# How do other planets reflect energy?

1. Apr 14, 2016

### Physics passion

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.

2. Apr 14, 2016

### Borg

Welcome to PF Physics passion. The surface can reflect energy also.

3. Apr 14, 2016

### SteamKing

Staff Emeritus
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.

4. Apr 14, 2016

### Physics passion

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.

5. Apr 14, 2016

### SteamKing

Staff Emeritus
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:

https://en.wikipedia.org/wiki/Earth's_energy_budget

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

https://en.wikipedia.org/wiki/Albedo

6. Apr 14, 2016

### Physics passion

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?

7. Apr 14, 2016

### SteamKing

Staff Emeritus
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.

8. Apr 14, 2016

### Tom.G

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.

9. Apr 14, 2016

### Baluncore

“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.

10. Apr 14, 2016

### davenn

hopefully this diagram will clear up your misconceptions

cheers
Dave

11. Apr 14, 2016

### Physics passion

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?

12. Apr 15, 2016

### Baluncore

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.

13. Apr 15, 2016

### Physics passion

What causes the planets to be in an equilbrium?

14. Apr 15, 2016

### Staff: Mentor

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.

15. Apr 15, 2016

### Baluncore

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”.

16. Apr 19, 2016

### kenyanchemist

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'

17. Apr 26, 2016

### D H

Staff Emeritus
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.

18. May 28, 2016

### Sherwood Botsford

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.)

19. Aug 18, 2016

### TattarBamse

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
20. Aug 18, 2016

### Staff: Mentor

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.

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