1 km not big enough...
Impact craters on Mars are ~ 15 times the size of the impactor (cf ~ 20 times on Earth)
It needs a 300-500 km class crater on Mars as the minimum to initiate an atmospheric event. This requires a 20-30 km impactor.
Haven't really been any of these since the end of the Noachian epoch, 3.7 Billion years ago on Mars.
(In fact, I suspect that it's the end of availability of such impactors that marks the end of significant atmospheres and widespread erosion on Mars, at the end of the Noachian)