Well excuse me, but 45 km deep is some f'ing big hole. We couldn't do that with our entire nuclear arsenal. Kim Stanley Robinson was only game to dig to 10 km in a limited shaft for his "Moholes" on Mars.

The only way I can imagine it happening is to repeat the formation of Hellas basin. That was originally ~40 km deep.

Except you need to do it in the northern lowlands so that it will actually drain the atmosphere into the plughole (Problems with the ramparts acting as a dam wall?)

Anyway, to blast a hole 2000 km across and up to 50 km deep, you need a 100 km diameter impactor. Phobos is WAY too small (27 x 19 km). There are around 26 Asteroids this size or larger, and probably quite a few KBO's.

All you need to do is get one to Mars and stand well back. :-)

(Earth would have spectacular meteor showers every night for decades, with free sample delivery from Mars - another bonus)