What makes the planets have different temperatures?

Answer 1

Their distance from the Sun and the composition of their atmospheres.

The terrestrial planets (Mercury, Venus, Earth, and Mars) receive heat from the Sun and radiate some back into space. Mercury has no appreciable atmosphere and experiences the highest range of temperatures simultaneously. Venus has a dense atmosphere that retains heat (too hot), and Mars has a thin atmosphere that loses most of its heat (too cold). The Earth is in the fortunate middle range for solar radiation and atmospheric density.

The gas giants (Jupiter, Saturn, Uranus and Neptune) receive comparatively little solar heat, which makes their outer atmospheres very cold. But their (deep and unseen) surfaces are heated to thousands of degrees by the massive pressure of their atmospheres.

Earth and Solar Radiation

The "Solar constant" is the average amount of sunlight that crosses a unit area every second. A recent estimate from satellite observations is 1,366 watts per square meter.

From the fact that Earth receives sunlight on its cross-sectional area (pi R^2), but has an actual area four times larger (4 pi R^2), we can see that the average sunlight falling on a square meter of the Earth is 341.5 watts per square meter. But the Earth reflects some of this right back out to space -- 30.6% according to NASA. So the sunlight entering the Earth's climate system averages 237 watts per square meter.

Energy is conserved. Earth must radiate back out as much as it receives. From the Stefan-Boltzmann relation, which says the flux density coming out of a radiating object is proportionate to the fourth power of temperature (times the "Stefan-Boltzmann constant" of 5.6704 x 10^-8 watts per square meter per kelvin to the fourth), that 237 watts per square meter corresponds to a temperature of 254 K. This is Earth's "radiative equilibrium temperature."

But water freezes at 273 K, so if equilibrium temperature were the whole story, Earth would be frozen over! But greenhouse gases in its atmosphere -- primarily water vapor and carbon dioxide -- bring the surface temperature up to 288 K, an increase of 34 K. Calculating how the greenhouse effect works is much more complicated than finding the radiative equilibrium temperature, but there are ways to do it.

For those who like formulas, here are a few to remember:

Flux input to the climate system: Fin = (S / 4) (1 - A) where S is the solar constant and A the planet's "bolometric Bond albedo."

Conservation of energy: Fin = Fout

Equilibrium temperature (also called effective temperature or emission temperature): Te = (Fout / sigma) ^ 0.25 where sigma is the Stefan-Boltzmann constant.

Greenhouse temperature increment: Tg = Ts - Te where Ts is the surface temperature.

Mars and Venus are good examples for comparison. For Venus, S = 2611 watts per square meter, A = 0.750 according to NASA, so Fin = Fout = 163 watts per square meter and Te = 232 K. For Mars, S = 589 W/m^2, A = 0.250, F = 110 W/m^2 and Te = 210 K. Note what a big difference the greenhouse effect can make: For Mars, Ts = 214 K, so Tg is 4 K, caused by Mars's thin carbon dioxide atmosphere. For Venus, Ts = 735 K (!), leading to Tg = 503 K, a huge difference due its very thick carbon dioxide atmosphere and sulfuric acid clouds.

The equilibrium temperature model breaks down for the giant planets, which have internal heat sources.