Since we are currently unable to visit some of the distant places in our solar system, we send probes to visit them for us. Only four spacecraft have visited Saturn, and for roughly 20 years some of the best data we had were images taken during the flybys of Pioneer 11, Voyager 1, and Voyager 2. That changed with the Cassini-Hyugens probe, which was launched from Cape Canaveral in 1997 to study Saturn and its surrounding moons. As a two-probe spacecraft, the Hyugens lander separated from Cassini in 2004 and was the first probe to land on the surface of a moon in the outer solar system. Titan, Saturn’s largest moon, is covered in a thick hazy atmosphere, but the Hyugens’ probe revealed rivers, lakes, and seas on its surface. As Cassini toured Saturn, it discovered jets of icy particles being released from another one of Saturn’s moons, Enceladus. These jets, called plumes, shoot ice hundreds of miles into space. Their origin is likely a large liquid ocean below the ice-covered surface.
Onboard the Cassini spacecraft is a specific type of instrument called a mass spectrometer, which can identify the particle composition. By passing through the plumes, Cassini became the first probe to sample an extraterrestrial ocean. While it found the ocean was mostly made of water, it also contained organic molecules such as carbon dioxide, methane, and ammonia. What these scientists were most interested in was the discovery of hydrogen particles, which could be evidence for much more complex processes. However, to make this conclusion, they first had to determine where all of it could come from.
Using models and other measurements, the scientists came up with several ideas. One thought was that the ocean or the icy shell acted as a sort of reservoir for hydrogen, which may have trapped the gas long ago and has been slowly releasing it over time. However, given the rate at which hydrogen was being expelled from the surface and the size of the moon, they realized that any hydrogen that may have been stored in the ocean would have been expelled after less than a million years. Therefore, any hydrogen that may have been trapped in the ocean before Cassini arrived would have been gone for quite some time. The scientists also determined that it is unlikely that the hydrogen arrived from the surrounding gas and dust in the solar system, given the moon’s low gravity and that the building blocks of Enceladus would not have been able to trap enough hydrogen during the moon’s formation. They concluded that Enceladus must be actively producing hydrogen.
The scientists predict that chemical processes could be the source of the hydrogen. However, most of them could not reproduce the conditions they found in the plume. This led the scientists to believe that hydrothermal reactions between water and rock, similar to what we see here on Earth, could provide a continuous source of hydrogen. This idea was supported by information that Cassini obtained from its other instruments.
Because hydrogen was found, the research team says it’s possible that microorganisms could live in those oceans and consume hydrogen. Just as we eat food for energy, microbes do the same thing. For example, when making bread dough, sugar is added to the yeast so that they will break it down and produce carbon dioxide and alcohol, causing the dough to rise. A special type of microorganism, known as a methanogen, can consume carbon dioxide and hydrogen to produce methane (see diagram above). However, even though these other molecules have also been found in the plumes, the scientists believe it is still not enough to say whether or not the process is actually occurring. Of course, it is enticing to consider the possibility that life may exist elsewhere in our solar system.