Pluto Atmospheric Observations: NH Post-Encounter Phase, 1 Aug 2015- TBD Options

[alan]

Quote from a recent NASA article

In these unwrapped images, the hazes appear brighter in the evening sky than in the morning sky, possibly suggesting that the hazes and their distribution are controlled by diurnal processes, becoming more concentrated over the course of Pluto’s long day and depleting during Pluto’s long night. Perhaps the haze particles gently rain down onto the surface through the night, staining Pluto with a distinctive reddish cast, or perhaps other atmospheric processes act to move and concentrate the haze.

https://blogs.nasa.gov/pluto/2015/09/25/pluto-at-twilight/

[stevesliva]

Those hazes are *blue*
http://pluto.jhuapl.edu/News-Center/News-A...p?page=20151008

[Nafnlaus]

And how blue they are

Such a simple image - there's not really much to comment about it from a scientific standpoint. But it really packs an emotive punch. Each time I look at that picture it makes me think, "why the heck haven't we visited this world sooner" and "wow, what else is out there in the kuiper belt?"

[ngunn]

And it vindicates the wonderful coloured version of the looking back landscape posted here earlier.

http://www.unmannedspaceflight.com/index.p...st&id=37888
I'm so relieved - I so much wanted to believe that scene was real but couldn't for sure till now.

[Bjorn Jonsson]

The blue color is interesting because it *may* mean that Mie scattering isn't as dominant as would have been case had the atmosphere been grayish (or not blue). Which in turn may mean that the atmosphere might be easier to see in low-phase views of Pluto. If I remember correctly, someone (Gennady maybe) had noticed what could be atmosheric limb hazes in low-phase LORRI images but it could also be due to scattered light - what's really needed are higher resolution low-phase images of Pluto's limb to distinguish possible hazes from scattering in LORRI's optics.

[scalbers]

Those hazes are *blue*
http://pluto.jhuapl.edu/News-Center/News-A...p?page=20151008

Interesting about the blue color. We can consider this in quantitative terms as to what the Angstrom Exponent would be. Typical aerosols on Earth range from 0-2 (gray to light blue). Air molecules are a value of 4 (more blue). What Bjorn is mentioning would be consistent as the smaller aerosols, having the higher Angstrom Exponent, would also have a flatter phase function with less brightness difference between forward and backward scattering. Thus the color can be correlated with both the size and with the phase function.

[remcook]

Though note that that fractal particles, such as found on Titan, can have a pretty high Angstrom coefficient AND are pretty forward-scattering.

[Ian R]

And it vindicates the wonderful coloured version of the looking back landscape posted here earlier.

http://www.unmannedspaceflight.com/index.p...st&id=37888
I'm so relieved - I so much wanted to believe that scene was real but couldn't for sure till now.

If anything, I was *too* conservative with the saturation of the blue hazes!

[scalbers]

Yes perhaps it should be bluer. I suppose we can sample the released RGB image and consider the color ratios, color space, tristimulus functions, gamma correction, and wavelengths of interest and even calculate the Angstrom Exponent.

[Bjorn Jonsson]

In hindsight this test render I posted 2 months ago is interesting:

http://www.unmannedspaceflight.com/index.p...st&p=225265

Not quite saturated enough since judging from the description I've seen of how the MVIC image was processed it should be very close to the true color and contrast of Pluto's atmosphere.

[Gennady Ionov]

Which in turn may mean that the atmosphere might be easier to see in low-phase views of Pluto. If I remember correctly, someone (Gennady maybe) had noticed what could be atmosheric limb hazes in low-phase LORRI images but it could also be due to scattered light - what's really needed are higher resolution low-phase images of Pluto's limb to distinguish possible hazes from scattering in LORRI's optics.

Yes, and I used "flat" phase function with forward/backward ratio equal to 6.
With regard to the published picture, I was confused by the fact that the color of a narrow crescent of reddish surface of Pluto are also blue.
Slightly corrected the color and get this:

[ngunn]

I think you both (Ian and Bjorn) were absolutely spot on with the colour. Colour saturation is a subjective physiological thing. When you look at the NH image the blue atmosphere occupies a very small fraction of your field of view. It looks strikingly blue, but if you actually went there so your whole sky in the direction of the sun was filled with it it would seem less so.

[fredk]

I was confused by the fact that the color of a narrow crescent of reddish surface of Pluto are also blue.

How narrow? Is the sunlit surface actually resolved, or are we just seeing mainly the brightest part of the sky (ie the largest phase angle part)? We may not expect to see any red if the sunlit surface is very narrow. Knowing the time the image was taken, it should be easy to simulate the view from NH and determine how many pixels (or what fraction of a pixel) the sunlit suface is wide.

Also, there will be some atmosphere "on top of" the sunlit surface adding blue light to the surface's reddish light (as much as half the optical depth of the pure-atmosphere lines of sight). Distant mountains on earth tend to look blue for this reason.

[Gennady Ionov]

I can simulate what portion of pixel crescent is sunlight reflected from the surface from the total brightness of the pixel, if it is known shooting time. But using previous experience of modeling LORRI images I can say that it is about 90%.

However, it is possible to analyze other more simple and reliable way without simulation of light tracing.
Unfold the image relative to the horizon

and find the dependence of the maximum brightness for each color channel from the position angle

blue channel overexposed in crescent region, but it does not hurt much to our analysis.
Night side takes an angle of approximately 155 to 335 degrees.
At this angles atmospheric glow reaches its maximum value and almost no change in the crescent. Thus the excess glow is associated with reflection from the surface of Pluto.
On the night side of the weakening of glow due to the fact that we see illuminated the higher layers of haze. Multiply colors on the night side by constant factors, so that at the border with crescent values in channels about the same.

Either the color of haze becomes more blue with height or the image have shifted zero at blue channel.
The color of the reflective surface is obtained as on the left side of picture, in the middle is avarage color of pluto surface:

[Gennady Ionov]

If we multiply the green channel by 0.9, and blue channel by 0.8, we get the color of the surface of Pluto as on the right part of picture:

RGB graph:

And corrected image in comparing to original: