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ngunn
Interesting news item:
http://www.spacedaily.com/reports/A_Summer...Triton_999.html
pjam
Thanks for posting the article link!
...There's also a recent UMSF discussion on Triton's atmosphere and aerobraking where this 40 year seasonal variability in atm pressure might play into plans for aerobraking a spacecraft!
I wonder what the mechanism is for seasonal growth of atmosphere -is it mostly from sublimation or is there a major role for the geysers?
-pjam
Explorer1
QUOTE (pjam @ Apr 18 2010, 05:04 PM) *
I wonder what the mechanism is for seasonal growth of atmosphere -is it mostly from sublimation or is there a major role for the geysers?


If it's anything like Enceladus (which is a stretch I admit), then their activity would effect the density of the atmosphere.
Of course, there are no other large moons in the Neptune system to to cause big tides, and there's almost no eccentricity either.
Drkskywxlt
QUOTE (pjam @ Apr 18 2010, 08:04 PM) *
I wonder what the mechanism is for seasonal growth of atmosphere -is it mostly from sublimation or is there a major role for the geysers?
-pjam


From the article:
QUOTE
As Triton's southern hemisphere warms up, a thin layer of frozen nitrogen, methane, and carbon monoxide on Triton's surface sublimates into gas, thickening the icy atmosphere as the season progresses during Neptune's 165-year orbit around the Sun. A season on Triton lasts a little over 40 years, and Triton passed the southern summer solstice in 2000.

Based on the amount of gas measured, Lellouch and his colleagues estimate that Triton's atmospheric pressure may have risen by a factor of four compared to the measurements made by Voyager 2 in 1989, when it was still spring on the giant moon.


I can't imagine what mechanism would make the geysers seasonal on their own. More likely they're due to internal processes.
Rob Pinnegar
QUOTE (Drkskywxlt @ Apr 19 2010, 05:21 PM) *
I can't imagine what mechanism would make the geysers seasonal on their own. More likely they're due to internal processes.


That's certainly possible; however, I seem to remember reading at one time that they could also be due to solar heating.

It's been a long time since then, but I think the basic idea was that some sunlight could be getting through the top layer of ice, and warming the nitrogen beneath it. Phil, do you remember this one?
john_s
That's right- in that case they would be quite similar to the "arachnoid" features seen on the subliming Martian polar cap.

The problem is that we have almost no data on the Triton plumes- just a handful of very low resolution images and some poorly-constrained models. My money is on the plumes being essentially meteorological (in the broad sense of including seasonal frost phenomena) rather than geological. I'm not yet ready to include Triton with Earth, Io, and Enceladus in the club of worlds proven to be currently geologically active.

John
Phil Stooke
"the basic idea was that some sunlight could be getting through the top layer of ice, and warming the nitrogen beneath it. Phil, do you remember this one?"

Yes, Rob - it was called a solid-state greenhouse, I think, though that term has been used for other places like Europa as well. I agree with John that it's premature to call Triton geologically active, though it's clearly been active in the not too distant past. It looks quite a bit older than Europa, but not as old as Ganymede (recognizing the limitations of our imaging coverage). But it's quite active in an atmosphere-surface-solar heating sense - there of course it's far ahead of Europa. Winds, plumes, frosts etc. Truly a fascinating place. Gotta go back!

Phil
tedstryk
QUOTE (Phil Stooke @ Apr 23 2010, 07:09 PM) *
I agree with John that it's premature to call Triton geologically active

Phil


Well, it does strain the category. It can definitely be said that it is premature to say that it has endogenically driven geologic activity. However, plumes shooting up out of the ground, even if exogenically powered, doesn't really fit into the category that we normally think of when we think of atmospheric activity either. To put it another way, Triton definitely seems to have some active geologic processes, but these processes may be exogenically powered. The best analog might be a comet.
nprev
Interesting distinctions emerging: geologically "exoactive" vs. "endoactive" worlds. Io is manifestly exoactive, so when discussing Triton aren't we talking about differences in magnitude of activity rather than anything else?

Seems as if endoactivity requires a body of a certain size. Earth & probably Venus are currently endoactive, Mars once was, might still be at very low levels. Io, Enceladus & Triton are exoactive, maybe Pluto will prove to be as well (solar heating + tidal effects from Charon?); Titan might be either a borderline or hybrid case.
ngunn
I agree these categories trigger interesting trains of thought. I suspect though the primary distinction for most planetary scientists would be between activity arising from an internal heat source, however generated, and processes driven by sunlight. True, tidal heating comes from outside, but in the widest sense so does the radioactivity from heavy atoms produced in a supernova. So I think there are two separate questions to ask of any candidate world:

When and how did/do you acquire the energy that drives your active processes?
What materials are involved in the activity and over what range of temperatures?

john_s
If we're lucky, New Horizons will find analogous plumes on Pluto and will give us much better data on them than Voyager could do at Triton. Though it's more likely that Pluto will come up with its own quite different but equally bizarre phenomena to amaze us.

John
nprev
Yeah...if there's anything like a consistent truism in planetary exploration it's that if we expect A, admit that B is possible, what we actually find is X... smile.gif
john_s
And then of course we find A somewhere else entirely. It's been said that all hypothesized solar system phenomena turn out to be real, just not necessarily on the worlds where they were first proposed. Thus we find Percival Lowell's canals on Europa, not Mars; Transient Lunar Phenomena on Io, not our own moon; lakes of exotic liquid on Titan, not Triton; high-albedo volcanic plains on Mercury, not our Moon, and so on.

John
ElkGroveDan
We'll probably discover the Sirens on Ceres then.
Bill Harris
Who? wink.gif

--Bill
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