Oddly this gave me a thought on the aliens themselves. They generate intense internal heat but cannot regulate it themselves. The cold environment is what keeps them down to a proper temperature. The development of a heat gun raises the temperature at a given point and the target broils their own body as they cannot shed the heat now.

Of course the device was not designed as a weapon. It was actually intended as a medical device to assist in mating. It was supposed to be a radiator to lower the body temperature to induce mating season. But the projector managed to melt one of the onlookers for the demonstration.

As to why they would try to attack Earth when it is too warm for them. They do not want Earth, they want to colonize Europa. But humanity might protest.