What Is a Comet Made Of?
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A comet is mostly ice, dust and rocky grains, which is why the old description of a dirty snowball stuck for so long. Comets spend most of their time frozen and invisible in the outer solar system, and only become spectacular when they get close enough to the Sun to start evaporating.
The nucleus at its core
The solid part of a comet is the nucleus, typically a few kilometres across. It contains water ice, frozen carbon dioxide, carbon monoxide, ammonia and methane, mixed with dust and rocky material.
The European Rosetta mission, which orbited comet 67P from 2014, refined the picture considerably. The nucleus turned out to be extremely dark, more porous than expected, and much less dense than solid ice, so a loosely packed rubble pile is closer than a snowball.
Why comets grow tails
Far from the Sun a comet is inert. As it approaches, solar heating turns the ices directly from solid to gas, a process called sublimation, releasing dust trapped inside.
This creates a large fuzzy cloud called the coma around the nucleus, which can be far bigger than the solid body. Radiation pressure and the solar wind then push material away from the Sun, forming the tail.
Two tails, not one
Comets usually have two distinct tails pointing in slightly different directions:
- •A dust tail, yellowish white, curved, made of grains pushed gently by sunlight
- •An ion tail, bluish and straight, made of charged gas swept by the solar wind
Where comets come from
Short period comets, which return within about two hundred years, mostly originate in the Kuiper Belt beyond Neptune. Halley's Comet, returning roughly every seventy six years, is the famous example.
Long period comets come from the Oort Cloud, a vast spherical shell of icy bodies thought to extend a substantial fraction of the way to the nearest star. These can take thousands or millions of years to complete one orbit.
Why scientists care about them
Comets are leftovers from the formation of the solar system, largely unaltered since. Studying them is close to sampling the original material that planets were built from.
They may also have delivered water and organic molecules to the early Earth, though the isotope ratios measured at comet 67P did not match Earth's oceans well, which pushed attention towards asteroids as a more likely source. The debate is unresolved and genuinely interesting.
Comets and meteor showers
Comets leave debris along their orbits, and when Earth passes through one of these trails the particles burn up in the atmosphere, producing a meteor shower.
This is why showers happen on predictable dates each year. The Perseids in August come from comet Swift-Tuttle, and the Leonids in November from comet Tempel-Tuttle. The particles are mostly the size of a grain of sand, and the streaks they produce are the result of them entering the atmosphere at tens of kilometres per second rather than of any great size.
The takeaway
A comet is a porous mix of ice, dust and rock that only becomes visible when solar heat turns its ices to gas, producing a coma and two separate tails, and it preserves material largely unchanged since the solar system formed.