The visible haze above a body, which is not the same thing as its atmosphere.
Atmosphere.ceiling is a physics number — where the drag model stops
integrating — and for a gas giant it is a thousand kilometers or more, because
there is no surface and the air just keeps going. Rendered as a shell that
thick, Saturn wears a halo 3% of its own radius and looks like a moon in a
jar. What you actually see above the cloud tops is a few hundred kilometers of
haze.
The colors are the licensed part, and docs/design/art.md says so
explicitly: scattering coefficients are "tuned within the real range for the
modeled composition". The hues are the published ones — Earth's limb is blue
and its terminator is orange because Rayleigh scattering says so; Titan's is
orange all the way round because its haze is tholins.
Members
colorreadonly
Scattering color looking straight down through it.
heightreadonly
Rendered thickness above the datum. Not Atmosphere.ceiling.
limbreadonly
Forward-scattered color at the terminator — the sunset seen from orbit.
thicknessreadonly
Visible optical thickness of the whole column, 0..1, where 1 is Earth-dense. Not pressure: it is how much the air shows. It is what separates Mars — whose 600 Pa limb stays a translucent butterscotch, because thin air never scatters its way to white — from Earth and Venus, whose dense limbs whiten with multiple scattering. Rendering with one constant here painted Mars with Earth's white halo.