The evolution of a post-impact plume on the surface of the Moon through a Smoothed Particle Hydrodynamics approach

Introduction: 
The emission of volatiles from the surfaces of planetary objects brings a lot of information concerning a variety of processes that shape their formation and evolution. The release of such material allows to constrain the physical properties and conditions of the interested target, such as its internal structure as well as its history. Furthermore, volatile emission is often associated with water-rich reservoirs, which are relevant from an astrobiological standpoint. 
The release of volatile materials and their complex phenomenology requires numerical models to characterize their dynamical and thermal behavior. 
In this work, we present preliminary results concerning the evolution of a plume as resulting from the impact of a small cometary-like object over the Moon’s surface. Such events could produce a transient atmosphere, eventually freezing into Permanently Shadowed Regions (PSRs), forming ice deposits and contributing to lunar water.

Methods: 
We adopt a Lagrangian method called Smoothed Particle Hydrodynamics (SPH) [1-3] to simulate the evolution of a post-impact plume on the surface of the Moon. Such a particle-based mesh-free approach uses volume elements and a statistical description to solve hydrodynamic equations, providing the spatial and temporal evolution of physical properties like velocity, density, and thermal energy. Our model was satisfactorily tested with the plumes of Enceladus [4-5], and considers a multi-component plume, made of water vapor and icy grains, thanks to a treatment for phase transitions. The viscous interaction between the two components shapes the dynamics of the material. Furthermore, the solar radiation alters the thermal distribution according to the optical properties of each component. We also account for thermal conduction with the Moon’s surface, which can lead to surface deposition of vapor. Such physical processes can affect the outcomes since they alter the thermal and dynamical behavior of the volatile material.

Results:
Our simulation results characterize the dynamical and thermal behavior of the volatile material, altered by the aforementioned processes.
Initial conditions: to describe the initial state of a post-impact plume, we simulate a fireball, that is a (spherical) distribution of material with high temperature and close to the surface, that is free to expand. The material is assumed to be initially made of water vapor at saturated conditions and located close to the South Pole of the Moon. We simulate 5400 s of evolution, that is approximately the time needed for the expanding water vapor to cover a spatial scale of the order of the Moon’s diameter.
Final snapshot: At the end of the simulation, after 5400 s, the material has covered the whole surface of the Moon. The illuminated areas are characterized by a transient atmosphere made of water vapor, while the shadowed regions show an efficient deposition of ice. We show in Fig. 1 the velocity distribution of water vapor (gas component) and ice (flying and deposited).

Figure 1: Velocity distribution of water vapor (red-yellow) and ice (blue-cyan) after 5400 s of evolution. The solar radiation is coming from the left.

The solar radiation effect and illumination conditions are responsible for the thermal behavior of each component. Indeed, the illuminated region favors sublimation, strongly enhancing the vapor component that produces a transient atmosphere. 
Ice deposition is more efficient in the shadowed region and at the poles, both in-flight and due to the interaction with the Moon’s surface. In particular, the area close to the initial spatial distribution of the material, namely the South Pole, hosts a much more consistent ice deposition, as reported in Fig. 2 where we show the mass density of each component.

Figure 2: Density distribution of water vapor (red-yellow) and ice (blue-cyan) after 5400 s of evolution. The shadowed region and the pole show a relevant deposition of ice (green-white) on the surface.

Ongoing work and conclusions:
We are further developing the model to account for the presence of a dust component. Furthermore, we are refining the introduced processes and considering the presence of a crater close to the impact area. Indeed, the ice deposits within Permanent Shadowed Regions (PSRs) can be stable to sublimation for a very long time, with temperatures not exceeding 110 K, the threshold temperature of the cold traps [6]. Thus, it is important to characterize the amount of material that can fall within PSRs due to close impacts, according to the properties of the material and the relative distance between the crater and the impact region, as well as the local morphology and thermal conditions.
Our work provides a numerical model applicable to different targets interested by volatile emissions events. Furthermore, it can work in connection with Eulerian methods [7-8], that characterize the surface and subsurface thermophysical behavior. This can allow a better description of volatile interaction with the surface, in particular on local scales. Thus, the model can also be applied to study the possible release of volatiles in mixtures of vapor-ice-dust, triggered by drilling activities [9], planned for ExoMars and Prospect missions.

References: 
[1] Gingold & Monaghan 1977, MNRAS, 181, 375. 
[2] Lucy 1977, AJ, 82, 1013.
[3] Monaghan 2005, Rep. Prog. Phys. 68, 1703. 
[4] Teodori et al. 2025, Icarus, under review. 
[5] Teodori et al. 2024, EPSC2024-55.
[6] Schorghofer et al. 2024, Planet. Sci. J. 5, 99.
[7] Formisano et al. 2018, J. Geophys. Res. 123, 2445. 
[8] Formisano et al. 2024, PSS, 251, 105969. 
[9] Maggioni et al., in preparation.

Acknowledgments: 
This work was supported by ISSI within the project “Thermophysical Characterization of Ice-Rich Areas on the Surface of Specific Planetary Bodies: Conditions for the Formation of a Transient Exosphere”.