In this study, plume experiments were conducted to mimic the thermodynamic conditions on Saturn's moon, Enceladus. The icy moon's subsurface ocean and cracks in the surface have been simulated using a liquid water reservoir and a narrow channel, while the low-pressure environment at Enceladus’ surface was achieved with a vacuum chamber. We aimed to examine how channel temperature affected the plume's temperature, solid fraction and velocity, testing two models with differing wall temperatures: room temperature and near 0 °C. The colder setup better replicated Enceladus’ plume, producing a saturated flow in which nucleation of icy particles is possible. A conservative 1.5%–3% minimum solid fraction is estimated from measurements and modelling. Pitot-tube measurements indicated velocities around 400–500 m/s at the channel outlet. Flow temperature and velocity are closely correlated with wall temperature, indicating effective heat transfer. With a plume model based on the energy conservation law, we concluded that supersonic plume velocities observed on Enceladus cannot be achieved with straight channels, i.e. without requiring extreme expansion ratios. Additionally, the research provides evidence of the relationship between the crevasse's expansion ratio and the temperatures of flow and crevasse walls.

Supersonic plumes of water vapour and icy particles have been observed by the Cassini spacecraft during several flybys over Enceladus. These plumes originate from the Tiger Stripes located in the South Polar Terrain (SPT), and indicate the presence of a subsurface ocean under the icy crust which is salty and contains complex organic molecules. Other characteristics of the plumes, such as the vent temperature, mass flow rate, velocity and mass fraction of icy particles can be used to determine the conditions in the channel, linking the subsurface ocean to the icy surface. In this paper, we developed a fluid dynamics model that accounts for nucleation, particle growth, wall accretion and sublimation. The channel behaves similarly to a converging–diverging nozzle, which forms supersonic plumes due to a pressure difference between the reservoir where the subsurface ocean is located and the exosphere. The geometry of the channel and its evolution with accretion of gas and sublimation of ice are studied to reproduce the characteristics of the plumes observed by Cassini. We first performed a parameter study on the channel geometry to determine how it influences the plumes’ velocity, solid fraction and exit temperature. Our results show that the size of the icy particles is primarily dependent on the length of the channel, indicating that large particles (∼75μm) must originate from within a kilometer below the surface, while smaller particles (∼3μm) can originate from only hundreds of meters below the surface. We further show that the velocity of the flow, exit temperature and nucleation depend directly on the exit-to-throat size ratio. We find that the channel geometry evolves within a few tens of hours until an equilibrium is reached, when considering the accretion of gas to the walls, or sublimation of ice from the walls. As the channel closes due to accretion, the flow becomes thinner, which in turn reduces accretion. After around 70 h, the accretion is sufficiently slowed such that the geometry does not evolve anymore. This equilibrium geometry produces higher Mach numbers and a larger particle size and solid fraction compared to the initial geometry.

The Kordylewski Dust Clouds (KDCs) are accumulation of dust particles located in the region of space surrounding the triangular Lagrangian points (L₄/L₅) of the Earth-Moon system. Despite their proximity to the Earth, the clouds' low density and variable distribution have made them difficult to study. DARKO (Dust Analysis and Remote sensing of KOrdylewski dust clouds) is a mission concept designed to overcome these limitations and bring breakthrough scientific information on the origin of these clouds. Orbiting around the 2 libration points, DARKO will use its imaging polarimeter to observe the changes in distribution and density of the clouds throughout one solar cycle. Moreover, the on-board dust analyser will determine the composition, charge and velocity of the dust particles. To work with both instruments and observe both the clouds, DARKO will orbit around L₄ for the first part of the mission, observing with the polarimeter the dynamics of the opposite cloud and performing in-situ measurements with the dust analyser in L₄ itself. The same is repeated when the spacecraft is moved in L₅ with a transfer orbit. The mission lifetime required is around 7 years, starting around 2033 to ensure the observation of the clouds' variability during the Solar Cycle 26 until its peak.