Editor’s note: A version of this has been previously published on the author’s website.
Saturn’s small moon Enceladus is probably one of the most likely places in the solar system, outside Earth, to harbor life. A tantalizing study released a few days ago makes the possibility seem good.
Enceladus has puzzled scientists for decades, according to NASA. They “didn’t know why Enceladus was the brightest world in the solar system,” according to an agency summary online at solarsystem.nasa.gov. With the arrival of NASA’s Cassini probe in 2004, the reason for the bright white glare was shown to be highly reflective ice that covers the surface.
The ice is gouged by cracks, grooves and lines, including four major gully-like depressions dubbed “tiger stripes.”
Over the 13 years Cassini spent in orbit around Saturn, it radioed back thousands of photographs of the planet, rings and moons. Hundreds of pictures of Enceladus, plus heat mapping and other studies, indicate that it has a subsurface ocean. Water vents from the ocean, blasting through the crust at 800 miles per hour, quickly freeze into ice grains. Warmer regions are “tiger stripes,” apparently where water has reached or neared the surface.
The heat source for the 310-mile diameter moon is gravitational friction with Saturn. In 2008, Cassini flew through gas and water icy plumes at distances ranging from thousands of miles to 120 miles above the surface, finding that “this tiny moon is so active, ‘hot’ and brimming with water vapor and organic chemicals,” wrote a NASA official.
A paper released in February 2019 on sciencedirect.com, based on gravity readings and analysis of Enceladus’ shape, indicates that the ice crust ranges from more than 19 miles to only about three miles thick. The ocean is estimated to be six miles deep and 200 degrees Fahrenheit at places (similar to Earth’s oceans with their deep-sea hydrothermal vents), according to a 2015 report in the journal Nature.
The Nature report says Enceladus has “global-scale geothermal activity that quickly transports hydrothermal products from the ocean floor at a depth of at least 40 kilometers (or 25 miles) up to the plume of Enceladus.” Among materials detected in the plume were silica from the ocean’s rocky floor. The ocean was determined to be moderately salty and alkaline, with hydrogen and methane present, as summarized by researchers from Germany and Colorado.
In 2017, the aging Cassini was directed to crash into Saturn’s clouds, where it quickly disintegrated. But the project continues to surprise researchers.
Oct. 2 brought the news that additional analysis of Cassini data found chemicals in the plumes that could be “ideal precursors for ... hydrothermal synthesis of biologically relevant organic compounds in the warm depths of Enceladus’ ocean.”
A team of researchers led by Nozair Khawaja of the Free University of Berlin used mass spectrometer readings from Cassini’s Cosmic Dust Analyzer (CDA) instrument, which characterized material spewed out by Enceladus. The study is titled “Low-mass nitrogen-, oxygen-bearing, and aromatic compounds in Enceladean ice grains” and was published by the Monthly Notices of the Royal Astronomical Society, based in London. Its nine co-authors include Sasha Kempf, associate professor at the University of Colorado, Boulder.
The newly-discovered organic compounds detected in ice grains are “the ingredients of amino acids ...
”On Earth, similar compounds are part of chemical reactions that produce amino acids, the building blocks of life. Hydrothermal vents on the ocean floor provide the energy that fuels the reactions. Scientists believe Enceladus’ hydrothermal vents may operate in the same way, supplying energy that leads to the production of amino acids,” says an analysis by the researchers at the Jet Propulsion Laboratory in Pasadena, California.
”One question I have goes to the distinction between biotic and prebiotic activity,” I wrote to Khawaja. “That is, if amino acids or other organic compounds indicating life had been present, would the mass spectrometer have identified them?”
Khawaja replied by email:
”Your question is very interesting. We are intensively working to simulate the identification of biologically-relevant organic compounds (amino acids and fatty acids) through mass spectrometer in the laboratory. The abundances of fragment ions and patterns of mass lines of amino acids and fatty acids can tell us whether their origin is biotic or abiotic.
”For example, fatty acid of biotic origin would give a different distribution of carbon numbers (i.e., features related to even number of carbon atoms dominates) as compared to fatty acids with abiotic origin, which show continuous Gaussian-like distribution of carbon atoms.
”However, just to clarify that, so far, we have not found any confirmed biotic related compounds (amino/fatty acids) in Enceladus’ ocean. May be such compounds are there (we do not know yet) but their detection requires more advanced mass spectrometers to do the job as compared to what we had onboard Cassini. Therefore, we need to go back to Enceladus with more sophisticated and advanced instrumentation/mass spectrometers to explore the exact nature of organic material in Enceladus’ ocean.”
Enceladus may have an ocean where communities of — animals? plants? something different? — derive nutrients from hydrothermal vents just as colonies of animals do around vents in Earth’s deep seas.
So let’s send a new probe to Enceladus.
Joe Bauman, a former Deseret News science reporter, writes an astronomy blog at the-nightly-news.com and is an avid amateur astronomer. His email is email@example.com.