Pluto’s Atmosphere Boasts Methane, Warmer Temps

Article written: 2 Mar , 2009
Updated: 24 Dec , 2015
by

 

Artist’s impression of how the surface of Pluto might look, if patches of pure methane rest on the surface. At the distance of Pluto, the Sun appears about 1,000 times fainter than on Earth. Credit: ESO

 

Pluto is certainly frigid, but new research has revealed its atmosphere is a bit warmer.

Astronomers using the European Southern Observatory’s Very Large Telescope have found unexpectedly large amounts of methane in Pluto’s atmosphere, which evidently helps it stay about 40 degrees warmer than the dwarf planet’s surface. The atmosphere warms to -180 degrees Celsius (-356 degrees Fahrenheit), compared to a surface that’s usually -220 degrees Celsius (-428 degrees Fahrenheit).

“With lots of methane in the atmosphere, it becomes clear why Pluto’s atmosphere is so warm,” said Emmanuel Lellouch of the Observatoire de Paris in France. Lellouch is lead author of the paper reporting the results, which is in press at the journal Astronomy and Astrophysics.

Pluto, which is about a fifth the size of Earth, is composed primarily of rock and ice and orbits about 40 times further from the Sun than the Earth.

It has been known since the 1980s that Pluto also has a thin, tenuous atmosphere. Abundant nitrogen, along with traces of methane and probably carbon monoxide, are held to the surface by an atmospheric pressure only about one hundred thousandth of that on Earth, or about 0.015 millibars. As Pluto moves away from the Sun, during its 248 year-long orbit, its atmosphere gradually freezes and falls to the ground. In periods when it is closer to the Sun — as it is now — the temperature of Pluto’s solid surface increases, causing the ice to sublimate into gas.

Until recently, only the upper parts of the atmosphere of Pluto could be studied. By observing stellar occultations, a phenomenon that occurs when a Solar System body blocks the light from a background star, astronomers were able to demonstrate that Pluto’s upper atmosphere was some 50 degrees warmer than the surface. Those observations couldn’t shed any light on the atmospheric temperature and pressure near Pluto’s surface. But unique, new observations made with the CRyogenic InfraRed Echelle Spectrograph (CRIRES), attached to ESO’s Very Large Telescope, have now revealed that the atmosphere as a whole, not just the upper atmosphere, has a mean temperature much less frigid than the surface.

Usually, air near the surface of the Earth is warmer than the air above it, largely because the atmosphere is heated from below as solar radiation warms the Earth’s surface, which, in turn, warms the layer of the atmosphere directly above it. Under certain conditions, this situation is inverted so that the air is colder near the surface of the Earth. Meteorologists call this an inversion layer, and it can cause smog build-up.

Most, if not all, of Pluto’s atmosphere is thus undergoing a temperature inversion: the temperature is higher, the higher in the atmosphere you look. The change is about 3 to 15 degrees per kilometer (.62 miles). On Earth, under normal circumstances, the temperature decreases through the atmosphere by about 6 degrees per kilometer.

The reason why Pluto’s surface is so cold is linked to the existence of Pluto’s atmosphere, and is due to the sublimation of the surface ice; much like sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto.

The CRIRES observations also indicate that methane is the second most common gas in Pluto’s atmosphere, representing half a percent of the molecules. “We were able to show that these quantities of methane play a crucial role in the heating processes in the atmosphere and can explain the elevated atmospheric temperature,” said Lellouch.

Two different models can explain the properties of Pluto’s atmosphere. In the first, the astronomers assume that Pluto’s surface is covered with a thin layer of methane, which will inhibit the sublimation of the nitrogen frost. The second scenario invokes the existence of pure methane patches on the surface.

“Discriminating between the two will require further study of Pluto as it moves away from the Sun,” says Lellouch. “And of course, NASA’s New Horizons space probe will also provide us with more clues when it reaches the dwarf planet in 2015.”

LEAD IMAGE CAPTION: Artist’s impression of how the surface of Pluto might look, if patches of pure methane rest on the surface. At the distance of Pluto, the Sun appears about 1,000 times fainter than on Earth. Credit: ESO

Source: ESO


25 Responses

  1. Jorge says

    On Earth, under normal circumstances, the temperature decreases through the atmosphere by about 6 degrees per kilometer.

    Only in the troposphere. The temperature profile is more complicated than that above 10 km altitude. In fact, in the thermosphere, the temperature skyrockets, if you forgive me for the bad pun.

    There’s one thing I don’t get. Knowing that the temperature is a measure of the average kinetic energy of some material’s constituent particles. wouldn’t it be expected that, in the absence of surface irradiation of heat and/or layers opaque to infrared radiation, the higher you get the higher the temperature, since those molecules found there would be the ones fast enough to get that high?

  2. @LouisS says

    I think they’ll have to re-think this whole “Pluto is not a planet” thing now. 😉

  3. Nephish777 says

    “The change is about 3 to 15 degrees per kilometer (about 3.3 feet). ” There is something wrong here. 1 meter is about 3.3 feet. 1 kilometer is about .62 miles.

  4. There must be a lot of cows, cyanobacteria, and dinosaurs living on Pluto to produce all that biogenic fossil fuel.

  5. Steve says

    Likely 3 – 15 degrees per kilometer. A 15 degree swing per meter is just to hard to believe. Imagine walking around in freezing temperatures it knee level and a sweater temperatures at your chest. Must be a kilometer

  6. Gary says

    Must be global warming. Can you see any cars or hairspray?

  7. Feenixx says

    Pluto’s highly eccentric orbit makes it believe that it’s a comet, so it tries to do some of the things comets do……

    😉

  8. Jesper says

    Recently there was this buzz about methane on Mars, which scientists think is either produced by geological or by biological activity.

    Where does the methane on Pluto come from? Does it work differently there than on Mars?

  9. Bill L. says

    Jesper;

    The reason that methane on Mars caused such a stir is because there was data that suggested it was being replenished somehow. Pluto, on the other hand, most likely has a more fixed amount, so it isn’t as exciting.

  10. Max says

    If I recall, the buzz about methane was due to it being a chemical that breaks down in direct sunlight.
    To find it on earth or mars means it would have to have been replenished over time from some source.

    On Pluto, where the Sun is barely brighter than other stars, theres probably not enough energy for the gas to decompose..

  11. Jorge says

    (snif… nobody answers my question… snif)

  12. Yael Dragwyla says

    There are good reasons not to “demote” Pluto and the other KBOs (although they’re far older than we are, and will go on existing after we become extinct, and simply don’t care about the issue, so “demotion” by us or anyone else isn’t an option): http://polaris93.livejournal.com/984977.html But none of those reasons have to do with emissions of methane into Pluto’s or any other planet’s atmosphere. Check out the URL above.

  13. RUF says

    Louis:
    “I think they’ll have to re-think this whole “Pluto is not a planet” thing now.”

    Why? no planetary criteria have changed.

    Does anyone know what state Pluto’s atmosphere will be in when New Horizons arrives? Will it befrozen? Sublimated? Somewhere in-between?

    We may have to sent three “New Horizons” to view Pluto in each state to get a good understanding of the dwarf planet.

  14. Feenixx says

    RUF says:
    “We may have to sent three “New Horizons” to view Pluto in each state to get a good understanding of the dwarf planet.”

    ideally, yes…. and with about 42 years in between each pair of missions… it’ll require some patience!

  15. Astrofiend says

    RUF Says:
    March 3rd, 2009 at 8:08 pm

    Does anyone know what state Pluto’s atmosphere will be in when New Horizons arrives? Will it befrozen? Sublimated? Somewhere in-between?

    It will not be frozen. Pluto is on the outward bound part of it’s orbit, but it’s atmosphere is not expected to be significantly frozen out for some time to come – I read it in an interview with Alan Stern. NH may glimpse the start of the process, but it won’t all be sitting in a heap of snow on the surface.

  16. Astrofiend says

    Jorge Says:
    March 2nd, 2009 at 6:18 pm

    “There’s one thing I don’t get. Knowing that the temperature is a measure of the average kinetic energy of some material’s constituent particles. wouldn’t it be expected that, in the absence of surface irradiation of heat and/or layers opaque to infrared radiation, the higher you get the higher the temperature, since those molecules found there would be the ones fast enough to get that high?”

    The temperature and kinetic energy of a particle do affect how they behave in the atmosphere, but it isn’t just a matter of ‘they have high KE and hence can shoot up higher’ – the mean free distance of a molecule in air is very short, so they tend to diffuse only fairly slowly. However, this says nothing about convection! Indeed – hot parcels of air do rise on Earth all the time due to their collective high KE – the constituent molecules have more kinetic energy and hence the gas becomes relatively less dense than that surrounding. They accordingly rise , but when they do the atmospheric pressure rapidly drops, and hence the air does work on it’s surroundings by expanding adiabatically – and the temperature of the parcel of air drops very quickly as a result!

    Hence, in a very (over) simplified model, thermal heating from the ground causes the atmosphere in close proximity to heat up and rise, which then cools by adiabatic expansion (look up orographic lift as an example of this process), and so the temperature profile decreases as we move higher in the atmosphere.

    Maybe.

  17. maudyfish says

    If Pluto was in a habitable orbit around the Sun would it be an ocean covered planet?

  18. Jorge says

    Ah! A reply! Great!

    I still have doubts, though.

    The temperature and kinetic energy of a particle do affect how they behave in the atmosphere, but it isn’t just a matter of ‘they have high KE and hence can shoot up higher’ – the mean free distance of a molecule in air is very short, so they tend to diffuse only fairly slowly.

    On Earth, as on any other planetary body with a thick atmosphere, sure, but I don’t know about something as thin as the atmosphere of Pluto… or Earth’s upper atmosphere, for that matter. I must admit my ignorance as to what kind of pressures would be needed for a gas molecule to become basically free to travel anywhere, though.

    However, this says nothing about convection! Indeed – hot parcels of air do rise on Earth all the time due to their collective high KE – the constituent molecules have more kinetic energy and hence the gas becomes relatively less dense than that surrounding. They accordingly rise , but when they do the atmospheric pressure rapidly drops, and hence the air does work on it’s surroundings by expanding adiabatically – and the temperature of the parcel of air drops very quickly as a result!

    Hm… yes, that’s true. The result would be an atmosphere which is hotter near the surface and colder higher up. However, in Pluto there is very little heat coming from the Sun to power that kind of system, which is probably also involved in the explanation of this “reversed” heat gradient they’ve observed out there.

    I very much doub’t it’s only one thing.

  19. Jorge says

    If Pluto was in a habitable orbit around the Sun would it be an ocean covered planet?

    No. It’s too small for that. Water would sublimate and be blown to interplanetary space, leaving behing only its rocky core.

  20. Bill Davis says

    It’s difficult to accept that the tenuous atmosphere, almost a vacuum, can cause a 40 deg rise at a distance of appx 4 billion miles from the only postulated heat source. That’s an enourmous temperature delta considering the tiny solar input. Let’s look to another mechanism. It does not appear that any prior calculation predicted this. Anomalous methane on Pluto…anomalous methane on Mars…but on Mars we instantly pounce on life as a serious possiblity of the source. How about a combining of carbon (from Mars’ CO2 atmosphere, and on Pluto, from organic (that’s Pluto’s red tint perhaps) and solar protons, add some electrons, to make the methane (CH4)?

  21. Vanamonde says

    Okay, that is THREE planets, sorry, two planets and a dwarf with greenhouse gas going on. I blame Gimli.

  22. maudyfish says

    What would happen to its two satellites, Jorge?

  23. Jorge says

    What would happen to its two satellites, Jorge?

    The same thing, basically. They’d partially evaporate and would remain only whatever rock they have. If Pluto was to replace the Earth, the outer satellites might also be lost due to gravitational interference by other bodies, since the inner system is much more packed than the Kuiper Belt. Perhaps. I can’t know for sure, because I don’t know how much mass Pluto would have without its volatiles, so I can’t say how tightly it’d grab Charon, Nyx and Hidra…

  24. RUF says

    Excellent. Thanks for the info Astrofiend! Seem like a good time to catch the dwarf (better than when it is completely frozen).

  25. maudyfish says

    The poor dwarf!!! You making fun of him.

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