Why is Venus Hotter Than Mercury?

Venus and Mercury are the closest planets to the Sun in our Solar System. So, Why is Venus Hotter Than Mercury? While Mercury is much closer, Venus has a higher average surface temperature. To be fair, Mercury is still screaming hot. Just not as sizzling as its neighbor.

Let’s explore why Venus is hotter than Mercury. And you might also be wondering just how hot Venus is anyway, so we’ll tackle that too.

The photo below is a Magellan radar image compilation with altered colors to see Venus’ surface differences. 

Why is Venus Hotter Than Mercury?
Compiled Venus Image: NASA

Venus, Mercury, and The Sun

The Sun’s impact on its nearest planets is significant regarding planetary temperature, atmosphere, and orbital dynamics.

How Far From the Sun Are Mercury and Venus?

The closest planet to the Sun is Mercury, with an average distance of 36 million miles (58 million kilometers.) However, Mercury’s proximity to the Sun varies due to its elliptical orbit. But even though it moves nearer or farther to the sun, it is still always closer to the Sun than Venus.

Venus averages about 67 million miles (108 million kilometers) from the Sun. And it also has an elliptical orbit but still stays further away than Mercury.

How Hot Are Mercury and Venus?

Venus is the exception to all planets in that the further the others get from the Sun, the colder they are. The mean temperatures of the Sun’s nearest and furthest planets are

  • Mercury: 333℉ (167℃)
  • Venus: 867℉ (464℃)
  • Neptune: Minus 330℉ (-200℃)
  • Dwarf Planet Pluto: Minus 375℉ (-225℃)
Messenger Mercury Image: NASA

What is Planetary Albedo and How Does it Affect Temperature?

Albedo refers to the ability of a surface to reflect sunlight. So higher albedo surfaces reflect more sunlight, while those with a lower albedo absorb it. For example, think of it in terms of car seats. Black seats get hotter during the summer, while white or tan seats stay cooler.

Planetary albedo plays an essential role in determining the temperature and climate of planets and other celestial bodies. For example, high-albedo planets reflect more sunlight and should have lower temperatures. In contrast, low-albedo planets absorb more sunlight and have warmer temperatures.

Mercury’s Albedo

Like other planets, Mercury’s albedo varies depending on the portion of its surface that scientists measure. However, its overall albedo comes in at an estimated at 0.142. So that means it reflects about 14.2% of the sunlight that falls on it and absorbs the remaining 85.8%.

For comparison, Earth has a geometric albedo of 0.434, so our planet absorbs about 60% of the sunlight it receives and reflects the other 40%.

Mercury’s Albedo
Studying Mercury’s Surface: NASA/Carnegie Institution of Washington/Johns Hopkins University Applied Physics Laboratory

Venus’ Albedo

On the other hand, Venus has a relatively high albedo of about 0.689. So it reflects approximately 70% of the sunlight it receives and only absorbs 30%. 

Scientists think the planet’s high albedo comes from Venus’ thick sulfuric-cloud atmosphere, reflecting a significant amount of sunlight and radiation back into space. So the resulting sunlight reflection is one of the reasons Venus appears so bright in the night sky.

So Why is Venus Hotter Than Mercury If it Absorbs Less Sunlight?

If a planet’s albedo rises and sends more light back to space, then planets generally get cooler. But Venus is hotter than Mercury.

  • Because of its thick, cloudy atmosphere, Venus absorbs less sunlight. 
  • However, the thick atmosphere also acts like a blanket trapping warmth inside. 
  • So just like a house with good insulation maintains a more even temperature than an uninsulated home, Venus has less temperature variation between its dayside and nightside.

It captures a smaller portion of the Sun’s heat than Mercury, but Venus keeps what it receives. And that’s part of why Venus is the hottest planet in our Solar System.

Let’s examine what Mercury and Venus are doing with the radiation they absorb.

Mercury’s Lack of Atmosphere

Mercury is closer to the Sun and receives and absorbs more sunlight than Venus. But it has a lower planetary temperature. And that’s because Mercury has a thin exosphere instead of an atmosphere.

Why is Mercury’s Exosphere So Thin?

Mercury’s exosphere mainly contains helium and hydrogen. But there are also traces of potassium, sodium, and oxygen. All these components get replenished by meteoroid strikes and solar winds blowing off the nearby Sun. 

Solar weather constantly bombards Mercury. In addition, the planet cannot retain a significant atmosphere due to its low gravity and small size. So any gases released from Mercury’s surface tend to escape directly into space.

Mercury’s lack of atmosphere and slow rotation also gives the planet a highly variable temperature profile. So dayside temperatures grow extremely hot (>8000℉) , while nightside temperatures can reach extreme cold (-290℉ (-179℃.)) 

Additionally, Mercury has a low surface pressure that is technically considered a vacuum. So overall, the absence of a significant atmosphere with no greenhouse gases means the planet doesn’t experience weather or atmospheric phenomena like clouds or winds. In other words, Mercury lacks a way to trap heat and warm its surface.

The Runaway Greenhouse Effect on Venus’ Atmosphere

Spacecraft probe measurements of Venus’ atmosphere show evidence of water’s molecular remnants. So at one point, the planet could control its temperatures. But the water evaporated when the planet got hotter, forming clouds that reflected the Sun’s radiation back into space.

On Earth, our plate tectonics help regulate and modulate the carbon. But Venus doesn’t have them.

Explanation of Greenhouse Effect

The Sun continues getting brighter as it ages, increasing its radiation intensity. And so, Venus could no longer cope with the increasing heat. As more water evaporated, more clouds formed, and more of the Sun’s radiation got trapped.

Eventually, the remaining water on Venus turned to vapor, making it possible for the Sun’s heat to break it up. Without water and its climate-balancing cycles, the atmosphere gathered carbon to suffocating extremes. And the planet could no longer rid itself of the Sun’s heat energy.

Venus Cloud Cover
Venus Cloud Coverage Image: NASA

Summary: Why is Venus Hotter Than Mercury?

Venus is hotter than Mercury because its atmosphere is full of greenhouse gases. Carbon dioxide build-up and thick, smelly sulfuric acid clouds trap heat. And even though the cloud coverage reflects the Sun’s brightness, the radiation Venus does absorb sticks around.

So you end up with a hellish furnace-like surface that looks similar to the red-hot glare of an artist’s blob of glass, heated for blowing. 

On the other hand, Mercury has a super thin atmosphere that is actually an exosphere. And it is constantly subjected to the Sun’s intense solar winds. So even though Mercury forms no natural weather systems on its own, the Sun’s weather dramatically impacts it.

In addition, Mercury’s low albedo allows the planet to absorb much of the Sun’s heat energy. But that thin exosphere lets the energy dissipate back into space. Since Mercury basically has no insulation, heat rises and drifts away.

Since Venus is so well-insulated, it absorbs and retains more of the Sun’s energy. And that makes Venus hotter than Mercury and every other planet in our solar system.