Modeling the Plumes on Saturn’s Moon Enceladus

  • 10-nm dust grain trajectories, plotted with the gas streamlines. They are affected by the gas flow the most, as can be seen from the spreading of the beam of grains towards the top. The grains are ice (density is 920 kg/m3) launched at the same speed as the gas at the vent. They are launched with only an upwards velocity. As the grains are moving up, they are affected by the gas flow and may be deflected.
    10-nm dust grain trajectories, plotted with the gas streamlines. They are affected by the gas flow the most, as can be seen from the spreading of the beam of grains towards the top. The grains are ice (density is 920 kg/m3) launched at the same speed as the gas at the vent. They are launched with only an upwards velocity. As the grains are moving up, they are affected by the gas flow and may be deflected.
  • The second figure is the 100-nm grain trajectories. They are still affected by the gas flow, but not as much as the 10-nm grains. The grains are ice (density is 920 kg/m3) launched at the same speed as the gas at the vent. They are launched with only an upwards velocity. As the grains are moving up, they are affected by the gas flow and may be deflected.
    The second figure is the 100-nm grain trajectories. They are still affected by the gas flow, but not as much as the 10-nm grains. The grains are ice (density is 920 kg/m3) launched at the same speed as the gas at the vent. They are launched with only an upwards velocity. As the grains are moving up, they are affected by the gas flow and may be deflected.
  • 1-micron grains are barely, if at all, affected by the gas flow. The larger the grains are, the more mass they have. As a result, the larger heavier grains are not affected by the gas flow as much as the smaller lighter ones. The grains are ice (density is 920 kg/m3) launched at the same speed as the gas at the vent. They are launched with only an upwards velocity. As the grains are moving up, they are affected by the gas flow and may be deflected.
    1-micron grains are barely, if at all, affected by the gas flow. The larger the grains are, the more mass they have. As a result, the larger heavier grains are not affected by the gas flow as much as the smaller lighter ones. The grains are ice (density is 920 kg/m3) launched at the same speed as the gas at the vent. They are launched with only an upwards velocity. As the grains are moving up, they are affected by the gas flow and may be deflected.
  • Velocity distribution of gas molecules in a two-phase flow at an altitude of 10 km. Both gas and dust grains exit the vent at the same speed, with the mass flow rate of the grains 10x that of the gas.
    Velocity distribution of gas molecules in a two-phase flow at an altitude of 10 km. Both gas and dust grains exit the vent at the same speed, with the mass flow rate of the grains 10x that of the gas.
  • This is a comparison between the simulated gas density data and the in-situ gas density data obtained by Cassini during one of the flybys.
    This is a comparison between the simulated gas density data and the in-situ gas density data obtained by Cassini during one of the flybys.
  • Images taken by the Cassini spacecraft of the south polar plume of Enceladus. Note that the visible plume is the ice grains
    Images taken by the Cassini spacecraft of the south polar plume of Enceladus. Note that the visible plume is the ice grains
  • The DSMC output is fed into the free-molecular model and the flow is propagated further into the far field. Eight point sources are placed on the planet surface according to Spitale and Porco. Free-molecular particles are launched from these sources with velocities obtained from the DSMC simulation. There are beams of particles shooting straight out from the sources, barely affected by the gas flow at all. The individual sources can easily be distinguished.
    The DSMC output is fed into the free-molecular model and the flow is propagated further into the far field. Eight point sources are placed on the planet surface according to Spitale and Porco. Free-molecular particles are launched from these sources with velocities obtained from the DSMC simulation. There are beams of particles shooting straight out from the sources, barely affected by the gas flow at all. The individual sources can easily be distinguished.
  • Images taken by the Cassini spacecraft of the south polar plume of Enceladus. Note that the visible plume is the ice grains
    Images taken by the Cassini spacecraft of the south polar plume of Enceladus. Note that the visible plume is the ice grains
  • This is the near-field gas number density contour of the flow out of the vent. The flow is axisymmetric, thus our simulation domain is a one-degree wedge instead of a 360-degree cylindrical domain. The gas is water vapor and it expands out of a 3-m vent at Mach-5. The gas number density drops as the flow expands into vacuum. Expansion waves emanate from the vent edges and turn the gas flow near the vent edges. Gas number density drops across these waves, creating a free-molecular (collisionless) region at the plume edges.
    This is the near-field gas number density contour of the flow out of the vent. The flow is axisymmetric, thus our simulation domain is a one-degree wedge instead of a 360-degree cylindrical domain. The gas is water vapor and it expands out of a 3-m vent at Mach-5. The gas number density drops as the flow expands into vacuum. Expansion waves emanate from the vent edges and turn the gas flow near the vent edges. Gas number density drops across these waves, creating a free-molecular (collisionless) region at the plume edges.
  • The DSMC output is fed into the free-molecular model and the flow is propagated further into the far field. Eight point sources are placed on the planet surface according to Spitale and Porco. Free-molecular particles are launched from these sources with velocities obtained from the DSMC simulation. The gas plume is diffused enough that the individual sources cannot be distinguished.
    The DSMC output is fed into the free-molecular model and the flow is propagated further into the far field. Eight point sources are placed on the planet surface according to Spitale and Porco. Free-molecular particles are launched from these sources with velocities obtained from the DSMC simulation. The gas plume is diffused enough that the individual sources cannot be distinguished.
  • This is the near-field gas translational temperature of the flow out of the vent. The translational temperature is defined based on the mean random kinetic (thermal) energy of the molecules. This temperature is equal to the thermodynamic temperature when the flow is in equilibrium. The translational temperature drops as the gas expands into vacuum. The flow is in equilibrium in the core of the plume where the density is high and collisions are frequent but is in non-equilibrium at the plume edges where density is low and collisions are few. This is expected since collisions are responsible for equilibrium.
    This is the near-field gas translational temperature of the flow out of the vent. The translational temperature is defined based on the mean random kinetic (thermal) energy of the molecules. This temperature is equal to the thermodynamic temperature when the flow is in equilibrium. The translational temperature drops as the gas expands into vacuum. The flow is in equilibrium in the core of the plume where the density is high and collisions are frequent but is in non-equilibrium at the plume edges where density is low and collisions are few. This is expected since collisions are responsible for equilibrium.
  • Images taken by the Cassini spacecraft of the south polar plume of Enceladus. Note that the visible plume is the ice grains
    Images taken by the Cassini spacecraft of the south polar plume of Enceladus. Note that the visible plume is the ice grains
  • This is the velocity distribution of the gas particles that escape the top of the inner DSMC domain as the gas flow becomes free-molecular. The tangential velocity is the velocity component tangential to the planet surface and the normal velocity is the velocity component normal to the planet surface.
    This is the velocity distribution of the gas particles that escape the top of the inner DSMC domain as the gas flow becomes free-molecular. The tangential velocity is the velocity component tangential to the planet surface and the normal velocity is the velocity component normal to the planet surface.
  • Images taken by the Cassini spacecraft of the south polar plume of Enceladus. Note that the visible plume is the ice grains
    Images taken by the Cassini spacecraft of the south polar plume of Enceladus. Note that the visible plume is the ice grains
  • The DSMC output is fed into the free-molecular model and the flow is propagated further into the far field. Eight point sources are placed on the planet surface according to Spitale and Porco. Free-molecular particles are launched from these sources with velocities obtained from the DSMC simulation. The far-field is less diffused and streakier. The individual sources are more distinguishable in this case.
    The DSMC output is fed into the free-molecular model and the flow is propagated further into the far field. Eight point sources are placed on the planet surface according to Spitale and Porco. Free-molecular particles are launched from these sources with velocities obtained from the DSMC simulation. The far-field is less diffused and streakier. The individual sources are more distinguishable in this case.

The Cassini spacecraft first detected a plume near the warm south pole of the Saturnian moon Enceladus in 2005. The discovery of the plume not only helped to explain some phenomena that have been puzzling scientists for a long time but also brought about the exciting possibility of finding liquid water on Enceladus, making it a possibly favorable environment for life. Therefore, more flybys have been made over the moon and have yielded spectacular images, details of the plume structure and composition, as well as the possible locations of the plume sources. Observations found that the plume is composed of gas (mostly water vapor) with tiny entrained ice particles. Based on the images and data from Cassini, we construct a hybrid model of the plume. Our model divides the plume into two regimes: the collisional flow in the near-source region and the collisionless flow in the far-field region. The direct simulation Monte Carlo (DSMC) method is used to simulate the collisional gas flow in the near-source region as the gas has only begun to expand and is therefore, still relatively dense and warm. Once the flow becomes collisionless further out, the DSMC output is fed into a computationally less expensive free-molecular model to propagate the flow further into the far field. The simulation results are directly compared to the in-situ measurements made by Cassini. Our objective is to attempt to deduce the nature of the plume sources and hopefully, answer the question of whether there is liquid water on Enceladus.