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Why can a comet suddenly become much brighter in an outburst?
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Comet
Comet McNaught with filamentary tail and the Moon over the Pacific Ocean, photographed from Paranal Observatory, Chile, January 2007.
A comet may experience a sudden, dramatic increase in brightnessâknown as an outburstâdue to a large, rapid release of dust and gas from its nucleus. While the exact mechanisms are still studied, astronomers point to two primary explanations:1
- Amorphous-to-Crystalline Ice Transition: Comets often contain water ice in an amorphous (randomly ordered) state. As the comet warms, this ice transforms into a crystalline structure. This process is exothermic, meaning it releases energy that can further drive the reaction and trigger a sudden surge in cometary activity.1
- Rotational Spin-up: As ices on the surface of an irregularly shaped nucleus sublimate, the resulting outgassing can create torques. These forces may cause the nucleus to spin faster, potentially leading to structural instability or the release of trapped materials.1
These outbursts can be significant; for example, the comet 17P/Holmes once brightened by 15 magnitudes in less than two days.1