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 6 2015, 08:31 PM
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Member Group: Members Posts: 244 Joined: 2-March 15 Member No.: 7408 |
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. 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. 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. |
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