How Enceladus' Ocean Spray Forms Diverse Ice Grains

Saturn's icy moon Enceladus hides a global ocean beneath its frozen surface. From fractures near its south pole, material from this ocean is ejected into space as a plume of water vapour and ice particles. These particles offer scientists a rare opportunity to investigate an extraterrestrial ocean without drilling through kilometres of ice.

Now, an international research team including scientists from the Earth-Life Science Institute (ELSI) at the Institute of Science Tokyo has uncovered how ocean water may be transformed on its journey from the subsurface ocean into the tiny ice grains observed in space.

From 2004 to 2017, the Cosmic Dust Analyser aboard the Cassini spacecraft measured the composition of individual ice grains in Saturn's E-ring, which is supplied by material ejected from Enceladus. Researchers led by Prof Frank Postberg at Freie Universität Berlin analysed 961 mass spectra of salt-rich grains, known as Type 3 particles. Rather than finding grains with broadly similar mixtures of ocean salts, they discovered striking chemical diversity.

Different grains were enriched in different salts, including sodium chloride, carbonates, phosphates and potassium chloride. In particular, chloride and carbonate were rarely found together in the same sodium-rich grain. This raised a question: if these particles originated from the same ocean, how did their compositions become so different?

To investigate, Professor Yasuhito Sekine and colleagues at ELSI conducted laboratory experiments using droplets designed to reproduce the major salt components expected in Enceladus' ocean. The team froze droplets of different sizes at different cooling rates and examined how their constituent elements were distributed after freezing.

The experiments revealed that cooling rate matters. In droplets around 200 micrometres across, salts became spatially separated when the droplets froze slowly, at approximately 10 K per minute or less. Faster freezing produced a much more uniform distribution.

"What surprised us was that the diversity seen by Cassini could emerge from droplets originating from essentially the same ocean water," said Sekine. "Our experiments show that when relatively large ocean droplets freeze slowly, different salts can separate within them. If those frozen droplets are later broken apart, they can produce much smaller ice grains, each with very different chemical compositions."

The slow freezing also provides clues to conditions inside Enceladus' icy crust. Previous research had generally assumed that seawater spray freezes rapidly, moving towards space fast after leaving the ocean. The new results instead suggest that droplets at the beginning travel more slowly through the subsurface vent system, potentially through more complex pathways within fractures, before reaching the surface.

The researchers propose a multi-stage journey. Ocean spray initially forms droplets tens to hundreds of micrometres across. These travel relatively slowly through deeper parts of the vents, allowing salts to separate as the droplets gradually freeze. Closer to the surface, the gas flow accelerates and the frozen droplets collide with the walls of narrower ice channels at high speeds, causing them to shatter. The resulting fragments can contain different salt-rich regions and are eventually carried into Saturn's E-ring.

"The Cassini data showed us that these salt-rich grains are far more chemically diverse than an average ocean composition would suggest," said Postberg. "Combining those observations with the freezing experiments gives us a physical explanation: Cassini may have sampled fragments of larger frozen ocean droplets, each preserving different components that became separated during their journey towards the surface. The abundance of each individual component in the ocean is then reflected in the number of fragments in which a particular component is found."

The findings have implications for future exploration of Enceladus. Freezing and fragmentation can concentrate particular compounds into different grains. Salts are not just separated from each other but also from organics, and previous analyses have also shown that many organic species show up separated from each other at elevated concentrations. Analysing particles individually could therefore make it much easier for future spacecraft detect compounds that are otherwise diluted in the ocean, mixed with many other compounds.

On Earth, for example, chemical laboratories go to great lengths to separate and concentrate the various components of a sample before analysis. Enceladus now conveniently performs both of these "sample preparation" steps for us: The chemical components are separated from one another and then appear in elevated concentrations in a fraction of the ice particles.

Slow freezing may also create small pockets of liquid brine between growing ice crystals, where salts and organic compounds become concentrated. Such concentration could be relevant to prebiotic chemistry, where bringing dilute organic molecules together is an important challenge. Since much of Enceladus' plume material falls back onto the moon, these processes could potentially occur repeatedly.

Understanding how these particles form, therefore, provides both a picture of the hidden environment beneath Enceladus' surface and a guide for interpreting material sampled by future missions searching for clues to the moon's habitability and signs of life.

Reference

Frank Postberg¹*, Zenghui Zou^2,1, Yasuhito Sekine^3,10,11, Minori Koga³, Jürgen Schmidt¹, Mark Fox-Powell⁴, Fabian Klenner^5,6, Jon K. Hillier¹, Nozair Khawaja¹, Toshihiko Kadono⁷, Melih Çakar^1,3, Sascha Kempf⁸, Ralf Srama⁹, Cassini CDA observes compositional segregation of Enceladus' ice grains from slow freezing and fragmentation of oceanic spray, Science Advances, DOI: 10.1126/sciadv.aee7256 　

1. Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany.
2. School of Mathematics and Physics, Qinghai University, Xining, China.
3. Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, Tokyo, Japan.
4. School of Environment, Earth & Ecosystem Sciences , The Open University, Milton-Keynes, UK.
5. Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA.
6. Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA.
7. Department of Basic Sciences, University of Occupational and Environmental Health, Kitakyushu, Japan.
8. Laboratory for Atmospheric and Space Physics, University of Colorado in Boulder, Boulder (CO), USA.
9. Institut für Raumfahrtsysteme, Universität Stuttgart, Stuttgart, Germany.
10. GENTEN Research Center, Institute of Science Tokyo, Tokyo, Japan.
11. Institute of Nature and Environmental Technology, Kanazawa University, Kanazawa, Japan.

More information

Earth-Life Science Institute (ELSI) is one of Japan's ambitious World Premiere International research centers, whose aim is to achieve progress in broadly inter-disciplinary scientific areas by inspiring the world's greatest minds to come to Japan and collaborate on the most challenging scientific problems. ELSI's primary aim is to address the origin and co-evolution of the Earth and life.

Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of Technology (Tokyo Tech), with the mission of "Advancing science and human wellbeing to create value for and with society."

World Premier International Research Center Initiative (WPI) was launched in 2007 by Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT) to foster globally visible research centers boasting the highest standards and outstanding research environments. Numbering more than a dozen and operating at institutions throughout the country, these centers are given a high degree of autonomy, allowing them to engage in innovative modes of management and research. The program is administered by the Japan Society for the Promotion of Science (JSPS).

Freie Universität Berlin (German: Freie Universität Berlin, often abbreviated as FU Berlin or simply FU) is a public research university in Berlin, Germany. It was founded in West Berlin in 1948 during the early Cold War period. The Free University's name referred to West Berlin's status as part of the intellectual continuum of the Western "Free World" in contrast to Soviet-controlled East Berlin. Its main campus is located in Berlin-Dahlem in the Steglitz-Zehlendorf district.