Recent computer simulations have shown that, during this early period, large amounts of material must have been exchanged between the two planets. Some of the material blasted from Earth in meteoritic impacts would have landed on Mars, and vice versa. Even today, a Martian meteorite hits Earth about once every three days, Sleep says. Several billion years ago, the impact rate was a thousand times greater. That means millions of Martian meteorites made the trip from Mars to Earth in just a few years, a short enough period for spores or possibly even intact microorganisms buried deep in the meteorites to survive, he calculates.
Sleep notes that even modest-sized impacts have been known to decimate life on Earth.
Yet, when it hit the Yucatan peninsula 65 million years ago, it ignited a fireball that spread out across most of North America and launched tremendous amounts of pulverized rock almost into orbit. As that rock dust fell back to Earth, enough heat was released to set forest fires worldwide and to evaporate about a meter of water from the world's oceans, Sleep says. That is mild compared to the damage from an object hundreds of kilometers across, like those that were hurtling through space about 4 billion years ago. Such tremendous heat would boil off all the world's oceans and create a steam atmosphere that would persist for about 3,000 years. That is long enough to sterilize the Earth's outer crust down to nearly one kilometer, according to Sleep and Zahnle.