Why does water ice in a comet sublimate instead of melting near the Sun?

Water ice in a comet sublimates rather than melts because of the environment in which the comet exists. Cometary nuclei are small, solid bodies with very low gravity, which is insufficient to maintain the atmospheric pressure required to support water in a liquid state.12

When a comet approaches the Sun, solar heat causes the ices on or near the surface to sublimate, transitioning directly from a solid to a gas. Because this process occurs in the vacuum of space, the evolving gases expand freely outward, carrying embedded dust and organic materials with them to form the cometary coma. In physics, sublimation is a phase change governed by the relationship between temperature and vapor pressure; without sufficient ambient pressure to keep water in a liquid phase, the ice bypasses the liquid state entirely.213