14. Comets

Anatomy of a comet

Fig. .. 

No two comets ever look identical, just as no two snowflakes are alike, but most comets have basic features in common. When they emerge from the deep freeze of outer space and move toward the Sun, the increased solar heat eventually causes their ices to sublimate and blow dust away with the escaping gas. The comet then becomes visible as an enormous moving patch of light. This glowing, misty ball of light is called the coma, the Latin word for ďhairĒ. One or more tails can eventually stream from the coma, in a direction away from the Sun.

Fig. .. 

Comet gas and dust are initially ejected primarily in the general direction of the Sun; solar forces push them into tails that flow away from the Sun. As a result, a comet travels headfirst when approaching the Sun and tail first when moving away from it.

At the heart of a cometís coma lies a nucleus of solid material, no more than 10 thousand meters across. The nucleus can be directly imaged from spacecraft that pass near it. This has been accomplished two times so far, measuring oblong shapes of about this size.

With a mass density somewhere between that of water ice and rock, or between 1,000 and 3,000 kilograms per cubic meter, the comet nucleus would have a mass of just 1015 kilograms or less. They are less than one billionth the mass of the Earth, which weighs in at 5.97 x 1024 kilograms. The visible coma, or head, is a spherical cloud of gas and dust that has emerged from the nucleus, which it surrounds like an extended atmosphere. The coma sometimes reaches a billion meters in size, which is about as large as the Sun, and they are usually become bigger than the Earth.

Fig. .. 

A vast hydrogen cloud, containing hydrogen atoms that emit ultraviolet radiation, envelops the coma and nucleus. Observations of this glow Ė invisible to the eye Ė indicate that the hydrogen halo can be ten billion, or 1010, meters across, or about ten times bigger than the Sun. The atomic hydrogen is produced when water molecules, released from the comet nucleus, are torn apart by energetic sunlight. The relatively light hydrogen atoms travel at high speed to great distances before they are also ionized by the Sunís energetic light and swept away by its winds.

Fig. .. 

Some comets show two types of tails at the same time. There are the long, straight blue ion tails and the shorter, curved yellow dust tails. The gases liberated by a comet nucleus become ionized by the action of solar ultraviolet radiation and emit a faint blue light by fluorescence. The dust tail shines only by reflecting yellow sunlight. Since the individual dust particles enter slightly different orbits of their own, the dust tail often spreads out into a fan shape. An individual comet may have a dust tail, an ion tail, both types of tail, and no tail at all.

But what are the solar forces that blow the gas and dust into comet tails? The gentle pressure of sunlight pushes the tiny, solid dust grains along curved paths as the comet moves through space. When the Sunís light bounces off the dust particles, it gives them a little outward push, called radiation pressure, and this forces them into the dust tails. For larger solid particles, comparable in size to sand or pebbles, the Sunís gravitational pull overcomes the radiation pressure, and so these particles stay near the orbital path of the comet and they do not enter the dust tails.

A solar wind of electrically charged particles and magnetic fields propels and constrains the ions on straight paths away from the Sun. The solar wind, which continuously flows away from the Sunís surface, also accelerates the ions to high velocities. Thus, the ion tail acts like a windsock and, in fact, the existence of the solar wind was hypothesized from observations of comets before the age of space exploration. Spacecraft have now confirmed these predictions, and have permitted measurements of the electrons and protons that are blown away from the Sun, carrying the solar magnetic fields with them.

Fig. .. 

The gas lost from a comet is ionized by ultraviolet sunlight, producing an ionosphere that envelops the comet nucleus. Magnetic fields carried by the solar wind are unable to penetrate the ionosphere, so they pile up in front of it and drape around it to form nearly parallel, adjacent magnetic field lines that point toward and away from the Sun. Guided and constrained by these folded magnetic field lines, the comet ions are pushed away from the Sun by the much faster solar wind particles, forming a straight, blue ion tail.

Fig. .. 

But the interplanetary magnetism extending from the Sun is divided into sectors that point in opposite directions, toward and away from the star. When a comet crosses from one sector to another, the magnetism that envelops its ion tail becomes pinched and the comet loses the tail, somewhat like a tadpole. But unlike a tadpole, the comet soon grows another ion tail.

Fig. .. 

To sum up, a cometís anatomy consists of a concealed nucleus, an Earth-sized or Sun-sized coma, a vast hydrogen cloud, and two types of tails, the dust and ion tails. But a cometís anatomy is not a static thing, for comets are always changing shape. All of the comet tails grow when the comet approaches the Sun, and shrink when the comet moves away from the Sun. There is no such thing as a typical comet tail. They differ in shape, size and structure. Some comets have multiple tails, some have only one tail, and others have no tail.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University