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How can a nonvolatile dust layer reduce a comet’s activity?

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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 nonvolatile dust layer reduces a comet’s activity by acting as an insulating barrier on the surface of the cometary nucleus.

As a comet approaches the Sun, solar heat causes the ices within the nucleus to sublimate, transitioning directly from solid to vapor. This process releases gas that carries away embedded silicate and organic dust particles. However, particles that are too large to be lifted by the escaping gases remain on the surface, accumulating into what is known as a lag deposit.1

Over time, this layer of nonvolatile dust can become sufficiently thick to effectively insulate the underlying icy components. By shielding these ices from solar radiation and heat, the dust layer prevents further sublimation, thereby suppressing the comet's overall activity.21