Phosphorus, an essential ingredient for life as we know it, has been detected for the first time in water samples that can be traced back to Enceladus, an ice-covered moon of Saturn.
The discovery, reported today in the journal Nature, lends further support to suggestions that life could lurk within Enceladus’ ice-covered oceans — and perhaps in similar environments elsewhere in the solar system.
Phosphorus-containing compounds, known as phosphates, provide the molecular backbone for DNA and RNA molecules. Adenosine triphosphate, or ATP, serves as the source of energy for living cells. This research marks the first time that phosphates have been traced to an extraterrestrial ocean. The Nature paper suggests that phosphate levels in Enceladus’ hidden seas could be hundreds or even thousands of times higher than what exists in Earth’s oceans.
“By determining such high phosphate concentrations readily available in Enceladus’ ocean, we have now satisfied what is generally considered one of the strictest requirements in establishing whether celestial bodies are habitable,” study co-author Fabian Klenner, a postdoctoral researcher at the University of Washington, said in a news release.
The team behind the research was led by Frank Postberg, a planetary scientist at Freie Universität Berlin. Klenner participated in the project during his studies at the German university, and started working at UW in May.
More than a decade ago, readings collected by NASA’s Cassini mission confirmed that water is spraying out of crevices in Enceladus’ surface — and that the grains of ice gravitate toward Saturn’s faint E ring. Cassini’s Cosmic Dust Analyzer sampled the spray coming from Enceladus, but didn’t register the presence of phosphorus.
“Previous geochemical models were divided on the question of whether Enceladus’ ocean contains significant quantities of phosphates at all,” Postberg said.
To address the question in a new way, Postberg and his colleagues looked at a set of readings from the Cosmic Dust Analyzer that focused on the material in the E ring.
“The E ring data that we looked at gives us better statistics, as compared to data from the few Enceladus flybys,” Klenner explained in an email exchange. “Phosphorus is hard to find because it’s the least abundant of the bio-essential elements, which are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.”
Klenner set up an experiment that mimicked the data generated by grains of water ice hitting Cassini’s detector. He tried different chemical compositions and concentrations, hoping to come up with readings that matched the chemical signatures recorded by Cassini.
“I prepared different phosphate solutions, and did the measurements, and we hit the bull’s-eye,” Klenner said. “This was in perfect match with the data from space.”
Klenner said the ice grains contain phosphates “because of water-rock interactions on the moon’s seafloor.”
“We show in the paper through geochemical experiments and modeling that high phosphate concentrations are an inevitable outcome from interactions of a carbonate-rich fluid that has an alkaline pH (= Enceladus ocean) with unaltered carbonaceous chondritic rock (= Enceladus rocky core),” he said via email.
Enceladus isn’t the only icy world that’s thought to harbor hidden seas. Scientists say that Europa, an ice-covered moon of Jupiter, appears to have an ocean that’s up to 10 times deeper than Earth’s. Other icy celestial bodies that may have subsurface seas include two other Jovian moons, Ganymede and Callisto; Titan, a smog-covered moon of Saturn; and the dwarf planets Ceres and Pluto.
Robotic space missions could well provide more insights into the composition of extraterrestrial oceans, and the prospects for habitability. NASA’s Europa Clipper and the European Space Agency’s Juice mission will take a closer look at Jupiter’s icy moons, while NASA’s Dragonfly mission will explore Titan.
Enceladus isn’t yet on the list for a future robotic visit, but several mission concepts have been proposed — including Enceladus Orbilander, Moonraker and Breakthrough Enceladus.
Postberg said the ultimate question about Enceladus still needs to be answered.
“Although we know now that Enceladus is a habitable place, we have no clue if it is actually inhabited,” he told GeekWire in an email. “We need a new mission to find that out.”
In addition to Postberg and Klenner, the authors of the paper published by Nature, “Detection of Phosphates Originating From Enceladus’s Ocean,” include Yasuhito Sekine, Christopher Glein, Zenghui Zou, Bernd Abel, Kento Furuya, Jon Hillier, Nozair Khawaja, Sascha Kempf, Lenz Noelle, Takuya Saito, Juergen Schmidt, Takazo Shibuya, Ralf Srama and Shuya Tan. The research was the subject of a presentation at the American Geophysical Union’s meeting in Chicago last December.
