Linking Organic Molecules to Hydrothermal Vents on Enceladus

Despite the vast distance between us and Saturn’s gleaming moon Enceladus, the icy ocean moon is a prime target in our search for life. It vents water vapour and large organic molecules into space through fissures in its icy shell, which is relatively thin compared to other icy ocean moons like Jupiter’s Europa. Though still out of reach, scientific access to its ocean is not as challenging as on Europa, which has a much thicker ice shell.

The presence of large organic molecules isn’t very controversial. But they don’t necessarily signify that something alive lurks in its ancient, unseen ocean. Instead, hydrothermal processes could produce them. The complexity arises because hydrothermal processes are also linked to the emergence of life.

Understanding the abiotic processes that produce these molecules is important not just for Enceladus. It could serve as a baseline for understanding the results of a future mission to the frozen moon and any biosignatures it might detect.

New research in the journal Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences examines this issue. It’s titled “Laboratory characterization of hydrothermally processed oligopeptides in ice grains emitted by Enceladus and Europa.” The lead author is Dr. Nozair Khawaja from the Institute of Space Systems (IRS) at the University of Stuttgart.

Scientists postulate the life on Earth got started at hydrothermal events on the ocean floor. These vents provide mineral-rich fluids. At deep ocean vents under extreme pressure, these minerals can react with seawater to produce the building blocks of life.

“In research, we also speak of a hydrothermal field,” explains lead author Khawaja. “There is convincing evidence that conditions prevail in such fields that are important for the emergence or maintenance of simple life forms.”

Much of what we know about Enceladus comes from the Cassini mission. Scientists are still working with Cassini’s data even though it ended in 2017. Although much of the data was low resolution, it’s still valuable.

Professor Frank Postberg from the Freie Universität (FU) Berlin is one of the study’s co-authors. “In 2018 and 2019, we encountered various organic molecules, including some that are typically building blocks of biological compounds,” Postberg said. “And that means it is possible that chemical reactions are taking place there that could eventually lead to life.”

There’s a missing link between the hydrothermal vents and the molecules vented into space. Scientists aren’t certain if the vents are responsible for the molecules or in what way. Is life involved?

To answer these questions, the researchers simulated an Enceladus hydrothermal vent in their laboratory.

“To this end, we simulated the parameters of a possible hydrothermal field on Enceladus in the laboratory at the FU Berlin,” said lead author Khawaja. “We then investigated what effects these conditions have on a simple chain of amino acids.” Amino acids are the basic building blocks of proteins and the basis of all Earth life. There are hundreds of them, and 22 of them are in all living cells. They’re the precursors to proteins and they show that life on Earth is all connected.

The researchers subjected amino acids to conditions thought to persist at Encledadus’ ocean floor. “Here, we present results from our newly established facility to simulate the processing of ocean material within the temperature range 80–150°C and the pressure range 80–130 bar, representing conditions suggested for the water-rock interface on Enceladus,” they write in their paper. Under those conditions, the chains of amino acids behaved characteristically.

But that’s in a lab. Can we devise a space probe that can detect these types of changes on Enceladus? The changes themselves are obscured, but do they produce byproducts or markers that are emitted into space?

Cassini’s Cosmic Dust Analyzer (CDA) detected the organic molecules in Enceladus’ plumes by watching collisions between rapidly moving particles that shatter molecules and vapourize their contents. Some particles, stripped of their electrons, become positively charged and are attracted to a negative electrode on the instrument. The less massive they are, the faster they reach the electrode.

By combining a large amount of this type of data, the CDA revealed a lot about the original molecules.

But this can’t be replicated in a lab.

“Instead, we employed an alternative measurement method called LILBID for the first time on ice particles containing hydrothermally altered material,” Khawaja explains. LILBID stands for laser-induced liquid beam ion desorption, a different type of mass spectrometry than the CDA performs. Though the method is different, it produces results similar to Cassini’s CDA instrument.

“This delivers very similar mass spectra to the Cassini instrument. We used this to measure an amino acid chain before and after the experiment. In the process, we came across characteristic signals that were caused by the reactions in our simulated hydrothermal field,” Khawaja said.

Specifically, the researchers examined the hydrothermal processing of the triglycine (GGG) peptide. GGG is a tripeptide, the most common one. Scientists often use GGG to study amino acids, peptides, and proteins, analyzing the molecular interactions and physicochemical parameters of all three.

“Differences observed between mass spectra of hydrothermally processed and unprocessed triglycine can be regarded as a spectral fingerprint to identify processed GGG in ice grains from icy moons in the solar system,” the authors wrote in their research.

“This delivers very similar mass spectra to the Cassini instrument. We used this to measure an amino acid chain before and after the experiment. In the process, we came across characteristic signals that were caused by the reactions in our simulated hydrothermal field,” Khawaja said.

The researchers intend to repeat this experiment with other organic molecules under extended geophysical conditions in Enceladus’ ocean. “With this new laboratory setup, we will simulate a range of hydrothermal conditions, from the high pressures and temperatures associated with greater depths into the core, to the milder conditions in the ocean water near the water-rock interface,” the authors write in their paper.

The results will allow them to search through Cassini’s data for similar markers. It can also work for future missions to Enceladus and would be further proof of hydrothermal activity on the frozen ocean moon.

If scientists can confirm hydrothermal vents on Enceladus, the excitement that moon generates will only increase.