Thermal Evolution and Interior Structure of Main-Belt Comet 133P/Elst-Pizarro

Volatile components in small bodies provide important clues on the evolution of the solar system and are of resource exploration interest. In recent decades, comet-like activities have been detected on several objects located in the main belt [1]. As the first recognized main-belt comet, 133P has been observed to be active for four consecutive perihelion passages, strongly suggesting the existence of ice in its body. Based on theoretical and numerical thermophysical analyses, prior studies predict that main-belt objects like 133P should have retained interior water ice at depths of a few hundreds of meters for over 4.5 Gyr [2, 3]. This result implies that a rather energetic impact must have occurred recently to expose the interior ice of 133P to account for the observed activity. An alternative scenario is that 133P is a recent-formed collisional fragment of its parent body given that its collisional lifetime is expected to be much shorter than the age of the solar system [4]. In this case, ice could exist a few meters beneath the surface [5]. Accurate modeling of the interior thermal evolution and gas diffusion within 133P is thus important for distinguishing the two evolutionary pathways.

In this study, we have developed a fully 3D thermal evolution model based on the generalized finite difference method for modeling long-duration heat conduction and rarefied gas flows in a porous body resembling 133P. Our simulation results reveal that the water gas production rate is on the order of 1027 mol/s initially when the surface is homogeneously covered by ice. The ice front near the equator rapidly retreats to a depth of ~1 cm after a perihelion passage, ~10 cm after ~1 kyr, and ~1 m after ~0.1 Myr. The production rate decreases substantially during this process and is reduced to about 1023 mol/s after the top 10 cm surface is depleted of ice. This production rate is well below the upper limit (~1024 mol/s) inferred from observations [1]. By comparing with the dust levitation gas density threshold, we find that subsurface ice at a depth <~1 m is necessary for 133P to eject μm-sized dust from its surface. Based on the results, we derive the possible interior ice content profile for 133P and constrain its history from its parent-body disruption.

References

[1] Snodgrass et al., A&A Rev 25:5 (2017). [2] Schörghofer, ApJ 682, 697-705 (2008). [3] Prialnik & Rosenberg, MNRAS 399, L79–L83 (2009). [4] Bottke et al. Icarus, 179, 63-94 (2005). [5] Schörghofer, Icarus 276, 88-95 (2016).

Acknowledgements

This work was supported by NASA Grant 80NSSC20K1079.