In 2017, a new way of understanding the cooling and heat transfer of terrestrial planets was proposed by a team of scientists from NASA and Louisiana State University.The theory was borne from observations of Jupiter’s tidally heated moon, Io shown in figure 2. The theory of heat-pipe cooling was developed to explain why Io has such a thick lithosphere that is consequently able to support its numerous mountains and calderas that result from its volcanism. If the lithosphere was not thick enough, any mountain formed on the moons surface would collapse under the stress. Scientists concluded based on observations that our solar systems terrestrial planets evolved in a manner consistent with heat-pipe cooling. In this way, the theory provides an explanation for Earth’s surface volcanic materials, its thick lithosphere, and its mountains.
Heat-pipe cooling/tectonics is a method of cooling for terrestrial planets wherein the main heat transport mechanism present on the planet is volcanism originating below the lithosphere, shown below in the top of figure 3. (stagnant-lid convection is discussed below as well). Melted rocks and volatile materials are moved from the liquid mantle through the lithosphere via vents and volcanic eruptions. These eruptions lead to global resurfacing of the planet by which older layers are buried and pushed down to form the thick, cooler lithospheres that contain the tectonic plates we are all familiar with.Since these first observations, scientists have hypothesized that this method of cooling has been involved in the evolution of all terrestrial planets, including Earth. They went a step further to say that heat-pipe cooling is the last significant endogenic (occurring below the surface of the planet) resurfacing process experienced by terrestrial bodies, and as such contains information from this period in their formation such as magnetic fields and gravitational anomalies. The time taken for a planet to cool via this method is directly related to its size, and as such, larger terrestrial planets in other solar systems may still be in heat-pipe cooling mode. The significance of this is that observing larger terrestrial planets still in their heat-pipe cooling mode may lead to a greater understanding of the role this physical concept may have played in the formation of Earth as we know it today. Unfortunately, all of the terrestrial bodies in our solar system, including the Moon show evidence of heat-pipe cooling in their past but are no longer actively undergoing this process.
The hallmark of heat-pipe cooling is the resultant strong lithosphere in addition to the constant resurfacing of the body due to persistent volcanic activity. The implications of heat pipes for the tectonic history of terrestrial planets are shown in figure 3 above. Planets that evolve through a heat-pipe cooling phase develop a thick lithosphere early in their history which subsequently thins as volcanism wanes and thickens as stagnant-lid convection takes over. This is where the surface of a terrestrial planet has no active plates and is instead locked into one giant plate, and the surface material does not experience subduction. Currently Earth does have active plates as evident by our abundant seismic activity, but this form of convection will eventually become dominant, and the lithosphere will no longer be recycled. At this stage, whatever condition the Earth’s surface is in will be preserved for extraterrestrials to view and study, similar to how we study other planets.