In the 17th century, Christiaan Huygens and Giovanni Cassini focused their telescopes on Saturn and realized its bright bands were not solid features. Instead, they identified immense, separate rings formed from innumerable nested arcs.
Centuries later, NASA's Cassini-Huygens (Cassini) mission pushed that exploration forward. Starting in 2005, it returned striking images that reshaped scientists' understanding of Saturn. Among the most dramatic findings were the towering geysers on the icy moon Enceladus that hurled debris into space and produced a faint sub-ring around the planet.
New Simulations: How Much Ice Escapes Enceladus
Recent supercomputer simulations from the Texas Advanced Computing Center (TACC), built on data from the Cassini spacecraft, refined estimates of how much ice Enceladus loses to space. The results support planning for future robotic exploration and deepen insight into conditions beneath the moon's surface, which could be suitable for life.
"The mass flow rates from Enceladus are between 20 to 40 percent lower than what you find in the scientific literature," said Arnaud Mahieux, a senior researcher at the Royal Belgian Institute for Space Aeronomy and an affiliate of the UT Austin Department of Aerospace Engineering & Engineering Mechanics.
Modeling the Plumes: DSMC Approach and Progress
Mahieux is the corresponding author of a computational study of Enceladus published August 2025 in the Journal of Geophysical Research: Planets. In this work, he and collaborators created Direct Simulation Monte Carlo (DSMC) models to better characterize the structure and behavior of the enormous plumes of water vapor and icy grains expelled from vents at the surface of Enceladus.
The study extends earlier research from 2019 led by Mahieux that first applied DSMC models to deduce the starting conditions for the plumes, including vent size, the ratio of water vapor to ice grains, temperature, and exit speed.
"DSMC simulations are very expensive," Mahieux said. "We used TACC supercomputers back in 2015 to obtain the parameterizations to reduce computation time from 48 hours then to just a few milliseconds now."
Using these mathematical parameterizations and Cassini's in-situ measurements as it flew through the plumes, the team calculated plume density and velocity for Enceladus's cryovolcanic activity.
"The main finding of our new study is that for 100 cryovolcanic sources, we could constrain the mass flow rates and other parameters that were not derived before, such as the temperature at which the material was exiting. This is a big step forward in understanding what's happening on Enceladus," Mahieux said.
A Small World With Powerful Jets
Enceladus spans only 313 miles across, and its weak gravity cannot fully retain the icy jets that erupt from its vents. The DSMC models account for this. Earlier techniques treated the physics and gas dynamics less rigorously than the DSMC method. What Enceladus does is akin to a volcano hurling lava into space -- except the ejecta are plumes of water vapor and ice.
The simulations track gas behavior at the microscopic level as particles move, collide, and exchange energy, much like marbles striking one another. They simulate several millions of molecules on microsecond time steps. The DSMC approach also enables calculations at lower, more realistic pressures than before, with longer travel times between collisions.
David Goldstein, UT Austin professor and study co-author, led development in 2011 of the DSMC code called Planet. TACC provided Goldstein with allocations on the Lonestar6 and Stampede3 systems through The University of Texas Research cyberinfrastructure portal, which supports researchers at all 14 UT system institutions.
"TACC systems have a wonderful architecture that offer a lot of flexibility," Mahieux said. "If we're using the DSMC code on just a laptop, we could only simulate tiny domains. Thanks to TACC, we can simulate from the surface of Enceladus up to 10 kilometers of altitude, where the plumes expand into space."
Ocean Worlds Beyond the Snow Line
Saturn lies beyond the solar system's "snow line," along with other giant planets that host icy moons, including Jupiter, Uranus, and Neptune.
"There is an ocean of liquid water under these 'big balls of ice,'" Mahieux said. "These are many other worlds, besides the Earth, which have a liquid ocean. The plumes at Enceladus open a window to the underground conditions."
What's Next: Missions and the Search for Life
NASA and the European Space Agency are preparing mission concepts to revisit Enceladus that extend beyond brief flybys. Plans include landing on the surface and drilling through the ice to sample the underlying ocean in a search for signs of life beneath miles of ice. By analyzing the plume material, scientists can assess subsurface conditions without penetrating the crust.
"Supercomputers can give us answers to questions we couldn't dream of asking even 10 or 15 years ago," Mahieux said. "We can now get much closer to simulating what nature is doing."
