Comets

Nucleus

At first glance, comets seem like nothing more than an icy body with a projected atmosphere(known as the coma) of gas and dust caused by the Sun. While this is true, there is a source that provides new material for the atmosphere to have because the atmospheres/comas of comets deplete due to strong gravity from other bodies in the Solar System. The source that refreshes the atmospheres of comets is known as the nucleus of a comet. The nuclei of most comets are made up of rock, dust, water ice, frozen carbon dioxide(CO₂), methane(CH₄), and ammonia(NH₃). Because of this combination of different ices, the model used to describe comet nuclei is often called the "dirty snowball" model. The surface of the nucleus of a comet is generally very rocky and dusty compared to the ices beneath.

According to relatively recent research, there have been traces of more hydrocarbons, organic compounds, and even amino acids(like glycine) found within the nuclei of comets.

Coma

The atmosphere of a comet is known as the coma, and it usually sublimates(goes from solid to gas) as the Sun warms it up to higher temperatures. Comas are primarily made up of water and dust, but most of the water outflows from the coma because sunlight causes it to undergo a process known as photodissociation. Through this process, photons interact with the water molecules and cause them to dissociate into ions which get released from the coma.

Tails

Comets also form notable tails as extensions of their coma, and so much of the water and dust found in the coma are projected out into space. The tails of comets always point away from the Sun due to radiation from the Sun and the stream of charged particles exerted from the Sun, known as the solar wind.

As you can see from the diagram above outlining the parts of a comet, the tails of comets have two distinctive components: the ionized gas tail, known as the ion tail, and the dust tail.

The ion tail is formed because the Sun's UV(ultra-violet) radiation ionizes the gas in the coma. The dust tail, however, is simply formed by the ejection of dust from the coma.

The dust tail curves significantly relative to the ion tail, which is projected straight out from the comet. This is because the dust tail follows the comet's orbits so many of the particles simply trace out the curve of the comet due to their inertia. This is no different from how tires from a vehicle make curved tracks in the road if you turn very hard. However, the ion tail directly interacts with the solar wind because its ions carry a net electric charge. This causes the ion tail to repel from the solar wind and project straight out from the comet.