Reproducing the blue halo of backlit Pluto turned out to be a challenge that was less easy than I thought at first.
Before getting to my results, let me recall the meaning of the various Mie theory parameters that need to be tuned:
Quoted from ChrisL from quite some time ago:
MieA float value giving the fraction of light scattered per kilometer at the planet's surface ('sea level')
MieAsymmetryHenyey-Greenstein phase function parameter for wavelength independent scattering. A floating point value between -1 and 1, with -1 indicating complete backscattering, 1 complete forward scattering, and 0 isotropy.
MieScaleHeightScale height for wavelength independent scattering particles. Height in kilometers at which wavelength independent scattering is 1/e of the value at the surface
RayleighWavelength dependent scattering coefficients. Three values giving fraction of red, green, and blue light scattered per kilometer at the planet's surface. To simulate Rayleigh scattering, they should be in ratios that fit the 1/wavelength^4 behavior, though this is not required.
RayleighScaleHeightScale height for wavelength dependent scattering. Currently ignored; MieScaleHeight is used for both Rayleigh and Mie scattering.
AbsorptionThree values giving the fraction of red, green, and blue light absorbed per kilometer at the planet's surface.
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Let me add some further qualitative remarks about the essential effects these parameters have:
- A larger (negative) MieAsymmetry than on Earth (e.g. -0.5.. -0.55)
is responsible for the conspicuous "ring of light"
when the atmosphere is backlit (cf TITAN!)
- the Rayleigh RGB values indicate the color components that are to be prominent in the highest part of the illuminated atmosphere
- the Absorption RGB values specify the amount of absorption as visible in the backlit color composition of the "ring of light". R=0.0 means red remains (i.e undergoes ZERO absorption)! Etc.
As you see there a quite a few free parameters to fix which is all but straightforward.
My (non-final) result is
Code:
Modify "Pluto:134340 Pluto" "Sol"
{
Atmosphere {
Height 130.0 # official value [km]
Lower [0.232 0.1929 0.1566]
Upper [0.07725 0.0643 0.0522]
Sky [ 0.3 0 0 ]
Mie 0.00012
MieAsymmetry -0.50
Rayleigh [0.000005 0.000054 0.000269542]
Absorption [0.000367 0.000245 0.0]
MieScaleHeight 49.0
}
}
Here is the corresponding backlit view of Pluto. It's not final but shows that I am on the right track
[To enlarge the image, click on it, and hit browser full screen (F11 key in Firefox)]Attachment:
blue_halo_.Sci_big.jpg [ 113.58 KiB | Viewed 5300 times ]
Implying
blue skies on Pluto!
[To enlarge the image, click on it, please]Attachment:
blue_halo_.Sci_zoom.jpg [ 94.21 KiB | Viewed 5303 times ]
and blue sunsets/sunrises
[To enlarge the image, click on it, and hit browser full screen (F11 key in Firefox)]]Attachment:
Blue_sunset_sunrise.jpg [ 67.69 KiB | Viewed 5300 times ]
Fridger