Why do comets develop tails when they are near the Sun?

Comets are often described as "dirty snowballs," composed of a nucleus of ice (water, carbon dioxide, etc.), dust, and rock. As a comet's orbit brings it closer to the Sun, the intense solar radiation heats the nucleus. This heating causes the ices to sublimate—transforming directly from a solid to a gas. This process, known as outgassing, releases large amounts of gas and trapped dust particles into space. The solar wind and radiation pressure then sweep this material away from the nucleus, forming the characteristic coma (the glowing head) and one or more tails that always point away from the Sun.

B. The tail is formed by material leaving the comet's own nucleus, not by an increased frequency of impacts from external debris.

C. Combustion is a chemical reaction requiring an oxidizer, which is not available in the vacuum of space. The glow is from fluorescence and reflected sunlight.

D. While a comet's velocity increases as it nears the Sun, the speed itself does not create the tail; the Sun's energy is the direct cause.

1. NASA Science, Solar System Exploration. (n.d.). Comets: In Depth. NASA. Retrieved from https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/in-depth/. In the "Anatomy" section, it states, "The solid nucleus or core of a comet consists mostly of ice and dust... As comets get closer to the Sun, the ice on the surface of the nucleus begins turning into a gas, forming a cloud known as the coma... Pressure from sunlight and high-speed solar particles (solar wind) can blow the dust and gas from the coma away from the Sun, forming a long, bright tail."

2. Fraknoi, A., Morrison, D., & Wolff, S. C. (2016). Astronomy. OpenStax, Rice University. Section 13.2, "The 'Long-Haired' Comets." This textbook explains, "As the comet absorbs ultraviolet light, chemical processes release hydrogen gas... The sunlight also pushes the dust particles away from the nucleus... The two tails—one made of gas and one of dust—point away from the Sun." (p. 446).

3. Department of Earth, Atmospheric and Planetary Sciences, MIT. (n.d.). 12.400x The Habitable Planet, Unit 2: Earth's Origin and the Solar System, Section 5: Small Bodies. MIT OpenCourseWare. This course material describes comets as icy bodies that "develop a coma and a tail of gas and dust when they travel into the inner solar system where the Sun's heat is strong enough to vaporize their surface ices."