Student Features

Transit of Venus: June 8, 2004
03.04.04
black and white photograph of the 1882 Transit of Venus
Photograph of Venus passing in front of the sun in the 1882 Venus transit
On December 6, 1882 the world 'went wild' when this event last occurred. Only five transits have actually been seen by humans in the course of history. Venus, a dark blotch set against the turbulent solar photosphere, is more than just a black spot. Yet, the image also reveals a pitiful black spot engulfed by the even more awesome countenance of the solar photosphere; a seething cauldron of gas so hot that even atoms lose their integrity. Studying Venus lets us better appreciate our own planet Earth, and why it has evolved into the planet that it now is.

full color satellite image of Earth
Earth
Earth 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. Its 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.

Venus image taken by a telescope
Venus
Venus Atmosphere
This planet is an awful place to visit. Its 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 that 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. With an axis tilt of nearly 90 degrees, Venus has no seasons in the same way that Earth does.

full color satellite image of Earth
Earth
Earth Magnetism
The solar wind never directly impacts the upper atmosphere because Earth has a powerful magnetic field. Closest to Earth, it is shaped like the field in a toy bar magnet, but farther away, it is shaped like a comet with Earth at its head. The 'magnetic tail' points directly away from the Sun and trails millions of kilometers into space beyond the orbit of the Moon. This field acts like an invisible umbrella in space to deflect most of the solar wind particles back into interplanetary space. Inside Earth's magnetic field are many complex systems of particles that have been trapped by our field, like fireflies in a bottle on a summer night. One of these systems is the Van Allen Radiation Belts. When our magnetic field is disturbed by a solar storm, the magnetotail snaps like pulled taffy and sends currents of particles streaming into the polar regions to cause the auroral lights.

Venus image taken by a telescope
Venus
Venus Magnetism
Because Venus has no magnetic 'umbrella' to shield itself, the solar wind penetrates deep into its 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 in about 700 million years!

To learn more about the Transit of Venus:
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Excerpted from http://image.gsfc.nasa.gov/poetry/venus/venus.html