By Jim Costello
Beautiful Venus, striking and bright in the western evening sky, brought wonder and awe to human imaginations before the concept of a planet existed. Today, knowing what we do about Venus, those thoughts are stretched even further with critically important questions for Earth and life itself.
When Venus is close to Earth, passing on an inside lane around the sun, it is by far the brightest star-like object in the sky. Then she follows the setting sun, first getting farther from the sun as days pass, and then getting closer to the sun. Finally, Venus disappears for a few days only to reappear and lead the sun in the eastern morning sky.
Many cultures thought Venus was two wandering stars – one called the morning star and the other the evening star. But the Greeks after Pythagoras (Circa 570 BC to 495 BC) understood that Venus was a single object, as did the Mayan people. Venus was important in Mayan religion, and the Mayan sacred calendar – as opposed to their agricultural calendar, was based on the approximately 20 lunar months between successive similar appearances of Venus.
Through a telescope, Venus is a disappointment compared to other planets, for one sees only smooth white clouds. No craters or hints of roughness like that of the moon or Mars can be seen. The Venusian surface is forever hidden from visual inspection by these thick clouds. It wasn’t until the invention of radar that the clouds were penetrated and rough surface features detected.
These clouds are not water vapor like Earth clouds, but mostly tiny droplets of sulphuric acid, reflecting 90 percent of the sun’s energy. As a consequence, the Venusian surface receives only two-thirds the solar energy that Earth gets, even though the sun’s intensity at the distance of the Venusian orbit is almost twice that of its intensity on Earth. Because so little solar energy reaches the surface, astronomers once believed Venus must be cold. Imagine how dark and cold Earth would be with a constant, complete cover of thick clouds and only 10 percent of the sun’s energy reaching the surface.
Astronomers sometimes spoke of Venus as a sister planet to Earth, as it is very nearly as large as Earth. But there are some peculiarities. Venus rotates backwards on its axis. This means the sun rises on Venus in the west – not that anyone could see it through the clouds. Venusian days and nights are very long because the planet rotates slowly, taking 243 Earth days to make one complete turn. It is thought that Venus must have been hit a hard glancing blow in the early solar system development by some large object, which effectively stopped a rotation that must have originally been similar in direction and speed to Earth’s.
In 1961, the Russian probe Venera perished almost immediately upon entering the atmosphere of Venus, and little was learned. The 1962 Mariner spacecraft mission verified an earlier suspicion based on microwave measurements. The surface of Venus is very hot – hot enough to melt lead! That horrendously high temperature reigns over the entire surface, even at the poles and on the night side, putting to rest hopes that Venus might harbor life. Even the thermal vents in the deep oceans of Earth that support primitive life forms are not as hot. Worse yet, there is no water anywhere on Venus.
Why is Venus so hot when many expected the surface to be cold? The Venusian atmosphere proved to be over 96 percent carbon dioxide. There is so much of it that the pressure at the surface is 92 times that on Earth, equivalent to the pressure more than half a mile beneath the surface of Earth’s oceans. The high temperatures are no mystery: They are the result of a huge greenhouse effect.
In 1896 a Swedish physicist/chemist, Svante Arrhenius, substantiated the importance of carbon dioxide, suggested earlier by others, in the warming of Earth’s surface. He undertook an involved arithmetic calculation, necessarily leaving out many important effects, but demonstrating how the absorption of infrared radiation by carbon dioxide would warm the Earth.
A carbon dioxide molecule is a carbon atom flanked by two oxygen atoms, resembling a baton. The baton can rotate and flex in various ways, and when bathed in long wavelength (infrared) radiation, it can absorb that radiation, soaking up energy.
Arrhenius answered the then-perplexing question about why Earth is as warm as we find it. Scientists in the 19th century applied their newly discovered laws of thermodynamics to Earth. They learned that the temperature on Earth, required to maintain a balance between energy coming in from the sun and energy going back to space, is only a very chilly 0 degrees F. If there were no carbon dioxide in Earth’s atmosphere, it would be an ice ball with little liquid water. Instead, Earth’s surface temperature is a balmy 57 degrees F and we are glad of it.
Living in Stockholm, Arrhenius did not view increases in atmospheric carbon dioxide as a bad thing, and suggested that industrial production of carbon dioxide might make Earth, or at least Stockholm, more pleasant.
A natural, and scary, next question is: Could Earth become like Venus? That nightmare scenario goes like this. Human discharges of carbon dioxide warm the Earth past a tipping point in which methane, also a greenhouse gas, is released from the continental shelves under the oceans. Methane ice has accumulated from decaying marine life over millions of years.
If this happens, then subsequent developments are completely out of our control. A second tipping point occurs when a temperature is reached at which the warming oceans and soil give up even more carbon dioxide. Experts disagree about the likelihood of such a scenario and whether or not it could proceed to Venus-like lengths. One can only hope that it is unlikely.
Part of that scenario played out 55 million years ago in an event called the Paleocene-Eocene thermal maximum. This occurred when the Indian subcontinent crashed into Asia, releasing methane from the subducting plates. Earth’s average temperature went up 22 degrees F, there was no permanent ice on the planet, and alligators lounged near the North Pole.
Venus has millions of times more carbon dioxide in its heavy atmosphere than does Earth. But we also know that Venus once had water and a much cooler, transparent atmosphere. Some physical process – perhaps volcanoes, the more intense energy from the sun, or a combination – started the runaway greenhouse effect, evaporated the liquid water and baked more carbon out of the surface material. On Venus, most of the planet’s carbon is in the atmosphere, while most of Earth’s carbon is still in carbonate rocks. We should hope it remains there.
Venus has gifted us yet again with a cautionary tale. When you next see her, think about this story. Its corollary is clear: We must drastically reduce adding carbon dioxide to Earth’s atmosphere.
Editor’s Note: The author of this essay, Jim Costello of Salida, approached me a few years back about an essay concerning Venus and global warming. Sadly, Jim passed away on June 21, 2013. A former physics instructor at Gustavus Adolphus College in St. Peter, Minnesota and later at Fort Lewis College in Durango, he was also a partner there in an early solar energy company, Tritec Solar Industries. His partner, Helen Brieske, contacted us after discovering the unpublished essay and submitted it to us. We present it to you now as part of a continuing series on the issue of climate change.
