Enceladus Plumes: A Key to Ocean Habitability Revealed

How can scientists estimate the pH level of Enceladus' subsurface ocean without landing on its surface? This is what a study recently posted to the arXiv preprint server hopes to address as a team of scientists from Japan investigated new methods for sampling the plumes of Enceladus and have provided more accurate measurements of its pH levels. This study has the potential to help scientists better understand the subsurface ocean conditions on Enceladus and whether it's suitable for life as we know it. For the study, the researchers propose using Raman spectroscopy for sampling Enceladus' plumes and evaluating the pH levels of the water being discharged from the plumes. The researchers note how Raman spectroscopy instruments have been used on planetary missions and provide unique capabilities for studying icy satellites like Enceladus. The goal of the study was to ascertain if Raman spectroscopy could be used to identify different pH levels, specifically weak and strong alkaline content (pH 8–12), which scientists have estimated to be the pH levels of the plumes. To accomplish this, the researchers conducted a series of laboratory experiments involving various samples of carbonate salty fluid and placed them in a vacuum chamber to evaporate the fluid and then freeze, which was designed to simulate the surface of Enceladus, leaving only the salt deposits remaining. Each sample was at a different pH level and the Raman spectroscopy instruments were configured to simulate their operation and they would work on a future space mission. Carbonates are types of chemical compounds that form when carbon dioxide interacts with water and rock. Scientists study carbonates due to their ability to estimate pH, water chemistry, and liquid water. The researchers aspired to ascertain if salty plume samples deposited on the surface of Enceladus could be analyzed and have their pH levels measured. In the end, the researchers found that the Raman spectroscopy instruments could successfully identify the different pH levels in each of the salt deposit samples. The researchers note in their conclusions, "These results demonstrate that Raman spectroscopy can identify carbonate minerals present on the surface of Enceladus. Furthermore, qualitative identification of surface carbonate minerals may enable the estimation of the pH of its subsurface ocean. Therefore, a Raman spectrometer could be an important analytical instrument for in-situ observations of surface materials on Enceladus." As the study notes, Raman spectroscopy instruments have been used on planetary science missions. An example of an active mission includes the SuperCam and SHERLOC instruments on the Perseverance rover, currently exploring Jezero Crater on Mars. Examples of planned use on future planetary missions include the Raman Laser Spectrometer (RLS) on the ESA's (European Space Agency) ExoMars Rosalind Franklin rover and the Raman spectrometer planned for JAXA's (Japan Aerospace Exploration Agency) Martian Moons eXploration (MMX) mission. For Perseverance, Raman spectroscopy is used for analyzing rock and mineral chemistry, and most importantly, detecting organics and biosignatures. Discovered by William Herschel in 1789, Enceladus was first imaged up-close by NASA's Voyager 1 spacecraft in November 1980. But the small moon's true time in the spotlight occurred during NASA's Cassini mission, which not only obtained stunning images of Enceladus' craterless surface, indicating resurfacing like Jupiter's moon Europa, but also discovered the plumes emanating from the south pole. During the mission, NASA made the bold attempt to fly Cassini through the plumes, which identified mostly water, but also found salt-rich ice grains, organic molecules, hydrogen gas, and heat, which indicated active geology. It is the vast subsurface ocean beneath Enceladus' icy crust that scientists hypothesize could contain life as we know it. Therefore, analyzing plume deposits on the surface could be an ideal method for determining more constrained estimates about the subsurface ocean's composition.