On Mon, Nov 24, 2014 at 07:16:53PM +0000, Timothy Collinson wrote:
> > On 24 Nov 2014, at 12:07, Tim <xxxxxx@little-possums.net> wrote:
> >  From much of
> > Triton's surface, Neptune would never be visible at all.
>
> "Much"?  Is it not simply 'half'?

Slightly less, since Neptune is rather large in Triton's sky.  From
some parts just inside the back half, part of Neptune would still be
visible above the horizon.  Then again from others just inside the
near half, an inconvenient terrain feature may obscure Neptune.

> Ok.  Between this and Richard's answer I'll have them visible.  Not
> that it's very crucial, just umm, well, atmosphere as it
> were. :-). Of course it might be fun to mention its dependence on
> where on its 'month' it is.  Nice.  Thank you.

They're almost transparent and very narrow, which is why I wasn't
sure.  The thickest two scatter only about 1% of the already fairly
dim sunlight, and have a width about 100 km viewed from a distance of
about 300 000 km.

I think that's enough to be visible, though much of the time they
might be lost in the millions of times brighter glare from Neptune
itself or worse, the Sun.

> Ah, that answers that question about the clouds I'd read about.  I
> didn't quite imagine that they'd be the white fluffy things of
> Earth!
>
> Sounds, however, if there's enough wiggle room for a bit of fiction
> if desired.

Yes, always some room for that.

> - would the frozen Nitrogen layer sit on the rock or water (etc)
>   below, or would there be any kind of gap (I'm specifically
>   thinking human sized!)

I'm not a planetary scientist or even a geologist, but I would expect
that the temperature would fairly gradually increase with depth, and
cause some sort of stratification.  My guess would be a surface crust
of solid nitrogen, water ice, and CO2 on top, giving way to mostly
water and CO2 ice with liquid nitrogen in pockets and crevices, and
maybe some slush mixture.  Going down further, any nitrogen would be
confined to pockets of high-pressure gas under ice, and further down
still, the same with CO2.  Eventually it may get hot enough to liquefy
water, and the icy crust may float on a mantle of liquid water under
immense pressure.  If so, it probably has a lot of dissolved CO2, with
some ammonia and methane as well.

So in short there would be some gaps (filled with gas), but almost all
at very high pressure confined by the enormous weight of ice above.

> - roughly how deep is the Nitrogen layer?

I don't think we have enough information to tell.  I would guess at
least 20-50 km, else the thermal gradient would significantly heat the
surface, which isn't observed.  The flow of interior heat reaching the
surface must average at most about 10 milliwatts per square metre
(Earth averages about 90 mW/m^2).  A thinner crust would mean a
greater temperature gradient and hence more heat flow.

Then again that thickness range is based on data I have for thermal
conductivity of relatively pure substances under lab conditions, not
some messy mixture in Triton's crust.

> - how long would it take to drill though (I'm thinking of a hole
> wide enough to take a human sized capsule)?  Or are we now in the
> realm of make up something that sounds reasonable?!

It would depend entirely upon the technology and economic resources
available in the setting.  Although the gravity is much less and
consequently the pressure does not increase nearly as fast as on
Earth, it's still a very long way down and an enormous amount of ice
to drill through.

- Tim