If we assume the scale height of Pluto's atmosphere is 60km and the aerosols have the same height as the gas, then I was able to get a few numbers in the course of comparing various airmass equations. Earth would be about 39 airmasses in the horizontal and Pluto would be 6.4. These numbers would be doubled when looking at grazing incidence from space as in the NH images. I'd still like to come up with a formula for an isothermal atmosphere (exponential density decrease with height) by integrating the thin shell relationship over height and to compare this with the other formulations in Wikipedia. On the other hand, the isothermal case is within just a few percent of the homogeneous (constant density with height) case.

To check the scale height and see why it is much higher than Earth, we might evaluate this expression for Earth and Pluto:

H = kT/mg

H is scale height
T is temperature (a representative value since this varies with height)
k is Boltzmann's constant
m is molecular mass
g is gravitational acceleration

The Wikipedia link above shows this worked example for Earth:

Taking T = 288.15 K, k = 1.3806488x10-13 J/K, m = 28.9644×1.6605×10−27 kg, and g = 9.80665 m/s2 yields H = 8345m

Roughly speaking, if pluto has .07 Earth's gravity and the same T and similar m we'd get about 120km scale height. If the scale height is 60km, then the temperature would still end up being ~140K. So we can check how much the temperature increases with height over the surface value of 44K.

There are other atmosphere posts in the Near Encounter thread as well (e.g. posts #1238 and #1252).

This topic is for discussion of data concerning Pluto's atmosphere (including exospheric phenomena) received after 1 Aug 2015.

As noted earlier:



And, as noted:




The Earle and Binzel article covers a lot, but there has been a wealth of study done on insolation and the atmosphere of Pluto over the past few years. Google "pluto insolation atmosphere" and start digging. The atmosphere of Pluto, which drives the weathering, erosion, transportation, deposition and induration processes of the surface is clearly solar-powered. I've not give much thought to the variations in insolation on a high axial-inclination planetary-body other than imagining that it is out of the ordinary. And the revelations on the structure of the Pluto atmosphere from the post-encounter occultations will drive more research.

--Bill

I meant to say that Earle and Binzel had sorted out the insolation, in the sense of watts per square metre at various latitudes, and including the orbital eccentricity, polar orientation etc. Of course the in situ measurements of the atmosphere and the images and composition of the surface will tie up theorists for years.
While the insolation is important, any other heat source will have interesting effects on such cold surfaces and such tenuous atmospheres.

Even though the NH encounter increased our knowledge of Pluto and Charon orders of magnitude and many orders of magnitude, respectively, this knowledge is in the form of snapshots of the system. Along with all of the prior data on Pluto the NH data is much like piles of jigsaw puzzle pieces on the table and we'll spend years seeing how the pieces fit together. Three weeks ago we didn't know what the surface of Charon even looked like but now the geomorph people are supposing terrains and processes. Even though Charon is a small airless world, it does have solids that turn to gases and back to solids, seasonally, which is something that the rocky and ice worlds don't have.

Interesting times, these.

--Bill

I've previously "unwrapped" the atmosphere/haze from two different sets of LORRI images from SOC.
The second set were nice because they covered the full disk, but this came with substantially lower resolution.
Since the Sun is lighting the haze unevenly around the limb, trying to merge data from around the disk to get a cleaner, higher-resolution profile of scattered light is not a simple task.
Here, I've taken the two LORRI JPEGs and, for each pixel, mapped the value of the sample (0-255) with (the center of) that pixel's distance from my best estimate of the center of the planet. What that gives me is a plot of the combined scattering gradients of all of the different Sun-haze-sensor geometries present in the LORRI frames.

Here are simple plots of that data for both LORRI images.
Click to view attachmentClick to view attachment
The vertical axis is sample value (0-255). The horizontal axis is distance (in pixels) from estimated Pluto center. The darkness is an indication of how many pixels are mapped to that spot. Each data point is rendered as a single pixel of black, but with subpixel precision, so the black pixel is distributed across as many as 4 pixels.
Shown in those plots are the mappings of about 32,000 pixels from each image. Specifically, all pixels that have a distance-from-center of between 105 and ~145.65 pixels.

Here is a combination of both images' data, with lor_0299323899's values scaled up slightly to correct for an apparent, small difference in brightness between the two JPEGs.
Click to view attachmentClick to view attachment

Pluto center used for lor_0299323899 (pixels): 412.125, 483.75
Pluto center used for lor_0299323929 (pixels): 337.125, 460
^{(center of image's top-left pixel is 0.5, 0.5)}


Edit (2015-08-10): Remade and replaced the "combined" images more carefully, by merging the original data, rather than combining images of both, and using a more precise subpixel render.

I don't have a solid explanation for some of the repeating patterns seen in some parts of the plots. They are not processing artifacts in the sense that they are present in a simple plot without any processing of the data, but given that they have a horizontal spacing of very close to one pixel of distance and occur very close to the mid-point between integer pixels of distance, it seems likely that they are in some way a result of the distribution of pixel distances near certain angles. That isn't to say that they don't represent the actual scattering gradient, but rather probably that near the vertical and horizontal, pixel center distance clusters, horizontally compressing the data in that plot due to a lack of pixels that fall at distances just below and above it near those angles.

To be seen the glow of the atmosphere due to Rayleigh scattering need to raise the sensitivity of the image in 8000 times. Then it will look something like this
Click to view attachment (old lovely version) Click to view attachment (fixed version, really Rayleigh glow is dimmer then Charonshine)
but we does not seen Charonshine on the LORRI frame
Click to view attachment
Thus the main optical depth falls on the aerosol.
If optical depth of aerosol in 1000 times bigger then for Rayleigh scattering and exponential scale equal to 20 km we obtain a proper image:
Click to view attachment

Are you saying that, for the geometry of that image, Charonshine on Pluto is very roughly of the same brightness as atmospheric Rayleigh scattering around the limb of Pluto?

What atmospheric parameters did you use for Pluto? And the intensity of Charonshine depends on knowing the absolute albedos of Charon and Pluto - what did you use?

Yes, Charonshine on Pluto is the same brightness as atmospheric Rayleigh scattering.
I used N2 atmosphere with scale 60 km and surface pressure 1.0 Pa.
Albedo of Charon = 0.375, and albedo of Pluto depends from a surface position. Pluto's albedo just is Pluto map.

Sure, Pluto's albedo depends on position, but how did you choose the absolute albedo corresponding to the Pluto map pixel values 1-255? I don't think anyone has made any claim for their map such as that 255 corresponds to albedo = 1 and 128 to 0.5, eg.

Of course, however you chose that shouldn't affect your conclusion that Charonshine is comparable to Rayleigh.

The resulting LORRI frames in addition to the absolute surface albedo of Pluto is influenced by many factors: the quantum yield of CCD, telescope aperture, mirrors albedo, etc. Consider all this I was not needed, because everything gives approximately linear contribution. Thus, for each of the simulated image, I introduced two parameters - the color of the background and gain. Varying its manually I achieved similarities with LORRI images. By the way, LORRI frames was pretreated and a value of zero and gain different for different images. Therefore, the correlation of the absolute value of the albedo to a pixel value on the map is not required.

By the way, the faint traces of the atmosphere can be seen even daily images of Pluto, if you know what to look for:
Click to view attachment Click to view attachment
Haze brightness in simulated pre-encounter frame is about 2 steps in 256 colours.
On difference (right picture) we can see, that haze glow almost completely compensate glowing on the source frame.

Normalizing to the LORRI sunlit images and taking into account the different exposure times means you can display the Charonshine as it would have appeared in the image (after scaling 8000x, of course). Without knowing the CCD QE etc, you don't know the absolute intensity of Charonshine. But what about the Rayleigh scattering? Is that calculated absolutely given the atmospheric parameters? There are no other images of Rayleigh scattering for reference, unlike the surface of Pluto. How do you decide how bright the scattering will appear on your image? I don't understand how you can compare the intensity of Charonshine and Rayleigh if you calculate the scattering absolutely but not the Charonshine.
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 checked my code and found that really I believe that value 255 corresponds to the albedo of 1.0 (in sense all falling light isotropically scatter in half-space). Charonshine light flow is determined relative the Solar light flow from angular size, phase and geometric albedo. Also, the intensity of the Rayleigh emission is determined with respect to the intensity of emission of the white body which isotropically scatter sunlight. Therefore it is possible to compare the Charonshine and Rayleigh glow.

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. Also one problem with the glow around Pluto in the farther-out images days before closest approach is that it may be caused partially by JPG compression artifacts in addition to scattering. Things really do not become completely clear until we see much higher resolution images of Pluto's limb taken before closest approach (i.e. not at high phase angles). Also let's not forget that a haze layer is visible in images of Triton where the phase angle isn't very high.

The general conclusion seems to be that Pluto's "sky" would be "space" during the day, but near sunrise and sunset the "haze" of the atmosphere should be discernible. That would be something to see, two "night skies" in one day.

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".

Whether visible or not, not all the atmosphere comes from the surface ices. Icy objects entering the atmosphere and "evaporating" could add significantly to Pluto's atmosphere over time. Its not going to replace 500 tonnes per hour thats for sure, but might help maintain the atmosphere's depth and seed ice crystals, via dust and organics, at higher levels of the atmosphere than those materials being transported from the surface reach. Pluto's equivalent of Noctilucent clouds. One might expect atmospheric dynamics would mix the two, but it is very cold and the evidence is that there are distinct layers within the atmosphere.

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.

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:

Click to view attachment

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.

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.}

Which is which?

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.

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.

Click to view attachment
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.

I made some modelling to estimate haze visibility in pre-encounter stage and post there blinking pair to compare.

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.

Yes, I agree. My scattering simulation in colour (colour does not simulated, just was set manually):
Click to view attachment

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
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.

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.


Colour does not simulated, just was set manually.
Click to view attachment

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.

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?

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

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

About the phase functions, here ( http://inspirehep.net/record/1127473/plots , first plot) is a plot of the size of the forward-scattering peak of the phase function as a function of wavelength for different solar system particles. Notice the log scale. Also, the Titan book chapter nicely shows how very much forward-scattering Titan particles are. The forward-scattering peak is much more intense than in the polar plot by Gennady. But we don't know whether that holds for Pluto of course.

Offhand, the phase function peak is more typically ranging from 20-1000 for various solar system hazes (including Earth). This agrees with the figure in 'remcook's link. The larger the particles, the larger the peak. So Gennady's plot value of 6 might indeed benefit from being increased. If we consider exactly where the sun is behind the disk of Pluto, then the asymmetry of the annular glow may help a bit in determining the phase function if shadowing effects of Pluto on its atmosphere are also considered. LORRI's field of view though is a bit small though for the scattering angle to vary much over the image.

Meteors may appear over Earth at altitudes where the pressure is less than the maximum surface pressure on Pluto, which suggests that visible meteors could be possible (were there anyone there to see them). However, recall that the velocity of meteors running into Pluto should be much less than the velocity of meteors hitting Earth's upper atmosphere. However: Water ice wouldn't remain ice when it was hot enough to glow, and would soon be a puff of vapor.

So, a visible meteor on Pluto would require an anomalously high velocity, and for the speck to be silicate rather than ice. I'll bet it's happened, but rarely.

If you mean the released LORRI post-encounter images, the sun is not behind Pluto. NH was 360 000 km from Pluto in the closer post-encounter frames, which is much farther than the Pluto-sun occultation distance (very roughly 50 000 km). Conversely, LORRI would only see a small fraction of Pluto during the occultation.

The variation in brightness we see around the limb in those shots is mainly due to the presence of a very slim sunlit crescent, though presumably the haze is somewhat in shadow on the opposite limb.

Edit: this assumes you meant "occulted by Pluto" when you said "behind Pluto".

Copied post from stevesilva over on the 'interesting/boring objects' thread that is relevant here:

Thank you, Explorer, Bill!

They do in the main post; I've copied the link locations:

http://pluto.jhuapl.edu/News-Center/News-A...p?page=20150812

https://blogs.nasa.gov/pluto/2015/08/10/atm...lutos-nitrogen/

Whups! Sorry. Thanks, Explorer.

Why not _fix_ the bad links in the earlier post? Leaving them dead, and finding the corrections two posts later, is noisy.

And possibly including a direct link to the PDF might be spiffy:
http://iopscience.iop.org/2041-8205/808/2/...5_808_2_L50.pdf

The evidence from military satellites is that numerous bolide airbursts, many probably from icy comet debris, occur tens of kilometres high in the atmosphere of Earth, where densities are comparable to those in Pluto's atmosphere. Some of these "explosions" are in the megatons of TNT range and at first were suspected as being atomic bombs. The well known Tunguska Event is thought to be the result of such an airburst a few kilometres above the surface. This paper and article by Gasperini et al on the possibility of the impactor being a comet and the morphology of a possible impact crater, suggests there are some applicable numerical models out there which could be used.

http://onlinelibrary.wiley.com/store/10.11...da53l2w8da6b86d

http://www.geotimes.org/feb08/article.html?id=nn_crater.html

The key here, as you suggest, is the inertia of the impactor. A sufficiently large icy object from the Oort Cloud travelling at interplanetary velocities impacting Pluto's atmosphere could be postulated to result in an atmospheric "airburst", one which, intuitively, would be a lot closer to the surface of Pluto given the very low atmospheric density.

If the Tombaugh Regio is the result of such a cometary impact, should we be looking for evidence of an accompanying "airburst"? This image by G.I. suggests there may be some visible along the South Western side of Tombaugh, (top right in the image). Together with the "snow cone" shape of the Western half of Tombaugh Regio, the depression at the base of the "cone" and the flattened terrain surrounding it, it seems worth further investigation when more topographic data is available.

http://www.unmannedspaceflight.com/index.p...st&id=37400

At Earth solar distances, we are not going to find icy debris particles. Fluffy refractory chunks are likely.

And watch out making too many Earth-analogies. Although cosmic uniformitarianism is a valid principle, Pluto is an strange alien world.

--Bill



AND, quite welcome Gennady!

Hmm, unexpected use my picture :-)
As for the Tombaugh Regio and of the equatorial darkening I have my own hypothesis, which appeared in early July. Too crazy to her voice.
I'm afraid that the moderator would not welcome her appearance here. At least I have encountered this in Russian forum.

Thanks for pointing this out. The blue enhancement may be less then due to the difference in the sharpness of the phase function peaks related to the size parameter. What is the scattering angle? We still have the Angstrom exponent caused blueness as a factor.

Surface reflectance and roughness properties then become of interest in determining how bright the crescent would be.

Here is an animation I did of the New Horizons flyby that includes atmospheric effects:

https://vimeo.com/136223988

I changed the atmospheric model to make the haze/aerosols gray. I'm using Mie scattering only and assuming that there is no wavelength dependence. The particles are gray which could easily be an incorrect assumption. The phase function I used is Cornette's improved version of the Henyey-Greenstein function. The parameters I used result in values ranging from 0.1 (phase angle 0°) to 27.2 (phase angle 180°):

Click to view attachment

The phase function is simply a guess and it works fairly well for terrestrial scenes but parameters almost certainly have to be adjusted once we see images of the haze/aerosols at a greater range of phase angles.

The scale height of Pluto's atmosphere is ~60 km according to pre-NH measurements. I'm using a scale height of 55 km for the aerosols which may or may not be correct. I tested other values; 30 km was visually different from the NH high-phase images and it was also not possible to increase the values to much more than 60 km without getting bad results.

Quite true Bill and I hesitate to hypothesise further without better evidence or access to the required models. I could be adding 2 and 2 to get 5.

I would add that chunks of icy material do cross the orbit of Earth on a regular basis in the form of comets and their debris, some detectable and some not.

Pretty dramatic flyby from Bjorn 2 posts back. Interesting to see the asymmetry in the atmospheric ring start to increase near the end as the sun pulls away. I was able to turn up my monitor brightness enough to see the Charonshine.

Nice video!
If you make time of Charon pass more slowly, it will look spectacular.
I'm probably wrong somewhere, but I have obtained similar to NH haze images at a scale of less than 30 km...
Yes! My mistake has a copy/paste nature. Haze scale variable does not used at all. Instead Rayleigh scale variable are used.
Its fit value is 48 km:
Click to view attachment

I should mention that my NH image measurements were rather crude and other things in the atmospheric models might be different (and I also cannot completely rule out subtle bugs in my code ). Also the haze/aerosols probably do not decrease uniformly with height, there could be one or more discrete layers of haze and the horizontal distribution of the haze could be non-uniform.

Bravo Bjorn! The animation is really great and helps put some things in a better perspective, namely the later shots.

Beautiful.

You say Charonshine is exaggerated - by what factor? And is the (far) post-encounter Mie scattered light intensity correct relative to the pre-encounter directly sunlit surface of Pluto? Ie, does the simulation use a constant "exposure" for each frame?

I had the impression that the scattered light must be much fainter than the sunlit surface, but now that I look at the LORRI exposures I see that they are comparable, so that means that the scattered light is remarkably bright.

Anyway, this leads me to wonder what the sky would look like from the surface (as others have speculated here already). But what I'm thinking about is the sky near the sun, which would correspond to Mie scattering at large phase angles, where we have a good handle on the phase function from LORRI. We now know that the sky near the sun would be quite bright - in particular, near sunrise/set (optical depth half what LORRI saw) the sky intensity would be comparable to that of the sunlit surface.

Of course our large uncertainty in phase function at smaller phase angles means we wouldn't be very certain how large the glow around the sun would be, and how faint it would get at large angles (eg 90 deg) from the sun. But pick a phase function out of the blue and you could render it...