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What causes jets of gas and dust to emerge from a comet’s nucleus?

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

Jets of gas and dust emerging from a comet’s nucleus are evidence of localized active areas on the surface. These jets occur when solar heat causes ices on or near the surface of the nucleus to sublimate, transitioning directly from a solid to a vapor.12

As these gases flow away from the nucleus, they carry along embedded silicate and organic dust particles. Because the nucleus has weak gravity, these materials expand freely into space. While the exact nature of the source vents remains unknown, these jets are often observed on the Sun-facing hemisphere of the nucleus.21

Additionally, sudden, large-scale releases of gas and dust, known as cometary outbursts, may be triggered by the exothermic transition of amorphous water ice into crystalline ice, or by rotational spin-up caused by torques from the outgassing process.2