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## 1. An Atmosphere

An atmosphere is a layer of gases around a celestial object, bound to the object by gravity. The atmosphere gets less dense as one goes higher. There is no sharp boundary between the atmosphere and interplanetary space.

Even around a celestial object with relatively weak gravity, such as the Moon or the rings of Saturn, there can be some gases that you would not find in interplanetary space in the same ratios and (higher) densities. You might call those atmospheres, though those atmospheres are so rarified that in a laboratory on Earth they'd be counted as a good vacuum.

Giant gas planets such as Jupiter and Saturn have very thick layers of gas that probably make up more than half of the diameter of the planet. Usually all of the gas of such a planet is counted as part of the atmosphere.

The relatively thin layer of a star from which most of the visible light comes that you can see from far away is sometimes called the atmosphere of that star. All the gas below that thin outer layer is not counted as part of the atmosphere.

## 2. The Earth's Atmosphere

The atmosphere of the Earth is traditionally divided into five layers:

Table 1: Layes of the Atmosphere of the Earth

Name altitude (km) temperature ()
Exosphere > 700 ≈ 1000
Thermosphere 80 − 700 −86 ↑ ≈ 1000
Mesosphere 50 − 80 −3 ↓ −86
Stratosphere 12 − 50 −57 ↑ −3
Troposphere 0 − 12 15 ↓ −57

The following table shows some properties of the atmosphere at various altitudes between 0 and 1000 km, based on the so-called "U.S. Standard Atmosphere 1976" //modelweb.gsfc.nasa.gov/atmos/us_standard.html. The shown characteristics are: temperature $$T$$, (air) pressure $$P$$, mass density $$ρ$$, particle density $$n$$, collision frequency $$ν$$, and particle mass $$μ$$ relative to that of a proton.

Table 2: Atmosphere

$${h}$$ $${T}$$ $${P}$$ $${ρ}$$ $${n}$$ $${ν}$$ $${μ}$$
km K
  ℃
Pa kg m⁻³ m⁻³ s⁻¹
0 288.1 15.0 1.01 × 10+05 1.23 2.55 × 10+25 6.92 × 10+09 28.8
2 275.1 2.0 7.95 × 10+04 1.01 2.09 × 10+25 5.55 × 10+09 28.8
4 262.2 −11.0 6.17 × 10+04 0.819 1.70 × 10+25 4.41 × 10+09 28.7
6 249.2 −24.0 4.72 × 10+04 0.660 1.37 × 10+25 3.47 × 10+09 28.8
8 236.2 −36.9 3.56 × 10+04 0.526 1.09 × 10+25 2.69 × 10+09 28.8
10 223.2 −49.9 2.65 × 10+04 0.414 8.60 × 10+24 2.06 × 10+09 28.8
12 216.6 −56.5 1.94 × 10+04 0.312 6.49 × 10+24 1.53 × 10+09 28.8
14 216.6 −56.5 1.42 × 10+04 0.228 4.74 × 10+24 1.12 × 10+09 28.8
16 216.6 −56.5 1.04 × 10+04 0.166 3.46 × 10+24 8.15 × 10+08 28.7
18 216.6 −56.5 7.56 × 10+03 0.122 2.53 × 10+24 5.96 × 10+08 28.8
20 216.6 −56.5 5.53 × 10+03 0.0889 1.85 × 10+24 4.35 × 10+08 28.8
22 218.6 −54.6 4.05 × 10+03 0.0645 1.34 × 10+24 3.17 × 10+08 28.8
24 220.6 −52.6 2.97 × 10+03 0.0469 9.76 × 10+23 2.32 × 10+08 28.8
26 222.5 −50.6 2.19 × 10+03 0.0343 7.12 × 10+23 1.70 × 10+08 28.8
28 224.5 −48.6 1.61 × 10+03 0.0251 5.21 × 10+23 1.25 × 10+08 28.9
30 226.5 −46.6 1.20 × 10+03 0.0184 3.83 × 10+23 9.22 × 10+07 28.8
32 228.5 −44.7 889. 0.0136 2.81 × 10+23 6.82 × 10+07 28.8
34 233.7 −39.4 663. 0.00989 2.06 × 10+23 5.03 × 10+07 28.8
36 239.3 −33.9 498. 0.00726 1.51 × 10+23 3.74 × 10+07 28.8
38 244.8 −28.3 377. 0.00537 1.12 × 10+23 2.79 × 10+07 28.8
40 250.4 −22.8 287. 0.00400 8.31 × 10+22 2.10 × 10+07 28.8
42 255.9 −17.3 220. 0.00299 6.23 × 10+22 1.59 × 10+07 28.8
44 261.4 −11.8 170. 0.00226 4.70 × 10+22 1.22 × 10+07 28.8
46 266.9 −6.2 131. 0.00171 3.56 × 10+22 9.32 × 10+06 28.8
48 270.6 −2.5 102. 0.00132 2.74 × 10+22 7.21 × 10+06 28.8
50 270.6 −2.5 79.8 0.00103 2.14 × 10+22 5.62 × 10+06 28.8
52 269.0 −4.1 62.2 0.000806 1.68 × 10+22 4.40 × 10+06 28.8
54 263.5 −9.6 48.3 0.000639 1.33 × 10+22 3.45 × 10+06 28.8
56 258.0 −15.1 37.4 0.000504 1.05 × 10+22 2.70 × 10+06 28.8
58 252.5 −20.6 28.7 0.000396 8.24 × 10+21 2.10 × 10+06 28.8
60 247.0 −26.1 22.0 0.000310 6.44 × 10+21 1.62 × 10+06 28.8
65 233.3 −39.9 10.9 0.000163 3.39 × 10+21 8.29 × 10+05 28.8
70 219.6 −53.6 5.22 8.28 × 10−05 1.72 × 10+21 4.08 × 10+05 28.8
75 208.4 −64.8 2.39 3.99 × 10−05 8.30 × 10+20 1.92 × 10+05 28.8
80 198.6 −74.5 1.05 1.85 × 10−05 3.84 × 10+20 8.66 × 10+04 28.8
85 188.9 −84.3 0.446 8.22 × 10−06 1.71 × 10+20 3.77 × 10+04 28.8
90 186.9 −86.3 0.184 3.42 × 10−06 7.12 × 10+19 1.56 × 10+04 28.7
95 188.4 −84.7 0.0760 1.39 × 10−06 2.92 × 10+19 6.44 × 10+03 28.5
100 195.1 −78.1 0.0320 5.60 × 10−07 1.19 × 10+19 2.68 × 10+03 28.2
120 360.0 86.9 0.00254 2.22 × 10−08 5.11 × 10+17 163. 26.0
140 559.6 286.5 0.000720 3.83 × 10−09 9.32 × 10+16 38.0 24.6
160 696.3 423.1 0.000304 1.23 × 10−09 3.16 × 10+16 15.0 23.3
180 790.1 516.9 0.000153 5.19 × 10−10 1.40 × 10+16 7.20 22.2
200 854.6 581.4 8.47 × 10−05 2.54 × 10−10 7.18 × 10+15 3.90 21.2
300 976.0 702.9 8.77 × 10−06 1.92 × 10−11 6.51 × 10+14 0.420 17.6
500 999.2 726.1 3.02 × 10−07 5.21 × 10−13 2.19 × 10+13 0.0160 14.2
1000 1000. 726.8 7.51 × 10−09 3.56 × 10−15 5.44 × 10+11 0.000750 3.9

### 2.1. Heat in the Thermosphere

[501]

The temperature is a measure for the amount of thermal energy in each atom or molecule. The temperature in the thermosphere is higher than at the surface, but there are a lot less molecules per unit volume in the thermosphere than at the surface, so the amount of thermal energy per unit volume is a lot less in the thermosphere than it is at the surface. In other words: the thermosphere is very hot but contains little heat.

The gas pressure is a measure for the density of thermal energy in the gas, and the gas pressure is very small in the thermosphere, because the thermosphere is almost a vacuum. 50% of the thermal energy of the atmosphere is at less than about 5 km altitude, 90% is lower than about 16 km altitude, 99% is below about 31 km. My calculations suggest that above 80 km altitude (including the thermosphere and the layers above it) there is only about 0.0008% of the thermal energy of the atmosphere, roughly as much as is contained in the bottom 0.06 meter (2 inches) of the atmosphere (if you regard a column of constant width from the surface to space).

If you could extract all thermal energy from a column of the atmosphere and put it into a pan of water of 10 cm (4 in) high (and equally wide as the column), then my calculations suggest that that water would get only about 20 hotter. With the 0.0008% of the thermal energy that is contained in the thermosphere and higher layers, the temperature rise would be only about 0.0002 ℃. I think that an astronaut or a space station would need more energy to raise its temperature by a degree than a pan of water of 10 cm high does, so the temperature of an astronaut or space station in the thermosphere would be even less. Also, an astronaut or space station won't receive all the heat from the whole thermosphere, so the effect of the high temperature of the thermosphere on astronauts or space stations within the thermosphere would be even a lot smaller than 0.0002 ℃.

## 3. Measuring Temperatures in the Thermosphere

[502]

You cannot measure the temperature of the thermosphere using a thermometer, because the measured temperature would then say more about the amount of radiation energy received from the Sun than about the very thin gas in the thermosphere. The temperature of the thermosphere is deduced from other things, such as the gas density, which itself is measured from the slowing down of satellites by friction with the gas.

I think that it should also be possible to deduce the temperature from measurements of light and other kinds of radiation coming from the thermosphere, similar to how we measure the temperature of stars, but then more difficult. I don't know if someone has ever done such measurements. They aren't very important, and are difficult to make and interpret, so perhaps it isn't even worth the trouble.

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Last updated: 2021-07-19