Enceladus Isn't Throwing As Much Ice Into Orbit As We Thought

Modeling something like geysers on a far-away moon seems like it should be easy. How much complexity could there possibly be when a geyser is simply a hole in some ice shooting superheated water through it? The answer is pretty complex, to be honest - enough that accurate models require a supercomputer to run on. Luckily, the supercomputing cluster at the University of Texas, known as the Texas Advanced Computing Center, gave some time to researcher modeling Enceladus’ ice plumes, and their recent paper in JGR Planets discusses the results, which show there might not be as much water and ice getting blown into orbit as originally thought.

Our most modern up-close data of Enceladus, as well as the rest of the Saturnian system, comes from Cassini, a probe that explored the system from 2004 to 2017. During that time it did 23 targeted flybys of its sixth largest moon, many of which went directly through the debris of the geysers. One was only 49 km (30 mi) above the surface of the moon, which itself is only 504 km (313 mi) in diameter.

While performing those flybys, it used three of its main instruments to great effect. The Ion and Neutral Mass Spectrometer (INMS) was designed to “sniff” gas as it flew through the plumes, and the Ultraviolet Imaging Spectrograph (UVIS) looked at the light from background stars to try to measure the density of the plumes. The Imaging Science Subsystem (ISS) individually marked out 98 separate geyser jets which then warranted further study.

Fraser discusses the difficult of getting samples from Enceladus' geysers.

The recent paper was an attempt to perform that further study. Led by Arnaud Mahieux of the Royal Belgian Institute for Space Science and UT Austin, the researchers were attempting to constrain some of the physical features of the vents, such as the mass flow rate, how big the ice particles in the geysers were, and the temperature they were released into space at. To do so, they came up with twelve parameters for each vent, which, when applied to all 98 vents on the surface, resulted in a whopping 1,176 variables to analyze and control for.

To solve control for some of those variables, the researchers used a technique called a Monte Carlo statistical analysis to isolate which parameters had the most impact on the feature they were looking for. The one that most consistently affected the values was the mass flow rate itself, with others, like the vent location, orientation, and radius only having an impact on some of the data, typically based on how the flyby took place. Cassini’s path compared to the “Tiger Stripes” - the fissures that break up its south polar region that plumes emanate from.

Even then, the computational time to model the entire ejection of a plume into space was prohibitively costly. To make it more tractable, the researchers used a series of parameters from earlier research to define what the plumes would look like 10 km above Enceladus’ surface. Then they ran the particle simulation, tracking each particle from all 98 vents simultaneously while under the gravitational influence of both Enceladus and Saturn.

Fraser makes the argument for why we need to go back to Enceladus

Their finding clarified some of the original calculations made from Cassini’s data. The mass flow rate of the geysers varied considerably between fly-bys, with the low end being around 109 kg/s and the high end being 486 kg/s. That’s still a pretty significant amount of material loss, but lower than originally thought. The exit temperature of the water ranged between 44K and 60K, which, while that might seem cold for water, makes sense for a gas expanding into the vacuum of space. Finally, the last parameter, the size of ice grains, was only able to be constrained for one particular vent (#29 in the paper). Its radius measured 1.1um on average, which is a very fine particle, about 1/50th the size of a human hair.

With this additional understanding of Enceladus’ plumes, mission planners and astrobiologists both would be better equipped to understand what they will meet if and when a mission is ever sent back to the icy moon. While there are plans in motion to do so, despite being a high priority in the most recent Decadal Survey. The Enceladus Orbilander is still on the drawing board though, and if it ever gets funded we might gain even more data into what is still one of the most interesting places in the solar system for astrobiological research.

Learn More:

University of Texas / TACC - Out-of-this-World Ice Geysers

A. Mahieux et al. - Enceladus Water Plume Modeling Using DSMC

UT - Enceladus, The Life Signs That Weren't

UT - Watch: 14 Hours of Enceladus Geyser Action