Previous calculations indicate that the Earth suffered impacts from objects up to Mars size. Such a giant impact may have produced a temporary ejecta‐based ring that accreted to form the Moon. To simulate the surface waves from such events we approximated the cratering source as a buried pressurized sphere. For a 1027J impactor we calculated the resulting surface wave using the mode summation method of Sato et al.. For such an impact, the solid Earth free‐surface velocity above, and antipodal to, the source achieves 2.6 and 1.9 km/s. Such large ground motions pump the atmosphere and result in upward particle motions which cause the atmosphere to be accelerated to excess of the escape velocity (11.2 km/s) at high altitudes. For a 1.3 × 1032J Moon‐forming impact we calculate that ∼50&percent; of the Earth’s atmosphere is accelerated to escape. © 2004 American Institute of Physics