Coma
Definition and meaning of Coma in chemistry.
Coma is the diffuse, roughly spherical cloud of gas and dust that surrounds the nucleus of an active comet, produced when solar heating drives volatile ices on the nucleus to sublime directly from solid to vapor.
In more detail
As a comet nears the Sun and its surface temperature rises, frozen volatiles such as water ice, along with lesser amounts of CO2, CO, NH3, and CH3OH, sublime and carry entrained dust grains away from the nucleus. Solar ultraviolet radiation then photodissociates and photoionizes these escaping molecules, generating radicals and ions (such as OH and CN) whose characteristic emission lines let astrochemists identify the coma's composition spectroscopically. The coma expands until gas density drops enough that radiation pressure and the solar wind take over, sweeping material into the separate dust and ion tails. Because sublimation is temperature-dependent, a comet typically develops a coma only within a few astronomical units of the Sun.
Key facts
| Field | Physical Chemistry |
|---|---|
| Formation mechanism | Sublimation of nucleus ices under solar heating |
| Typical diameter | 10^4 to 10^6 km |
| Dominant species | H2O, CO2, CO, NH3, CH3OH |
The coma of comet 67P/Churyumov–Gerasimenko was sampled directly by the Rosetta spacecraft's ROSINA mass spectrometer, which detected H2O, CO2, CO, H2S, and trace organics including the amino acid glycine.
Frequently asked questions
How is the coma different from a comet's tail?
The coma is the roughly spherical envelope of gas and dust close to the nucleus; the tail forms when solar radiation pressure and the solar wind push coma material outward into elongated dust and ion streams pointing away from the Sun.
Why doesn't a comet have a coma far from the Sun?
Sublimation requires enough solar heating to vaporize surface ices; beyond roughly 2 to 3 astronomical units, nucleus temperatures are too low, so the ices stay frozen and no coma forms.