Venus, the second planet from the sun and Earth’s closest neighbor is the brightest object in the night sky after the moon. Although Venus is twice the distance away from the sun as Mercury is, it is our solar system’s hottest planet.How is that possible?
Let’s find out!
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This article explores Venus’ temperature. We study its CO2-filled atmosphere, its weather and climate. Together, these will explain the factors that make Earth’s twin sister the hottest planet in our system.
How Hot is Venus?
The average temperature of the Venusian atmosphere hovers around 863°F (462°C). At 872°F (467°C), its surface temperature is hot enough to melt lead!
Venus’ atmosphere is so reflective that the planet absorbs only 2.5% of solar energy (Earth absorbs 50%). Since very little of it reaches the surface, there is little variation in temperature across the Venusian globe as it orbits the sun.
The atmosphere acts like a thick blanket around the planet and prevents heat from escaping back into space. Moreover, since Venus is only titled 3° with respect to the sun, there is also a consistency in high surface temperatures during days and nights across all latitudes.
Why Isn’t Mercury the Hottest Planet in the Solar System?
Since Mercury lacks a thick atmosphere, it reflects most of the received solar energy back into space. Venus’ thick, CO2-filled atmosphere prevents the heat from escaping, thus maintaining a consistent 863°F across latitudes at all times. This is higher than Mercury’s 800°F, leading Venus to be the hottest planet in the solar system.
Let’s look at this in a little more detail.
Reflecting and Absorbing Sunlight
When planets receive sunlight, they absorb some of it and reflect the rest back into space, a property known as albedo.
The exact amounts of absorption and reflection depend on atmospheric composition and its thickness but, generally speaking, the higher the albedo, the lower the rate of absorption.
Mercury’s albedo is 0.119, meaning only 11.9% sunlight that hits it gets reflected, whereas the remaining 88.1% is absorbed. Venus’ albedo is 0.9—the highest of all planets in our solar system, meaning 90% of the received sunlight is reflected.
That’s why, when we observe Venus, it’s so bright in our telescopes.
Even though Venus reflects much more of the sun’s energy it receives than Mercury, it’s hotter. That shows us that Venus must keep the heat it receives, which we’ll examine next.
Role of Atmosphere in Trapping Heat
If Mercury both receives four times more sunlight and absorbs more of it compared to Venus, why is Mercury’s temperature less than Venus? This is because Mercury lacks a critical aspect: a thick atmosphere.
Mercury’s thin exosphere consists of hydrogen, helium, potassium, and oxygen—elements blown upwards from its surface by strong solar winds. Due to this, Mercury sends most of its received sunlight back into space.
Venus’ atmosphere, on the other hand, consists primarily of carbon dioxide and nitrogen, which is much denser and hotter than Mercury’s atmospheric elements.
Additionally, the highly reflective cloud layers (a major cause for the planet’s brightness) trap the absorbed sunlight, preventing the heat from escaping into space.
While Mercury absorbs heat during the day and dissipates most of it at nights, Venus’ atmosphere maintains the planet’s warmth, as shown in this explanatory video.
The Answer is in the Atmosphere!
Since Mercury is closest to the sun at 58 million kilometres, it is natural to assume Mercury to be the hottest planet in our solar system. But, if we compare Venus to Mercury, there are some key differences.
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It takes only 88 Earth days to complete an orbit but 176 Earth days to rotate once around its axis. At this slow rate, the planet’s dayside temperatures reach a maximum of 800°F (427°C) while on the nightside they drop sharply to -280°F (-173°C).
On the other hand, Venus resides 108 million kilometres from the sun, takes 255 Earth days to complete an orbit at twice the distance from the sun as Mercury, and absorbs only 2.5% solar energy.
Even so, the planets’ thick insulating atmosphere causes surface temperatures to be a consistent 863°F at all times, making it hotter than Mercury!
How Does Venus’ Atmosphere Compare to Earth’s?
Venus’ atmosphere is 100 times hotter and thicker than Earth’s!
It primarily consists of carbon dioxide whereas nitrogen dominates Earth’s atmosphere. There is also a lack of water in the Venusian atmosphere, which is very different from Earth.
The air pressure on Venus’ surface is 92 times that on Earth. To match Earth’s atmospheric pressure and temperature, a person would have to be 50 kilometres above the surface.
The atmospheric pressure is so high on Venus that no spacecraft landing there has lasted more than an hour.
The high pressures led to an increase in temperatures, creating a runaway greenhouse effect (similar to global warming on Earth, only much worse).
While Earth has clouds made of water vapor, Venus’ sky is filled with sulfuric acid clouds. On the surface, the environment remains very hot and dry.
What Gases is the Venusian Atmosphere Composed of?
The most abundant gas is carbon dioxide (CO2), accounting for 96.5% of the Venusian atmosphere. The next major gas is nitrogen (N2) at 3.5%. Traces of sulfur dioxide, carbon monoxide, argon, and neon make up 0.5% of the atmosphere.
Where Did the Atmosphere Come From?
After the lighter gases were blown away by the solar wind during Venus’ evolution, leftover carbon dioxide and nitrogen dominated its atmosphere.
Venus, like all planets, started out with an atmosphere containing hydrogen and helium. These light elements and liquid water that was once stable in Venus’ atmosphere evaporated.
Carbon dioxide made up an ever-larger proportion of the atmosphere, leading to a sharp increase in the greenhouse effect. This heated up the Venusian interior, which generated even more carbon dioxide that currently constitutes 96% of the Venusian atmosphere.
How Do We Study the Venusian Atmosphere?
There are two main ways in which we study the atmosphere of Venus: space probes and earth-based observatories.
Space probes sent explicitly to Venus like Pioneer Venus orbit the planet and sometimes attempt a landing to. All the while they send crucial information data to Earth.
Other missions, like Venera 2, are designed to plunge through the atmosphere and send back data to Earth before they perish due to atmospheric pressures.
Spacecraft bound for the solar system’s outer planets, like Galileo and Cassini, perform flybys of Venus and collect what information they can.
On the ground, here on Earth, are missions that send radar signals to Venus and study the planet based on the received echoes.
Transits of Venus
Every 115 years, Venus’ orbit places it on the same plane as the Earth and the sun as it passes between them.
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Known as a Transit of Venus, this placement provides astronomers a rare opportunity to study Venus’ atmosphere.
Using a method called spectroscopy, astronomers study how light is scattered by Venus’ layers of atmosphere. Since different elements react differently to light, this method is used to learn the atmospheric elements and their quantity.
Weather and Climate on Venus
We’ll finish our investigation of why Venus is so hot with a look at the weather and climate on our nearest planetary neighbor.
Super Rotation of Atmosphere
Venus is a hot, dry planet. Thick layers of clouds form an opaque blanket, so much so that Venus’ surface is not visible from the outside.
Different layers of clouds move at varying speeds. The top-most layer rotates around the Venusian globe within 4 Earth days, a speed that is 60 times the speed of Venus’ rotation.
Wind speeds in the upper layers reach 190 mph (300 kph). These winds are in retrograde motion and are faster near the poles compared to the equator.
However, near the surface, there is almost zero wind due to the phenomenally high air pressure.
Water
There is almost no water in the Venusian atmosphere.
Since solar winds eroded water from Venus’ surface, the planet now does not experience storms and precipitation (water is required to evaporate into water vapor and condense to form clouds).
However, there is evidence that the sulfuric acid clouds produce lightning.
Seasons
The planet’s lack of axial tilt prevents it from experiencing seasons as we know them here on Earth. If Venus was tilted more than the current 3.39° (Earth’s tilt is 23.4°), its two hemispheres would receive differing amounts of sunlight throughout the year, leading to seasons.
Added to the lack of tilt is the fact that Venus’ orbit is nearly circular, meaning it is always at a similar distance from the sun and receives the same amount of light. So there is nothing eccentric about its orbit which may have forced different temperatures throughout its year.
Diurnal Temperature Variation
Not only are there no seasons on Venus, but there is very little diurnal temperature variation either, i.e. day and night temperatures are the same.
This is because Venus’ winds move heat around the planet but the atmosphere prevents the planet from dissipating it into space.
So, in addition to lack of seasons, the planet Venus does not cool down at night.
Conclusion
Even though Venus is called Earth’s twin sister, it is a very different place.
Temperatures are high, seasons do not exist, and wind speeds are faster than the planet’s rotation itself.
Venus’ thick atmosphere traps most of the received solar heat, increasing its temperature through the greenhouse effect. The temperatures are so high currently that they can melt lead.
While most planets cool down during the night, Venus does not.
For all of these reasons, Venus is the hottest planet in our solar system.
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