Astronomy Question: Average Temperatures in the Solar System?

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Alex asks:

Dear Astronomer, What are the average high and low temperatures of the planets in our solar
system?

Great question Alex!

Given the different distances and compositions of objects in our Solar System, there is a wide range of temperatures. What is very interesting about the range of temperatures is how a planet’s atmosphere (or lack of) can affect it’s temperature, along with the surface composition.

Keep reading for a full list of high and low temperatures across our Solar System, along with what factors can affect said temperatures.

I’ll start with a simple table of temperatures, then I’ll explain the factors that affect temperatures. Keep in mind these figures are “ballpark” figures, listed for the sake of comparison.

Mercury: 100 – 700 Kelvin ( -170 – 420C )

Venus: Average temp of 740 Kelvin ( 460C)

Earth: 180 – 330 Kelvin (-90 – 56C)

Moon: 100 – 400 Kelvin (-170 – 56C)

Mars: 180 – 300 Kelvin (-90 – 26C)

Dwarf Planet Ceres (Asteroid Belt) : 160 – 240 Kelvin (-120 – -30C)

Jupiter: Atmospheric Temperatures of around 150 Kelvin (-120C)

Europa (Moon of Jupiter) :50 – 120 Kelvin (-220 – -150C)

Saturn: Atmospheric Temperatures of around 120 Kelvin (-150C)

Titan (Moon of Saturn) :About 90 Kelvin (-180C)

Uranus: Atmospheric Temperatures of around 60 Kelvin (-210C)

Neptune: Atmospheric Temperatures of around 60 Kelvin (-210C)

Triton (Moon of Neptune/Possible captured Kuiper Belt Object) :About 40 Kelvin (-230C)

Dwarf Planet / Kuiper Belt Object Pluto: 30 – 50 Kelvin (-240 – -220C)

Dwarf Planet / Kuiper Belt Object Eris: 30 – 50 Kelvin (-240 – -220C)

Unofficial Dwarf Planet Sedna: 10 Kelvin (-260C)

Based on the above list, it’s fairly intuitive that distance from our Sun plays an important role in the temperatures of objects in our Solar System. A few important considerations though are the reflectivity of an object, and its atmospheric composition. I could make a completely separate post on the albedo and atmospheric properties of all the major objects in our Solar System, but for the purposes of this post, I’ll keep it to the basics.

As we know, things like ice reflect sunlight, whereas dark sand and rock tend to absorb sunlight. Interestingly enough, as bright as our Moon is during its full phase, lunar reflectivity is about the same as asphalt pavement. On the opposite end of the spectrum, astronomers estimate that Eris is nearly as reflective as a household mirror.

So, reflectivity plays a role in surface temperature, but how does an atmosphere (or lack of) affect surface temperature?

The basics are that a thick atmosphere acts like a blanket. Similar to how cloud cover on Earth can trap heat (hence why deserts can get really cold at night), an atmosphere can retain heat, given sufficient density and composition. A thick atmosphere can also stabilize the surface temperature and prevent wild variations from freezing cold to boiling hot.

Compare Mercury to Venus. Sure, Mercury is a lot closer to the sun, but it’s lack of a significant atmosphere results in a wide temperature variation. The thick, crushing atmosphere of Venus traps heat, and also distributes it more or less evenly, so Venus doesn’t experience much variation in surface temperature.

Go a bit farther from the Sun, and compare Earth, the Moon, and Mars. Interestingly enough, despite being farther from the Sun, Mars has a minimum temperature similar to Earth, of course the thin Martian atmosphere doesn’t retain much of the heat, so surface temperatures rarely exceed the freezing point of water. Even Dwarf Planet Ceres (in the Asteroid belt) has somewhat comparable temperatures to Mars!

Once you get out to Jupiter and beyond, things get cold… really, really cold. Even with an atmosphere thicker than Earth’s, Titan only hovers around -200C. Once you are out at the fringes of our Solar System objects aren’t much warmer than the Cosmic Microwave Background Radiation of ~2.7 Kelvin. For reference, helium boils at about 4 Kelvin. As shown above, temperature estimates of Sedna place its surface temperature at around 10 Kelvin… brrrrr indeed!.

I hope I’ve given you a good idea of the “climate” of our Solar System. Stay tuned, as I’ll most likely follow up this post with an article on the effects of atmospheric and surface compositions on the temperature of a body in our solar system.

Thanks for reading!

Ray Sanders is a Sci-Fi geek, astronomer and blogger. Currently researching variable stars at Arizona State University, he writes for Universe Today, The Planetary Society blog, and his own blog, Dear Astronomer

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