When the Cassini spacecraft launched in 1997, scientists weren’t sure what they’d find around Saturn’s orbit, but it they certainly weren’t expecting what’s now being described as a “candy shop” for microbes. With a lot of instruments to study the moon Titan, it was certainly a surprise when the icy moon Enceladus was seen spraying columns of water into space. Liquid water isn’t something to be scoffed at, especially 888.2 million miles from the Sun, and so subsequent flybys of Enceladus were been arranged, with the latest in 2015 taking the probe within 30 miles of the moon’s surface. With these latest samples, researchers have found that the geysers not only contain water, but also significant amount of molecular hydrogen (H2) and carbon dioxide (CO2), both of which indicate that Enceladus’ oceans may indeed be habitable.
(Relatively) heaping helpings of hydrogen
Detecting molecular hydrogen in Enceladus’ geysers matters in a couple of ways. Hydrogen doesn’t usually like to bond to itself, and so to find H2 in quantities even as low as 0.4 percent by volume indicates a significant amount of chemical reactions taking place below Enceladus’ icy shell. These amounts were considered to be out of equilibrium for a more stabilized environment, and so there’s likely a lot of energy being fed into the system to keep churning the hydrogen out. The current best guess for how this system works is that geothermal activity in the rocky core of Enceladus is creating hydrothermal vents at the ocean’s floor which then lead to chemical reactions between rocks and water, not unlike the thermal vents you find on Earth.
This then leads to the second significance of hydrogen, which is that it’s the aforementioned candy in the ‘candy shop.’ Microbes on Earth metabolize H2 and CO2, making methane as a byproduct. This form of harvesting energy may have been the first source of food for early life on our planet, and so people are wondering if it’s taking place on Enceladus as well.
Nobody is declaring the existence of alien life yet though. While Cassini has detected water, hydrogen and carbon, as well as methane as a possible waste-product, we haven’t seen other elements like phosphorus or nitrogen yet, so our usual ingredients list for life isn’t completely filled out. Some researchers also wonder if we’re seeing too much hydrogen, since microbes would theoretically be eating it up instead of allowing it to escape, although perhaps it’s just produced faster than they can eat it. It’s hard to answer some of these questions with Cassini though, as the spacecraft wasn’t designed to look for all these signs of life, and is running out of fuel. To protect what may or may not be on Enceladus, NASA actually plans to crash the spacecraft into Saturn later this year rather than risk losing control of it.
Establishing the presence of geysers on Europa
Those of us wanting more may be in luck though, because more evidence is piling up of similar phenomena on Jupiter’s moon, Europa. The Hubble telescope recently captured new images of what seems to be a similar geyser erupting through that moon’s icy shell. What’s more, the location of that geyser matches up with earlier images, as well a relatively hot patch on the moon, according to heat maps made by the Galileo spacecraft in the 1990s. Scientists suspect that a similar cycle of venting is happening on Europa, and that it might be a better candidate to find microbial life, largely because it’s billions of years older than Enceladus and thus would have more time to have life evolve.
While Cassini may be leaving us with a bit of a cliffhanger here, we might be able to get some definitive answers about Europa. The Europa Clipper mission that is currently being planned will be sent to the moon fully loaded to hunt for microbial life, and the hydrogen candy it loves so much.
Source: NASA Missions Provide New Insights into "Ocean Worlds" in Our Solar System, NASA News & Features