How did the 1986 spacecraft encounters support the dirty snowball model of comets?

The 1986 spacecraft encounters with Halley’s Comet—specifically by the Giotto probe and the Soviet Vega spacecraft—provided definitive evidence that the comet nucleus was a solid body, confirming the "dirty snowball" (or "icy conglomerate") model proposed by American astronomer Fred Whipple in 1950.12

These missions successfully imaged the nucleus, revealing a dark, solid, potato-shaped object composed of a mixture of water and other volatile ices, along with rocky silicates and carbon-rich organic dust. This discovery effectively disproved the competing "sandbank" model, which had been promoted by English astronomer R.A. Lyttleton and suggested that comets were merely loose clouds of meteoritic particles with adsorbed gases.123

The data confirmed that the nucleus was a solid, cohesive body, with approximately 70 percent of its surface covered by a dark, insulating crust of organic compounds and silicates, while the remaining active areas produced bright jets of gas and dust as the ices sublimated.1