[Bear_in_RoseBear]

Uh, one of the characteristics of a comet is that it travels in a highly eliptical orbit. This means that, for part of its orbit, it moves very, very close to the sun.

Perhaps the substances formed during that part of the comet's orbit.

[S0122017]

But not all comets get close to the sun, these ones aren't supposed to have done that.