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 5 2015, 05:20 PM
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Senior Member Group: Members Posts: 3009 Joined: 30-October 04 Member No.: 105 |
As noted earlier:
That has had me doing back-of-envelope scribbling, too. I dunno. At the Solstices, one pole is 100% in sunlight (with varying sun elevations) for 1/2-year while the other pole is 100% in stygian darkness for 1/2-year. The Equator, OTOH, varies from 1/2-day sunlight _at_ the Equinoxes to 100% sunset (ie, zero Sun elevation, on the horizon) at the Solstices. For arm-waving simplicity, I'm assuming 90 deg polar inclination instead of the 119.xx deg it is. And add to that the variation of solar intensity due to the orbital eccentricity. The insolation at any point will be the product of the solar intensity (x) a function of the solar elevation (x) a factor of the Sun's time above the horizon (x) whatever I've not thought of. I suspect that we'd need some sort of Calculus to describe that, with sine/cosine wheels to vary things. Dang. What a can o'worms. Much easier to crank out purty pictures, but less fun. --Bill And, as noted: "Pluto’s insolation history: Latitudinal variations and effects on atmospheric pressure" by Earle and Binzel in April's edition of Icarus is probably the last word on Pluto's insolation history. They've done all the calculus so we don't have to! For those without access there are various papers on Arxiv which touch on the subject. The Earle and Binzel article covers a lot, but there has been a wealth of study done on insolation and the atmosphere of Pluto over the past few years. Google "pluto insolation atmosphere" and start digging. The atmosphere of Pluto, which drives the weathering, erosion, transportation, deposition and induration processes of the surface is clearly solar-powered. I've not give much thought to the variations in insolation on a high axial-inclination planetary-body other than imagining that it is out of the ordinary. And the revelations on the structure of the Pluto atmosphere from the post-encounter occultations will drive more research. --Bill -------------------- |
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