"Obviously, impacts could provide local heating in the past as well, but it is not too relevant today as the planetary bombardment by asteroids is rather sporadic these days! "

It could be relevant today. I believe it's roughly true that if Earth was at Mars distance there would twice a many impactor hitting Earth as compared to what we get currently. And what we get now on Earth is space rocks impacting the Earth's atmosphere on a weekly basis and larger rocks on longer time scales: about 30 impactor per year creating nuclear size explosions in your high atmosphere and in hundreds of year time scale we get space rocks big enough that they punch through our atmoshere impacting at surface or near surface.

With Mars there is no heavy Earth type atmosphere in which impactor can somewhat harmlessly explode into, so on time scale weeks to a month or two small impactor will be hitting the surface or very near surface of Mars, creating relatively minor heating such as much as of the size of nuclear bomb dropped on Japan. And over the timescale of hundreds to a thousand years you will get impact events which are more substantial in terms of having a geological affect.

Considering how weak Mars is in term of volcanic activity, in the last billion year, it seems possible that impactor have played a more significant role in terms of local heating.