On the charge of nanograins in cold environments and Enceladus dust

TL;DR

This paper investigates the charging behavior of nanometer-sized dust grains in cold planetary environments, revealing that grains around 1 nm typically carry one electron and are influenced by polarization effects, affecting their stability.

Contribution

It provides analytical models for nanograin charging in cold dense environments and applies these to Enceladus dust, highlighting the charge distribution and stability of nanodust.

Findings

Most 1 nm grains carry one electron.

A significant fraction are positively charged by ion impacts.

Electrostatic stresses likely destroy smaller grains.

Abstract

In very-low energy plasmas, the size of nanograins is comparable to the distance (the so-called Landau length) at which the interaction energy of two electrons equals their thermal energy. In that case, the grain's polarization induced by approaching charged particles increases their fluxes and reduces the charging time scales. Furthermore, for grains of radius smaller than the Landau length, the electric charge no longer decreases linearly with size, but has a most probable equilibrium value close to one electron charge. We give analytical results that can be used for nanograins in cold dense planetary environments of the outer solar system. Application to the nanodust observed in the plume of Saturn's moon Enceladus shows that most grains of radius about 1 nm should carry one electron, whereas an appreciable fraction of them are positively charged by ion impacts. The corresponding electrostatic stresses should destroy smaller grains, which anyway may not exist as crystals since their number of molecules is close to the minimum required for crystallization.