Neutral gas coma dynamics: modeling of flows and attempts to link inner coma structures to properties of the nucleus

TL;DR

This paper reviews models of neutral gas coma flows around comets, emphasizing how inner coma data can reveal surface properties and discussing the limitations and applications of different modeling approaches.

Contribution

It provides a comprehensive overview of both heuristic and physically consistent models for inner coma gas flow, highlighting their strengths and limitations.

Findings

Inner coma data can inform about nucleus surface properties.

Different models vary in complexity and applicability.

Understanding coma flow models aids interpretation of remote sensing data.

Abstract

Deriving properties of cometary nuclei from coma data is of significant importance for our understanding of cometary activity and has implications beyond. Ground-based data represent the bulk of measurements available for comets. Yet, to date these observations only access a comet's gas and dust coma at rather large distances from the surface and do not directly observe its surface. In contrast, spacecraft missions are one of the only tools that gain direct access to surface measurements. However, such missions are limited to roughly one per decade. We can overcome these challenges by recognising that the coma contains information about the nucleus's properties. In particular, the near-surface gas environment is most representative of the nucleus. It can inform us about the composition, regionality of activity, and sources of coma features and how they link to the topography, morphology, or other surface properties. The inner coma data is a particularly good proxy because it has not yet, or only marginally, been contaminated by coma chemistry or secondary gas sources. Additionally, the simultaneous observation of the innermost coma with the surface provides the potential to make a direct link between coma measurements and the nucleus. If we hope to link outer coma measurements obtained by Earth-based telescopes to the surface, we must first understand how the inner coma measurements are linked to the surface. Numerical models that describe the flow from the surface into the immediate surroundings are needed to make this connection. This chapter focuses on the advances made to understand the flow of the neutral gas coma from the surface to distances up to a few tens of nuclei radii. We describe both simple/heuristic models and state-of-the-art physically consistent models. Model limitations and what they each are best suited for are discussed.