Earth’s habitable loop: Water, atmospheric structure, the geomagnetic field and plate tectonics

Abstract

Complex and intelligent life developed on Earth because it has retained sufficient water, primarily in liquid form, for several billion years. The loss of Earth’s water to outer space is limited by a cold trap at the top of its thick troposphere. Earth maintains a thick troposphere because the dipolar geomagnetic field deflects the solar wind, preventing ionized particles from heating the upper atmosphere and lowering the tropopause. Earth’s magnetic field is sustained by the dynamo action of convective motions within the outer core, which require a large flux of heat from core to mantle. Plate-tectonics, which provides the required rate of planetary cooling over billions of years, requires the presence of liquid water to make the plates ductile and to lubricate the plate boundaries. The sustained, coupled interaction of liquid water, atmospheric structure, the geomagnetic field and plate tectonics forms a feedback-loop which maintains Earth’s long-term habitability.