Why did Doomguy shoot Mars?

Why did Doomguy shoot Mars?

This gate became crucial to the Doom Slayer’s efforts, as he needed to reach Sentinel Prime where Deag Grav was located. Since there was no other way to reach Hebeth, the Slayer utilized the BFG-10000 and fired it at Mars’ surface, breaching the crust and exposing the planet’s core.

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