Cassini data reveals the presence of phosphorus in Enceladus’ ocean plumes

by Haygen Warren

A team of scientists, using archival data from NASA’s now-retired Cassini mission, has discovered the presence of phosphorus in the massive plumes of water being ejected from the southern regions of Saturn’s moon Enceladus. Phosphorus — a chemical element essential for life and often considered a building block for life — is locked inside of salt-rich ice grains ejected in these plumes.

The plumes themselves are thought to be caused by water from a subsurface ocean within Enceladus. The water from the ocean makes its way to space through several fissures in the southern surface regions of Enceladus. The resulting plumes have been studied by several spacecraft — including Cassini — and even created a ring around Saturn (known as the E ring).

Orbiting Saturn from 2004 to 2017, Cassini was a mission that set out to discover Saturn and reveal some of its most captivating mysteries. One such mystery was the existence of a subsurface ocean underneath the icy crust of Enceladus, which Cassini found evidence for when it discovered the massive geyser-like plumes at Enceladus’ south pole. Since the discovery of the plumes, scientists have used data from Cassini to determine that, if a subsurface ocean does indeed exist, the ocean would be around 26 to 31 kilometers deep — significantly deeper than Earth’s 3.7-kilometer-deep oceans.

To determine the makeup of the plumes, Cassini flew through the plumes, as well as Saturn’s E ring, several times while at Saturn. From the data collected by Cassini’s fly-throughs, scientists were able to determine that the ice grains ejected in the plumes contain an array of minerals and organic compounds that are associated with life. Some of the minerals and organic compounds found even form the ingredients for the formation of amino acids.

However, scientists were not able to find any phosphorus within the compounds initially discovered within the plumes — until now.

Phosphorus is one of the least abundant elements of the essential elements for life — which makes its discovery even more exciting and mesmerizing. Not only is phosphorus a building block for DNA and present within mammals, cell membranes, and ocean plankton, but it is also an important part of all energy-carrying molecules in all life on Earth. Basically, without phosphorus — life wouldn’t be possible.

“We previously found that Enceladus’ ocean is rich in a variety of organic compounds. But now, this new result reveals the clear chemical signature of substantial amounts of phosphorus salts inside icy particles ejected into space by the small moon’s plume. It’s the first time this essential element has been discovered in an ocean beyond Earth,” said lead author Frank Postberg of Freie Universität in Berlin, Germany.

Since the detection of sodium, potassium, chlorine, and carbonate-containing compounds in the plumes, Enceladus has been noted as one of several icy moons in our solar system that could host life. For Enceladus specifically, life would have to exist within its subsurface ocean, which computer modeling suggests is of moderate alkalinity. The alkalinity of the ocean and the compounds detected in the plumes make Enceladus one of the most habitable places in our solar system outside of Earth.

Graphic showing Enceladus’ massive subsurface ocean. (Credit: NASA/JPL-Caltech)

As mentioned, Postberg et al. found phosphorus in the plumes by analyzing archived Cassini data. They accessed the data through NASA’s Planetary Data System, which is a long-term archive of data collected by various NASA planetary science missions.

Specifically, Postberg et al. researched data collected by Cassini’s Cosmic Dust Analyzer (CDA), an instrument that analyzed dust particles by determining the particle’s charge, speed, size, and direction, and by breaking down the particle to determine its composition. Instead of analyzing CDA data from when Cassini flew through the plumes, the team analyzed CDA data from when Cassini flew through Saturn’s E ring, which is the ring that is created by Enceladus’ plumes. Interestingly, more ice particles were analyzed by the CDA when Cassini flew through the E ring than when the spacecraft flew through Enceladus’ plumes.

In the CDA’s E ring data, Postberg et al. discovered a high concentration of sodium phosphates in the ice grains.

Additional analysis of phosphorus by team members in Europe and Japan shows that Enceladus’ ocean most likely has phosphorus inside different water-soluble forms of phosphate. What’s more, the team determined that the phosphate could be found in concentrations of at least 100 times that of Earth’s oceans.

In this image from Cassini, Enceladus can be seen in the center, with the massive E ring extending above and below the moon. (Credit: NASA/JPL-Caltech/Space Science Institute)

The discovery of phosphate in Enceladus’ plumes and its existence within the moon’s subsurface ocean means that other icy ocean moons in our solar system (i.e. Ganymede, Europa, etc.) likely have phosphorus within their oceans. In fact, if an icy moon formed from primordial ice that contained carbon dioxide, the likelihood of that moon containing phosphorus is significantly increased.

“High phosphate concentrations are a result of interactions between carbonate-rich liquid water and rocky minerals on Enceladus’ ocean floor and may also occur on a number of other ocean worlds. This key ingredient could be abundant enough to potentially support life in Enceladus’ ocean; this is a stunning discovery for astrobiology,” said co-investigator Christopher Glein of the Southwest Research Institute.

While the detection of phosphorus on Enceladus is certainly exciting, the discovery does not mean that life itself has been found on the moon.

“Having the ingredients is necessary, but they may not be sufficient for an extraterrestrial environment to host life. Whether life could have originated in Enceladus’ ocean remains an open question,” said Glein.

Postberg et al.’s research is yet another example of how Cassini’s incredible mission continues to impact planetary science and astrobiology. Cassini’s mission ended in 2017 when the spacecraft purposely entered and burned up Saturn’s atmosphere. Though scientists had already made groundbreaking discoveries using Cassini data when the spacecraft’s mission ended, the scientific community still had — and still has — massive amounts of data to analyze.

“This latest discovery of phosphorus in Enceladus’ subsurface ocean has set the stage for what the habitability potential might be for the other icy ocean worlds throughout the solar system. Now that we know so many of the ingredients for life are out there, the question becomes: Is there life beyond Earth, perhaps in our own solar system? I feel that Cassini’s enduring legacy will inspire future missions that might, eventually, answer that very question,” said Cassini project scientist Linda Spilker of NASA’s Jet Propulsion Laboratory.

Postberg et al.’s research was published on June 14 in the journal Nature.

(Lead image: Cassini image of Enceladus’ southern plumes. Credit: NASA/JPL-Caltech/Space Science Institute)

Related Articles