Convection is one of the ways heat is transferred both within and on the surface of planetary bodies.  As fluid gets warmer it becomes less dense and thus more buoyant than the cooler fluid around it.  In “Convection Cells” a vessel of fluid is heated from below, causing the fluid nearest the heat source to become less dense and rise.  When the warmer, denser fluid reaches the top of the vessel, some of its heat is transferred to the glass covering and into the room.  Having released heat, the cooler top layer of fluid becomes denser again and sinks into the warmer fluid.  As the fluid circulates in a continuous cycle, it organizes itself into pockets called convection cells.  The darker areas flowing outward and downward are the cooler, denser, sinking material.  They surround a column of warmer, rising cells.  Click here to run a simulation of thermal convection.

Photo Credit: Benjamin Burress

This type of convection—thermal convection—is the primary means by which planets lose their internal heat, and has played an important role in the evolution of the planets.  Small, solid planets like the Earth were formed from clouds of rock, metal, and ice that became compacted under the pull of gravity.   Intense heat was generated by this compaction.  Additional heat resulted from the decay of heavy radioactive elements, which sank to the Earth’s core.  That heat is slowly being radiated back into space through the Earth’s surface.  The process of convection is moving the heat to the surface, causing the cooling of the Earth.

Each planet or moon has a certain volume of mass in which heat is stored and a certain surface area through which heat is released.  The rate of heat loss through convection is proportional both to the surface area and to the amount of heat originally stored within the planet or moon.  Larger planetary bodies possess a greater volume relative to their surface area than smaller ones.  As a result they are able to store a greater volume of heat and release it more slowly than smaller planets and moons, which cool more quickly.  The smaller planetary bodies, such as our Moon, have cooled to the point where the interior has frozen, and convection has ceased.

Convection occurs in many guises on planets and stars throughout the universe. It affects tectonic movement on planets in the Solar System.  For instance, huge convection cells, as big as continents, form in the mantle under the Earth’s crust.  The slow but inexorable motion of those cells is responsible for the movement of continental plates on the Earth's surface, and thus for the earthquakes in California and throughout the world.

Photo Credit: SVST