[SunkenCiv]

I don't think there's enough mass in all the asteroids, combined, to raise Mars' mass that much.

[ http://www.nineplanets.org/earth.html ] Earth mass: 5.972e24 kg

[ http://www.nineplanets.org/mars.html ] Mars mass: 6.4219e23 kg

Okay, now I'm 90 per cent sure.

[Old Student]

We don't have to match Earth's mass. Just enough to hold an atmosphere. Venus is 1.1e24 kg smaller than earth, and it holds an atmosphere just fine! I don't have the math to figure out how much mass we'd need, but I'm sure someone out there does. About 3e16 kg of it is hanging there in orbit right around Mars, for that matter. Several of the larger moons of Jupiter are within about 1 order of magnitude of the mass needed to match Earths, for that matter. We can do it. Not easily or quickly, but we can.