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How does a comet’s low gravity affect its atmosphere?

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Comet Comet McNaught with filamentary tail and the Moon over the Pacific Ocean, photographed from Paranal Observatory, Chile, January 2007.

Because cometary nuclei are small and possess very low gravity, they are unable to retain an atmosphere. As a result, when ices on or near the surface of the nucleus sublimate due to solar heat, the resulting gases and entrained dust particles expand freely into the vacuum of space.12

This outflowing mixture of gas and dust forms the cometary coma, which serves as the comet's atmosphere. Rather than being held in a stable layer, this material flows radially outward from the nucleus. The shape of this atmosphere is governed by the "fountain model," where material is liberated from the Sun-facing hemisphere and eventually influenced by solar radiation pressure, which slows the dust and accelerates it in the anti-Sun direction to form the comet's tail.213