Comets are some of the solar system’s most beautiful and colorful objects. They remain a mystery not only to people who study them but also to their appearance. Scientists know that a comet’s color depends on the dust and gas particles in its tail.
Comets are celestial bodies with elliptical orbits that take them close to the sun. When a comet’s orbit brings it within the sun’s gravitational field, it often “blows off” gas and dust from its surface in various directions—the gas and dust form into two different tails. The tail nearest to the sun, called the ion or plasma tail, is made of electrically charged particles spewed by the sun’s solar wind. As these charged particles interact with an object’s magnetic field, they form beautiful structures like in many comets.
Reason Why Comet Heads Green – But Not Their Tails
A comet’s ion tail is made of electrically charged particles. The sun’s solar wind is an example of this charge. Since a comet was first discovered in 1704, scientists have wondered about its color. They also tried to determine what produced the ion tail but to no avail. As a result, they have yet to find the answer to why comets have such different colors. However, just recently, almost 90 years after that discovery, they found an explanation.
In 1928, Swiss-American astronomer Percival Lowell published his claims that comets had a green ion tail. German physicist Carl Gustav Auer first proposed the idea of a green ion tail. In the early 1900s, he wanted to find a way to explain why the Northern Lights were different colors and even tried to measure their greenish-blue color using a spectroradiometer. He interpreted their color as being due to sunlight scattering off oxygen ions in the Earth’s atmosphere. It means that for nearly 100 years, scientists have been trying to find an explanation for the origin of this ion tail—still no answer.
It was in 2011 that they found the answer. After analyzing pictures and data from the SOHO (Solar and Heliospheric Observatory) spacecraft, scientists discovered that “an accidental discovery caused the green ion tail.” The discovery involved a camera on the SOHO spacecraft that measures solar winds. The instrument analyzed a tail and spotted an enormous, greenish-blue cloud of gas. It is called a coronal cloud because it resides in the solar corona, the outermost part of the sun’s atmosphere. This coronal cloud is made of charged particles cooled down by the sun’s extreme ultraviolet radiation.
The discovery of this green ion tail was an accident, but it was an accident that changed the way scientists view comets. This discovery proved that the ion tail found on a comet is not a result of the solar wind but rather due to a cloud of cold, charged particles residing high above the sun. On the other hand, the head of a comet is colored by different ejected materials. Comets get their sparkly appearance from dust and ice particles (called volatiles) forming in their necks as they approach the sun. Before re-entering the solar system, these ice and dust particles are ejected from the head of a comet. The green color of the ion tail is caused by oxygen ions that are present in their vaporized state.
The ion tail does not only cause a comet to be green but is also responsible for giving comets their seats. As already stated, the solar wind carries charged particles into its ion tail when a comet approaches the sun. When these ions are heated up by sunlight, they emit light that makes them appear green. Scientists believe that this radiation also warms up material within the cometary nucleus. It causes the material to boiling off rapidly, creating a cloud of vaporized materials that carry dust particles. These dust particles form a coma that surrounds the comet’s nucleus. We see this light-colored head as a comet approaches the sun.
The study of comets has been ongoing for centuries. Modern cometary science has led scientists to discover an enormous coronal cloud containing oxygen ions above the sun’s surface. As a result of this discovery, they found that the tail on a comet is not caused by the solar wind but rather by oxygen ions within that coronal cloud.
