Poison gas indicates potential for life on an ocean moon of Saturn

Scientists have discovered a poison among the spray of molecules coming from a small moon of Saturn. That adds to the existing intrigue about the possibility of life there.

The poison is hydrogen cyanide, a colorless, odorless gas that is fatal to many terrestrial creatures. But it could have played a key role in chemical reactions that created the ingredients that set the stage for the arrival of life.

“It’s the starting point for most theories about the origin of life,” says Jonah Peter, a biophysics student at Harvard. “It’s kind of like the Swiss army knife of prebiotic chemistry.”

Mr Peter was therefore excited when he found hydrogen cyanide near Enceladus, an icy moon of Saturn about 500 kilometers across. It has a subsurface ocean making it one of the most promising places to look for life elsewhere in the solar system.

Mr. Peter and his collaborators, Tom Nordheim and Kevin Hand of NASA’s Jet Propulsion Laboratory in California, reported their findings in a paper published Thursday in the journal Nature Astronomy.

They also found organic molecules such as acetylene, propylene and ethane that could power chemical reactions to provide energy to microorganisms living in the Enceladus ocean. The data also indicates the presence of an alcohol such as methanol, although the researchers could not definitively identify which alcohol.

Chemical experiments have shown that hydrogen cyanide could be an important precursor to the molecules that needed to be present for life to emerge. “It can be combined in different ways to produce amino acids, which are precursors to proteins, as well as nuclear bases and sugars needed to make RNA and DNA,” Mr Peter said.

Twenty years ago, Enceladus was seen as a mostly boring ball of ice.

But in 2005, planetary scientists were startled when NASA’s Cassini spacecraft spotted plumes of vapor and ice crystals shooting out of fractures near the moon’s south pole. Saturn’s tidal forces pull and compress Enceladus’ interior, and the friction generates enough heat to melt the ice.

The first analysis identified not only water, but also carbon dioxide, methane, hydrogen and ammonia. The eruptions indicated hydrothermal reactions beneath the surface, where hot rocks meet liquid water.

The subsequent examination of data from the Cassini mission, which ended in 2017, has only increased curiosity about what lies beneath. Scientists now believe that Enceladus possesses not only a pool of liquid water beneath its south pole, but also a global ocean of salt water beneath an outer layer of ice.

Earlier this year, another team of scientists reported that ice particles in the Enceladus plumes contained phosphates, which also pointed to geochemical interactions between the ocean and the bedrock. Phosphorus is another important element considered essential for life.

“Indeed, the prospects for the development of life on Enceladus are improving,” said Frank Postberg, professor of planetary sciences at the Free University of Berlin who led the phosphate research.

In the latest work, Mr. Peter and his collaborators have once again delved into the data from the Cassini flybys. The amount of hydrogen cyanide is too small to be observed directly. Instead, the researchers started with a list of 50 compounds that they thought might be present in Enceladus. They then constructed models of 10 to 15 of those compounds, and tested which models best fit what Cassini observed.

“It’s a nice analysis done to get a little more information about what we can see in the plumes,” said Kathleen Craft, a planetary scientist at the Johns Hopkins Applied Physics Laboratory in Maryland.

For Dr. Craft, Enceladus is an intriguing place, but not the only place. Other moons such as Europa, which orbits Jupiter, also have oceans beneath the ice. An upcoming NASA mission, Europa Clipper, will carry an instrument similar to Cassini’s that could make similar discoveries.

“All ocean worlds are extremely exciting,” said Dr. Craft. “They’re all a little different from each other, but they have a lot of similarities.”

The presence of hydrogen cyanide and the other newly reported organic compounds in Enceladus’ plumes “does not reveal the source of complex organic matter in the ocean,” said Alfonso Davila, a researcher in the exobiology division of NASA’s Ames Research Center in California.

“But it brings us an inch closer to the answer,” he said.