Coma
Definition and meaning of Coma in chemistry.
In chemistry, a coma is the glowing, spherical cloud of gas and dust that surrounds a comet. It forms when intense heat from the Sun causes frozen chemicals to turn directly into gas. This massive cloud gives the active comet its famously fuzzy appearance in the dark night sky.
In more detail
A comet spends most of its life far away as a frozen block of ice and dirt. When it eventually travels closer to the Sun, its surface temperature begins to rise very rapidly. This sudden heat causes frozen chemicals like water, carbon dioxide, and solid ammonia to sublime.
Sublimation means these solid ices turn straight into expanding gases without ever becoming a liquid first. As these high-speed gases rush away from the comet surface, they carry tiny dust grains with them. Intense ultraviolet light from the Sun then strikes these newly released gas molecules in space.
The powerful light energy breaks chemical bonds and rips outer electrons away from the individual atoms. This violent process creates highly reactive chemical fragments and charged ions that glow brightly. Scientists on Earth can analyze this glowing light to figure out exactly what chemicals are present.
The coma keeps expanding outward until the forceful solar wind eventually blows it firmly backward. This steady push of solar wind sweeps the glowing coma material away to form the long tails.
Key facts
| Field | Physical Chemistry |
|---|---|
| Formation mechanism | Sublimation of comet ices from solar heat |
| Typical diameter | 10,000 to 1,000,000 kilometers |
| Dominant species | H2O, CO2, CO, NH3, CH3OH |
| Typical shape | Roughly spherical cloud |
| Glow source | Sun's ultraviolet light energizing molecules |
In recent years, scientists sent the Rosetta spacecraft to closely study a large comet called 67P. The robotic space probe flew directly into the comet's coma to chemically test the surrounding gas. Scientists expected to find plenty of water and carbon dioxide vapor inside this rapidly expanding cloud. However, they were totally amazed to also discover complex organic molecules floating freely in the coma. They even found glycine, which is a crucial amino acid used to build proteins for life.
Frequently asked questions
How is a comet's coma different from its tail?
The coma is the roughly spherical cloud of gas and dust immediately surrounding the nucleus. The tail forms when the solar wind pushes that coma material backward into a long stream.
Why doesn't a comet have a coma when it is far from the Sun?
The freezing temperatures in deep space keep the comet's chemicals trapped as solid ice. The comet must get close to the Sun to get enough heat for those ices to sublime.
What is sublimation?
Sublimation is a phase change where a solid turns directly into a gas. It skips the liquid melting phase entirely, which is very common in the vacuum of outer space.