Lesson 2, part 4: Earth vs. Venus

By: Shaun Tanner , 9:58 PM GMT on October 19, 2011

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In the last lesson, you learned about the greenhouse effect and how it is essential for life on Earth. The greenhouse effect is created by greenhouse gases such as carbon dioxide and methane that form a blanket around the surface. This blanket allows the warmth emitted by the surface to remain in the atmosphere, keeping the surface warmer than it would be otherwise.

This lesson, however, will give you a practical application of the greenhouse effect gone wild. It is also a near-Earth warning shot of the direction the Earth could be headed if we continue to pump greenhouse gases into the atmosphere.

Earth vs. Venus

Our solar system is made up of nine planets. True, Pluto was recently relegated to a dwarf planet, but I still call it a planet because that is how I grew up. Old school. Earth is the third planet from the Sun, making it one of the warm planets. Earth's nearest neighbor is Venus, which is the second planet closest to the Sun. Thus, it makes a pretty good comparison when comparing the atmosphere of the two planets.

There are three main factors that determine the surface temperatures of both Earth and Venus.

1. Which planet is closer to the Sun?
2. What planet is more reflective?
3. Which planet has a larger ability to absorb and retain solar energy?

Let's take a look at each of these step-by-step.

Which planet is closer to the Sun?

Since Venus is closer to the Sun, you would obviously expect it to receive more solar radiation than Earth. In fact, sunlight striking Venus is 93% stronger than sunlight striking Earth, thus we would expect Venus to be warmer. 93% is considerably more solar radiation. That is almost double what the Earth receives.

Thus, based only on the distance from the Sun, the average surface temperature of Venus should be much warmer than Earth. Keep in mind, this takes into account that ALL of the solar radiation that strikes the planets is absorbed by the respective surfaces. If this were to happen, the surface temperature of Earth would be 5°C and the surface temperatures of Venus would be 55°C. That is a big difference. However, based on the last lesson when you learned about the greenhouse effect, you know that the average surface temperature of Earth is around 15°C. Thus, something is up.

What planet is more reflective?

One of the things that is "up" is albedo. Albedo is the percentage of solar radiation that is reflected back to space. The higher the albedo, the more radiation is reflected. When solar radiation is reflected, it is not absorbed and thus takes no part in warming the surface. Thus, if the albedo of a planet is 100%, then it reflects ALL of the incoming solar radiation and thus SHOULD be a pretty cold place because solar radiation is not warming the surface.

The albedo of the Earth is 30%. This means that nearly a third of the Sun's radiation is bounced back out to space by light-colored things on Earth such as ice, snow, deserts, and clouds. If we wanted to increase the albedo of the Earth, we could cover it in snow. This would cool the planet.

The albedo of Venus is 70%. This means that the atmosphere of Venus reflects a whopping 70% of the solar radiation that hits its atmosphere and TAKES NO PART IN WARMING OF THE PLANET. That is key. Remember that as we move forward.


Figure 1. Venus on June 25 2005. Note the high reflectivity of Venus.


Figure 2. A picture of Earth from space.

Which planet has a larger ability to absorb and retain solar energy?

So what on Venus does all the reflecting? Well, remember, the "stuff" that does the reflecting on Earth are the light-colored things like ice, snow, etc. Well, Venus is quite literally covered with a thick layer of clouds made of sulfuric acid. This acid is very highly reflective and does the majority of the solar radiation reflection.

So, let's re-examine the numbers:

If the amount of solar units hitting Earth is 100 units, then based on the fact that Venus is closer to the Sun, it receives 193 units. Yet Earth absorbs 70% of these units and Venus absorbs 30% of these units.

Earth absorbs:

100 X 0.70 = 70 units

and Venus absorbs:

193 X 0.30 = 58 units

Thus, based on the distance from the Sun AND the albedo of both planets, Venus should be COLDER than Earth because it actually absorbs a lot less solar radiation.

Here's the rub. The average temperature of Earth is 15°C, while the average surface temperature of Venus is........480°C! What is up with that? Something else must be going on.

The answer is in the respective atmospheres. Earth's atmosphere is primarily made up of Nitrogen (78%) and Oxygen (21%). There are also trace amounts of carbon dioxide, water vapor, methane, etc. Venus has a much simpler atmosphere that is made up mostly of carbon dioxide and nitrogen. The thing is, the atmospheric content of carbon dioxide on Venus is 96%. The remaining 4% is nitrogen.

Also, because carbon dioxide is a much heavier gas than oxygen and the other gases in Earth's atmosphere, Venus' atmosphere is much more massive than that of Earth. So much so that the mass of just the carbon dioxide in Venus' atmosphere is almost 100 times greater than the TOTAL mass of the Earth's atmosphere! That is amazing.

Here is where we put it altogether. Carbon dioxide is a greenhouse gas. We learned this in the last lesson. Carbon dioxide is very good at trapping radiation from the surface. The more carbon dioxide in the atmosphere, the more radiation it will trap and the hotter the surface will be. Venus' atmosphere contain around 25,000 times as much greenhouse gases as the Earth's atmosphere. Thus, it is much more efficient at trapping radiation and accounts for the vast difference between Earth's surface temperature and Venus' surface temperature.

Thus, Venus has a much hotter surface that Earth, despite the fact that Venus absorbs much less sunlight that Earth does.

I wanted you to know this because if we wanted to debate what happens when we pump more carbon dioxide into our own atmosphere, we look no further than our nearest celestial neighbor. It has a scientific story to tell. That story is also a warning.

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5. BirdsAreOff
5:07 PM GMT on October 22, 2011
The Earth is so beautiful. Thank you and all the folks at WU for trying to keep it so. And for explaining the science.
Member Since: August 29, 2011 Posts: 0 Comments: 2
4. BaltimoreBrian
5:09 AM GMT on October 22, 2011
David Harry Grinspoon wrote an excellent book about Venus called "Venus Revealed". Engagingly written! It came out in 1996 but incorporates all the Magellan observations.
Member Since: August 9, 2011 Posts: 26 Comments: 8714
3. Patrap
2:48 AM GMT on October 21, 2011
NASA sent Mariner 2 to Venus in 1962... in the first-ever close planetary encounter. Its instruments showed that Venus is nothing at all like Earth. Rather, it's extremely hot, with an atmosphere made up mostly of carbon dioxide.

The data showed that Venus rotates very slowly... only once every 243 Earth days... and it goes in the opposite direction. American and Soviet scientists found out just how strange Venus is when they sent a series of landers down to take direct readings.

Surface temperatures are almost 900 degrees Fahrenheit, hot enough to melt lead, with the air pressure 90 times higher than at sea level on Earth. The air is so thick that it's not a gas, but a "supercritical fluid." Liquid CO2. On our planet, the only naturally occurring source is in the high-temperature, high-pressure environments of undersea volcanoes. It comes in handy for extracting caffeine from coffee beans... or drycleaning our clothes.

You just wouldn't want to have to breathe it. The Soviet Venera landers sent back pictures showing that Venus is a vast garden of rock, with no water in sight. In fact, if you were to smooth out the surface of Venus, all the water in the atmosphere would be just 3 centimeters deep. Compare that to Earth... where the oceans would form a layer 3 kilometers deep.

If you could land on Venus, you'd be treated to tranquil vistas and sunset skies, painted in orange hues. The winds are light, only a few miles per hour... but the air is so thick that a breeze would knock you over. Look up and you'd see fast-moving clouds... streaking around the planet at 300 kilometers per hour. These clouds form a dense high-altitude layer, from 45 to 66 kilometers above the surface.

The clouds are so dense and reflective that Venus absorbs much less solar energy than Earth, even though it's 30% closer to the Sun.

Member Since: July 3, 2005 Posts: 426 Comments: 129089
2. 1911maker
2:36 AM GMT on October 21, 2011
interesting stuff

Do you have data on the Temp of Venus over the same time frame that Earth has been rising? IE if Venus went up a similar amount as earth we could blame the temp rise on the sun.

If Venus stayed "flat", then the sun can be ruled out.

Or am I really missing some thing and over simplifying?
Member Since: February 25, 2011 Posts: 0 Comments: 474
1. Angela Fritz , Atmospheric Scientist (Admin)
12:58 AM GMT on October 20, 2011
A-run run run run runawaaaayy....

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About shauntanner

Shaun Tanner has been a meteorologist at Weather Underground since 2004.

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