Why are temperatures on the gas giants so low? Saturn, Jupiter, Uranus, and Neptune, the gas giants, are the largest planets in our solar system and have much lower average temperatures than their rocky counterparts.
Now, the question is, why do gas giants generally have lower average temperatures?
No, it is definitely not because those planets want to impress you with their coolness.
The true explanation is far more empirical in nature, and we are here to probe the frozen secrets of these massive planets.
Mainly, the hydrogen and helium in their atmospheres plus their great distance from the Sun are the reasons why gas giants are much cooler than Earth.
Why are Temperatures on the Gas Giants So Low?
The Jovian gas planets, Jupiter, Saturn, Uranus and Neptune inhabit the furthest reaches of our solar system.
Their atmospheres are mostly hydrogen and helium, although Uranus and Neptune, also have methane.
This thick gas blanket blocks out what little sunlight reaches them so that temperatures fall as low as -224oC.
So, to understand those lower temperatures, you need to get an idea about the atmosphere and composition of those gas giants.
Overview of the Gas Giants’ Atmosphere and Composition
Although the Jovian planets share some characteristics, they are different in size, mass, and density and are located millions if not billions of miles apart.
Even so, the mix of hydrogen and helium is present in a similar ratio to the Sun with Saturn being the most hydrogen rich at 98% and Neptune the least at 83%.
All have different compounds, including:
- Ammonia
- Ammonia hydrosulfides
- Methane
- A little water vapor
- … And silicon-based compounds
Fact: Mercury and Venus do not have any moons because they are so close to the Sun and its gravitational pull.
Atmospheres of Gas Giants
Saturn and Jupiter’s atmospheres are similar, although Jupiter also has traces of oxygen, carbon and nitrogen along with other heavy elements.
It is thought that these elements are delivered by asteroid or comet impacts but most of Jupiter’s mass is a liquid metallic hydrogen that produces a magnetic field.
Although distant from each other, Uranus and Neptune have similar hydrogen helium methane atmospheres but Uranus is incredibly cold, even for a planet in the outer solar system.
Moreover, Neptune has very strong winds, resulting in its lower temperatures.
The Gas Giants’ Distance from the Sun Compared to that of the Terrestrial Planets
The distance from the Sun will always have an impact on the temperature on any planet, and the same holds true for the gas giants.
The enormous distances between the planets are hard to comprehend.
But it helps to think of a planet as being either part of the inner system and close to the Sun or of the outer system closer to the Kuiper belt.
The four planets closest to the Sun, Mercury, Venus, Mars and Earth are called the terrestrial planets with solid cores.
Of the four, only Earth is in the Goldilocks Zone, that is, at a distance from the Sun where temperatures are conducive to life.
Fact: The Kuiper belt is an orbiting collection of ice, dust and rock, at the end of our solar system.
The Greenhouse Effect and its Role in Temperature Regulation
Just as with many other planets, including Earth, the atmospheres of the gas giants contain gases that create a greenhouse effect that warms the surface of the planet.
These gases form thick layers that allow heat from the Sun to pass through but trap it at the surface as infrared radiation.
Moreover, the heavy cloud cover prevents it to escape and reflects it back onto the surface to raise temperatures. Saturn appears to be an exception.
Although warm, it does not warm up excessively, despite absorbing and emitting infrared radiation as the other gas giants do.
The Greenhouse Effect and Lower Temperatures
Despite the greenhouse warming effect provided by the dense cloud cover, the gas giants remain extremely cold.
This is because they are so far from their Sun. Neptune is 3530 below zero and almost three billion miles away.
Jupiter is the closest, but its surface temperature is still 2300 degrees below zero.
The further away from the Sun a planet is, the less it feels its effects. And all the gas giants are far away from their suns.
This is because they are formed by capturing the available gases and dust that are expelled and pushed out far into space as their star is created.
The heavier material does not make it out that far.
An Important Consideration
At the birth of their star, there is sufficient heat to kick start the process of planet formation even at vast distances.
But this quickly dissipates leaving the gas giant with a small core or perhaps no core at all.
Internal Heat Sources and How They Contribute to Low Temperatures
With their enormous mass, the gas giants can maintain an internal heat source.
Jupiter and Saturn are so efficient that they emit more heat than they receive.
It is this thermal radiation that drives the planets’ winds and weather.
Pockets of differing densities and pressures affect temperatures so they fall.
The Role of Internal Heat Sources in Planetary Temperature Regulation
Saturn uses helium condensing as rain as an internal heat source.
The mechanism increases temperatures as kinetic energy is transferred to the atmosphere by the motion of the falling drops.
Now thermal energy convection circulates back through the atmosphere where some are lost to space as radiated heat.
However, helium is left over from planet formation and is a finite resource so climatic conditions will eventually change.
Neptune’s internal heating source is the result of tidal torque from its moon, Triton.
Much of this thermal energy is lost which is forcing Triton into a decaying orbit as a consequence.
The Effect of Gravitational Contraction and Residual Heat
Except for Neptune, the gas giants also emit internal heat via gravitational contraction.
The largest giant, Jupiter, is contracting at a rate of about 1mm per year.
The gravity of their own mass compresses their atmosphere, increasing pressure which releases residual heat.
The three planets also emit heat created by radioactive decay as individual atoms still excited by reactions of planet formation spontaneously give off radiation and heat as they attempt to stabilize.
Fact: Observing and modeling different processes confirm to scientists that these gas giant’s formation is not yet complete, as it seems likely for Neptune.
More about Atmospheric Circulation and Low Temperatures
For Jupiter, Saturn, and Uranus the result is a dynamic and turbulent atmosphere capable of very high winds and blistering heat.
As pressure, density, and temperatures interact in the atmosphere powerful circulating storms are created.
Some last decades, or as in the case of Jupiter’s Red Spot, hundreds of years.
The Role of Atmospheric Circulation in Temperature Regulation
The thick vaporous clouds of gas giant storms like those observed in the Red Spot, not only determine the chemical makeup of the atmosphere, but also its dynamics.
Depending on their density, the clouds reflect, absorb or scatter the weak light from the Sun and other stars affecting the temperature, density and pressure within the cloud.
Precipitation occurs, although what falls may not be pure water even though water vapor plays a role in cooling the atmosphere as it condenses out the clouds.
As the atmosphere circles the planets in jet streams and winds heat is distributed planet-wide.
Circulation Patterns and How They Contribute to Low Temperatures
Much of the activity takes place along the gas giants equator forming belts and zones as observed on Jupiter.
How far the activity within them reaches down to the center of the planet is not known, but peaks and troughs in the layers can be seen clearly, as can patterns in the circulation.
Scientists have observed day/night and seasonal variations activity related to lowering temperatures.
Uranus, for example, has unusually cold temperatures with an atmosphere that is relatively calm until its orbit takes it closer to the Sun.
This movement causes uneven changes in temperature, which shifts warm air currents into colder areas.
Fact: Jupiter is the largest among the gas giants and has 79 moons.
Takeaway
Why are temperatures on the gas giants so low? It is true that gas giants are far cooler than rocky planets.
That is mainly because they are further from the Sun, and they do not get as much heat from it.
Also, they do not have the same ability to retain heat as rocky planets because their atmospheres are mostly hydrogen and helium.
And what little sunlight does make it through the atmosphere’s heavy gas cover causes freezing conditions.
