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: 1670 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 8 2015, 11:09 PM
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IMG to PNG GOD Group: Moderator Posts: 2256 Joined: 19-February 04 From: Near fire and ice Member No.: 38 |
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.
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Oct 9 2015, 08:46 PM
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Junior Member Group: Members Posts: 82 Joined: 13-July 15 Member No.: 7579 |
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: |
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Oct 9 2015, 10:51 PM
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Senior Member Group: Members Posts: 4260 Joined: 17-January 05 Member No.: 152 |
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. |
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