| http://www.w3.org/ns/prov#value | - [13][14] .These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple's dirty snowball model, which correctly surmised that Halley would be composed of a mixture of volatile ices???such as water, carbon dioxide and ammonia???and dust.^ They are composed of rock, dust, ice and frozen gases such as carbon monoxide, carbon dioxide, methane, a
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