Comets are delicate and shortlived. They are frozen wanderers that originated in the outer fringes of our Solar System, but were infortunately evicted from their dimly-lit frigid home. As a result, these outcast celestial snowballs have come sweeping into Earth’s inner region of our Solar System, that is very far from their cold shadowy cradles beyond the outermost major planet, Neptune. Many astronomers think that comets hold in their frozen centers the purest traces of the primeval substances that went into the formation of our Solar System. These ancient ingredients have been kept in a deep dark “freezer” that is far from the light and heat of our Star. Hence, understanding the composition of the comets translates into identifying the ingredients that created our Solar System.
The comets are actually icy planetesimals. Planetesimals are the building blocks of planets, and in the ancient Solar System they merged together to eventually form the quartet of majestic, giant, gaseous planets that dwell in the outer Solar System as we see it today. The icy planetesimals were the relatively small seeds that merged together to create Jupiter, Saturn, Uranus, and Neptune, as well as their frozen entourage of mostly icy little moons. The rocky and metallic asteroids, that are primarily found in the Main Asteroid Belt, are similar to the rocky and metallic planetesimals that went into the formation of the quartet of inner solid planets: Mercury, Venus, Earth and Mars. In the early Solar System, rocky and icy planetesimals collided with one another and often merged to create ever larger and larger bodies–ultimately giving rise to planets.
The icy, dusty comets streak into Earth’s warm inner region of the Solar System, arriving from two distant domains. The first domain has been named the Kuiper belt, and it circles our Star beyond Neptune’s orbit. The Kuiper belt is the source of short-period comets, which are those that zip into the dazzling light and melting heat of the inner Solar System more frequently than every two centuries. The second domain, that has been named the Oort cloud, is an immense sphere of icy objects that encircles our entire Solar System. The Oort cloud is the very remote home of the long-period comets, which come shrieking into the inner Solar System more frequently than every two centuries. Since Earth is closer to the Kuiper belt than it is to the Oort cloud, short-period comets have had a greater influence on our planet’s history than long-period comets.
Every time a traveling comet enters the inner Solar System, it loses a small amount of its mass. This “shedding” results from the sublimation of its surface ices to water vapor. Indeed, one of the most famous icy vagabonds from far, far away–Halley’s Comet–has been calculated to have a lifetime of less than 100,000 years–a brief period on celestial timescales. The comets that we can observe today, as they fly across our sky with their fiery tails flashing, will eventually melt away and vanish due to the sublimation of ices to gas. However, they will be replaced by a new generation of comets that are destined to continue the legacy of those that have vanished. A sparkling population of fresh, new comets will be jostled out of the cold darkness of the Kuiper belt and Oort cloud, and they will wend their weary way ever closer and closer to our Sun.
The frozen core of a comet is termed its nucleus, and it is made up mostly of ice and dust that is well-coated with a dark organic substance. The ice is mostly frozen water, but other ices may also be present. These additional frozen substances are methane, ammonia, carbon monoxide and carbon dioxide ices.