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Temperature Inversion on Pluto

With an atmospheric pressure one hundred thousand times less than that on Earth, Pluto becomes an even more intriguing object than usual when it moves closer to the Sun in its 248-year orbit. This period, occurring now, causes the temperature of the surface to increase, and that causes what had been frozen nitrogen (with trace amounts of methane and, probably, carbon dioxide) to sublimate into gas. Studying these matters with ESO’s Very Large Telescope, astronomers have now found unexpectedly large amounts of methane in that atmosphere.


Image: Artist’s impression of how the surface of Pluto might look, according to one of the two models that a team of astronomers has developed to account for the observed properties of Pluto’s atmosphere. The image shows patches of pure methane on the surface. At the distance of Pluto, the Sun appears about 1000 times fainter than on Earth. Credit: ESO/L Calçada.

A second discovery: The atmosphere of the distant ice world is some forty degrees Celsius hotter than the surface, so that the global weather on Pluto seems to be characterized by a thermal inversion, with warm air sitting above cold (this is the kind of pesky weather situation on Earth that can trap smog and sharply reduce visibility). Sublimation seems to be the cause of this inversion. Think of winter snow evaporating even when it never moves through a liquid state, or comets approaching the Sun, their coma and tails forming through the effects of sublimating ice.

Warming the atmosphere, sublimation in turn cools the surface. The model for all this draws on spectrographic observations that show methane represents about half of one percent of the atmosphere. The team, led by Emmanuel Lellouch, believes that two different models can explain these findings at Pluto, one involving a thin layer of methane at the surface, the other invoking distinct areas of pure methane. From the paper:

Two scenarios… have been described to explain this elevated methane abundance (i) the formation, through surface-atmosphere exchanges, of a thin methane-rich surface layer (the so-called “detailed balancing” layer), which inhibits the sublimation of the underlying, dominantly N2, frost, and leads to an atmosphere with the same composition as this frost (ii) the existence of geographically separated patches of pure methane, warmer than nitrogen-rich regions, and which under sublimation boost the atmospheric methane content.

New Horizons should be able to tell us more when it reaches Pluto/Charon in 2015. Until then, I’m inclined to agree with Hans-Ulrich Käufl, a co-author of the paper on this work, as he describes the team’s use of the CRIRES spectrograph on the VLT: “It is fascinating to think that with CRIRES we are able to precisely measure traces of a gas in an atmosphere 100,000 times more tenuous than the Earth’s, on an object five times smaller than our planet and located at the edge of the Solar System.” Indeed.

The paper is “Pluto’s lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations,” in press at Astronomy & Astrophysics (abstract).

Comments on this entry are closed.

  • James M. Essig March 3, 2009, 14:17

    Hi Paul;

    The above artist impression is beautiful. When humans one day set foot on pluto, the images will be the stuff of story book dreams.

    Learning all we can about pluto is important, since methane which appears to be a major component of Pluto’s makeup, is a hydrogenic gas and as a rtesult, Pluto is potentially a great source for fuelling hydrogen fusion space craft especially given it small gravity well.



  • george scaglione March 3, 2009, 15:34

    jim you are not wrong the views seen upon our setting foot on pluto will indeed be dreamlike.and…refueling on pluto – !! good idea!! funny i just saw something about the kuiper belt this morning also makes this article all the more timely.i eagerly await new horizons revealing more to us about pluto and the belt! not impossible that many of us here will see mankind arrive on pluto.also i’d like to adress a remark to everybody who stops by here also…does not our administrator work overtime to bring us some great ideas to conjure with!!? cool stuff! i’ll drop by again soon.thanks jim thanks paul -thanks everybody else too i appreciate being part of this group and having the chance to read soooo many good ideas! your a bunch of geniuses as far as i can tell. your friend george

  • Didac March 3, 2009, 16:55

    Moreover, the perspectives of future Pluto bases are also valid for other Kuiper Belt Objects. Perhaps there will be a place to build future manned interstellar probes.

  • Procyan March 3, 2009, 22:02

    I know this discussion is about KBO’s but it triggered this thought…If our system’s Oort cloud has a diameter on the order of a light year, and alpha centauri likewise, then these bodies could be in gravitational contact. If an extra solar comet were sufficiently disturbed to fall sunward from alpha C, would we be able to somehow deduce its origin? Or at least know that it is not one of ours? Perhaps, somewhat as tektites are to nearby planets, certain special comets are a poor civilization’s first opportunity to sample material from nearby solar systems?

  • Administrator March 4, 2009, 9:12

    Procyan, you might find this interesting re your question on interstellar comets:

    “One comet with a radically modified orbit is Periodic Comet Machholz 1 (also known as 96P/Machholz 1), discovered by amateur comet hunter Don Machholz in 1986 using 29×130 binoculars. It has an orbital period of 5.3 years and a high inclination with respect to the plane of the solar system. A new study finds that its chemical makeup is unique among 150 comets analyzed to date. It must have had a very unusual origin and might even have formed in another solar system where things were quite different than here.

    However, dynamical studies have found that the chance of the solar system capturing and holding a passing interstellar comet is very slim.”

    This is drawn from a Sky & Telescope story on this topic:


    See also this Centauri Dreams entry from last December (look at the third entry down):


  • ljk March 4, 2009, 11:06

    While that is a very nice bit of artwork depicting Pluto, I was
    under the impression that things were a lot darker out there
    than this art is implying.

    When Voyager 2 flew by Neptune in 1989, scientists said the
    light levels at that distance from Sol were equivalent to twilight
    on Earth.

    And as for objects in our Sol system having originated from
    far, far away:


  • Ron S March 4, 2009, 14:09

    I remember some time back noting that the purported outer radius of the Oort cloud is approximately half the distance to Alpha Centauri. I imagine this is a hypothesis only since I don’t believe there’s supporting observational data. Even so, it makes sense to me. It also implies that exchange of Oort cloud material with other stellar systems is routine over the life of the solar system. My thinking is as follows.

    Draw a sphere of equal-gravitational potential around the sun and another, with the identical potential, for a passing star (like A Cen). Choose the potential such that the spheres intersect. Then have a body orbit on the surface of one of those spheres.

    When that body passes the intersection point with the other sphere it is now drawn more towards the second star. It won’t necessarily go into orbit since its velocity could well put it on a hyperbolic path. This is much like crossing a high point of land into a different watershed. The body on falling toward the other star (and almost certainly oblique to the plane of its native planets and other stuff) could become a comet or simply fly off into the distance.

    Since the orbital velocity out 2 ly from the sun is (much?) lower than the velocity of the passing stellar system, only a small portion of the Oort cloud would be affected by such an encounter, although there could be many similar encounters over the eons. Therefore we don’t deplete our Oort cloud but we do exchange material with other systems. Just as we must now be doing with the A Cen system.

    No, I haven’t studied this so I could be making some really awful errors in this scenario.

  • george scaglione March 4, 2009, 15:03

    didac,re the above comment i can only say – YES!!!!101% correct!!! thank you very much, your friend george and procyan that too is a good idea.thank you g paul,thank you too for what you added on interesting comets above after procyans comment. ljk maybe you are right about the level of light but still the picture fires the imagination! well every body i really shouldn’t take the shortcut of answering three or four people in one paragraph,but this time the device just seemed to lend itself.respectfully to all your friend george

  • Adam March 5, 2009, 19:22

    Hi Ron S

    The main data for the Oort Cloud is the distribution of orbital periods of long period comets. When allowances are made for perturbations from Jupiter the comets all cluster around a semi-major axis of 22,000 AU for their orbits. Since visible comets have to be perturbed sunwards that means an original aphelion of about 44,000 AU. So the distance of the comet cloud is 2/3 of a light year. Oort’s old data gave an aphelion of about twice that – 100,000 AU – which is a bit more than 1/3 of Alpha Centauri’s 275,000 AU. Galactic tides are probably the main perturber and an inner Oort Cloud is probably needed to resupply comets lost to stellar near-misses and Giant Molecular Cloud flybys. The inner Cloud is more like a fat Disk and extends from ~ 1,000-10,000 AU. Alan Stern suggests that a few Mars-to-Earth mass proto-planets might’ve been perturbed into the Inner Cloud by the formation of the giant planets, so there could be quite a few worlds yet to explore in the extreme Outer System. A thousand Pluto/Eris scale worlds might be out there, which would give ambitious Orboformers a lot of mass to play with.

  • ljk March 5, 2009, 22:25

    Still a planet in Illinois

    State senators uphold the honour of Pluto-discovering Illinoisian…


  • ljk March 19, 2009, 13:49

    Pluto and “Mickey” from the 200-inch Hale Telescope in 2006:


  • ljk June 24, 2009, 15:51

    Buoyancy waves in Pluto’s high atmosphere: Implications for stellar occultations

    Authors: W. B. Hubbard (1), D. W. McCarthy (2), C. A. Kulesa (2), S. D. Benecchi (3), M. J. Person (4), J. L. Elliot (4,5), A. A. S. Gulbis (6) ((1) LPL, U of Ariz., (2) Steward Obs., U of Ariz., (3) Space Tel. Sci. Inst., (4) EAPS, MIT, (5) Physics, MIT, (6) S. African Astron. Obs.)

    (Submitted on 22 Jun 2009)

    Abstract: We apply scintillation theory to stellar signal fluctuations in the high-resolution, high signal/noise, dual-wavelength data from the MMT observation of the 2007 March 18 occultation of P445.3 by Pluto.

    A well-defined high wavenumber cutoff in the fluctuations is consistent with viscous-thermal dissipation of buoyancy waves (internal gravity waves) in Pluto’s high atmosphere, and provides strong evidence that the underlying density fluctuations are governed by the gravity-wave dispersion relation.

    Comments: Accepted 18 June 2009 for publication in Icarus

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0906.4141v1 [astro-ph.EP]

    Submission history

    From: William Hubbard [view email]

    [v1] Mon, 22 Jun 2009 22:27:35 GMT (777kb)


  • ljk September 4, 2009, 10:19

    Review: Pluto Confidential

    The recent IAU General Assembly has come and gone without any changes in the definition of “planet” or Pluto’s classification. Jeff Foust reviews a new book that takes yet another look at the controversy surrounding Pluto’s status and how it compares to previous planetary controversies.

    Monday, August 31, 2009


  • ljk November 17, 2009, 21:23

    Review: The Case for Pluto

    More than three years after Pluto was “demoted” from planetary status, the decision remains controversial. Jeff Foust reviews the latest book to examine the debate and how Pluto should fit into the solar system’s pantheon of worlds.