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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Aug 14 2015, 12:42 PM
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Junior Member Group: Members Posts: 82 Joined: 13-July 15 Member No.: 7579 |
I calculate the average over the angle values of haze brightness in dependance on the height above the Pluto surface on lit side limb (on the illuminated side the haze glow is practically independent from angle).
Such calculation I made for the original LORRI images and for simulated pictures. Also I estimate PSF of LORRI from far Pluto snapshots (it can be verified from the images of Charon and other satellites). Now I take PSF as radial function exp(-r*r/1.44)+0.0008*exp(-r*r/100) divaded by norm. You can get the results in ftp://gionov:NG@46.45.15.20/_Data/_LORRI/Haze/ For example: For simulate of haze I used scale 48 km, Rayleigh scale is 60 km. 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. It can be seen that only on the distant image lor_0298996664_0x630_sci_2 haze glowing a little less than light scattering inside the optics. The picture lor_0299148263_0x632_sci_3_Haze shows appear as the effect of scattering near the limb, and haze glowing far away from the limb. |
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