secef button
top banner
Return to: Home, For Educators: Venus Transit Theme: Background Reading

Some materials require the Flash 6 plug-in
Get the Flash Player

red bar


In later years, during the 20th century astronomers continued to study Venus and found it to be a much different world than Earth. Venus is often called a 'Twin' to Earth, but we know that this really isn't the case except for its diameter and mass. Ancient observers once called it Lucifer that later became a synonym for Satan the ruler of Hell. In fact given the surface temperature of Venus, it is not a bad analogy for this horrific abode within our own solar system. Rather than a twin of Earth, it is more like the antithesis of Earth in nearly every way we can now scientifically quantify.

Why do we still want to study Venus? Because it lets us think about what a planet looks like that is otherwise like Earth, but has a different atmosphere, distance from the sun, and a 'missing' magnetic field. From these differences we can learn more about Earth and how it may change in the future.


Atmosphere: Earth is the garden spot of the solar system, at least for organic life! It has a nitrogen-oxygen atmosphere with an average temperature of 50 F. The pressure at the surface is completely bearable by humans and living organisms. The trace amounts of carbon dioxide keep it comfortably warm thanks to a very mild greenhouse effect. It's axis is tilted at 23.5 degrees to its orbit plane, which gives us the cycle of the seasons. Liquid water is abundant on its surface, and could cover it to a uniform depth of several miles. There are also two forms of water: fresh and sea water. Few organisms can consume the salt and mineral-rich sea water directly without damaging their internal organs. Most of our water is believed to have come from ancient comet impacts.

Atmosphere: This planet is an awful place to visit. It's atmosphere is more than 95% carbon dioxide, and it has rain clouds that produce droplets of sulfuric acid. The surface pressure from this dense atmosphere is so thick you would be crushed by a half-ton per square inch. The greenhouse effect has trapped so much solar infrared radiation that the surface temperature is over 800 F, far above the setting on your kitchen oven! The surface seems to be completely free of craters more than 700 million years old, unlike Earth. Some scientists have suggested that the entire planetary surface was 'erased' or 'recycled' in some cataclysmic event at about that time.

planetary comparison Planetary Comparison Animation (36 MB) The Venus, Earth, Mars animation begins with an Orrery view of the solar system followed by a more realistic view of the inner planets orbiting around the Sun. We then zoom in for a closer view of our Sun and with a Coronal Mass Ejection in progress! You will then see how that CME impacts Venus, Earth and finally Mars.

Magnetism: The solar wind never directly impacts the upper atmosphere because Earth has a powerful magnetic field, which acts like an invisible umbrella in space to deflect most of the solar wind particles. Cosmic rays, which are other forms of high-energy particles are either absorbed by Earth's dense atmosphere, or are deflected back into space by its magnetic field. Even when Earth loses its main magnetic field, it still has a weak crustal field, although this field is much less effective in steering solar particles away from Earth. Fossil and other evidence, however, shows that even when Earth's field weakens, there are no measurable effects at the Earth's surface that would affect living organisms. Compare this to the bizarre 'predictions' by such popular movies as 'The Core'.

Magnetism: Because Venus has no magnetic 'umbrella' to shield itself, the solar wind penetrates deep into its upper atmosphere, as do cosmic ray particles. This direct contact causes solar wind particles to collide and eject gas from the atmosphere of Venus into space, forming an invisible comet-like tail behind the planet. Astronomers have studied the Sun-Venus interaction through computer models and satellite flybys since the 1960's. There is still much we don't understand. For example, why does Earth have such a strong magnetic field and Venus has none? Is it because Venus rotates 10 times slower, or is it that there is something very different about the core of Venus that prevents magnetic fields from forming? Learning about Venus may help us better understand Earth's magnetism, and what may be in store for our planet in the far future...say 700 million years!

Planets Beyond the Solar System.

The transit of Venus is exciting for other reasons too.

Astronomers can now study planets around other stars by waiting for some of them to pass in front of their star. One planet called HD 209458b is the first known 'transiting extra-solar planet'. Scientists turned this transit to their advantage in 2001. Just as Lomonosov detected the atmosphere of Venus during a transit, modern astronomers have used spectroscopes to detect the atmosphere of this distant Jupiter-like world as it passed in front of its star over 100 light years from Earth.


The good news is that this transit allowed astronomers to detect sodium in the planet's atmosphere. The bad news is that further studies show that this planet, orbiting once every three days at over 4 million miles from its star, is evaporating at the rate of nearly 100,000 tons per day, and has a temperature near 3000 F! In a few billion years, it may be gone.

The NASA Kepler and Navigator missions will study thousands of other stars, searching for the tell-tale dips in their spectrum that can identify their atmospheric gases. We hope that one day, soon, we will find a planet with oxygen, which will be the first signs of living creatures beyond Earth.

For answers to your questions about the Transit of Venus, visit Ask the Space Scientist at

Responsible NASA Official: Dr. James Thiemann
Web Manager/Curator: Troy Cline
Web Designer/Curator: Lori Ann Persichitti Lopez
Additional Credits

NASA Privacy, Security, Notices