Sublimation and Comet Tails

What is sublimation?

Sublimation is the process of a solid turning directly into a gas without going through the a liquid state first.

We all know that water freezes at 0 degrees C and boils at 100 degrees C. But this is only true if the air pressure is at 1 atmosphere - e.g. at sea level. If you went up a mountain and boiled water to cook some food, then you would find that the water boils at a lower temperature because of the lower air pressure. The higher you go, the lower the pressure and the lower the temperature the water boils at.

If you went to the extreme and took your pan of room temperature water into the vacuum of space, then you would find that it would boil away very rapidly without heating. This is because liquid water cannot exist in a vacuum. If water molecules have enough energy break the intermolecular bonds that would hold them together as a solid, then, without any air pressure pushing the water molecules closer together, they also have enough energy to escape each other altogether.

Do comets melt or sublimate?

To be precise, Comets do not melt. Melting is the process of ice turning into water. Because comets are in space, as the sun warms the ice to a sufficient temperature, the ice sublimates - meaning it turns directly into a gas.

What is a comet tail?

A comet tail is formed when a comet approaches the Sun. As the heat of the Sun warms the comet the volatile compounds - such as water ice begin to sublimate which results in the ejection of gases and dust particles that were held in suspension within the ice. These gases create a very large, but thin, atmosphere about the comet called the coma.

There are three phenomenon created by the release of material from the sun as follows:

Ion or Gas Tail

As the solar wind (a stream of charged particles from the Sun) hits the coma, the ionised gases in the coma are accelerated directly away from the Sun (often for tens of millions of miles) causing the first of the comet tails called the Gas or Ion Tail. The action of the solar wind causes the gases of the ion tail to flouresce, often with a eery blue colour. To explain: Ions (charged molecules) flouresce when they lose their positive charge by absorbing a negatively charged electron. The excess energy from this interaction is emitted as a photo of light.

The tail can also be affected by the suns magnetic field causing to to twist and occasionally break.

Dust Tail

Smaller particles of dust (about this size of smoke particles) released from the comet are affected by the pressure of the light from the Sun and so are also pushed directly away from the Sun. These particles are not accelerated as fast as teh ionised gas and so tend to form a curving arc behind the comet. Dust tails are observable from the reflected sunlight rather than the ion glow.

Dust Trail

The comet also releases larger particles of dust which may vary between sand grain/gravel sized rocks. These particles are mostly unaffected by the solar wind and so continue to travel in approximately the same orbit as the comet.

Over the years these particles tend to spread out all along the orbit of the comet. When the earth passes through the path of a comet these particles are experienced as a meteor shower as the dust impacts the earth’s atmosphere.