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
Post
#1
|
|
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 ]
|
|
|
Aug 11 2015, 05:36 PM
Post
#2
|
|
Senior Member Group: Members Posts: 4256 Joined: 17-January 05 Member No.: 152 |
Sure, Pluto's albedo depends on position, but how did you choose the absolute albedo corresponding to the Pluto map pixel values 1-255? I don't think anyone has made any claim for their map such as that 255 corresponds to albedo = 1 and 128 to 0.5, eg.
Of course, however you chose that shouldn't affect your conclusion that Charonshine is comparable to Rayleigh. |
|
|
Aug 11 2015, 06:12 PM
Post
#3
|
|||
Junior Member Group: Members Posts: 82 Joined: 13-July 15 Member No.: 7579 |
Sure, Pluto's albedo depends on position, but how did you choose the absolute albedo corresponding to the Pluto map pixel values 1-255? I don't think anyone has made any claim for their map such as that 255 corresponds to albedo = 1 and 128 to 0.5, eg. Of course, however you chose that shouldn't affect your conclusion that Charonshine is comparable to Rayleigh. The resulting LORRI frames in addition to the absolute surface albedo of Pluto is influenced by many factors: the quantum yield of CCD, telescope aperture, mirrors albedo, etc. Consider all this I was not needed, because everything gives approximately linear contribution. Thus, for each of the simulated image, I introduced two parameters - the color of the background and gain. Varying its manually I achieved similarities with LORRI images. By the way, LORRI frames was pretreated and a value of zero and gain different for different images. Therefore, the correlation of the absolute value of the albedo to a pixel value on the map is not required. By the way, the faint traces of the atmosphere can be seen even daily images of Pluto, if you know what to look for: Haze brightness in simulated pre-encounter frame is about 2 steps in 256 colours. On difference (right picture) we can see, that haze glow almost completely compensate glowing on the source frame. |
||
|
|||
Aug 11 2015, 07:31 PM
Post
#4
|
|
Senior Member Group: Members Posts: 4256 Joined: 17-January 05 Member No.: 152 |
Varying its manually I achieved similarities with LORRI images. Normalizing to the LORRI sunlit images and taking into account the different exposure times means you can display the Charonshine as it would have appeared in the image (after scaling 8000x, of course). Without knowing the CCD QE etc, you don't know the absolute intensity of Charonshine. But what about the Rayleigh scattering? Is that calculated absolutely given the atmospheric parameters? There are no other images of Rayleigh scattering for reference, unlike the surface of Pluto. How do you decide how bright the scattering will appear on your image? I don't understand how you can compare the intensity of Charonshine and Rayleigh if you calculate the scattering absolutely but not the Charonshine.By the way, the faint traces of the atmosphere can be seen even daily images of Pluto, if you know what to look for: That glow around Pluto is probably just scattered light inside the camera. You can see a similar glow in images taken much farther out before closest approach, but the glow's thickness in those images is much thicker relative to the angular (or pixel) size of Pluto. That strongly suggests camera scattered light. The real atmosphere should always appear the same thickness relative to the size of Pluto.
|
|
|
Lo-Fi Version | Time is now: 26th September 2024 - 01:00 PM |
RULES AND GUIDELINES Please read the Forum Rules and Guidelines before posting. IMAGE COPYRIGHT |
OPINIONS AND MODERATION Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators. |
SUPPORT THE FORUM Unmannedspaceflight.com is funded by the Planetary Society. Please consider supporting our work and many other projects by donating to the Society or becoming a member. |