The icy crust at the south pole of Enceladus exhibits large fissures that allow water from the subsurface ocean to spray into space as geysers, forming a plume of icy particles. NASA’s Cassini spacecraft captured images of this phenomenon in 2009, and by sampling the particles it revealed the chemicals contained in the ocean. With the data collected by Cassini, an international team of scientists has discovered phosphorus – an essential chemical element for life – hidden within salt-rich ice grains that have been ejected into space from Enceladus.
The small moon of Saturn, Enceladus, is known to have a subsurface ocean, and this water is thought to erupt from cracks in the icy crust as geysers at the south pole. This creates a plume that feeds Saturn’s E ring with icy particles. Cassini flew through this plume and E ring multiple times during its mission from 2004-2017, and analysis revealed that the ice particles contain a variety of minerals and organic compounds, including those essential for the formation of amino acids.
The discovery of phosphorus as one of the essential elements necessary for biological processes could not have come at a better time. As the least abundant of these elements, phosphorus is a vital component of life on Earth as it is a building block for DNA, which forms chromosomes and carries genetic information. Furthermore, it is present in the bones of mammals and cell membranes, as well as ocean-dwelling plankton, and is an integral part of energy-carrying molecules in all living organisms. Without phosphorus, life as we know it would not exist.
In a study published in Nature, planetary scientist Frank Postberg led a research team to find that Enceladus’ ocean is rich in a variety of organic compounds. The team further discovered the clear chemical signature of phosphorus salts and concluded that the small moon’s plume ejects substantial amounts of phosphorus into space. This is the first time an essential element has been found beyond Earth in an ocean. Previous studies of Enceladus’ ice grains revealed concentrations of sodium, potassium, chlorine, and carbonate-containing compounds. Computer modeling showed that the subsurface ocean is of moderate alkalinity – all suggesting conditions which could support life.
For this study, the authors utilized data from NASA’s Planetary Data System, an archive of digital products from planetary missions. In particular, they accessed data collected by Cassini’s Cosmic Dust Analyzer instrument when it sampled ice particles from Saturn’s E ring. This data set was much larger than that collected when Cassini flew only through the plume, allowing the scientists to analyze a greater number of compositional signals. Their analysis revealed high concentrations of sodium phosphates – molecules of sodium, oxygen, hydrogen, and phosphorus – in some of the grains. This archive is actively managed by planetary scientists to ensure its usefulness and usability for the global community and is a great example of how powerful this kind of archival data can be in furthering scientific inquiry.
In this discovery, co-authors from Europe and Japan have conducted laboratory experiments to show that the ocean of Enceladus contains phosphorus in concentrations of at least 100 times that of our planet’s oceans. This is due to interactions between carbonate-rich liquid water and rocky minerals on the ocean floor. Further geochemical modeling by the team demonstrated that an abundance of phosphate is possible on other icy ocean worlds in the outer solar system, particularly those that formed from primordial ice containing carbon dioxide.
The implications of discovery are far-reaching for astrobiology, as it suggests that the key ingredient necessary to support life may be abundant enough in Enceladus’ ocean to make it a viable environment. “This is a stunning discovery for astrobiology,” said Christopher Glein, a planetary scientist and geochemist at Southwest Research Institute in San Antonio, Texas. With further research, these findings could lead to a better understanding of life beyond our own planet.
The science team was excited to find the building blocks of life on Enceladus, but Glein stressed that life has yet to be discovered anywhere beyond Earth in the Solar System. Despite having the necessary ingredients, it’s still an open question whether life could have originated in Enceladus’ ocean. The Cassini mission, launched to explore Saturn, its rings, and moons, ended in 2017 with the spacecraft burning up in Saturn’s atmosphere. The data it collected on its mission has continued to be a major resource for planetary science and has made discoveries that have far-reaching impacts.