Few places in our Solar System have drawn as much attention as Enceladus, a medium-sized (500 km in diameter) moon located near Saturn’s main rings. Enceladus has been a subject of intense research, particularly since the Cassini spacecraft discovered jets of water vapour and icy particles ejected from four main faults at its south pole. The presence of this plume has allowed for sampling of the ocean beneath the icy crust and has raised intriguing questions about the moon’s geological activity and the potential habitability of its subsurface ocean.
Yet, the complex dynamics that drive the varying eruption activity remained enigmatic. By combining cutting-edge numerical models of Enceladus’ tidal deformation with detailed simulations of the processes within its fault zones, we have uncovered new insights into how the observed activity varies over one orbit and what this implies for the ejection of fresh oceanic materials to space.
The Enigma of Plume Variability
Since the discovery of the eruptive plume on Enceladus, researchers have observed that the intensity and timing of its brightness vary along the orbit in a way that seems to be controlled by tides raised by Saturn. This tidal interaction with the period of 32.9 hours, causes Enceladus’ ice shell to flex, leading to a mutual displacement of faults’ walls and inducing stresses within the moon’s south-polar region. However, the precise relationship between these tidal forces and the observed plume activity has remained elusive.
Previous studies have suggested that the variations in plume brightness—essentially how much material is being ejected into space—are tied to the changing stresses within the moon’s ice shell. Yet, the observed pattern, with two distinct peaks in activity, and their specific timing, did not align with any of the proposed models. This discrepancy hinted that more complex processes are at play.
A New Approach to Understanding Enceladus
Fig. 1 Numerical simulation of the tidal deformation of Enceladus’ outer ice shell throughout its orbit around Saturn (exaggerated 1000x). The view is centered at the south pole, showing the prominent faults nicknamed “Tiger Stripes”. Colours indicate the magnitude of displacement.
In our recent study, we have approached this puzzle by combining global and local modelling perspectives. On the global planetary scale, a three-dimensional numerical model simulates how Enceladus’ ice shell deforms under the influence of varying tides raised by Saturn (Fig. 1, Fig. 2a). The simulations allowed us to calculate the stresses and displacements along the moon’s south-polar faults.
Fig. 2 Summary of our activity model for diurnal variations combining the global and local components. The 3D global model (panel a) describes the tidal deformation of a fractured ice shell with realistic thickness variations. Kinematic and dynamic quantities are averaged along the faults and over the depth across the shell and used as input for the local model. The local 1D model (panel b) combines a model for liquid water hydraulic motion (I, white), and a model for the transport of vapour and solid grains above the water table (II, blue) with a near-surface aperture model (III, brown) comprising two mechanisms: a slip-controlled mechanism corresponding to macroscopic jet flow (panel c) and a normal-stress-controlled mechanism corresponding to ambient (more diffuse) vapour flow (panel d). Numbers indicate the typical magnitude of the various length scales.
These results are linked to a one-dimensional model that focuses on the local processes within the fault zones (Fig. 2b-d). This local model quantifies how liquid water, vapour and solid ice particles move and propagate through the shell within the fault zones, accounting for both the direct jet-like flow of material and a more diffuse ambient flow that seeps through small-scale cracks and porous materials.
Key Findings and Their Implications
One of the main outcomes of our study is the identification of two distinct mechanisms that feed the plume and control its variability. The first mechanism is driven by slip along the faults (Fig. 2c, red curve in Fig. 3), i.e., the mutual horizontal displacement of the opposing ice blocks. The slip creates, through geometric incompatibilities, pathways for vapour and ice grains to escape to the surface. This process is particularly effective at producing jet-like emissions.
The second mechanism is governed by the normal stress acting on the faults, which affects how material is transported through more diffuse channels. This mechanism helps us explain the more gradual ambient emission of the plume material (Fig. 2d, yellow curve on Fig. 3).
Fig. 3 Our activity model prediction (black solid line) compared to the rescaled data obtained by Cassini (grey dots) reveals the details of the Enceladean “plumbing system”. The normalized activity signal, shown here as a function of the Enceladus’ position along the orbit, is composed of the slip-controlled (red dashed) and the normal-stress-controlled (gold dash-dotted) components. The former can explain the two peaks at around 40°-50° MA and 200° MA, while the latter can explain the ramp around 80°-140° MA and the increase of activity beyond 300° MA (MA denoting the mean anomaly with MA=0° corresponding to the pericenter).
By combining these two mechanisms, our model successfully replicates the observed pattern of plume activity (grey dots and black curve in Fig. 3), including the two distinct peaks, their timing, and relative amplitudes. This represents a significant step forward in our understanding of Enceladus, offering a more complete picture of how its faults’ dynamics influence the above surface and plume activity.
Looking Ahead: What’s Next for Enceladus Research?
Our findings not only shed light on the current behaviour of Enceladus’ plume but also have broader implications for future exploration. Understanding the precise mechanisms behind plume activity is crucial for planning future missions to this icy moon. For instance, the timing of plume emissions determines the optimal scheduling of flybys or lander missions, maximizing the chances of sampling fresh material from Enceladus’ subsurface ocean.
Furthermore, our study highlights the importance of continued observations, particularly with instruments like the James Webb Space Telescope (JWST), which can provide high-resolution data on Enceladus’ plume from afar. These observations could further test our model’s predictions and refine our understanding of Enceladus’ mysteries.