Slide 1
A comet’s nucleus can grow a temporary atmosphere
A comet contains ice, dust and rocky material. As it approaches the Sun, released gas and dust can surround the solid nucleus with a temporary atmosphere.
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That surrounding cloud is called the coma. It can dominate a telescope view, so the bright head of a comet should not be confused with the much smaller solid body supplying the material.
Slide 2
Sublimation turns ice directly into gas
Heating can turn cometary ice directly into gas, a process called sublimation. Escaping gas can carry dust away from the surface.
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Activity depends on which volatile materials are present and how they are exposed. A comet’s changing brightness is therefore not just a simple measure of its distance from Earth.
Slide 3
Dust and charged gas form different tails
Dust and ionized gas, gas containing electrically charged particles, respond differently to sunlight and the solar wind. A comet can develop separate dust and ion tails.
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Radiation pressure pushes dust while charged particles interact with the solar wind’s magnetic environment. The resulting tails need not have the same curvature or appearance, even though they come from one nucleus.
Slide 4
Tails usually point away from the Sun, not behind the motion
Comet tails generally extend away from the Sun. Their direction is controlled by solar effects rather than simply trailing behind the comet’s motion.
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A comet can travel outward while material stretches ahead of it relative to its orbital motion. Thinking about the Sun’s position explains this better than treating the tail like exhaust behind a moving vehicle.
Slide 5
Each active passage removes some of the comet’s material
A comet loses material as it becomes active. Repeated passages can alter its surface and the supply of accessible ice.
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Some comets return on relatively short orbits, while others take far longer. Their histories of solar heating differ, so not every comet has experienced the same number or intensity of active episodes.
Sources
- Comet Facts
NASA
- The Long-Haired Comets
OpenStax, Rice University