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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.
