Saturn’s moon Enceladus couldn’t look less Earth-like. Instead of an atmosphere and oceans warmed by the Sun, it has a thick shell of ice that covers a global sea, likely kept liquid by tidal squeezing from its host planet. And yet Enceladus is one of the likeliest candidates for life beyond Earth in the solar system, an intriguing status that has become even more possible thanks to a new discovery.
Using archival data from the Cassini mission, researchers uncovered evidence of phosphorus in the form of sodium phosphates in Enceladus’s subsurface ocean, the first time the chemical has been measured in a liquid environment beyond Earth. Sodium phosphates are a family of molecules that combine sodium (Na+) and phosphate ions (PO43−) with various other elements.
Phosphorus is the rarest of elements necessary to life as we know it. It’s often found locked up in rocks, unavailable for organisms to use. The presence of phosphorus in water at more than 100 times greater abundance than on Earth is suggestive of how available it might be through the entire outer solar system.
“The previous concern that phosphorus might be a bottleneck for the emergence of life on Enceladus is gone,” said Frank Postberg, a planetary scientist at the Free University of Berlin who coled the new study published in Nature.
Phosphorus is one of six elements present in the proteins and genetic molecules—DNA and RNA—of all known life. Of the six, carbon, hydrogen, nitrogen, and oxygen are common in the solar system and beyond, with sulfur being rarer and phosphorus rarer still. Much of the challenge for determining the origins of life is tracing how these elements (often written as CHNOPS and pronounced “schnapps”) came together and the conditions necessary to build the first recognizable biochemicals.
“We already knew that Enceladus has a number of things we tend to think of as requirements for life,” said Sarah Hörst, a planetary scientist at Johns Hopkins University who was not involved in the study. Her work uses Cassini data as well as laboratory experiments. “Now we see that there are phosphorus-containing compounds, which gets us closer to the possibility of the chemistry of life as we know it.”
The Probe That Keeps On Giving
The Cassini mission was launched in 1997 and ended in 2017 when the spacecraft’s controllers deliberately crashed it into Saturn. That was, in part, to prevent the probe from possibly contaminating Saturnian moons with elements from Earth.
When Cassini first flew past Enceladus in 2005, researchers spotted plumes of ice jetting from surface cracks near the moon’s south pole. Over the next decade, controllers directed the spacecraft through the plumes several times to collect material for analysis.
Data collected by the craft’s Cosmic Dust Analyzer revealed salty, alkaline water and organic molecules in the plumes, opening the possibility of finding life.
Of course, organic molecules by themselves don’t mean life: Astrochemists have found them in comets and interstellar clouds, neither of which are hospitable. However, Cassini revealed that Enceladus has hydrothermal activity and an ocean underneath its ice, which provides shelter from radiation and an energy source—things the atmosphere and the Sun provide on Earth to allow life to flourish.
“Enceladus has been very helpfully throwing its material into space where it is much easier to measure with spacecraft,” Hörst said. “The Cosmic Dust Analyzer obviously wasn’t designed to measure Enceladus because we didn’t know about the plumes, but it’s become a really powerful tool for measuring the composition of Enceladus’s ocean. Enceladus is awesome.”
Biochemical Riddles in the Dark
The data set obtained from Cassini was so huge it couldn’t be fully processed while the probe was operational.
It took 3.5 years to find sodium phosphates, Postberg said. “We didn’t look for phosphates or anything specific; we just wanted to look for something new.”
Even then, he and his colleagues had to answer two important questions: Was this analysis reliable, and how could Enceladus have 100 times more phosphorus in its oceans than Earth does?
Over the next year and a half, part of the team performed geochemical experiments and modeling to answer those questions. They showed that assuming the rocky seabed of Enceladus has a chemical makeup similar to most meteorites and many comets, an alkaline ocean like Enceladus’s would dissolve the amounts of phosphates measured in the plumes.
Postberg cautioned that none of the data so far have contained clear signatures for the sixth essential element for all known life: sulfur.
As both Postberg and Hörst pointed out, Enceladus bears similarities to many other icy moons in the outer solar system, including Saturn’s moon Titan and Jupiter’s moon Europa. If the geological and chemical conditions for life are widely present on these worlds, that raises the distinct possibility that life-bearing worlds very different from Earth are common in the cosmos.
—Matthew R. Francis (@DrMRFrancis), Science Writer