The Thermal Equator on Earth and Mars

Abstract The thermal equator (also known as the heat equator) is the circumplanetary set of points that represent the highest mean annual temperature at each longitude. Recent high precision global datasets for Earth and Mars provide a basis for a detailed calculation of the thermal equator on these worlds. On Earth, the temperature values that comprise the thermal equator range from 25.85° to 34.75°C, with a mean of 27.75° ± 1.3°C, and extends in latitude as high as 20°N in Mexico and 29.3°N in the Indian subcontinent. The maximum southern extent is 20°S in Australia. On Mars, lacking oceans, the thermal equator takes a simpler track and is roughly parallel to the equator, and displaced 5°–10°S. However, there is a region of longitude on Mars where the thermal equator becomes bimodal with a northern branch centered at 10°N and a southern branch centered at 20°S. Significance Statement Temperature is one of the factors that affects tropical ecosystems. However, the proximity of an ecosystem to geographical equator is not necessarily an indication of the temperature. The hottest annual average surface temperature at each longitude defines the thermal equator. Using new and detailed global temperature data, we have determined the precise path of the thermal equator on Earth and on Mars. We find that on both planets, the thermal equator moves well above and well below the geographical equator. On Mars, this may affect the distribution of ground ice. On Earth, this matters for understanding ecosystems in the tropics. Future studies might find aspects of tropical ecosystems that correlate with the distance from the thermal equator.