QUESTION:
Why does the "blue" image of Neptune look smaller than the "red" image?

Answer from Heidi Hammel on May 16, 1996:
That is a very astute observation, and a very good question!  First, let
me remind you that the way we see Neptune is by how much sunlight is
scattered off its atmosphere back to our telescopes here near the Earth.

In the "blue" images (images taken through a blue filter), the edges of
the planet are dark.  This is a very normal thing, and has to do with the
way reflected sunlight scatters near the edge of the planet.  We call this
"limb darkening," which should not be too surprising.

But something special is going on in the "red" images.  These images are
taken through a filter that lets through red light, and this light is easily
absorbed by methane gas in Neptune's atmosphere.   There is only a little
bit of methane gas there, but that methane gas is very likely to absorb
this red light.

That means that Neptune is usually very dark in the red, because it does
not reflect much sunlight (the sunlight is absorbed, or kept in the
atmosphere, by the methane).  However, if Neptune has a hazy cloud layer
high up in its atmosphere, then some light can get scattered by to our
telescopes by the haze particles.  Now, if we look through the haze near the
center of Neptune's disk, the haze is very very thin, and most of the light
goes through it and gets absorbed by the methane.  BUT near the EDGES of the
planet, the *apparent* amount of haze is bigger (we say that you are looking
through a "longer pathlength" of haze).   Therefore, there is more scattering
from haze particles near the edge of the planet.  Thus the planet looks
brighter near the edges.  And that is why Neptune looks bigger in the red,
because the edges of the planet are brighter than the center.

The methane gas does not absorb blue light, so this doesn't happen in the
"blue" Neptune image, so it has regular "limb darkening" and looks smaller.

To understand this "pathlength" business, think of a soap bubble.  You can
see through the center of it, but the edges look thicker.  But it is just
an optical effect, because the soap bubble has the same thickness all
around. It is just that the very edges, from your point of view, look
thicker.  Now imagine that the bubble is -- instead of soapy water -- a
layer of haze, and that it is so big that Neptune will fit *just* inside
it (that is one big bubble!).  The edges of this Neptune bubble will
scatter the sunlight much better than the center of the Neptune bubble.
Thus you will have bright edges, limb brightening. In fact, we will use
the amount of "limb brightening" in these red HST images to measure the
amount of Neptune's haze and the haze's thickness!