Which of the following is not a characteristic of comets?

Comets are primarily composed of ice, dust, and various gases, which is why they are often referred to as "dirty snowballs." The presence of ice is a defining characteristic of comets, as they typically originate from the outer regions of the solar system where conditions allow for the accumulation of frozen gases. Similarly, comets contain dust particles that are released when the comet approaches the Sun and the ice begins to sublimate, contributing to the development of their tails.

While methane can be found in some comets, it is not a universal or defining characteristic. The composition of comets varies widely, and not all comets contain significant amounts of methane. However, metal composition is not typical for comets. Comets are not primarily composed of metals but rather a combination of ice and organic compounds. In contrast, asteroids, which are often found in the inner solar system, have a higher metal content. Therefore, the statement about metal composition not being a characteristic of comets is accurate, as it distinguishes them from other celestial bodies, particularly asteroids.