'Our Moon From Formation to Asteroid Target: Message for Life on Earth'

The present Earth-Moon system formed in the aftermath of the impact of a Mars-sized body on our planet. The Earth was then mostly melted and the Moon accreted from a ring of vapor and liquid orbiting the Earth. Part of the impactor's core ended up in the Moon-forming disk around the Earth. Iron metal within the disk was partly oxidized by ferric iron and water. Metallic iron remained and this formed our Moon's small core, and about 2% of the impactor's core ended up within Earth's mantle.

It is conceivable that early asteroid bombardment on the Earth was relatively benign and that planet sterilizing impact never occurred. A dense CO2 atmosphere blanketed Earth within about 10 million years of the impact, and a solar-heated greenhouse maintained 200 degrees C temperatures at the surface. Earth did not become habitable until the CO2 subducted into the mantle. Subducted oceanic crust carried carbonates into the mantle, which partially melted beneath island arcs to form alkaline CO2-rich lavas. Groundwaters within these lavas are an attractive prebiotic environment. By the time of Earth's earliest sedimentary record at about 3.8 billion years ago, the surface was clement, the ocean was near its current pH about 8, and the CO2 pressure in the air was comparable to the modern value. By then, life had already begun to modify the composition of the Earth's surface, and even its mantle.

Speaker: Norman Sleep, Stanford University