From Icy Plumes to Extraterrestrial Life: Saturn’s Icy Moon Holds the Key

Enceladus’ ice plumes may hold the building blocks of life.

As astrophysics technology and research continue to advance, one question persists: is there life elsewhere in the universe? The Milky WayThe Milky Way is the galaxy that contains our Solar System and is part of the Local Group of galaxies. It is a barred spiral galaxy that contains an estimated 100-400 billion stars and has a diameter between 150,000 and 200,000 light-years. The name "Milky Way" comes from the appearance of the galaxy from Earth as a faint band of light that stretches across the night sky, resembling spilled milk.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>Milky Way galaxy alone has hundreds of billions of celestial bodies, but scientists often look for three crucial elements in their ongoing search: water, energy and organic material. Evidence indicates that SaturnSaturn is the sixth planet from the sun and has the second-largest mass in the Solar System. It has a much lower density than Earth but has a much greater volume. Saturn's name comes from the Roman god of wealth and agriculture.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>Saturn’s icy moon Enceladus is an ‘ocean world’ that contains all three, making it a prime target in the search for life.

Cassini’s Discoveries on Enceladus

During its 20-year mission, NASAEstablished in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. Its vision is "To discover and expand knowledge for the benefit of humanity." Its core values are "safety, integrity, teamwork, excellence, and inclusion." NASA conducts research, develops technology and launches missions to explore and study Earth, the solar system, and the universe beyond. It also works to advance the state of knowledge in a wide range of scientific fields, including Earth and space science, planetary science, astrophysics, and heliophysics, and it collaborates with private companies and international partners to achieve its goals.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>NASA’s CassiniThe Cassini–Huygens Mission, generally called Cassini, was a joint mission between NASA, the European Space Agency and the Italian Space Agency to study the Saturn system. Launched in 1997, Cassin provided astronomers with a massive amount of data about Saturn and its rings, its magnetosphere, and its moons. Cassini reached the end of its journey in 2017 when it deliberately dived into Saturn's atmosphere, where it disintegrated like a meteor.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>Cassini spacecraft discovered that ice plumes spew from Enceladus’ surface at approximately 800 miles per hour (400 m/s). These plumes provide an excellent opportunity to collect samples and study the composition of Enceladus’ oceans and potential habitability. However, until now it was not known if the speed of the plumes would fragment any organic compounds contained within the ice grains, thus degrading the samples.

(This artistic rendering shows ice plumes being ejected from Enceladus at speeds of up to 800 miles/hour. Credit: NASA/JPL-Caltech

Breakthrough in Laboratory Research

Now researchers from the University of California San Diego have shown unambiguous laboratory evidence that amino acids<div class="cell text-container large-6 small-order-0 large-order-1">
<div class="text-wrapper"><br />Amino acids are a set of organic compounds used to build proteins. There are about 500 naturally occurring known amino acids, though only 20 appear in the genetic code. Proteins consist of one or more chains of amino acids called polypeptides. The sequence of the amino acid chain causes the polypeptide to fold into a shape that is biologically active. The amino acid sequences of proteins are encoded in the genes. Nine proteinogenic amino acids are called "essential" for humans because they cannot be produced from other compounds by the human body and so must be taken in as food.<br /></div>
</div>” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>amino acids transported in these ice plumes can survive impact speeds of up to 4.2 km/s, supporting their detection during sampling by spacecraft. Their findings were published in The Proceedings of the National Academy of Sciences (PNAS).

Unique Aerosol Impact Spectrometer

Beginning in 2012, UC San Diego Distinguished Professor of Chemistry and Biochemistry Robert Continetti and his co-workers custom-built a unique aerosol impact spectrometer, designed to study collision dynamics of single aerosols and particles at high velocities. Although not built specifically to study ice grain impacts, it turned out to be exactly the right machine to do so.

“This apparatus is the only one of its kind in the world that can select single particles and accelerate or decelerate them to chosen final velocities,” stated Continetti. “From several micron diameters down to hundreds of nanometers, in a variety of materials, we’re able to examine particle behavior, such as how they scatter or how their structures change upon impact.”

A custom-built aerosol impact spectrometer from UC San Diego Professor of Chemistry Robert Continetti’s lab. Ice grains impact the microchannel plate detector (far right) at hypervelocity speeds, which can then be characterized in-situ. Credit: Robert Continetti lab / UC San Diego

The Europa Clipper Mission

In 2024 NASA will launch the Europa Clipper, which will travel to JupiterJupiter is the largest planet in the solar system and the fifth planet from the sun. It is a gas giant with a mass greater then all of the other planets combined. Its name comes from the Roman god Jupiter.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>Jupiter. Europa, one of Jupiter’s largest moons, is another ocean world, and has a similar icy composition to Enceladus. There is hope that the Clipper or any future probes to Saturn will be able to identify a specific series of molecules in the ice grains that could point to whether life exists in the subsurface oceans of these moons, but the molecules need to survive their speedy ejection from the moon and collection by the probe.

Artist’s rendering of NASA’s Europa Clipper spacecraft. Credit: NASA/JPL-Caltech

Innovative Experimentation Techniques

Although there has been research into the structure of certain molecules in ice particles, Continetti’s team is the first to measure what happens when a single ice grain impacts a surface.

To run the experiment, ice grains were created using electrospray ionization, where water is pushed through a needle held at a high voltage, inducing a charge that breaks the water into increasingly smaller droplets. The droplets were then injected into a vacuum where they freeze. The team measured their mass and charge, then used image charge detectors to observe the grains as they flew through the spectrometer. A key element to the experiment was installing a microchannel plate ion detector to accurately time the moment of impact down to the nanosecond.

Significant Findings in Astrobiology

The results showed that amino acids — often called the building blocks of life — can be detected with limited fragmentation up to impact velocities of 4.2 km/s.

“To get an idea of what kind of life may be possible in the solar system, you want to know there hasn’t been a lot of molecular fragmentation in the sampled ice grains, so you can get that fingerprint of whatever it is that makes it a self-contained life form,” said Continetti. “Our work shows that this is possible with the ice plumes of Enceladus.”

Broader Implications in Chemistry

Continetti’s research also raises interesting questions for chemistry itself, including how salt affects the detectability of certain amino acids. It is believed that Enceladus contains vast salty oceans — more than is present on Earth. Because salt changes the properties of water as a solvent as well as the solubility of different molecules, this could mean that some molecules cluster on the surface of the ice grains, making them more likely to be detected.

“The implications this has for detecting life elsewhere in the solar system without missions to the surface of these ocean-world moons is very exciting, but our work goes beyond biosignatures in ice grains,” stated Continetti. “It has implications for fundamental chemistry as well. We are excited by the prospect of following in the footsteps of Harold Urey and Stanley Miller, founding faculty at UC San Diego in looking at the formation of the building blocks of life from chemical reactions activated by ice grain impact.”

Reference: “Detection of intact amino acids with a hypervelocity ice grain impact mass spectrometer” by Sally E. Burke, Zachary A. Auvil, Karl A. Hanold and Robert E. Continetti, 4 December 2023, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2313447120

This work was supported by the Air Force Office of Science Research (MURI-22, grant FA9550-22-0199) and the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (grant 80NM0018D0004).