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

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 or even the outgassing layer. In contrast, spacecraft fly-by
and rendezvous missions are one of the only tools that gain direct access to surface measurements. However,
these 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 (e.g., from icy grains released into the coma), and can retain fine structure which need time
to dissipate. Additionally, when possible, 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. The current
state of research on linking the inner gas coma properties and structures to the nucleus is explored, describing both
simple/heuristic models and state-of-the-art physically consistent models. The model limitations and what they
each are best suited for is discussed. In the end, the different approaches are compared to spacecraft data, and the
remaining knowledge gaps and how best to address them in the future are presented.