Presentation #1253 in the session “Open Engagement Session C”.
Whether a planetary body can introduce material—and volatiles in particular—to its surface and then return that material to its interior plays a major role in its geochemical evolution, surface conditions, and even its habitability. Plate tectonics on Earth is the dominant process by which volatiles including water and carbon are conveyed to the surface and back to the mantle via volcanism and subduction, helping the planet to regulate its temperature for the last several billion years. This foundational process of Earth has distinctive morphological signatures, in the form of elevated, extensional spreading ridges where new oceanic crustal material is generated, and deep trenches where thrust faults conduct that crust down into the planet’s interior (often with associated arcs of volcanoes). Additionally, major strike-slip fault systems accommodate the motion of oceanic plates as they move across the surface of ellipsoidal Earth.
In marked contrast to the mosaic of plates that make up the Terran lithosphere, there is no morphological evidence on Mercury, Mars, the Moon, or Io for spreading ridges, subduction zones, or large-scale strike-slip fault systems. There are major thrust faults on Mercury and Mars, but those structures are tied to global contraction from secular interior cooling. Enormous mountain blocks on Io are likely formed by compression of the lithosphere resulting from the continued burial of material by that moon’s prodigious volcanic activity. Extensional systems are widespread on Mars, but are mainly concentrated around the Tharsis volcanic province and probably reflect shallow intrusive activity. Extensional faults are largely absent on Mercury and the Moon.
Venus lies somewhere in between: there is certainly no interconnected network of tectonic plates as on Earth, but the Venus lithosphere shows far more tectonic deformation than the static lithospheres of Mars, Mercury, or the Moon. No spreading ridges are as yet recognized on Venus, but there are deep, arcuate trenches that morphologically resemble subduction zones on Earth. Indeed, localized plume-induced subduction may operate on the second planet, returning crustal material to the interior. Venus’ oldest terrain, the tesserae, show amounts of tectonic deformation that rival those of ancient continental interiors on Earth—which are shaped by convergent tectonic plates. And Lakshmi Planum, at high northern latitudes on Venus, is bounded by mountain chains that bear a striking resemblance to the Himalaya, formed when the Indian plate collided with Asia. It may be then that large-scale horizontal translations of crustal material was once a feature of Venus’ active geology, with plate tectonics or some cognate mechanism responsible for such motion. If so, then Venus would have been capable of far more crustal recycling than at present. Whether this prospect is true remains a major open question of Venus science.
Together, the rocky worlds of the Solar System offer us a framework for understanding how volatile cycling may work on terrestrial planetary bodies generally. Volcanic activity is the rule on all these Solar System worlds, either dominantly from radiogenic heating (Earth, Venus, Mars, Mercury, and the Moon) or tidal interactions (Io). The transfer of volatiles from a planetary interior to the surface, therefore, appears to be a characteristic of rocky planets. But efficiently returning volatiles to the mantle via plate tectonics appears to be the exclusive purview of Earth, at least in the present. Venus may once have hosted this same process, and today localized sites of subduction may sustain limited crustal recycling. Mars, Mercury, the Moon, and Io seem never to have had meaningful lateral mobility of their lithospheres. To first order, then, the larger the body, the greater the likelihood of satisfying the conditions required for plate tectonics.
But the story doesn’t end there. Even without plate tectonics, it is possible to return material once at the surface to the subsurface. Much of Mercury’s surface is interpreted as volcanic in nature, comprising presumably many kilometers of material erupted over the first few hundred million years of that planet’s history. The same formational history likely pertains to much of Mars, including the geologically younger lava plains in the northern hemisphere as well as the more ancient, rugged southern uplands. The mountains on Io reflect the horizontal compressive stresses that result from the burial of material once erupted onto the surface and now at considerable depth. And a conspicuous absence of large impact basins on Venus points to a prolonged history of volcanic resurfacing. Together with crustal delamination—where hotter, lower crustal material detaches from the cooler, more rigid upper crust, and which operates on Earth and has been proposed for Venus and Mars, too—the burial of surficial materials by volcanic eruptions (and perhaps early in Solar System history by vast ejecta deposits from huge impacts) represents additional means by which volatiles can be cycled back into a planetary interior.