeit difficult to quantify, evidence is the presence of scars on Europa’s surface that look a lot like cracks in a sheet of ice, exactly what they are theorized to be. These linae, as they are technically called, are indications of deformities in Europa’s icy shell, caused by the tidal stresses imposed on Europa’s not insignificant mass. Another important piece of evidence is the 2010 flyby of NASA’s Galileo spacecraft, which detected changes in Europa’s magnetic field, indicating the presence of a layer of an electrically conductive material, namely, an ocean of salt water.