Question

Describe what happens to the nucleus of a comet as it moves from 10 AU from the Sun to 0.5 AU from the Sun.

Short answer

The nucleus sublimates, forming a coma and tails as it nears the Sun.

Step-by-step solution

Understanding AU

AU stands for Astronomical Unit, which is the average distance from the Earth to the Sun, approximately 149.6 million kilometers. It is a useful distance measurement in our solar system.

Comet Nucleus at 10 AU

At 10 AU from the Sun, the nucleus of a comet is relatively inactive. It consists mostly of ice, dust, and rocky material. Due to the low temperature in this region, the nucleus remains mostly frozen, and there is minimal cometary activity observed from afar.

Approaching the Sun

As the comet travels closer to the Sun, the temperature increases. Around 3-5 AU from the Sun, the energy from the Sun begins to significantly heat the nucleus, sublimating its ices, which are converted directly from solid to gas. This process forms a surrounding cloud of gas and dust around the nucleus called the coma.

Comet Nucleus at 0.5 AU

At 0.5 AU from the Sun, the comet experiences much stronger solar radiation and increased temperature. This intense heating causes more rapid sublimation of the ices in the nucleus. This results in the prominent formation of tails, consisting of a dust tail and an ion tail, which can extend millions of kilometers away from the nucleus, always pointing away from the Sun due to the solar wind.

Outcome and Effects

As the comet nears 0.5 AU, it may lose a significant amount of its mass due to sublimation. The nucleus can become more porous and fragile, potentially fragmenting. These changes are what often create the spectacular displays associated with comets as they near the Sun.

Key concepts

Astronomical Unit (AU)

The term "Astronomical Unit" (AU) is crucial when talking about distances in space. An AU is defined as the mean distance between the Earth and the Sun. This is about 149.6 million kilometers or roughly 93 million miles.
Using AU helps astronomers simplify the vast distances in our solar system. It serves as a basic unit of measurement, allowing easier calculation and comparison of distances to celestial bodies, enhancing clarity and precision.
In the context of comets, understanding distances in AU helps anticipate how changes in distance from the Sun affect a comet's behavior, particularly as its nucleus warms and begins to interact with solar radiation.

Sublimation of Comet Ices

As a comet approaches the Sun, it undergoes a fascinating transformation due to sublimation. Sublimation is the transition of a substance from a solid state directly into a gas without passing through a liquid phase.
Comet nuclei are rich in various ices, primarily water, carbon dioxide, and other frozen compounds. At about 3-5 AU from the Sun, the increased heat causes these ices to sublimate. This change creates a cloud of gas and dust around the nucleus called the coma.
Key points about sublimation:

• The surface layers of the comet begin vaporizing as thermal energy from the Sun hits them.
• The sublimation process can accelerate as the comet draws nearer to the Sun, fueling the generation of gas and dust.
• This event becomes pivotal in forming the visible atmosphere and tail of the comet.

Understanding sublimation helps explain the dynamic transformations comets experience and predicts their visible characteristics as they travel through the solar system.

Comet Tails Formation

A signature aspect of comets is their striking tails, which are formed as a consequence of sublimation and solar effects. Once the comet is around 0.5 AU from the Sun, this feature becomes very prominent.
A comet typically develops two kinds of tails: dust tails and ion tails.

• Dust Tail: Formed by small particles released from the nucleus as the ices sublimate. These particles reflect sunlight, giving the tail a bright, curved appearance.
• Ion Tail: Created when solar wind interacts with gases in the coma, stripping electrons and creating ions. This tail is often straighter and has a bluish tint.

Both tails tend to point away from the Sun. This opposition is due to the solar wind – a stream of charged particles emitted by the Sun's atmosphere, pushing material outward.
Through understanding tail formation, we gain insights into both the internal composition of comets and the dynamics of their orbits as they approach and recede from the Sun.