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How do other planets reflect energy?

  1. Apr 14, 2016 #1
    Hi all

    Today in high school we learned about the procedure by which the earth reflects the energy given to it by the sun. It did so by the atmosphere, clouds, ect.. Anyways, apparently the process works in such a way that all energy which is absorbed is eventually reflected back into space. If this didn't occur, the Earth would continue heating up forever, at least until the sun was decimated. I was wondering how this would work for other planets because they don't contain atmospheres.
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  3. Apr 14, 2016 #2


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    Welcome to PF Physics passion. The surface can reflect energy also.
  4. Apr 14, 2016 #3


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    The small rocky worlds and asteroids don't have much in the way of an atmosphere.

    Mars' atmosphere has a very low surface pressure.

    The large gas giants in the outer solar system, Jupiter, Saturn, Neptune, and Uranus, have quite healthy atmospheres, although these are filled with gases which you wouldn't want to breathe.

    Venus has enough atmosphere to supply a couple of worlds, it is so dense and hot.
  5. Apr 14, 2016 #4
    I guess what I'm wondering is if it's common for planets to reflect all their energy back into space in the way that the earth does it. I know the atmosphere is not the only thing that reflects energy, but many other planets don't have atmospheres and so it seems like they shouldn't be able to reflect all the energy like the Earth does.
  6. Apr 14, 2016 #5


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    The earth does not reflect "all" of the solar energy back into space that it receives from the sun. If it did, things would get very cold very quickly on the surface.

    Instead, there is something called an "energy budget" which describes how much energy the earth receives and how much it loses or reflects back into space:


    The characteristic of a planet which describes how much radiation it reflects back into space is called its "albedo":

  7. Apr 14, 2016 #6
    Thanks for the reply. I guess I wasn't clear in asking my question. It is my understanding that the speed in which the earth receives energy from the sun is equal to the speed of how it gives off and reflects energy. If it did not do this then the earth would receive energy faster or slower than it gives off and the earth would increase/decrease in heat overtime. Is earth unique in this or is it common?
  8. Apr 14, 2016 #7


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    It's complicated.

    Because the earth revolves on its axis, the radiation it receives varies during the day. Water, land, and cloud cover all influence the amount of radiation which is trapped and absorbed or re-radiated back into space at night.

    It's also hard to generalize about this, since we're able to study in detail what happens to the planets in our own solar system.
  9. Apr 14, 2016 #8


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    Essentially all planets are in equilibrium with their surroundings, at least in the long term. The hotter something gets the more energy it sheds. This is a self-balancing operation. Take an ordinary incandescent light bulb as an example. The bulb will heat up until it is hot enough to shed the energy (Wattage) being put into it, the. A 4 Watt nightlight can be carfully handled when lit, but if you try to handle a 100 Watt bulb when it's on you will be quite uncomfortable. That's because the 100 Watt bulb has to get hotter to shed all the energy going into it. The same is true for planets.
  10. Apr 14, 2016 #9


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    “Reflection” suggests an immediate turn around of photons without change of wavelength. What actually happens is that much incident energy is absorbed temporarily, then re-radiated after some delay at longer wavelengths, usually IR.
  11. Apr 14, 2016 #10


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    hopefully this diagram will clear up your misconceptions


  12. Apr 14, 2016 #11
    Thanks for the responses.
    Although unfortunately I have just one more question.
    Are all other planets in a state of equilibrium like the Earth is?
  13. Apr 15, 2016 #12


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    Yes. If that was not the case those bodies would be changing temperature until they again reached an equilibrium. Equilibrium is simply the situation where the energy accounts balance.
  14. Apr 15, 2016 #13
    What causes the planets to be in an equilbrium?
  15. Apr 15, 2016 #14


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    It is common -- I'd go so far as to say a requirement of conservation of energy. If the star and planet have been around for a while, the orbit is stable and the star's output is stable, the planet will reach an equilibrium.
    Any process will either move toward or away from an equilibrium depending on the details of the process. Since heat transfer rises as temperature difference rises, it finds an equilibrium. All it takes is time.
  16. Apr 15, 2016 #15


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    Each planet will have an equilibrium. If the planet surface was very cold, then it would absorb more energy than it radiated, so it would accumulate energy and so rise in temperature. Once it's surface temperature rises sufficiently to radiate all the energy it receives, the temperature of the planet will have reached equilibrium with it's orbital environment.
    Equilibrium is an accounting term that relates the equality of the “energy in” to the “energy out”.
  17. Apr 19, 2016 #16
    Let me put it this way,
    Every body (planets, asteroid, comet, moon) whether having an atmosohere or not has that 'comfort zone'
    That temperature range that it mostly oscilliates in
    Several factors affect the temperature e.g. distance from the sun, atmosphere thickness and composition, time of 'day' etc
    But at the end of the day all bodies have a fixed range...an 'equilibrium'
  18. Apr 26, 2016 #17

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    I think your confusion is in part from the title of the question. The Earth reflects about 30% of the incoming solar radiation. The other 70% is absorbed rather than reflected. What makes the Earth be in equilibrium is that the Earth also radiates energy, but in the thermal infrared. If one looks across the electromagnetic spectrum at the energy emitted by a planet, you'll see two peaks, one in the visible due to reflected sunlight, and the other in the thermal infrared due to the planet not being at absolute zero.

    Every object that is not at absolute zero emits thermal radiation. One thing that happens as an object gets warmer is that the frequency at which this thermal radiation increases linearly with increasing temperature. You can't see the thermal radiation emitted by rock at room temperature, but you can see the thermal radiation emitted by very hot rock (e.g., lava). Another thing that happens as an object gets warmer is that the amount of energy radiated per unit time also increases with temperature, but now the increase is quartic (proportional to absolute temperature raised to the fourth power) rather than linear.

    It is this thermal radiation that causes planets to be in equilibrium. Suppose a planet is receiving more energy from absorbed sunlight than it is emitting via thermal radiation. This energy imbalance will make the planet warm up, which will increase the amount of thermal radiation emitted. Eventually the planet will reach a point where the radiated thermal energy equals the absorbed solar energy. If on the other hand, the planet is above its equilibrium temperature, it will radiate more energy thermally than it receives from the Sun. The planet will cool until it reaches that equilibrium temperature.
  19. May 28, 2016 #18
    Very common, and not quite.

    Consider: Radiation goes up rapidly with temperature. If planet X loses less heat than it gains it warms up. The amount of heat lost by radiation increases, until there is balance between heat in vs heat out.

    The not quite happens because the reaction to changing circumstances is not instant. There will be a lag of some time while the temperature warms up enough to make the required change in radiation.

    On Earth radiation is complicated. Some is direct from the surface. Some is from the tops of clouds. Some is the net balance of warm air masses and cold air masses radiating at each other, but ultimately the heat radiates into space.

    On an airless body, such as the moon, the process is more direct. During the day, the surface rock reflects some, absorbs some, heats up, radiates into space. At night it continues to radiate and cools off. Because there is no air, the temperature swings of that pebble on the surface are pretty extreme.

    Right now we are experimenting with this effect on the Earth. We've added a bunch of stuff to the air that slows down the radiation from the surface. The current estimate of the differential between in and out is about 1W/m2, or about 0.1% So the temperature of the planet is slowly rising. Depending on what we do about the stuff, we may end up with palm trees in arctic regions within a thousand years. (In my home province of Alberta, ecozones are moving north at an average of 10 km per year. 2080 for us is 5-9 C warmer than at present. The joys of living at higher latitudes -- stronger climate effects.)
  20. Aug 18, 2016 #19
    But temperature is instantaneous, as it is practically the same thing as emitted energy considering that the fourth power of degrees Kelvin is what defines the amount of energy leaving matter at a certain temperature. Isn´t it a bit of a contradiction to talk about lag when explaining changes in temperature?`

    When temperature rises it always is caused by an increase in energy going into a system. The change in input is instantaneous and the matter that receives that energy changes instantaneously. Warming is the result of an increased amount of heat going into a system continously.

    The lag you are talking about, isn't that the time it takes for the increased amount of energy to get evenly distributed throughout the system eventually reaching a steady state?

    Excited matter at a given temperature is totally dependant on constant input of energy, in cases like this when matter gets heated from the outside. Any change in the amount of input will immediately give an equal change in excitation.

    If input was shut off entirely to earth, how long time would pass before the planet reach equillibrium in cold space?

    More than other planets? In what way?

    How can radiation be slowed down? Doesn´t radiation, if we talk about speed, have the same speed as the photon? Doesn´t all photons, both IR and SW, have the speed of light?

    Did you mean that it takes a longer route, travelling through more molecular absorption in greater numbers and decreasing in energy with every emission that results from added absorbing molecules? It takes longer time for a single photon to travel from the surface to space, if more molecules absorb the photon before it reaches the boundary. But slowing it down, is that really what happens?

    Does anyone know how long time we are talking about? How much longer does it take for a photon to leave the atmosphere and enter space, when there are a certain amount or a certain increase in absorbing molecules when the photon travels at light speed? Is it minutes, seconds, hours or what?

    Since photons reach earth in ten minutes when they travel from the sun, it seems like the short distance in the atmosphere would be done very fast even with an absorbing gas in the way.

    Water can hold lots of absorbed heat radiation over time, but even water emits that energy and drops in temperature quickly if the input or surrounding temperature drops. Other absorbing gases doesn´t hold the energy for very long at all as radiated photons. As far as i know, they emit it practically instantaneously, and if they don´t, they will instead collide with the other gas molecules and release it equally fast in the process of transformation to kinetic energy, heat.

    Isn´t it so, that most of the transfer of energy in the atmosphere is by kinetic transfer?
    I know that I have read that close to the surface there is no radiative heat transfer and it is entirely a process where conduction moves heat through convection, transferring it in moving matter that expands as it warms up. Of course there must be radiation from the surface somewhere sometimes, but the main mechanism of heat transfer is done by conduction and convection close to the surface, is that right?

    Convection is a slower form of moving heat than radiative transfer of individual photons, but it compensates by being more effective. With added mechanisms like waters phase change to gas form when it´s energy content drives individual molecules to carry the heat away from the surface, I have been told that it is preferred method of heat transfer when possible. Mass that carries the energy as kinetic energy or as absorbed photons. Isn´t that right?

    It´s weird to use terms as "preferred" when talking about heat transfer. It´s more like, when conduction and convection is possible, it will happen before radiative transfer, as I have understood.

    It seems a bit misleading when the slowest form of transfer as energy contained in matter carrying the heat away from the surface, is the dominant form of heat transfer from the surface, and the argument is that radiative transfer of photons that travels at light speed is slowed down in a process that also attenuates it. And also, the slowest way of energy moving to space, seems to be the most effective method of cooling the surface. Slowing down radiation is a strange argument when considering the speed of photons and the very effective but slow transfer by convection.

    There must be a better way to explain it.

    And you also write that there is an inbalance in energy of 1W/m^2. The radiated energy that is lowered by 1W/m^2 in output, is equal to a lowered temperature by the same amount of energy. This is confusing.

    Temperature is equal to the energy emitted and the fourth power gives us W/m`^2. When you say that a radiative imbalance that has decreased by 1W/m^2, which is the same as saying that the emitted radiative temperature has dropped with the same amount of energy, how do you mean that it is connected to a rising temperature?

    Does the temperature drop first at higher altitude and then the surface heats up? Or does the temperature rise at the surface first and that causes a drop in temperature at higher altitude?

    In both cases it seems like they cancel out. As I know heat transfer, a drop in temperature is not something associated with increasing temperature and energycontent in a system. Actually, it never happens that temperature drops in the same process that temperature increase in different locations in a system. At least in the heat transfer literature I have read.

    It would be nice to get some clarification of these processes and how earth´s temperature is governed by the gasses in the atmosphere.
    Last edited: Aug 18, 2016
  21. Aug 18, 2016 #20


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    The lag is from the time taken to change the temperature of an object. For example, you can't just instantly boil a pot of water the moment you put in on a hot stove. You need to add a certain amount of energy, which takes time. We tend to talk about objects as a whole, not solely about the surfaces, so it takes time to increase the temperature of the object. Note that temperature is about the behavior of a large number of particles, not the behavior of a small number near the surface. You might not even be able to define the temperature of these particles if there are too few of them.

    The point isn't about the speed of the radiation itself. It's about energy transfer. The radiation coming up from the surface is absorbed by molecules in the atmosphere and then re-radiated a short time later. But the key is that the re-radiation happens in all directions and not solely upward. So some of that radiated energy goes back down towards the surface to be re-absorbed and re-emitted again. The net effect is that the temperature required for the Earth to reach equilibrium with the incoming solar radiation has to increase versus an Earth with no atmosphere.

    The energy leaving the planet is less than the energy incoming to the planet, and this imbalance leads to an increase in the temperature until it reaches a point that the outgoing energy is equal to the incoming energy.
  22. Aug 18, 2016 #21
    I agree, that was my point also. The lag is the time needed to distribute an increase in the energyinput throughout the total amount of matter in the system. But I still must say that temperature always is a number that relates to an instantaneous mechanism. It is only a product of instantaneous flow of energy at the point where temperature is measured. Over time we get other units like kWh, and that is not something that we should mix up with the state of excitation in matter at a given time.

    I agree that we usually use temperature for an amount of some substance like a pot of water. But it is a bit misleading when we try to describe radiation and heat transfer. I think we need to be aware of that temperature is measured level of the energy flowing in matter and is totally dependant on the input to the system. The input decides all temperatures in the heated system.

    I´m very aware of the processes that is described in the atmospheric transfer of heat. But there are some strange differences to how we usually describe heat transfer in a system like earths.
    As I wrote before, I´m pretty sure that conduction and convection is the dominant processes for transfer of heat from the surface. Then how can we fit that mechanism with radiative transfer from the atmosphere to the same surface. I think that the same mechanisms that transfer heat away from surface must also dominate the transfer in the opposite direction when it happens at the same wavelenghts(roughly). Why would there be radiative transfer in one direction and convective transfer in the opposite direction. It seems to me that we have a layer near the surface that would eliminate radiative transfer to small amounts through massive kinetic transfer by rapidly moving molecules that distribute heat by collisions instead of radiated photons. The pressure difference and higher density makes it unlikely that there would be any photons reaching the surface and completing the radiating transfer to the surface.

    On top of that, at every absorption and re-emission in the gasmolecules, the radiation is lowered to half the amount since it is radiated in all directions. The probability seems low for any significant amount of radiation to reach the surface before attenuation.

    And as far as I know, heat transfer is not something that can be calculated with fotons included, and the absorption leading to re-emission from any molecule is a process of loss, isn´t it?

    Re-emitted heat radiation is decreased in it´s intensity and has lost some of it´s heat, or thermal energy, am I right?

    If we look at the troposphere temperature distribution, what layers at what altitude contributes to surface temperature, and how large distances can radiative transfer cover, from an altitude of say 5km at a temperature of 255K downwards through increasing density and increasing intensity in kinetic energy with rising temperature, in layers where there is massive upwards convective heat transfer?

    And how does this radiative heat transfer from higher altitudes at lower temperatures relate to the distribution of excited states in matter in the maxwell-boltzmann description of the relationship between temperature and distribution of probable states of excitation in matter at a certain temperature?

    Is it the amount of photons or the level of excitation in the amount of photons that determines the excitation in absorbing matter, which leads to a certain temperature?

    I have not seen photons included in heat transfer between gasses and surfaces before. The calculation from difference in temperature is the way that I know to calculate the transfer at a given location and point in time.

    I understood that. But my question was, the change that has appeared as a decrease by 1W/m^2 that you mentioned, is an equal change in energy that is proportionate to a decrease in temperature where it is measured as the radiation leaving the atmosphere. Do you mean that this decrease in temperature at high altitude is part of a mechanism that simultaneously raise temperature at the surface?

    Is there a direct connection of cause and effect, so that if we could hypothetically manipulate temperature at high altitude, there would be a direct and proportio nate change at the surface?

    I can´t see how it works, how can a decrease in temperature at high altitude at already very low temperature, cause a rising temperature at surface over long distance with varying compositions, density and temperatures in between?

    And how is this transfer of energy calculated in relation to how we calculate heat transfer otherwise?

    The only way I know of a connection between a low temperature to high temperature where a similar change is connected to increasing temperature, is a refridgerator. But that needs an added amount of energy performing work on the system to make this happen. With added work from added energy I can understand how a decrease in temperature is connected to an increased temperature at another point in the system. Is this mechanism where a decreasing temperature in the atmosphere is part of the same function that raise surface temperature related to what happens in a refridgerator? Can it be done without work being included in the process?

    Can you provide any links to litterature describing this transfer of energy, it is not something like the ways of heat transfer I am used to. Usually transfer is relative to difference in temperature and can give the rate of transfer between points of different temperature. With only transfer in one direction towards the lower temperature, and the source of the higher temperature emitting independently of any lower temperatures in the absorbing matter. It clearly cannot be applied here, I want to know why and how this transfer works.
  23. Aug 18, 2016 #22


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    Unfortunately I don't think I can answer most of your questions. My understanding of all this is rudimentary at best.
  24. Aug 19, 2016 #23


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    When a lower level is heated by radiation from above, it will increase in temperature and so reduce density. That drives convective processes that then carry the energy back up.

    There is a thermal gradient from about +6000°C at the centre of the Earth, to about 25°C at the surface, it falls to -57°C at 10 km, to a -93°C minimum at 90 km. It then rises up through +1000°C at 200 km, to 1200°C in the ionosphere, then all the way down to -270°C in outer space.
    Each layer in the Earth's thermocline model is coupled to it's adjacent layers.
    Radiation can couple separated layers in the atmosphere, for example the ozone reduction over the poles in winter results in more UV at lower altitudes in spring and summer.

    You might do better thinking of temperature as being the average kinetic energy of the molecules in a sample. The average kinetic energy is a state variable, it is not an energy flow. An energy flow is necessary to change the temperature.
    Last edited by a moderator: Aug 19, 2016
  25. Aug 19, 2016 #24
    I´m very interested in the total distribution in the whole system, I think that it must provide a more complete model for distribution of excited states in the system. The atmosphere and the solar irradiation must interact with earth´s internal gradient as well, even though the transfer across the surface is very small. But in absence of solar energy there would have to be a larger transfer from earth´s inner structure cooling it to lower temperature. Do you agree?

    The coupling of layers is how heat transfer happens, a change in output is always caused by a change in input caused by a higher temperature either increasing or decreasing.

    But I cannot understand how the coupling of layers can include a function where a lower temperature would cause an increase in excitation of matter at a higher temperature. I think it is a contradiction of itself
    That is exactly the point I am trying to make. That temperature is a measured state of energycontent in matter,

    But when determining temperature we do it by measuring heat transfer from matter to a thermometer, or by spectral analysis of radiated energy. That is a measurement of flow, it exactly determines the rate of transfer as a function of difference in temperature, and from that we can get the temperature in units of Kelvin, which determines the level of excitation in the emitting matter.

    Temperature and emitted energy are both measurements of the level of excitation in the radiating field. The level of excitation determines everything else and is a product of the intensity of the input at the system boundary. Input is equal to the level of excitation needed for every temperature inside the system. They are part of the same field of radiation and all change in temperature is caused by an equal change in input. The flow of energy and the temperature are both a description of the same mechanism that is the radiation field.

    When in a state of energy balance, the result is increased entropy in the transfer. The energy available to do work is decreased and partially lost in an irreversible process. When earth emits the absorbed energy at 240W/m^2, the ability to raise temperature in matter is decreased to 1/4.

    Every rise in temperature is inevitably a result of an increase in surrounding higher temperatures when measured at any point inside the system. A decrease in radiation to space by 1W/m^2 must be caused by an equal decrease in input.

    Average excitation decrease when absorbing matter increase ind input is constant. That is my interpretation of a mechanism based on increase in the amount of absorbing matter coupled with decrease in output of 1W. Adding absorbing matter without increasing the available energy will give a lower average state of excitation. Since the available energy for the atmospheric absorption is limited to the emitted energy from the surface. I see no other way of increasing the surface temperature other than a direct increase in absorbed solar radiation.

    At the same temperature, the transfer is equal in all directions.

    That is why I wonder what function that makes spontaneous increase in excitation possible coupled with decreasing temperature in absorbing matter. A lower temperature always results in increased transfer in one direction. A decrease of 1W OLR would inevitably lead to an increased rate of transfer directed towards that point where it is measured. But you say the opposite.
  26. Aug 19, 2016 #25
    How does that relate to the fact that shorter wavelengths increase in radiative transfer and radiation from the atmosphere is transformed into longer wavelengths when re-emitted at lower temperature?

    I thought that the higher intensity in shorter wavelengths is part of the reason for radiative transfer through the atmosphere, as it is intensity at a level that is not corresponding to the states of excitation possible in the atmospheric gasses. Is it both longer and shorter wavelengths than what the surface emits that can penetrate the atmosphere?

    From what I have seen in absorption spectrum of the atmosphere, longer wavelengths at lower temperatures than the surface is absorbed to 100% by waters rotational bands. There was nothing being transmitted through that part of the spectrum. Which again questions radiative transfer from the atmosphere to the surface and the relation to increasing temperature.
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