Pluto Atmospheric Observations: NH Post-Encounter Phase, 1 Aug 2015- TBD |
Pluto Atmospheric Observations: NH Post-Encounter Phase, 1 Aug 2015- TBD |
Jul 31 2015, 02:57 PM
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Senior Member Group: Members Posts: 1669 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
A neat paper by Jonathan Fortney shows this ratio to scale (approximately) with sqrt(Rp/H), with Rp being the planet radius and H the scale height. Both indeed decrease this effect for Pluto. 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). -------------------- Steve [ my home page and planetary maps page ]
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Oct 10 2015, 12:32 AM
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Junior Member Group: Members Posts: 82 Joined: 13-July 15 Member No.: 7579 |
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: |
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Oct 11 2015, 03:41 PM
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Senior Member Group: Members Posts: 4256 Joined: 17-January 05 Member No.: 152 |
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. |
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