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==== Problems for uniformitarian theories ====

==== Problems for uniformitarian theories ====

Many comets have life spans less than 10,000 years. According to the [[nebula theory]], comets formed with the rest of the solar system, 4.6 billion years ago. Adherents of the interstellar capture theory try to connect the origin of cometary matter with the [[big bang]], which, they say, happened about 13.7 billion years ago. But in that case, all the short-period comets ought to have disappeared. This is especially true of the Jupiter family. Even if the Kuiper belt is the source of short-period comets, such comets would have to lose much kinetic [[energy]] in order to settle into the short-aphelion orbits of Jupiter's family. This begs the question of how and where they lost this energy.<ref name=Brown/>

Many comets have life spans less than 10,000 years. According to the [[nebula theory]], comets formed with the rest of the solar system, 4.6 billion years ago. Adherents of the interstellar capture theory try to connect the origin of cometary matter with the [[Big Bang]], which, they say, happened about 13.7 billion years ago. But in that case, all the short-period comets ought to have disappeared. This is especially true of the Jupiter family. Even if the Kuiper belt is the source of short-period comets, such comets would have to lose much kinetic [[energy]] in order to settle into the short-aphelion orbits of Jupiter's family. This begs the question of how and where they lost this energy.<ref name=Brown/>

Brown lists many other problems for various uniformitarian theories posed by comets:

Brown lists many other problems for various uniformitarian theories posed by comets:

# Comets form by accretion of water ice and other materials. This explains why comets usually have the consistency of snowballs and not of hard blocks of ice. Accretion from a solar nebula has never been satisfactorily modeled. Accretion requires the sudden release of a stream of matter beyond the sphere of gravitational influence of another body, and into a region in which the matter can form its own, rapidly growing sphere of influence. The putative conditions of the solar nebula do not meet this requirement.

# Comets form by accretion of water ice and other materials. This explains why comets usually have the consistency of snowballs and not of hard blocks of ice. Accretion from a solar nebula has never been satisfactorily modeled. Accretion requires the sudden release of a stream of matter beyond the sphere of gravitational influence of another body, and into a region in which the matter can form its own, rapidly growing sphere of influence. The putative conditions of the solar nebula do not meet this requirement.

# Large quantities of water ice have been observed on the poles of the [[moon]], the planet [[Mercury (planet)|Mercury]], and now, most recently, on [[Mars]]. Why this ice has not evaporated, particularly from the moon and Mercury, where atmospheric pressures are negligible, has never been explained.

# Large quantities of water ice have been observed on the poles of the [[moon]], the planet [[Mercury]], and now, most recently, on [[Mars]]. Why this ice has not evaporated, particularly from the moon and Mercury, where atmospheric pressures are negligible, has never been explained.

# Crystalline dust might form from an exploded planet, but it would not be likely to form in any of the other proposed models.

# Crystalline dust might form from an exploded planet, but it would not be likely to form in any of the other proposed models.

# If comets formed in the region of the putative Oort cloud, they would not attain near-parabolic orbits.

# If comets formed in the region of the putative Oort cloud, they would not attain near-parabolic orbits.