![]() Jupiter and Neptune have ratios of power emitted to solar power received of 2.5 and 2.7, respectively. This corresponds to a ratio between power emitted and solar power received of ~2.4, indicating a significant internal energy source. įor example, on Saturn, the effective temperature is approximately 95 K, compared to an equilibrium temperature of about 63 K. These internal processes will cause the effective temperature (a blackbody temperature that produces the observed radiation from a planet) to be warmer than the equilibrium temperature (the blackbody temperature that one would expect from solar heating alone). Orbiting bodies can also be heated by tidal heating, geothermal energy which is driven by radioactive decay in the core of the planet, or accretional heating. Again, these temperature variations result from poor heat transport and retention in the absence of an atmosphere. The same process would be necessary when considering the surface temperature of the Moon, which has an equilibrium temperature of 271 K (−2 ☌ 28 ☏), but can have temperatures of 373 K (100 ☌ 212 ☏) in the daytime and 100 K (−173 ☌ −280 ☏) at night. Consequently, in order to derive a meaningful mean surface temperature on an airless body (to compare with an equilibrium temperature), a global average surface emission flux is considered, and then an ' effective temperature of emission' that would produce such a flux is calculated. This is significant because our understanding of planetary temperatures comes not from direct measurement of the temperatures, but from measurements of the fluxes. ![]() according to the Stefan-Boltzmann law), temperature variations propagate into emission variations, this time to the power of 4. Assuming the planet radiates as a blackbody (i.e. ![]() Because of a relative lack of air to transport or retain heat, significant variations in temperature develop. There are large variations in surface temperature over space and time on airless or near-airless bodies like Mars, which has daily surface temperature variations of 50-60 K. On airless bodies, the lack of any significant greenhouse effect allows equilibrium temperatures to approach mean surface temperatures, as on Mars, where the equilibrium temperature is 210 K (−63 ☌ −82 ☏) and the mean surface temperature of emission is 215 K (−58 ☌ −73 ☏). The surface temperatures of such planets are more accurately estimated by modeling thermal radiation transport through the atmosphere. Similarly, Earth has an effective temperature of 255 K (−18 ☌ −1 ☏), but a surface temperature of about 288 K (15 ☌ 59 ☏) due to the greenhouse effect in our lower atmosphere. For example, Venus has an effective temperature of approximately 226 K (−47 ☌ −53 ☏), but a surface temperature of 740 K (467 ☌ 872 ☏). Consequently, such planets have surface temperatures higher than their effective radiation emission temperature. Planets with substantial greenhouse atmospheres emit more longwave radiation at the surface than what reaches space. In the greenhouse effect, long wave radiation emitted by a planet is absorbed by certain gases in the atmosphere, reducing longwave emissions to space. ![]() There are several reasons why measured temperatures deviate from predicted equilibrium temperatures. The equilibrium temperature is neither an upper nor lower bound on actual temperatures on a planet. The amount of radiation arriving at the planet is referred to as the incident solar radiation, I o Caveats The star emits radiation isotropically, and some fraction of this radiation reaches the planet. Calculation of equilibrium temperature Ĭonsider a planet orbiting its host star. Planetary equilibrium temperature differs from the global mean temperature and surface air temperature, which are measured observationally by satellites or surface-based instruments, and may be warmer than the equilibrium temperature due to the greenhouse effect. The effective radiation emission temperature is a related concept, but focuses on the actual power radiated rather than on the power being received, and so may have a different value if the planet has an internal energy source or when the planet is not in radiative equilibrium. Other authors use different names for this concept, such as equivalent blackbody temperature of a planet. In this model, the presence or absence of an atmosphere (and therefore any greenhouse effect) is irrelevant, as the equilibrium temperature is calculated purely from a balance with incident stellar energy. The planetary equilibrium temperature is a theoretical temperature that a planet would be if it was in radiative equilibrium, typically under the assumption that it radiates as a black body being heated only by its parent star. Temperature of a planet when approximated as radiating as a black body
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