A better question is what would make a comet out beyond the orbit of Mars suddenly start outgassing "water vapor" i.e. "steam" when the temperature is still far below the melting point of ice, much less the boiling point, even in a vacuum, even the triple point. Why would they come close to grazing the Sun and still survive as a "dirty snowball" to make multiple passes as we know Halley's Comet has and still make a spectacular show? And, as you say, why were they not blown to smithereens, or sublimated to nothing by the vacuum of space causing them to outgas on their own at the triple point (to directly to gas, never stopping at the liquid stage). It makes no sense.
Here we have a body that has craters indicating it has been hit at high velocity by another object. The sheer mechanical energy should have generated sufficient heat to have melted any "ice" near the crater, but there are no signs of such molten materials. Instead we see jagged edges and broken rock. No ice, no water, no dirty snowball.
We DO, however, see material that looks exactly like the remnants left from "electrical machining" where material is carried off by the discharge of electrons.
Comets are not the only astronomical objects with tails:
Galaxy 3C 129 with a 1.5 million light-year long plasma tail.
Earth and all of the other planets have cometary plasma tails as well. . .
As for charges not being in space:
Io is in close orbit with Jupiter, so intense electromagnetic radiation bombards its surface, removing approximately one ton per second in gases and other materials. Io acts like a generator as it travels through Jupiter’s plasmasphere. More than four hundred thousand volts at three million amperes of current flows into the electric environment of Jupiter from Io.
The above is a measured current flow.