NASA Study Finds Life-Sparking Energy Source and Molecule at Enceladus

Discovery at Enceladus - 1 - Eyes on the Solar System - NASA/JPL

A study zooms in on data that NASA’s Cassini gathered at Saturn’s icy moon and finds evidence of a key ingredient for life and a supercharged source of energy to fuel it.

Scientists have known that the giant plume of ice grains and water vapor spewing from Saturn’s moon Enceladus is rich with organic compounds, some of which are important for life as we know it. Now, scientists analyzing data from NASA’s Cassini mission are taking the evidence for habitability a step further: They’ve found strong confirmation of hydrogen cyanide, a molecule that is key to the origin of life.

The researchers also uncovered evidence that the ocean, which is hiding below the moon’s icy outer shell and supplies the plume, holds a powerful source of chemical energy. Unidentified until now, the energy source is in the form of several organic compounds, some of which, on Earth, serve as fuel for organisms.

“Our work provides further evidence that Enceladus is host to some of the most important molecules for both creating the building blocks of life and for sustaining that life through metabolic reactions.”

The findings, published Thursday, Dec. 14, in Nature Astronomy, indicate there may be much more chemical energy inside this tiny moon than previously thought. The more energy available, the more likely that life might proliferate and be sustained.

“Our work provides further evidence that Enceladus is host to some of the most important molecules for both creating the building blocks of life and for sustaining that life through metabolic reactions,” said lead author Jonah Peter, a doctoral student at Harvard University who performed much of the research while working at NASA’s Jet Propulsion Laboratory in Southern California. “Not only does Enceladus seem to meet the basic requirements for habitability, we now have an idea about how complex biomolecules could form there, and what sort of chemical pathways might be involved.”

Versatile and Energetic

“The discovery of hydrogen cyanide was particularly exciting, because it’s the starting point for most theories on the origin of life,” Peter said. Life as we know it requires building blocks, such as amino acids, and hydrogen cyanide is one of the most important and versatile molecules needed to form amino acids. Because its molecules can be stacked together in many different ways, the study authors refer to hydrogen cyanide as the Swiss army knife of amino acid precursors.

“The more we tried to poke holes in our results by testing alternative models,” Peter added, “the stronger the evidence became. Eventually, it became clear that there is no way to match the plume composition without including hydrogen cyanide.”

In 2017, scientists found evidence at Enceladus of chemistry that could help sustain life, if present, in its ocean. The combination of carbon dioxide, methane, and hydrogen in the plume was suggestive of methanogenesis, a metabolic process that produces methane. Methanogenesis is widespread on Earth, and may have been critical to the origin of life on our planet.

 The new work uncovers evidence for additional energy chemical sources far more powerful and diverse than the making of methane: The authors found an array of organic compounds that were oxidized, indicating to scientists that there are many chemical pathways to potentially sustain life in Enceladus’ subsurface ocean. That’s because oxidation helps drive the release of chemical energy.

“If methanogenesis is like a small watch battery, in terms of energy, then our results suggest the ocean of Enceladus might offer something more akin to a car battery, capable of providing a large amount of energy to any life that might be present,” said JPL’s Kevin Hand, co-author of the study and principal investigator of the effort that led to the new results.

Math Is the Way

Unlike earlier research that used lab experiments and geochemical modeling to replicate the conditions Cassini found at Enceladus, the authors of the new work relied on detailed statistical analyses. They examined data collected by Cassini’s ion and neutral mass spectrometer, which studied the gas, ions, and ice grains around Saturn.

By quantifying the amount of information contained in the data, the authors were able to tease out subtle differences in how well different chemical compounds explain the Cassini signal.

“There are many potential puzzle pieces that can be fit together when trying to match the observed data,” Peter said. “We used math and statistical modeling to figure out which combination of puzzle pieces best matches the plume composition and makes the most of the data, without overinterpreting the limited dataset.”

Scientists are still a long way from answering whether life could originate on Enceladus. But as Peter noted, the new work lays out chemical pathways for life that could be tested in the lab.

Meanwhile, Cassini is the mission that keeps giving – long after it revealed that Enceladus is an active moon. In 2017, the mission ended by deliberately plunging the spacecraft into Saturn’s atmosphere. “Our study demonstrates that while Cassini’s mission has ended, its observations continue to provide us with new insights about Saturn and its moons – including the enigmatic Enceladus,” said Tom Nordheim, a JPL planetary scientist who’s a co-author of the study and was a member of the Cassini team.

Life might have been possible just seconds after the Big Bang

Life has found a home on Earth for around 4 billion years. That's a significant fraction of the universe's 13.77-billion-year history. Presumably, if life arose here, it could have appeared anywhere. And for sufficiently broad definitions of life, it might even be possible for life to have appeared mere seconds after the Big Bang.

To explore the origins of life, first we have to define it. There are over 200 published definitions of the term, which shows just how difficult this concept is to grapple with. For example, are viruses alive? They replicate but need a host to do so. What about prions, the pathogenic protein structures? Debates continue to swirl over the line between life and nonlife. But for our purposes, we can use an extremely broad, but very useful definition: Life is everything that's subject to evolution.

This definition is handy because we'll be exploring the origins of life itself, which, by definition, will blur the boundaries between life and nonlife. At one point, deep in the past, Earth was not alive. Then it was. This means that there was a transition period that will naturally stretch the limits of any definition you can muster. Plus, as we dig deeper into the past and explore other potential options for life, we want to keep our definition broad, especially as we explore the more extreme and exotic corners of the universe.

With this definition in hand, life on Earth arose at least 3.7 billion years ago. By then, microscopic organisms had already become sophisticated enough to leave behind traces of their activities that persist to the present day. Those organisms were a lot like modern ones: They used DNA to store information, RNA to transcribe that information into proteins, and the proteins to interact with the environment and make copies of the DNA. This three-way combo allows those batches of chemicals to experience evolution.

But those microbes didn't just fall out of the sky; they evolved from something. And if life is anything that evolves, then there had to be a simpler version of life appearing even earlier in Earth's past. Some theories speculate that the first self-replicating molecules, and hence the simplest possible form of life on Earth, could have arisen as soon as the oceans cooled, well over 4 billion years ago.

And Earth may not have been alone — Mars and Venus had similar conditions at that time, so if life happened here, it may have happened there, too.

The first life among the stars 

But the sun was not the first star to ignite into fusion; it is a product of a long line of previous generations of stars. Life as we know it requires a few key elements: hydrogen, oxygen, carbon, nitrogen and phosphorus. With the exception of hydrogen, which appeared in the first few minutes after the Big Bang, all of these elements are created in the hearts of stars during their life cycles. So, as long as you have at least one or two generations of stars living and dying, and thereby spreading their elements out into the wider galaxy, you can have Earth-like life appearing in the universe.

This pushes the clock back on the possible first appearance of life to well over 13 billion years ago. This era in the history of the universe is known as the cosmic dawn, when the first stars formed. Astronomers aren't exactly sure when this transformative epoch took place, but it was somewhere within a few hundred million years after the Big Bang. As soon as those stars appeared, they could have started creating the necessary elements for life.

So, life as we know it — built on chains of carbon, using oxygen to transport energy, and submersed in a bath of liquid water — may be much, much older than Earth. Even other hypothesized forms of life based on exotic biochemistries require a similar mixture of elements. For example, some alien life may use silicon instead of carbon as a basic building block or use methane instead of water as a solvent. No matter what, those elements have to come from somewhere, and that somewhere is in the cores of stars. Without stars, you can't have chemical-based life.

The first life in the universe 

But perhaps it's possible to have life without chemistry. It's hard to imagine what these creatures might be like. But if we take our broad definition — that life is anything subject to evolution — then we don't need chemicals to make it happen. Sure, chemistry is a convenient way to store information, extract energy and interact with the environment, but there are other hypothetical pathways.

For example, 95% of the energy contents of the universe are unknown to physics, literally sitting outside the known elements. Scientists aren't sure what these mysterious components of the universe, known as dark matter and dark energy, are made of. 

Perhaps there are additional forces of nature that work only on dark matter and dark energy. Maybe there are multiple "species" of dark matter — an entire "dark matter periodic table." Who knows what interactions and what dark chemistry play out in the vast expanses between the stars? Hypothetical "dark life" may have appeared in the extremely early universe, well before the emergence of the first stars, powered and mediated by forces we do not yet understand.

The possibilities can get even weirder. Some physicists have hypothesized that in the earliest moments of the Big Bang, the forces of nature were so extreme and so exotic that they could have supported the growth of complex structures. For example, these structures could have been cosmic strings, which are folds in space-time, anchored by magnetic monopoles. With sufficient complexity, these structures could have stored information. There would have been plenty of energy to go around, and those structures could have self-replicated, enabling evolution. 

Any creatures existing in those conditions would have lived and died in the blink of an eye, their entire history lasting less than a second — but to them, it would have been a lifetime.

Essential life molecules may originate near new stars and planets

New research postulates that basic amino acids could have “formed alongside stars or planets within interstellar ices.”

Scientists have long been in the quest to uncover the source from which Earth obtained its essential ingredients of life. 

Since the detection of organic molecules in a Murchison meteorite that landed in Australia in 1969, they have been captivated by the prospect that the fundamental components of life may have originated in outer space.

The question remains: Where and when did these crucial molecules come into existence before finding their way to Earth preserved within meteorites?

Now, new research postulates that fundamental amino acids, such as carbamic acid, could have "formed alongside stars or planets within interstellar ices."

Amino acids play a vital role in life as they are the fundamental building blocks of proteins, carrying out diverse functions within living organisms.

Model of interstellar ice reveals key details

Multiple theories propose scenarios explaining how Earth acquired the building blocks of life. 

One predominant theory suggesting the origin of life centers on the concept of a "primordial soup." It basically refers to a mixture of organic molecules that may have evolved in Earth's early oceans as a result of the reaction of simple chemicals.

An alternate theory posits that meteorites could have delivered amino acids to the Earth's surface during its forming years. 

As per the official release, these celestial bodies, or space rocks, could have accumulated the molecules from dust or interstellar ice, which consist of water and other gases frozen into solid form due to the extremely cold temperatures of outer space. 

However, the mystery lies in the fact that since meteorites originated from distant regions in the universe, scientists are perplexed about the precise location of the formation of these molecules. The question remains: Where and when did these crucial molecules come into existence before finding their way to Earth preserved within meteorites?

The researchers from the University of Hawaii went on to obtain a better grasp of the situation, concentrating on the chemical processes that may have occurred in interstellar ices that were once present in the vicinity of newly forming stars and planets.

They created a model including interstellar ice containing ammonia and carbon dioxide to study this. These ices were then progressively heated after being put on a silver substrate.

The team employed Fourier transform infrared spectroscopy to understand the interstellar ice better. 

The formation of amino acids

The findings revealed that carbamic acid began to form at -348 degrees Fahrenheit, while ammonium carbamate initiated its formation at -389 degrees Fahrenheit.

"These low temperatures demonstrate that these molecules — which can turn into more complex amino acids — could have formed during the earliest, coldest stages of star formation," revealed the press release. 

"In addition, the researchers found that at warmer temperatures, similar to those produced by a newly formed star, two carbamic acid molecules could link together, making a stable gas," the release added. 

The team put forward a hypothesis suggesting that these molecules might have become part of the basic constituents of the solar system. After its formation, comets or meteorites, acting as cosmic deliverers, could have transported these molecules to early Earth.

The results could serve as valuable insights for training sophisticated telescopes such as the James Webb Space Telescope to explore distant, star-forming regions of the universe in search of prebiotic molecules.