20 October, Tuesday, 7:45 PM, Presidio Observation Post, Building 211
“Our Moon From Formation to Asteroid Target: Message for Life on Earth”
Norman Sleep, PhD, Geophysics Department, Stanford University
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.
Norman Sleep graduated from Michigan State in 1967. He completed his PhD on island arcs at MIT in 1973. He was assistant professor of Geology at Northwestern from 1973 to 1979. He has since been on the Geophysics faculty at Stanford. His work applies heat and mass transfer to physical processes within the Earth and other planets. He is currently studying the aftermath of the moon-forming impact and its implications to early life. He also has interests in earthquake seismology including nonlinear rock failure during strong shaking. This work includes determining maximum past and future levels of shaking within Greater Los Angeles.