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Pluto Atmospheric Observations: NH Post-Encounter Phase, 1 Aug 2015- TBD
fredk
post Aug 11 2015, 09:52 PM
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About the visibility of aerosols/haze in the pre-encounter (low phase angle) images, now that we have observations of that component post-encounter (at high phase angle), could we (Gennady?) do some modelling to estimate their visibility pre-encounter? Of course, we only have observations at high phase angle, and would need to extrapolate to estimate the back-scattered visibility. You'd need to state your assumptions about that, and even better include a range of possibilities as an error estimate.

Even if that turned out to be far too faint to see pre-encouter, you might still find localized bands/clouds of higher density haze.
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Bjorn Jonsson
post Aug 11 2015, 10:16 PM
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I have been attempting to model Pluto's atmosphere to get reasonably good 3D renders at high phase angles. This is a simulation compared to image lor_0299323899_0x630_sci_2.jpg:

Attached Image


The simulated image doesn't have the same orientation as the NH image. I'm using Mie scattering only since Rayleigh scattering is insignificant in Pluto's very thin atmosphere. I suspect that the haze really isn't bluish (my guess is that it's grayish) but MVIC images are needed to know its real color. The corresponding haze brightness in simulated pre-encounter images is ~3 (range 0-255) next to Pluto's limb and this seems plausible but the problem is that I don't know about a possible contrast stretch applied to the JPGs at the NH website. I suspect they are not stretched and one thing implying that they are at least not severely stretched is that there are many intensity levels in the haze in the NH images.


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Herobrine
post Aug 11 2015, 10:21 PM
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QUOTE (Sherbert @ Aug 11 2015, 05:08 PM) *
This being the case would "shooting stars" be visible? That is, is the atmosphere "thick" enough to "burn up" incoming debris? Its not very dense, but it is "tall".

Not having done any calculations, personally, I'd be surprised if you could ever get a visible "shooting star" at Pluto. I suppose it wouldn't be too terribly difficult to calculate approximations of how hot/bright a large, stony meteor would get, crossing through the densest part of Pluto's atmosphere at 100,000 m/s or something. I just can't imagine it's enough to generate the heat required for it to become visible to the human eye, even as sensitive as that is.
I remember reading about a meteor believed to have been imaged by one of the Mars rovers, but while Mars' atmosphere is something like 1/100th as dense as Earth's, Pluto's is like a thousand times less dense than that.
If someone does the math and it suggests it's possible, I'll believe it, but in my mind, Pluto's atmosphere is so thin, you could scoop it away at 1,000 km/s with a nix-sized scoop without generating enough heat to really glow.
I'm frequently wrong, though.
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ngunn
post Aug 11 2015, 10:32 PM
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QUOTE (Bjorn Jonsson @ Aug 11 2015, 11:16 PM) *
The simulated image doesn't have the same orientation as the NH image

Which is which?
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scalbers
post Aug 11 2015, 11:00 PM
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QUOTE (Bjorn Jonsson @ Aug 11 2015, 10:16 PM) *
I have been attempting to model Pluto's atmosphere to get reasonably good 3D renders at high phase angles.

Looks pretty interesting. Are you making a certain assumption about things like the phase function, and by extension particle sizes? I agree it wouldn't likely be blue especially with this forward scattering, unless it were just the right size distribution.

This relates to fredk's last post, and to the earlier comment about Triton's haze being visible at various phase angles.


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fredk
post Aug 12 2015, 12:25 AM
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QUOTE (Bjorn Jonsson @ Aug 11 2015, 11:16 PM) *
The corresponding haze brightness in simulated pre-encounter images is ~3 (range 0-255) next to Pluto's limb
This is interesting - I wouldn't've guessed it might get that high. (I assume you mean ~3/255, not between 0/255 and 255/255.) But of course this will be strongly dependent on the assumptions - particle size distribution, refractive index, etc.
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Gennady Ionov
post Aug 12 2015, 07:43 AM
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QUOTE (Bjorn Jonsson @ Aug 12 2015, 01:34 AM) *
There is definitely scattered light around a bright target like Pluto but I wouldn't completely rule out a very faint layer of aerosols/haze in addition to the scattering.

Attached Image

Haze brightness in simulated pre-encounter frame is about 2 steps in 256 gray steps.
On difference we can see, that haze glow almost completely compensate glowing on the source frame.
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Gennady Ionov
post Aug 12 2015, 07:45 AM
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QUOTE (fredk @ Aug 12 2015, 12:31 AM) *
That glow around Pluto is probably just scattered light inside the camera. You can see a similar glow in images taken much farther out before closest approach, but the glow's thickness in those images is much thicker relative to the angular (or pixel) size of Pluto. That strongly suggests camera scattered light. The real atmosphere should always appear the same thickness relative to the size of Pluto.

I made some modelling to estimate haze visibility in pre-encounter stage and post there blinking pair to compare.
QUOTE (fredk @ Aug 12 2015, 02:52 AM) *
About the visibility of aerosols/haze in the pre-encounter (low phase angle) images, now that we have observations of that component post-encounter (at high phase angle), could we (Gennady?) do some modelling to estimate their visibility pre-encounter? Of course, we only have observations at high phase angle, and would need to extrapolate to estimate the back-scattered visibility. You'd need to state your assumptions about that, and even better include a range of possibilities as an error estimate.

I adjust only optical depth of the haze (its about 0.003 in zenith direction) and used scattering with particle phase function (1 + 5 * mu * mu * mu * mu * mu * mu * mu *(3 + 4 * mu))*(9 / 58.0) which obtain in approximation of the light scattering from Chelyabinsk bolide on winter atmospheric haze. (Rayleigh low is (1 + mu*mu)*(3 / 8.0))
Where mu is cosine of scattering angle. So we have equal scattaring in opposite and forward direction in the case of Rayleigh low and have factor 6 in case aerosol scattering.
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Gennady Ionov
post Aug 12 2015, 08:02 AM
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QUOTE (Bjorn Jonsson @ Aug 12 2015, 03:16 AM) *
I suspect that the haze really isn't bluish (my guess is that it's grayish) but MVIC images are needed to know its real color.

Yes, I agree. My scattering simulation in colour (colour does not simulated, just was set manually):
Attached Image
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remcook
post Aug 12 2015, 10:51 AM
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Bjorn, Scalbers: why not blue? It will be blue-ish if the particles are small compared to the wavelength. To have grey scattering, you need fairly large particles, which might be tricky to form at such low pressures. Forward-scattering also tends to strongly increase towards shorter wavelengths. My bet would be blue smile.gif
To Bjorn, Gennady: what did you assume for the particle phase functions (as a function of wavelength), or refractive indices? They look like really cool simulations.
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Gennady Ionov
post Aug 12 2015, 11:51 AM
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QUOTE (remcook @ Aug 12 2015, 03:51 PM) *
Bjorn, Scalbers: why not blue? It will be blue-ish if the particles are small compared to the wavelength. To have grey scattering, you need fairly large particles, which might be tricky to form at such low pressures. Forward-scattering also tends to strongly increase towards shorter wavelengths. My bet would be blue smile.gif

I believe that the haze color is grayish for two reasons:
1) Scattering by haze particles are not so much shifted into the blue area as Rayleigh scattering.
2) At least 30% of the scattered light flux is reflected from the reddish surface of Pluto.

QUOTE (remcook @ Aug 12 2015, 03:51 PM) *
To Bjorn, Gennady: what did you assume for the particle phase functions (as a function of wavelength), or refractive indices? They look like really cool simulations.

Colour does not simulated, just was set manually.
Attached Image
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remcook
post Aug 12 2015, 02:59 PM
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Thanks! So, the colour from your colour simulation is basically picked manually. Very interesting about the surface contribution. I guess that will depend a lot on how forward-scattering the particles really are. Looking forward to seeing more data from New Horizons when it will be sent.
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HSchirmer
post Aug 12 2015, 04:04 PM
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QUOTE (fredk @ Aug 11 2015, 04:39 PM) *
... Charonshine on Pluto is very roughly of the same brightness as atmospheric Rayleigh scattering around the limb of Pluto?

...And the intensity of Charonshine depends on knowing the absolute albedos of Charon and Pluto ...


Quick question- just how intense IS Charonshine? Enough to make a 1 or 2 kelvin difference?

Triton's geysers may be powered by as little as 4 kelvins difference in temperature.
The area on Pluto that is illuminated/heated by Charonshine does not have an ice cap.
The area opposite Charon that faces deep space and is not illuminated/heated by Charonshine
has the Tombaugh ice cap.

Is that a coincidence? Or, could the heat of reflected light from tidally locked Charon influence the
location of the equatorial ice cap?
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scalbers
post Aug 12 2015, 04:14 PM
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Good discussion of the additional factors about the haze color. We might further consider what we think the Angstrom Exponent is, a number telling how blue the preference is for scattering that becomes more important if the haze particles are smaller than the wavelength of light. A second factor is that the peak of the phase function can be sharper for bluer light (size parameter varies with wavelength), so would make a bluish cast at the small scattering angles. Perhaps there's some analogy with the particle size distribution from Triton and Titan (or even Mars upper atmosphere) that can help with determining the phase function and how it varies with wavelength.

Here is a paper about Titan's aerosols: http://www.ciclops.org/media/sp/2010/6514_15623_0.pdf


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Gennady Ionov
post Aug 12 2015, 04:26 PM
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QUOTE (HSchirmer @ Aug 12 2015, 09:04 PM) *
Quick question- just how intense IS Charonshine? Enough to make a 1 or 2 kelvin difference?

Charon angular radius is about 600 km / 20000 km = 0.03, its square relative sky hemisphere equal pi*0.03*0.03 / (2*pi)=0.00045, and albedo 0.35, and avarage phase is 0.5.
So Charonshine irradiation only 0.00045*0.35*0.5=8e-5 from Solar irradiation. But Solar irradiation we must divide by 2 due to night time.
In total its make relative temperature difference about 0.00008*2*4=0.00064 (factor 4 from derivative of Stephan-Bolzman low).
In absolute value it is about 0.00064*40K=0.0256 K
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