h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The Earth, like all the planets, gets bathed in solar radiation every day - enough to power great storms, ocean currents, keep temperatures high in many places and fuel life on the planet. However all this extra energy must be lost somehow, or else our planet would have continued to become warmer and warmer and warmer, ending up a bit like Venus I guess. So how do planets lose all (or at least most) of the energy they gain each day from the Sun, so that the heating effects and the cooling effects remain (pretty much) in equilibrium?

I think of it as the Goldilocks scenario, the is just the right distance from the sun so as not to freeze or fry.

The conditions are just right for life to evolve with a symbiotic relationship between plant life and carbon-based life-forms.

I am grateful for my life, and I enjoy the beauty of the planet.

Best wishes Woodpigeon, for a good day, and I hope someone more knowledgable than myself pops along and answers your question soon.

A very small amount of the energy is used to power plants: it is stored as chemical energy in plant tissue. This can then be used by the plants to grow, by animals that eat the plants, and by the petroleum industry when the plants form into oil over millions of years.

But most of it is radiated by the Earth into space as heat.

Basically, the energy input from the sun is *roughly* fixed, depending on the sun's strength and distance, the reflectivity of the earth and its atmosphere, and the cross-sectional area of the earth. Some extra input is provided by heat generated within the earth, but again, that is fairly constant.

The rate at which the earth loses heat [to space] depends largely on the temperature of the earth's surface.

Imagining an earth starting off very cold, it would initially lose little energy to space due to its low temperature, and so it would heat up via solar input until it reached a temperature where the overall energy loss balanced the energy input from the sun. Likewise, a very hot earth would cool until its rate of heat loss was balkanced by solar input.

In practice, there are other factors - the reflectivity of the earth's surface (and clouds) can play a part in how much energy is absorbed, or radiated straight back into space - an ice-covered world would not have the same effective energy input due to much solar input being reflected back into space, but these are *relatively* small factors most of the time on earth.

If one could sit 'behind' the earth in space, and one had infra-red sensitive eyes, one would see it glowing very strongly. It's really the fact that we only see visible light, and the equilibrium temperature of the earth is not enough to cause it to glow visibly that stops us realising how bright the earth actually is as a thermal heat source at night.

This is where people get confused between the standard greenhouse effect (a good thing) and a runaway greenhouse effect (a very bad thing). Greenhouse gases (carbon dioxide, methane etc.) keep the heat bouncing around between the earth and the atmosphere for a bit before it radiates off into space, keeping the biosphere at what we consider noraml temperatures. Turning up the greenhouse effect appears to cause global warming, while the planet Venus shows the result of a runaway greenhouse effect with surface temperatures of more than 400 Celsius.

Azara

Presumably Venus has also now stabilised in terms of absorbtion of energy versus emission - in the infrared it must shine like a halogen!

Another question. Since planets, comets and asteroids essentially must release as much energy through the infrared as they receive from the sun, does this not make infrared telescopes particularly useful in spotting planetary bodies?

Good question and one I hadn't thought about before. I don't know the answer.

IR telescopes are certainly potentially useful in extrasolar planet-spotting - a Jupiter type body may be bright even on the scale of the star it orbits.

However, for solar minor planets, our position (relatively) close to the sun does mean that bodies in significantly wider orbits tend to have a face towards us that is usually almost completely sun-lit, so optical scopes can be particularly useful, especially given the transparency of the Earth's atmosphere to visible light.

PS
Objects that are quite reflective may be much brighter in optical terms than in the infrared, since they may reflect much more energy than they absorb and subsequently re-emit as infrared.

Duracell put it in their batteries

Here's what Andromeda looks like in IR
http://antwrp.gsfc.nasa.gov/apod/ap051020.html

Ste <= LOVES APoD