MadSci Network: Earth Sciences

Re: What is the temperature just above the earth, in space,...

Date: Mon Apr 20 19:26:41 1998
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Earth Sciences
ID: 890407668.Es

The very concept of temperature is not a fundamental one like mass or 
charge. It is actually a measure of the average kinetic energy of all of 
the particles in a system. It does not have very much meaning except when a 
system is itself at thermal equilibrium. And its value depends very much on 
the nature of the thermal contact between a system and its surroundings.

Interplanetary space is a fairly good vacuum. There is little opportunity 
for heat to move to or from an object by conduction or convection. Small 
amounts of energy will be transferred in occasional encounters with the 
molecules of interplantetary space. Most heat transfer to or from a body in 
interplanetary space will be through radiation.

I presume that you are asking the following question: suppose that we were 
to take an object to the edge of the earth's atmosphere, where the 
background molecule concentration was similar to that in interplanetary 
space. We leave it there for a long time until it achieves some sort of 
thermal equilibrium with its surroundings. What temperature will we find it 

There is a need to be even more precise in the first part of your question. 
Something in direct sunlight has a sunny side and a shady side; at most one 
half of it is in direct sunlight. So I will presume that your object is 
made of metal (so that heat will be rapidly conducted through the whole 
object), or rotating rapidly with an axis perpendicular to the plane of the 
earth's orbit, or both.

Finally, if we are looking at radiation transfer into and out from an 
object, it is important how much of the radiation is absorbed, and how much 
is simply reflected away from the surface. The proportion of incoming 
sunlight that is reflected directly back into space by an object is known 
as its albedo. Initially, we will assume that your object is a perfectly 
black body, that is, its albedo is 0.

In this case, the required datum can be found in the CRC Handbook of 
Chemistry & Physics, in a table entitled 'Physical Data for the planets, 
their satellites, and some asteroids' in a column headed 'Average 
Temperature (K)' and sub-headed 'equilibrium'. In my volume (56th Ed) it is 
on page F-176.

The equilibrium temperature of a black-body in direct sunlight in 
interplanetary space near the earth would be 394 K = 121 deg C.

If the body is not black, a good approximation can be obtained simply by 
multiplying (1 - albedo) by the absolute temperature. For the earth itself, 
the albedo is about 0.36. 36% of total incident sunlight is reflected back 
into space, which means that 64% is absorbed by the Earth/atmosphere 
system. The equilibrium temperature for a body of this brightness would be 
0.64 times 394 K = 252 K = -21 deg C.

The equilibrium temperature for a body as bright as the earth in direct 
sunlight in interplanetary space near the earth would be 252 K = -21 deg C.

The earth itself is some 35 degrees warmer than this because of the natural 
greenhouse effect due to the water vapour and carbon dioxide in its 

The second part of the original question is much more difficult to answer 
in any meaningful way at all. An answer I can give is 'definitely less than 
100 K = -173 deg C, and definitely more that 2 K = -271 deg C'. 

If direct radiation from the sun is not part of the energy input, then what 
is? To what extent does the object causing the 'shading' block access of 
energetic solar particles? Is the object itself exposed to bright 
'earthlight' -- reflected sunlight from the earth? It is 'just above the 
earth', but how much of the infrared emission of the earth's own radiation 
it will receive depends exactly on just how far above the earth.

The figure of 100 K is the temperature of the night side of the moon. 
Basically that is 'an object just above the earth in the shade'. But this 
temperature will be a fairly drastic over-estimate, because much of it will 
be due to a residual warmth from when the local surface was on the sunlit 
side of the moon a fortnight previously. The temperature of 2 K is the 
temperature of the background microwave radiation that is believed to be 
the echo of the 'big bang'. As the whole universe is bathed in this 
radiation, it probably represents the lowest equilibrium temperature that 
any object can achieve in the universe.

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