Pluto's Expanding Atmosphere Confounds Researchers, Pluto Atomosphere Options

[bagelverse]

Pluto's Expanding Atmosphere Confounds Researchers

http://news.sciencemag.org/sciencenow/2011...onf.html?ref=hp

Could these be evidence of geyers like on Triton?

[Drkskywxlt]

Pluto's atmosphere is (almost certainly) hydrodynamically escaping. Basically, the solar wind blows the atmosphere off the planet. So, that leads to a very large height for the exobase of the atmosphere (several Pluto radii potentially). That coupled with the seasonal cycle when the atmosphere is thicker anyway, and you get a very extended, low density atmosphere.

[Rob Pinnegar]

Pluto's atmosphere is (almost certainly) hydrodynamically escaping.

If gas escape is occurring, would the region of greatest gas escape tend to concentrate around the anti-Charon point?

With such a huge scale height to the atmosphere, centripetal acceleration (due to Pluto's motion around the system's barycentre) might be just enough to give gas molecules the heave-ho on the anti-Charon side.

[Drkskywxlt]

would the region of greatest gas escape tend to concentrate around the anti-Charon point?

Hmmm...not sure the answer to that. Charon is about 12 Pluto radii away. All of the literature that I've seen regarding models/simulations of hydrodynamic escape of Pluto's atmosphere have ignored contributions from Charon's gravity. Remember the r^2 law...even if gas is escaping at several Pluto radii above the surface, the gravitational influence from Pluto is still going to be dramatically larger than that of Charon.

With such a huge scale height to the atmosphere, centripetal acceleration (due to Pluto's motion around the system's barycentre) might be just enough to give gas molecules the heave-ho on the anti-Charon side.

Again I haven't seen any discussion of this in the literature. Pluto's rotation rate is quite slow so I don't think this would add appreciable energy. The energy that really drives this process is heating by solar EUV and UV. That's a much bigger energy source than centripetal acceleration.

[Rob Pinnegar]

Again I haven't seen any discussion of this in the literature. Pluto's rotation rate is quite slow so I don't think this would add appreciable energy. The energy that really drives this process is heating by solar EUV and UV. That's a much bigger energy source than centripetal acceleration.

That's probably true. I just did a back-of-the-envelope calculation to that effect (literally... it actually *was* on the back of an envelope).

Pluto's orbital speed around the system barycentre turns out to be only about 23 metres per second (a bit slower than normal highway driving speed). The centripetal acceleration from that amounts to around 2-3 cm/s^2. This is quite a bit less than Pluto's surface gravity, which is about 66 cm/s^2.

Of course, at really high altitudes the gravity would be a bit lower. Also, I've based my centripetal-acceleration calculation on the distance from the system barycentre to Pluto's centre... so that effect would be a tiny bit larger as well at high altitudes. But either way, we're talking a difference of a few percent.

I don't know enough about this topic to say whether a difference of a few percent would translate into a measurable difference in gas escape.