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Origin of the Bright Ejecta (Faculae) on Ceres

Presentation #102.07 in the session Asteroids: Objects of Interest (Oral Presentation)

Published onOct 23, 2023
Origin of the Bright Ejecta (Faculae) on Ceres

1. Introduction

Faculae are bright spots (formed of bright matter) observed on the surface of Ceres. They are of four types: (a) floor faculae, (b) faculae on Ahuna Mons, (c) rim/wall faculae found on craters’ rims or walls, (d) faculae in the form of bright ejecta blankets. We use here term “salt” for bright material [1].

Faculae (a) are usually attributed to evaporation of brines. However, the recent existence of water reservoirs on the surface of Ceres is unlikely. Our experiments has suggested another hypothesis [2, 3]. It assumes that initial evaporation and/or sublimation of water left grains of salt dispersed in regolith (see similar processes on Mars [4]). The later concentration of salt on the surface (forming present faculae) was a result of interaction of gas jets and grains.

2. Model of formation of faculae of type (d)

We consider here the process of formation of type (d) faculae. We assumes that faculae are results of interaction of gas jets and grains during impacts. Formation of impact craters proceeds in 3 phases [5]: (1) contact and compression of meteoroid and surface of planet, (2) excavation and (3) modification of the crater. Rapid vaporization of some volatiles takes place due to the energy of the impact. Eventually, the expanding gas interacts with a cloud of grains ejected from the crater.

We present calculations indicating segregation of grains. In our numerical model we assume a hemispherical shape of the gas cloud and its radial expansion. There is a strong segregation of grains depending on the density, size and shape. Segregation of ejecta during crater formation has been observed for some terrestrial craters [6]. The presence of volatiles in the regolith is important for this process. We now considering similar models for faculae type (c).

References [1] Raponi, A., et al., 2019. Icarus 320, 83–96. [2] Czechowski, L., 2023. j.icarus.2023.115473 [5] Melosh, H.J., 2011. Planetary surface processes. Cambridge Univ. Press. [6] Sturm, S., et al. 2013. Geology; 41: 531-534

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