Is Pluto warming up? Options

[ups]

With any luck the atmosphere will continue to warm and thicken as New Horizons makes its pass.

Thoughts?
_______

Pluto thought to be warming up

Astronomers at the University of Tasmania have found that the solar system's smallest planet is not getting colder as first thought and it probably does not have rings.

Dr John Greenhill has collected observations from last month's event when Pluto passed in front of a bright star, making it easier to study.

French scientists have shared the measurements they took in Tasmania that night, which indicate that the planet is unlikely to have rings.

Dr Greenhill says the results are surprising because they show Pluto is warming up.

"It looks as though the atmosphere has not changed from 2002, which is pretty surprising because we expected the atmosphere would freeze out as the planet moved further away from the Sun," he said.

"But so far, if anything, the atmosphere has gotten even denser."

abc

[Guest_AlexBlackwell_*]

With any luck the atmosphere will continue to warm and thicken as New Horizons makes its pass.

Thoughts?

Thanks, ups. This is interesting, though I'd be interested in seeing exactly what Greenhill et al. are claiming and, moreover, what they manage to get through peer review.

That said, it's probably good that this press release didn't come out before New Horizons was launched.

[Guest_Richard Trigaux_*]

Ah, global warming extending to Pluto, even Greepeace did not expected that


The curious thing is that the atmosphere seems denser when they expected it would become lighter, from increasing cold. This is perhaps from some cryovolcanic activity. We can imagine a surface layer of frozen volatiles, which would warm up from bottom by geothermal heating. At moments, deep pockets of gasses would erupt to the surface, thickening the atmosphere. And then, they would freeze again, forming surface layers. So the overal frozen gas layer would slowly turn over itself.

[Big_Gazza]

Ah, global warming extending to Pluto, even Greepeace did not expected that
The curious thing is that the atmosphere seems denser when they expected it would become lighter, from increasing cold. This is perhaps from some cryovolcanic activity. We can imagine a surface layer of frozen volatiles, which would warm up from bottom by geothermal heating. At moments, deep pockets of gasses would erupt to the surface, thickening the atmosphere. And then, they would freeze again, forming surface layers. So the overal frozen gas layer would slowly turn over itself.

Geothermal heating? In a small icy body 40AU from the sun? Unlike Europa, Enceladus or Triton, Pluto is not in orbit around a large parent capable of invoking tidal forces, and any heat from radioactive decay in its rocky core would be insufficient to drive any appreciable activity. Charon no doubt contributes some tidal heating, but it would surely be negligible.

Having said that, the HST images show striking albedo variations, so clearly the surface demonstrates variety. I expect that we will see evidence of past geological activity, but long since ceased, and of course the expected seasonal frosts waxing & waning with orbital distance.

Can't wait for NH to arrive!! Lets hope Pluto is more than a distant cousin of boring Rhea! That would be a cause for much grief and a gnashing of teeth

[Alan Stern]

I'll still put my money on it being a simple case of thermal inertia, i.e., a phase delay between
perihelion and the warmest day. Almost 20 years ago we modelled this effect in a paper
led by Larry Trafton and predcted phase lags of up to 17 years after perihelion (1989.7)
before the cooling begins. The situation is rather anlagous to the reason the hottest days of
summer are some weeks after the summer solstice, but Pluto's 248 year orbital timescale stretches
everything out from weeks to decades.

[Phil Stooke]

You beat me to it, Alan, but that was my feeling as well.

Phil

[djellison]

Just out of interest...I set up a spreadsheet..

All units are just random really - I set up a body with a thermal capacity which was added to at a rate that followed the inverse square of an orbit from 90 to 110 random units..but that energy was lost at a constant rate

The attached shows the 'range' to the sun in blue, and the 'temperature' of the body in pink - and it lags behind the range by quite a bit. I probably screwed up the maths somewhere, but it showed that 'thermal lag' of an object behind the seasonal temperature.

Attached File(s)

 range_thermal_lag.pdf ( 119.67K ) Number of downloads: 357

 

[Rob Pinnegar]

Charon no doubt contributes some tidal heating, but it would surely be negligible.

Actually, it wouldn't. Pluto and Charon both keep the same face towards the other all of the time, so there wouldn't be any tides in the system at all. With nothing to force Charon's orbit, it would circularize and then stay like that for eons.

On second thought, I've forgotten for the moment about Nyx and Hydra. They'd contribute some tidal action, partly from their own gravity acting on Pluto, and partly from keeping Charon's orbit a bit elliptical (assuming they are in resonance with Charon). But geez, it couldn't amount to much.

As for thermal inertia, methane's a pretty strong greenhouse gas, isn't it?

[Guest_AlexBlackwell_*]

I'll still put my money on it being a simple case of thermal inertia, i.e., a phase delay between
perihelion and the warmest day. Almost 20 years ago we modelled this effect in a paper
led by Larry Trafton and predcted phase lags of up to 17 years after perihelion (1989.7)
before the cooling begins. The situation is rather anlagous to the reason the hottest days of
summer are some weeks after the summer solstice, but Pluto's 248 year orbital timescale stretches
everything out from weeks to decades.

Alan, I'm not sure if you were aboard at the time but other Yahoo! planetary_sciences alumni may recall a rather long thread "The Plutonian Atmospheric Collapse Debate" from 2002, which I started at message #3960. I'll quote that post below verbatim and without any editing, which means I'll probably be embarrassed Oh, and don't assume the four-year old links still work.

Well, I just had the chance to re-read:

Emissivity and the Fate of Pluto's Atmosphere
J. A. Stansberry and R. V. Yelle
Icarus Vol. 141, No. 2, October 1, 1999, pp. 299-306
(doi:10.1006/icar.1999.6169)
Abstract http://www.idealibrary.com/links/doi/10.1006/icar.1999.6169

which was offered as support for the recent AAS/DPS press release,
which, while endorsing New Horizons, downplayed the significance of
the criticality of meeting the 2006 JGA launch window in order to
arrive before Pluto's predicted atmospheric collapse. As Bruce noted
in his recent SpaceDaily piece, the DPS's position in this regard is
a minority one; the Pluto SDT and the planetary sciences community at
large have identified Plutonian atmospheric studies *before* collapse
as a Group 1 science objective (i.e., one that is integral to the
mission's justification). However, Stansberry and Yelle [1999] offer
a different view of the predicted Plutonian atmospheric collapse, one
that is at odds with the prevailing one. As the abstract to their
paper indicates, their model is based on "... the potential
importance of the solid-state phase transition between [alpha-
nitrogen] a-N2 and [beta-nitrogen] B-N2 ... [and] shows that under
simplified but not unreasonable assumptions Pluto may have nearly the
same atmospheric pressure at aphelion as it does now, near
perihelion." In other words, no freezing out. At the outset,
Stansberry and Yelle's model assumes that the nitrogen ice on Pluto's
surface has a globally uniform temperature (i.e., is isothermal),
which they label T(N2), or at least does not vary "within a small
fraction of a Kelvin." They base this assumption on the efficiency
in common volatile/energy redistribution processes (e.g.,
sublimation, condensation, atmospheric transport, etc.), which the
presume does not alter the overall energy balance. To this end, they
utilize theoretical models and laboratory studies of the nitrogen a-B
phase transition, which occurs at a temperature (TaB) of 35.6 K, and
note the "sudden change" (or contrast) observed in bolometric
emissivity (thermal reradiation) at TaB. Those who have read the
paper may note that the Stansberry and Yelle [1999] borrows from and
extends the results of an earlier paper in Planetary and Space
Science:

Stansberry, J.A., D.J. Pisano, and R.V. Yelle
The emissivity of volatile ices on Triton and Pluto
Planet. Space Sci. 44, 945-955 (1996).
Abstract
http://www.elsevier.com/gej-ng/10/37/40/36...9/abstract.html

With that said, Stansberry and Yelle [1999] essentially argue that
even as Pluto recedes towards aphelion (i.e., as insolation per unit
area decreases) the equilibrium temperature of the nitrogen surface
ice-nitrogen vapor atmosphere (Teq), which they treat as a single
system, adjusts itself itself to Tab and remains constant. While
this system is treated singly, however, their model assumes the
presence of both phases of solid nitrogen (i.e., a single mixed-
phase) on Pluto's surface. Due to the emissivity contrast between
the two phases at Tab, the resulting latent heat fluxes (from
adsorption and release) tend to move the individual Teq for both
phases (either of which may be enriched locally) towards Tab. Since
Tab (35.6 K) is above the temperature at which the global
distribution of nitrogen ice is predicted to become non-isothermal
(31 K from Spencer et al., 1997 in "Pluto and Charon"), this keeps
the atmosphere from developing pressure (and temperature) gradients.
Stansberry and Yelle state that as long as T(N2)=Tab at aphelion
remains above 31 K, the predicted atmospheric pressure "will always
be larger than 1 ubar, and ... the atmosphere will remain in its
hydrostatic state."

It is an intriguing argument, though admittedly based on a couple of
assumptions: (1) the volatile tranport mechanisms on Pluto are
efficient enough in redistributing energy that imbalances (gradients)
do not arise; (2) though their paper is nominally directed towards
emissivities, as a practical matter (due to the difficulty in working
with and measuring the emissivity of a sufficiently large quantity of
nitrogen ice) their work has to rely on the simpler process of
measuring adorption coefficients combined with radiative transfer
theory.

That said, I believe this a slender reed for the DPS to hang a
cautionary recommendation of launching New Horizons by 2006. Indeed,
even Stansberry and Yelle note in their Icarus paper that other
published models (including one of their own) have different outcomes.

[hendric]

Also, Pluto's albedo isn't uniform, so is it possible that as it heads away from perihelion darker parts of the planet are preferred and keeping it warmer longer?

Also, I was wondering if the light/dark albedos could be a self-reinforcing effect? IE, the dark units stay warmer longer, preventing the atmosphere from condensing on them and lightening them up?

[JRehling]

Also, Pluto's albedo isn't uniform, so is it possible that as it heads away from perihelion darker parts of the planet are preferred and keeping it warmer longer?

Could be, although roughly speaking, Pluto has bright poles and a dark equator. You'd think that the equinox would provide maximum exposure of the dark areas, and that progressing towards a solstice would increase the explosure of bright, reflective pole surface. On the third hand ( ), that may defrost a pole out, increase the greenhouse effect, and/or decrease the albedo.

This is a complex dynamical system, and the increase in thickness now makes me a little nervous that there could be a sudden collapse before NH gets there. The more uphills you encounter before reaching your destination, the more downhills are waiting for you in the successive portions of your voyage. Of course, it may be a simple phase shift with the thermal seasons lagging behind the solar seasons.

Just to detail the Earth's continental climates as an example, St. Louis has a maximum average high sometime around July 22 (yes, this week) and low around January 12. Those average to a lag of about 25 days behind the solstice, or 6.8% of the year. The same proportion on Pluto (and there's no reason why it would be the same) would be 17 years. With an equinox at 1988, that would correspond to 2005, and lo, that's the same number Alan mentions and gibes with the sparse data showing that it's warmer in 2006 than 2002. If everything were symmetric, NH's arrival would be isothermal with 1995, and there was no freeze-out then.

[Guest_Richard Trigaux_*]

JRehling, Doug and Alan Stern, thermal inertia is a function of time, not of phase. And I would be very surprised that the small Pluto has a 17 year effect, when in is counted in days on Earth. In more, most of Earth's inertia is caused by the oceans, a very efficient retarder that Pluto don't have (nothing can remain liquid on Pluto surface). If Earth had a 248 years orbit, everything else the same, the lag would still be about 22 days, very far from 17 years.

So we cannot make a proportion in a matter of a fraction of the year (phase angle) but in absolute duration. So if we assume Pluto has the same inertia than earth, it is still 22 day, not 17 years. And even if some process would give a much larger inertia to Pluto than on Earth, 17 years make about four orders of magnitude more than Earth's ocean, I realy wonder what would store that much heat on Pluto than our oceans.

hendric, I think your idea of self reinforcing effect is interesting: dark places would be hotter, thus avoiding the condensation of gasses, while clear places would remain cold.

Eventually the evaporation of gasses could happen by bursts, suddenly cleaning a place from gasses and explaining surges in atmospheric pressure.

Big_Gazza, I still hold to the idea of geothermal heating. Of course Pluto is not Io, and even not Enceladus. But only a very small amount of geothermal heating would be enough to evaporate some frozen gasses in a pocket, in a time scale of centuries. It is much like geothermal melting of Antarctic ice cap, from the bottom, which is very weak, but on all the surface it is enough to form rivers. A geothermal flux as weak as we can imagine (due to a remnant or radioactivity, or tidal heating by the satellites) could, by accumulation over centuries or millenia, evaporate significant amounts of gasses, which could erupt violently, at times. The weaker the geothermal flux, the less eruptions, but the geothermal flux cannot be just zero.

[djellison]

Why do you suggest it wouldn't scale to be a longer duration effect with a longer duration orbit?

Doug

[ermar]

So we cannot make a proportion in a matter of a fraction of the year (phase angle) but in absolute duration. So if we assume Pluto has the same inertia than earth, it is still 22 day, not 17 years.

This doesn't make sense to me. If, for the sake of argument, the Earth were in a 44-day orbit, would the warmest day fall at the winter solstice 22 days later? Further, I think not much thermal inertia may be required to account for the Plutonian warming - with temperature differentials between seasons smaller and seasons longer than on Earth, a lesser thermal inertia could account for the effect. Also, nitrogen sublimation into the atmosphere is a cooling effect around perihelion... perhaps this cooling weakens as more polar nitrogen ices are consumed, helping to account for the warming?

[The Messenger]

The heat capacity of water - the oceans - is much greater than that of liquid nitrogen - also, the heat of evaporation of water is much higher than almost anything else. So as Richard has argued, it is difficult to see how the thermal inertia at pluto could create a significant lag between the heating and cooling cycle.

One other thought on the subject: In the dry artic air of Fairbanks Alaska, the mean temperature peaks earlier in July than in more temperate regions.

http://www.met.utah.edu/jhorel/html/wx/climate/maxtemp.html