Mars has a mass of about 6.4 × 10^23 kilograms, roughly a tenth of Earth’s mass, very close to the estimated size of Theia, the Mars‑like body thought to have formed the Moon by colliding with the early Earth. If such a planet struck Earth at a typical giant‑impact speed of around 12 kilometers per second, the kinetic energy released would be about 4 × 10^31 joules.
That figure is not a guess; it comes directly from the standard kinetic energy relation using the reduced mass of the two‑planet system and a conservative, realistic impact velocity in the same range used in modern simulations of the Moon‑forming impact. Even if we vary the velocity within plausible bounds for such encounters, the result stays in the same order of magnitude: a few times 10^31 joules, not less.
To feel what this means, compare it to the Chicxulub impact that helped end the age of the dinosaurs, estimated at about 10^23 joules. A Mars–Earth collision would therefore release roughly one hundred million times more energy than that extinction event. It would also exceed humanity’s total yearly energy use by about eleven orders of magnitude.
In such a cataclysm, crusts melt, mantles mix, atmospheres are blown away and rebuilt. Planets do not merely collide; they are reforged.