Comets are bodies of ice, dust, and rock that orbit the Sun and exhibit a coma (or atmosphere) extending away from the Sun as a tail when they are close to the Sun. They have orbital periods that range from a few years to a few hundred or even thousands of years. Short-period comets have orbital periods of fewer than 200 years, and most of these orbit in the plane of the ecliptic in the same direction as the planets. Their orbits take them past the orbit of Jupiter at aphelion, and near the Sun at perihelion.
Long-period comets have highly elongated or eccentric orbits, with periods longer than 200 years and extending to thousands or perhaps even millions of years. These comets range far beyond the orbits of the outer planets, although they remain gravitationally bound to the Sun. Another class of comets, called single-apparition comets, have a hyperbolic trajectory that sends them past the inner solar system only once, then they are ejected from the solar system. Before late 20th-century space probes collected data on comets, comets were thought to be composed primarily of ices and to be lone wanderers of the solar system.
Now, with detailed observations, it is clear that comets and asteroids are transitional in nature, both in composition and in orbital character. Comets are now known to consist of rocky cores with ices around them or in pockets, and many have an organic-rich dark surface. Many asteroids are also made of similar mixtures of rocky material with pockets of ice. There are so many rocky/icy bodies in the outer solar system in the Kuiper belt and Oort Cloud that comets are now regarded as the most abundant type of bodies in the universe. There may be one trillion comets in the solar system, of which only about 3,350 have been cataloged. Most are long-period comets, but several hundred short-period comets are known as well.
The heads of comets can be divided into several parts, including the nucleus; the coma, or gaseous rim from which the tail extends; and a diffuse cloud of hydrogen. The heads of comets can be quite large, some larger than moons or other objects including Pluto. Most cometary nuclei range between 0.3 and 30 miles (0.5–50 km) in diameter and consist of a mixture of silicate rock, dust, water ice, and other frozen gases such as carbon monoxide, carbon dioxide, ammonia, and methane. Some comets contain a variety of organic compounds including methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane, as well as complex hydrocarbons and amino acids. Although some comets have many organic molecules, no life is known to exist on or be derived from comets.
These organic molecules make cometary nuclei some of the darkest objects in the universe, reflecting only 2–4 percent of the light that falls on their surfaces. This dark color may actually help comets absorb heat, promoting the release of gases to form the tail. Cometary tails can change in length, and can be 80 times larger than the head when the comet passes near the Sun. As a comet approaches the Sun, it begins to emit jets of ices consisting of methane, water, and ammonia, and other ices.
Modeling of the comet surface by astronomers suggests that the tails form when the radiation from the Sun cracks the crust of the comet and begins to vaporize the volatiles like carbon, nitrogen, oxygen, and hydrogen, carrying away dust from the comet in the process. The mixture of dust and gases emitted by the comet then forms a large but weak atmosphere around the comet, called the coma. The radiation and solar wind from the Sun causes this coma to extend outward away from the Sun, forming a huge tail.
The tail is complex and consists of two parts. The first part contains the gases released from the comet forming an ion tail that gets elongated in a direction pointing directly away from the Sun and may extend along magnetic field lines for more than 1 astronomical unit (9,321,000 miles; 150,000,000 km). The second part is the coma, or thin atmosphere from which the tail extends, which may become larger than the Sun. Dust released by the comet forms a tail with a slightly different orientation, forming a curved trail that follows the orbital path of the comet around the Sun. Short-period comets originate in the Kuiper belt, whereas long-period comets originate in the Oort Cloud.
Many comets are pulled out of their orbits by gravitational interactions with the Sun and planets or by collisions with other bodies. When these events place comets in orbital paths that cross the inner solar system, these comets may make close orbits to the Sun, and may also collide with planets, including the Earth. Several space missions have recently investigated the properties of comets. These include Deep Space 1, which flew by Comet Borrelly in 2001. Comet Borrelly is a relative small comet, about 5 miles (8 km) at its longest point, and the mission showed that the comet consists of asteroid-like rocky material, along with icy plains from which the dust jets that form the coma were being emitted.
In 1999 the National Aeronautics and Space Administration (NASA) launched the Stardust Comet Sample Return Mission, which flew through the tail of comet Wild 2 and collected samples of the tail in a silica gel and returned them to Earth in 2006. Scientists were expecting to find many particles of interstellar dust or the extrasolar material that composes the solar nebula, but instead found little of this material; instead they found predominantly silicate mineral grains of Earthlike solar system composition.
The samples collected revealed that comet Wild 2 is made of a bizarre mixture of material that includes some particles that formed at the highest temperatures in the early solar system, and some particles that formed at the coldest temperatures. To explain this, scientists have suggested that the rocky material that makes up the comet formed in the inner solar system during its early history, then was ejected to the outer bounds of the solar system beyond the orbit of Neptune, where the icy material was accreted to the comet.
Calciumaluminum inclusions, which represent some of the oldest, highest temperature parts of the early solar system, were also collected from the comet. One of the biggest surprises was the capture of a new class of organic material from the comet tail. These organic molecules are more primitive than any on Earth and than those found in any meteorites; they are known as polycyclic aromatic hydrocarbons. Some samples even contain alcohol.
These types of hydrocarbons, thought to exist in interstellar space, may yield clues about the origin of water, oxygen, carbon, and even life on Earth. Comets are rich in water, carbon, nitrogen, and complex organic molecules that originate deep in space from radiation-induced chemical processes. Many of the organic molecules in the coma of comets originated in the dust of the solar nebula at the time and location where the comets initially formed in the early history of the solar system. Comets are relatively small bodies that have preserved these early organic molecules in a cold, relatively pristine state. This has led many scientists to speculate that life may have come to Earth on a comet, early in the history of the planet. Clearly, comets both delivered organic material to the early Earth and also destroyed and altered organic material with the heat and shock from impacts. Numerical models of the impact of organic-rich comets with Earth show that some of the organic molecules could have survived the force of impact.
The organic molecules in comets may be the source of the prebiotic molecules that led to the origins of life on Earth. Studies of the chemistry and origin of the atmosphere and oceans suggest that the entire atmosphere, ocean, and much of the carbon on Earth, including that caught up in carbonate rocks like limestone, originated from cometary impact.
The period of late impacts of comets and meteorites on Earth lasted about a billion years after the formation of Earth, before greatly diminishing in intensity. Life on Earth began during this time, hinting at a possible link between the transport of organic molecules to Earth by comets, and the development of these molecules into life.
The early atmosphere of Earth was also carbon dioxide–rich (much of which came from comets), however, and organic synthesis was also occurring on Earth. In addition to bringing organic molecules to Earth, the energy from impacts certainly destroyed much of any biosphere that attempted to establish itself on the early Earth. Even the late, very minor KT impact at Chicxulub had major repercussions for life on Earth.
Certainly the early bombardment characterized by many very large impacts would have had a more profound effect on life. Any life that had established itself on Earth would need to be sheltered from the harsh surface environment, perhaps finding refuge along the deep sea volcanic systems known as black smokers, where temperatures remained hot but stable, and nutrients in the form of sulfide compounds were used by early organisms for energy.
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