And if a comet is rock, what’s all that stuff streaming off it?
Refer to your first question. . .
Second question: it's plasma. every one of the comets we've flown by or visited or impacted has been indistinguishable from the asteroids we've visited or flown by. . . their only distinction is their extreme orbits. Do you know that largest astronomical objects in the solar system, other than the heliosphere, can be the comae and tails of comets? Some of them reach sizes larger than the sun, and can stretch from one planetary orbit to another, which expand that large in just months! Did you know that planets have tails? Mercury, Venus, and even our Moon have cometary tails. . . and their tails certainly are not water. The Moon's is a Sodium plasma.
In addition, ordinary asteroids in ordinary orbits have suddenly, for reasons unknown to orthodox cosmologists, when solar wind activity is high (i.e. High energy plasma streaming from the sun), spouted cometary comae and tails!
Collage of four previously visited comets, rocks, not dirty snowballs. If they were ice, the impact craters, if they were impact craters, would be surrounded by shattered ice and cracks in distinctive patterns. They're not.
The orthodox cosmologists believe that gravity drives everything in the Universe. This has resulted in absurdities such as claiming that the ice is hidden inside the comets (!) and only comes out to play when close to the sun. But that requires the sun to heat up millions of tons of rock (by the way, think about the insulating qualities of a coating of rock dust on the surface of the comet with vacuum separating the dust particles, and a 1.5mm/sec2 gravity to force them into contact with each other to transfer heat mechanically) to beyond the boiling point of water to super heat the ice to create the coma of a comet, which becomes, after the heliosphere, the second largest object in the solar system.
It also doesn't explain how this solar heating can cause comets to start their flare ups beyond the orbit of Jupiter where the solar energy can heat the surfaces of the comets UP to an amazing 140° below zero! how is that going to generate steam jets? Think about the energy required to do that. From what source does that come? Exactly how many Watts per meter of sunlight fall on the surface of a two or three kilometer in diameter comet beyond the orbit of Jupiter?
Given all that can you seriously believe the out-gassing of comets is from steam power? Or is the spread of the coma and tail to humongous size is because of Boyle's Law? What force, causes that spread so quickly? Gravity? Anyone? Perhaps it's like charges repelling each other.