Discovery of Phosphates in an Ocean Outside of Earth: Chemical Measurements of Enceladus’ Ocean

An artist’s impression of the view of Saturn from Enceladus. Credit: NASA/Michael Carroll.

The Cassini-Huygens joint NASA/European Space Agency (ESA)/Agenzia Spaziale Italiana (ASI) mission surveyed Saturn and its moons, including Enceladus, its sixth-largest moon. This mission led to the discovery of a hydrothermally active, ice-covered ocean on Enceladus that was rich in Na+, K+, Cl, HCO3, and CO32- and had an alkaline pH. An ocean and the presence of carbon, oxygen, nitrogen, hydrogen, phosphorus, and sulfur are considered the key ingredients to the origins of life. However, until now, phosphorus, the least abundant of the primary elemental building blocks required for life, has yet to be detected in an ocean outside of Earth.

The Cassini spacecraft used its Cosmic Dust Analyzer (CDA) to study the chemical composition of the ocean on Enceladus by analyzing materials ejected from the moon by cryovolcanic plumes and previously ejected salt-rich ice particles that were trapped in Saturn’s rings. Using a time-of-flight spectrometer, the CDA analyzed cations generated by high-velocity impacts of individual grains hitting the instrument’s rhodium target. As the CDA directly measured aerosolized water from Enceladus’ subsurface ocean, this instrument provided important information on the chemical composition of the ocean. Frank Postberg from Freie Universität Berlin and his team recently identified sodium phosphates in spectra from the CDA that measured salt-rich ice grains from Saturn’s rings. These results, combined with recently refined modeling of mineral solubilities and laboratory analogues, indicate that Enceladus’ ocean could have phosphorus concentrations 100 times greater than in Earth’s oceans. The authors propose that the presence of sodium phosphates results from a higher solubility of calcium phosphate compared to calcium carbonate in moderately alkaline fluids rich in carbonate or bicarbonate ions. The discovery of phosphorus in an ocean outside of Earth for the first time demonstrates the potential for habitable rocky bodies within our solar system. READ MORE