It has been almost six years since NASA's Cassini probe performed its "Grand Finale" dive into Saturn, but scientists are still making discoveries thanks to the wealth of data that was beamed back. A team led by planetary scientist Frank Postberg from the Freie Universität Berlin reports the discovery of phosphorus on the watery moon Enceladus, marking the first time this essential biological element has been confirmed in an alien body of water.
Enceladus gained fame during the Cassini era thanks to the stunning images of the moon's geysers. Tidal heating from Saturn keeps the moon's interior warm enough to support a salty subsurface ocean. This water leaks out through fissures in the surface and leaks into space around Saturn. Plumes from Enceladus feed Saturn's outermost E ring (see below), which allowed Cassini to fly through the material and analyze it with its Cosmic Dust Analyzer (CDA).
Cassini detected numerous organic molecules, including amino acids, during its multiple flybys of Enceladus. There was one vital ingredient for life that didn't appear in the data: phosphorus. As we gaze out across the universe, Phosphorus appears to be the least common of these essential ingredients. It forms the structural backbone of DNA, and without it, life as we know it is impossible. So, finding phosphorus on Enceladus is quite interesting.
The German team accessed Cassini's archived data in the NASA Planetary Data System. Cassini analyzed more molecules in the E ring than in the plumes, so that's where they focused their efforts. Upon re-analysis, they discovered clear evidence of sodium phosphates, molecules that feature phosphorus bound to oxygen, hydrogen, and sodium. Collaborators in Japan and Europe conducted laboratory experiments based on that data. They determined that the moon's subsurface ocean likely has water-soluble phosphate molecules at concentrations at least 100 times higher than Earth's.
Cassini is just part of the reason scientists are becoming more interested in the moons orbiting gas giants. Evidence is mounting that many of these worlds could have subsurface oceans, and the discovery of phosphorus on Enceladus increases the likelihood that something could be alive out there. "This key ingredient could be abundant enough to potentially support life in Enceladus’ ocean; this is a stunning discovery for astrobiology," says co-investigator Christopher Glein from the Southwest Research Institute. There are no missions currently en route to moons in the outer solar system, but NASA hopes to launch Europa Clipper late this year, and JPL is designing a snake-like robot that could one day explore the surface of Enceladus.
