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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Sep 15 2015, 11:33 PM
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Member Group: Members Posts: 244 Joined: 2-March 15 Member No.: 7408 |
Looks like ninjas are nearby, but here's an unwrapping of lor_0299236719, lor_0299236749, lor_0299236779, and lor_0299236809. No correction for distortion was performed. A minor level adjustment was performed on some frames to make the brightness curve consistent across all frames. According to the metadata, there was a 0.16% difference in distance-to-target between the first and last frames, amounting to over a pixel of difference in Pluto's diameter, so each image was scaled based on the distance-to-target value in the metadata to correct for this. Alignment was manual, done at 4x scale to allow for some sub-pixel precision. Dimensions scaled-to were 4096, 4098, 4100, and 4103, from nearest to farthest. After alignment, Pluto center was estimated with reasonable confidence that the estimate was within 1 pixel of center at the original scale. The scaled frames were cropped to the region they all shared, yielding a set of 3829x4032 images, which were lightly sharpened with a simple unsharp masking algorithm with radius 6 (1.5 at the original scale) in lieu of deconvolution (because I'm lazy), after which each pixel from each of the four resulting frames (61,754,112 samples in all) was measured for distance from estimated Pluto center and angle from +X relative to Pluto center, and then 2-dimensionally binned in 1 pixel scaled radius x 0.1 degree angle bins (lists of integer sample values, actually, which were later averaged to a floating-point value). The 4000(only 2914 used)x3600 bins were rendered to a 3600x4000 image, which was then cropped to 3600x1748 (to remove incomplete radius space high above the planet), horizontally shifted 25% to place the limb-bound section near the center, gamma-adjusted, and finally scaled to half size (1800x874) to be small enough to upload here.
The horizontal axis spans 360 degrees around the center of the planet, with each horizontal pixel equaling 0.2 degrees. The vertical axis is image-space distance from the center of the planet, with each vertical pixel equaling 0.5 pixels at the scale of lor_0299236719. The bottom of the image is the estimated center of Pluto. Artefacts near the bottom are the result of a shortage of pixels for binning at different angles very near the center. Pluto average radius appears to be about 618 pixels (314 pixels at the scale of lor_0299236719). Here's a sharpened, level-adjusted, cropped, shifted, and scaled version, including just the limb/terminator and surrounding bright areas. |
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