Astronomy
The Composition and Structure of Cometary Nuclei
Quick fact
The nucleus of a comet is typically only a few kilometres across – smaller than a city – yet it can produce a coma larger than the Sun.
Why this is interesting
Every so often, a cosmic snowball plunges toward the Sun, sprouting a glowing tail that stretches millions of miles. But what exactly is that ‘dirty snowball’ at its heart?
Read the full explanation
Understanding The Composition and Structure of Cometary Nuclei
Imagine a filthy snowball left over from the making of the solar system. That’s essentially a comet’s nucleus: a solid, irregularly shaped body made of water ice, frozen gases like carbon dioxide and carbon monoxide, dust, and dark organic compounds. It does not shine on its own; we see it only when sunlight warms the surface. As the comet nears the Sun, the ice turns directly into gas (sublimation), blasting off dust and gas to form a glowing cloud around the nucleus – the coma – and long tails pushed away by the Sun’s light and wind. The nucleus itself remains small and solid, but its surface is active, with gas jets erupting from cracks and vents.
A deeper explanation
The structure of a cometary nucleus is complex, far from a simple ice ball. Observations from space missions like Rosetta and Deep Impact reveal a low-density, fragile body. The interior is porous and likely contains a mixture of ice and dust – the ‘primordial ice cream’ of the solar system. The surface is mostly a dark crust, rich in organic materials and silicates, that insulates the interior. When a comet approaches the Sun, solar heat penetrates through this crust, sublimating ices beneath. Pressure builds, and gas escapes through weak points, carving jets and carrying dust out. This process doesn’t just create tails; it also shapes the nucleus over time, leading to fractures, erosion, and even splitting. The composition and structure hold because cometary nuclei are relicts from 4.6 billion years ago – untouched by processes like planetary melting. Thus, they are time capsules, preserving the original material of the solar system and potentially the building blocks of life on Earth.