Pluto Atmospheric Observations: NH Post-Encounter Phase, 1 Aug 2015- TBD Options

[scalbers]

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).

[fredk]

Are you saying that, for the geometry of that image, Charonshine on Pluto is very roughly of the same brightness as atmospheric Rayleigh scattering around the limb of Pluto?

What atmospheric parameters did you use for Pluto? And the intensity of Charonshine depends on knowing the absolute albedos of Charon and Pluto - what did you use?

[HSchirmer]

... Charonshine on Pluto is very roughly of the same brightness as atmospheric Rayleigh scattering around the limb of Pluto?

...And the intensity of Charonshine depends on knowing the absolute albedos of Charon and Pluto ...

Quick question- just how intense IS Charonshine? Enough to make a 1 or 2 kelvin difference?

Triton's geysers may be powered by as little as 4 kelvins difference in temperature.
The area on Pluto that is illuminated/heated by Charonshine does not have an ice cap.
The area opposite Charon that faces deep space and is not illuminated/heated by Charonshine
has the Tombaugh ice cap.

Is that a coincidence? Or, could the heat of reflected light from tidally locked Charon influence the
location of the equatorial ice cap?

[Gennady Ionov]

Quick question- just how intense IS Charonshine? Enough to make a 1 or 2 kelvin difference?

Charon angular radius is about 600 km / 20000 km = 0.03, its square relative sky hemisphere equal pi*0.03*0.03 / (2*pi)=0.00045, and albedo 0.35, and avarage phase is 0.5.
So Charonshine irradiation only 0.00045*0.35*0.5=8e-5 from Solar irradiation. But Solar irradiation we must divide by 2 due to night time.
In total its make relative temperature difference about 0.00008*2*4=0.00064 (factor 4 from derivative of Stephan-Bolzman low).
In absolute value it is about 0.00064*40K=0.0256 K

[HSchirmer]

Quick question- just how intense IS Charonshine? Enough to make a 1 or 2 kelvin difference?

...
In total its make relative temperature difference about 0.00008*2*4=0.00064 ...
In absolute value it is about 0.00064*40K=0.0256 K

Just occurred to me that tiny temperature differences on Pluto that might be enought to drive weather
Those amazing terminator images may have caught "sideways thunderstorms" convecting heat from the days side to the night side.

First bizzare concept - Pluto's N2 ices and N2 atmosphere create a planet-wide equal temperature/pressure bath.
Think of a giant pitcher of ice water, melting or freezing water in a pitcher of ice water does not change the system temperature until all the water is in one phase. N2 ice and N2 atmosphere should be a solid-fluid constant pressure/temperature system.
I've read papers that say Pluto's N2 atmospher and N2 ices should form a contant temperature system, or a constant pressure system , but that idea just sunk in- you literally can not heat up local patches of N2 ice on pluto, it just moves somewhere else, so that all N2 ice on Pluto should be at the same temperature. Instead, you end up heaing up or cooling down ALL surface N2 ice of Pluto. It's sort of a thermodynamic verision of "sea level" on earth.

Second bizzare concept- Tiny variations in heating/sublimation might drive amazing storms in Pluto's atmosphere.
On Earth, a 1% difference in atmospheric pressure creates a high or low pressure system. A 5% difference is a hurricane, a 10% difference is a tornado. Estimates for Pluto are a 1K temp increase doubles the N2 vapor pressure. Wow, a 1K temperature increase creates a 100% pressure difference. So, something as tiny as a .025 K increase in temperature due to relflected light from Charon, might create a 2.5% atmospheric pressure difference- that percentage difference on earth is sufficient to drive a tropical storm.