If you didn’t know, it’s hot in North America at present. Really hot. As hot melt cables. The Pacific Northwest and Canada were the hottest they have ever been. Cities accustomed to summer temperatures reaching nearly 32 ° C (90 ° F) experience temperatures above 43 ° C (110 ° F). This is all part of climate change on our planet, and while not the hottest temperatures on Earth ever, this heat covers a remarkable part of the planet.
What controls the surface temperature?
Now the surface temperature of planets and moons is really controlled by three things. The first is the amount of solar radiation (called insolation) it receives, which is more or less dictated by the distance from our Sun. The closer to the sun, the hotter you could be.
The second is albedo, or the reflectivity of the object’s surface. Most rocky planets are made of dark rock which absorbs much of the energy it receives from the sun. If you’re lighter in color, maybe if your surface is covered in ice, you’re reflecting a lot of the energy back into space. So the darker the surface, the more heat absorbed, the higher the surface temperature.
Albedo only gives you part of the way. The third thing is the Greenhouse effect. Gases like carbon dioxide, methane, water vapor, and ozone will pass solar radiation as visible light, but then trap infrared radiation that the planet’s surface emits after absorbing sunlight. Sun. This means that an atmosphere containing these gases will warm up during the day, but will now cool well below zero at night as the greenhouse effect traps heat which re-emits near the surface.
The surface temperatures of the planets and moons in our solar system are all affected by these three factors. Let’s take a tour!
The inner planets
The most intimate planet, Mercury, get hot during the day thanks to its proximity to the sun … and by hot I mean a hot 800 ° F (430 ° C). Mercury’s dark, basaltic surface absorbs much of this energy, creating a roasting surface. However, Mercury does not have a real atmosphere, so when the Sun is not shining on the planet’s surface, it quickly cools down to -290 ° F (-180 ° C). That’s over 1000 ° F of temperature variation!
Venus Express image showing the planet’s clouds (lower half) and infrared radiation in the clouds (upper half) which show the high temperatures of the planet’s atmosphere. Credit: ESA
Venus, although further from the Sun than Mercury, is actually warmer on its surface. The temperature reaches an incredible 880 ° F (471 ° C) on Venus and it is thanks to the uncontrollable greenhouse effect of our sister planet. The thick atmosphere of carbon dioxide traps all thermal energy emitted into space and the surface never cools. The greenhouse effect is so powerful that daytime and nighttime temperatures hardly differ. These hostile conditions mean that the only landers we have on Venus have been operating for a few hours at most. We don’t know what sent Venus on this path to the carnage in the greenhouse, but I hope that the three missions headed this way over the next two decades will help unravel this mystery.
Earth to that “right” combination distance from the Sun, albedo and greenhouse effect. The average surface temperature is around 60 ° F (15 ° C), so we have liquid water. If our atmosphere were devoid of greenhouse gases, this average temperature would be below the freezing point of water, so we need a greenhouse effect. Still, it’s a precarious position, so we want to avoid overloading this greenhouse.
Further into the solar system, March lack of heat. The thin atmosphere of the Red Planet means it has a weak greenhouse effect in place, so although much of the surface is covered with rocks that absorb heat, the average temperature at the surface is only -20 ° F (-28 ° C) farthest distance from the Sun. Perhaps earlier in the history of Mars, it could retain more of its heat with a thicker atmosphere, but these days Mars is near a frozen wasteland.
Titan’s thick atmosphere, seen in this mosaic of Cassini images, prevents the surface from becoming as cold as the other moons around Saturn. Credit: NASA.
As we move towards the distant solar system, most rocky or icy objects lack the atmosphere to conserve the meager energy they receive from the Sun. In the Jovian system, moons like Ganymede warm to -171 ° F (-112 ° C) because it received only 1 / 30th of the energy the Earth receives from the Sun. Out to Saturn, a moon like Enceladus is at -330 ° F (-201 ° C) and by the time you get to Pluto, this dwarf planet experiences surface temperatures of -388 ° F (-233 ° C).
The only exception to the truly freezing temperatures of the outer solar system is Titan. It has a thick atmosphere that allows the surface to remain soft at -290 ° F (-179 ° C), which is hotter than it could possibly be (see Enceladus) but still cool enough for the liquid on its surface is methane and ethane instead of water.
So, as you sit in the scorching summer heat of Portland or Seattle, remember that much of the solar system is much colder… or at least we are not on Venus or Mercury!