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

[scalbers]

I thought it would be interesting to compare simulated blue skies on Earth using my ray tracing software to what we see on Pluto. In a rather clean atmosphere case, I show Earth skies to have a G/B ratio of about .71. This is after converting the radiances to an RGB image for display. Meanwhile the released image has a somewhat variable G/B ratio from roughly 0.68 to 0.81. I wonder why this ratio should vary with the intensity if all of the color balancing is done correctly? Nonetheless if we take a "typical" value in the released image of .76 this would work out to an Angstrom Exponent of about 3.3. This is on a par with the bluest of hazes we see on Earth though not quite a pure Rayleigh scattering value of 4.

If the vertical color gradient in the released image is real, then we might surmise the Angstrom Exponent increases with altitude and the aerosol particle size also decreases with altitude.

[fredk]

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.

It's completely unclear what you're saying here. On the night side, as I think you say, we're seeing pure atmosphere and no sunlit crescent. On the day side we're seeing atmosphere plus thin sunlit crescent.

But, as I think you also say, the atmosphere we see on the night side is typically higher than the atmosphere we see on the day side. The low-elevation atmosphere is in the shadow of Pluto on the night side. But on the day side we can see the atmosphere right down to the surface of Pluto. This means that, even if the suface of Pluto was perfectly black, we'd expect the day side atmospheric glow to appear brighter than the night side glow!

Of course Pluto is not black and there will also be a contribution to the day side from the thin sunlit crescent. But here's the key point: in the absence of modeling, we don't know how much of the excess brightness between night and day is due to the fact that we see lower atmosphere on the dayside, and how much is due to the fact that we also see sunlit crescent on the dayside! In other words, we don't know how much the sunlit surface contributes to the day side brightness, so we can't set colours according to the average surface colour!

In addition, as scalbers pointed out, the day-night colour differences you're seeing may simply be due to the atmosphere being bluer higher up, which is consistent with small particles clumping into larger particles as they fall, as we'd expect. Or they may be due to nonlinearity of the RGB levels, due maybe to gamma adjustment.

To sum up, we have no reason to mistrust the image we've been given and nothing upon which to base a quantitative adjustment of it.

[scalbers]

That's largely true though I'd be interested to refine the assessment of the maximum blueness in the upper atmosphere. The image drops to a G/B ratio of around 0.6 in the top and faint part. In theory it should stay above about 0.70 at least from my various assumptions.

[Gennady Ionov]

It's completely unclear what you're saying here. On the night side, as I think you say, we're seeing pure atmosphere and no sunlit crescent. On the day side we're seeing atmosphere plus thin sunlit crescent.

But, as I think you also say, the atmosphere we see on the night side is typically higher than the atmosphere we see on the day side. The low-elevation atmosphere is in the shadow of Pluto on the night side. But on the day side we can see the atmosphere right down to the surface of Pluto. This means that, even if the suface of Pluto was perfectly black, we'd expect the day side atmospheric glow to appear brighter than the night side glow!

Yes, it is obvious that the night side is darker than day side. I tryed to say that in the case of the black surface of Pluto on the day side will be almost constant brightness. And the deviation from a constant glow is caused by the surface, which is due to the optical blur mixed to atmospheric glow. So it can be used of caliberate of color chanels.

[Gennady Ionov]

That's largely true though I'd be interested to refine the assessment of the maximum blueness in the upper atmosphere. The image drops to a G/B ratio of around 0.6 in the top and faint part. In theory it should stay above about 0.70 at least from my various assumptions.

Yes, if we multiply blue channel by 0.8 than the result will be more consistent with the theory.

[fredk]

I tryed to say that in the case of the black surface of Pluto on the day side will be almost constant brightness.

Do you mean constant brightness as a function of position angle around the disk, on the day side? I don't see why that should be constant. The phase function should peak at the largest phase angles, so the atmosphere should be fainter towards the ends of the day side. (Of course the important question is how fast the phase function drops, which is model-dependent.)

This could be easily checked by rendering the atmosphere with a black surface, keeping in mind that we don't know the phase function so a range of possibilities should be considered.

[Gennady Ionov]

Do you mean constant brightness as a function of position angle around the disk, on the day side?

Yes! Exactly!

I don't see why that should be constant. The phase function should peak at the largest phase angles, so the atmosphere should be fainter towards the ends of the day side. (Of course the important question is how fast the phase function drops, which is model-dependent.)

If distance at the shooting time was about 50,000 km, the change in the scattering angle was about 1 degrees with an angle value about 15 degrees. The reason I say that is ALMOST constant.

This could be easily checked by rendering the atmosphere with a black surface, keeping in mind that we don't know the phase function so a range of possibilities should be considered.

I try to do it for MVIC:

Distance to Pluto is 175000 km. Take image time is 15:21:30 UTC July 14 if original image does not scaled.

If we turn on surface with map http://laps.noaa.gov/albers/sos/pluto/pluto_rgb_cyl_16k.png
we obtain

To comparing original MVIC image and with multiplied blue by 0.8 and green by 0.9:

[remcook]

A tweet from the Pluto session at the DPS meeting: Hayley Williamson ‏@hayleynw92 :

"Pluto haze is probably aggregate particles to allow for blue color (small particles) and forward scattering (large particles). #DPS15"
Basically follows the discussion here.

[alan]

Other results from the conference showed that Pluto’s tenuous atmosphere — its surface pressure is only ten millionths that of the Earth’s atmosphere — is colder and more compact than initially expected. That suggests that far less of the atmosphere is escaping to space than expected.

“This changes our thinking of the long-term evolution of Pluto and its atmosphere,” said Leslie Young of the Southwest Research Institute. That includes losing far less of the ice on Pluto’s surface than originally thought. “The atmosphere has some huge implications for the history of the geology of Pluto.”

http://spacenews.com/new-horizons-reveals-...es-about-pluto/

[scalbers]

On an AGU poster I recall something like a .0045 optical thickness (tau) for the haze with a scale height of ~50km. It's interesting to note this tau is similar to what we see on Earth for stratospheric aerosols when they are enhanced somewhat due to volcanic emissions (such as presently). This relates to the brightness of twilight on Earth about 25-30 minutes after sunset.

When we're talking about "strong" forward scattering, I wonder what the peak of the phase function is for a scattering angle of zero degrees (180 phase angle)? The phase function is a quantity that integrates to 1 over the sphere. For the analogous atmosphere of Titan, this function is shown in figures 12.20 and 12.21 here: http://www.ciclops.org/media/sp/2010/6514_15623_0.pdf. Scaling factors are used in the figures to help separate the individual plots. The peak is thus nearly 1000. This indeed is what we'd see on Earth for pretty large particles in the haze, as would happen in a dusty day with a condensed bright aureole around the sun with around half of the scattered light contained in just a few degrees of radius.

[Ron Hobbs]

This morning New Scientist pinged me about an article: Exclusive photos: Clouds seen on Pluto for first time

It is a short article, but an interesting insight into a discussion that has been occurring between members of the NH team and people at New Scientist. It also looks they might be releasing a new photo through them.

Pluto: Curiouser and curiouser!

[MOD NOTE]: Interesting article, but reminder to all to consider rule 1.9 during discussion. Thanks!

[fredk]

It should be simple to look at the putative cloud locations in other images to check whether they might be stationary surface features rather than clouds. Of course even the lack of a feature in a different image doesn't prove it's a cloud if the lighting is very different, since we've seen what look like specular-ish reflections from smooth surfaces already.

[alan]

Pluto's hybrid interactions with the solar wind:

Previously, most researchers thought that Pluto was characterized more like a comet, which has a large region of gentle slowing of the solar wind, as opposed to the abrupt diversion solar wind encounters at a planet like Mars or Venus. Instead, like a car that’s part gas- and part battery-powered, Pluto is a hybrid, researchers say.

Like Earth, Pluto has a long ion tail, that extends downwind at least a distance of about 100 Pluto radii (73,800 miles/118,700 kilometers, almost three times the circumference of Earth), loaded with heavy ions from the atmosphere and with “considerable structure.”

Pluto’s obstruction of the solar wind upwind of the planet is smaller than had been thought. The solar wind isn’t blocked until about the distance of a couple planetary radii (1,844 miles/3,000 kilometers, about the distance between Chicago and Los Angeles.)

Pluto has a very thin boundary of Pluto’s tail of heavy ions and the sheath of the shocked solar wind that presents an obstacle to its flow.

http://www.nasa.gov/feature/pluto-s-intera...que-study-finds

[elakdawalla]

New Horizons has been downlinking lots of departure phase 1 images recently, showcasing the shrinking "O" of the atmosphere. It'll make a nice animation, something I plan to do....unless someone else beats me to it

http://planetary.s3.amazonaws.com/data/nh/dp1.html

(New items downlinked in May are highlighted in yellow)

[Astroboy]

Aligned on the stars: