How hot is it on our nearest planets?

Temperature swings of over 500°C in a single day? Beyond Earth, the weather forecast is mostly terrifying.

Outside of Earth, there are very few places in the Solar System where a human would not be frozen solid or turned into steam in a matter of seconds. It can be as hot as 400°C on Mercury and Venus and as cold as below -200°C on the distant planets.

Apart from how close you are to the Sun, the strength of the atmosphere and the speed of the planet’s rotation can have a huge effect.

The Sun's energy

The Sun emits radiation through all the electromagnetic spectrum from radio-waves to X-rays. This radiation spreads out through the whole Solar System. The planets which are close to the Sun receive more heat than those further away. This simple fact explains broadly the range of surface temperatures for the planets according to their distance from the Sun. The actual situation is more complicated.

Planets with no atmosphere

For planets with no atmosphere, all the Sun's radiation will strike the surface. Some of this will be reflected away from the planet but the rest will be absorbed. For planets like Mercury, this results in a very hot surface where the Sun is shining (more than 400°C) but very cold on the night side, where the radiation from the surface rapidly cools it to -180°C.

The Moon is similar in many ways to Mercury. The night-side of the Moon is at almost the same temperature as that of Mercury but the day-side reaches 110°C.

Planets with atmospheres

The Earth is, of course, a planet with an atmosphere and we can use it as an example. A fair proportion of the Earth is covered by clouds, which reflect a lot of the Sun's radiation back into space.

The atmosphere affects the radiation emitted in turn by the warm Earth and traps some of the heat by the ‘greenhouse effect’. Carbon dioxide is the main gas involved and increasing levels are causing global warming of the Earth and changing its climatic patterns.

The planet Venus is an extreme example of the ‘greenhouse effect’. Venus is surrounded by clouds, preventing a lot of the Sun's radiation from reaching the surface and so we might expect the surface to be cool. However, the atmosphere of Venus is largely composed of carbon dioxide, trapping most of the radiation from the planet's surface. This is so effective that the surface is heated to 470°C!

The planet Mars has an atmosphere but this has a surface pressure less than one hundredth of the Earth's. It thus has only a small effect and the surface of Mars can vary between 0°C in summer and -100°C in winter.

The giant planets

The giant planets all have small solid cores surrounded by enormously thick layers of liquid forms of substances that on Earth we encounter as gases. The giant planets receive only a small amount of radiation from the Sun and this is insufficient to raise their temperatures above the point at which these gases liquefy or freeze.

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