Coma of the comet

The comet’s coma or head of the comet . is a fuzzy mist surrounding the actual nucleus of the comet. Including the tail, the coma is all we actually see, watching the comet from the Earth.

The shape and structure of the comet’s coma

The shape of the coma can vary from comet to comet and for the same comet during its passage, depending on its distance from the Sun and the corresponding quantities of dust and gas. Pale or bright comets producing little dust and often have a round shape, and comets that produce a significant amount of cometary dust have fan-shaped or parabolic shape.

This is due to released different-sized dust grains: those that are bigger are moving to the left along the trajectory of a cometary orbit, and the smaller particles are repelled from the Sun using light pressure.

Coma has 2 main components: gas-shell and shell dust. In fact, scientists believe that comets produce almost the same amount of gas and dust.

The gas coma

The gas coma consists of molecules, separated from the comet’s nucleus due to solar heating and sublimation (transition of the molecules from solid to gas). After molecules have left the core, in the shell, they are exposed to direct sunlight and can be damaged in different ways: joint action of these reactions,most of the molecules break apart (dissociated) soon after leaving the nucleus.

Coma of comet Siding spring

The most familiar of these reactions is Photodissocialion molecule (the so-called “parent molecule”) absorbs a Photon in the form of solar radiation and is divided into 2 parts (called “daughter molecules”). The resulting child molecules are very easy to spot, because they have strong spectral lines in the optical range.

In addition to Photodissociation, the comet gas can also be ionized (it loses one or more electrons). The formed ions are affected by magnetic field of the Sun . carried out by the solar wind. Consequently, the ions are arranged radially with respect to the Sun in a long distinctive tail, called the ion tail.

The rate of outflow of the coma combined with the lifetime of a molecule enables to determine the size scale of the coma. In a single day at a speed of 1 km/s, the molecule can cover the distance of about 50 thousand km. This distance is the approximate radius of the gas shell.

Coma of dust

Motes, forming a coma of dust released from the nucleus during sublimation gas. The speed of these particles will be higher, but it mainly depends on their size, since small dust particles are easier, and therefore move faster due to gas. The dust can become part of the shell, if I reach a speed higher than the speed of the comet’s nucleus. Particles that are too heavy to be thrown back to the surface of the nucleus and may again become part of the nuclear cover. Particles that are easier, leave the core and remain in free flight around the Sun (because the nuclear gravity of the comet is very weak).

Cometary dust particles come in different sizes. Particles with a diameter of 0.001 mm are the most visible to the eye, but particles with diameter of a few tens of centimeters can be ejected by the gas flow is very active comets close to the Sun. Many of these particles are transferred from the tail to the interplanetary medium, giving rise to the meteorites, whose flash can be seen at night.

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