lpetrich
Contributor
"Toffee Planets" Hint at Earth's Cosmic Rarity - Scientific American
At high enough pressures and temperatures, rock will flow, though more like super thick syrup than like water. In the Earth's interior, rock starts doing that at roughly 25 km (15.5 mi) beneath its surface, its brittle-ductile transition (BDT) point. Above it is the "lithosphere", including the crust and uppermost mantle, and below it is the "asthenosphere", including most of the mantle.
Paul Byrne and his colleagues have considered where the BDT might be in super-Earth exoplanets, using 300 megapascals (3 kilobar) pressure to get the maximum depth and that pressure with a temperature gradient of 25 K/km to get the minimum depth. This is roughly consistent with experiments on deformation of presumably typical rocks. This research, presented at the 50th Lunar and Planetary Science Conference (2019), finds much thinner lithospheres for some super-Earths, around 3 to 5 km.
Here in the Solar System, Venus likely has a similarly thin lithosphere, and the Earth likely had a similarly thin lithosphere early in its history. Venus does not have very elevated topography, and the early Earth likely also did not have very elevated topography.
A super-Earth planet close to its star or with a very thick and hot atmosphere or with lots of heat flux from its interior is a planet that may have no lithosphere, thus being a "toffee planet".
Testing this hypothesis will be very challenging. I've found Simulations of Light Curves from Earth-like Exoplanets - Planetary Habitability Laboratory @ UPR Arecibo, but that does albedo features, not topography, and some large albedo features are not associated with prominent topography, albedo features like the Moon's maria -- big flood basalts. The Earth's continents are both topographic and albedo features, with their surfaces averaging about 4.5 km above the ocean floors.
At high enough pressures and temperatures, rock will flow, though more like super thick syrup than like water. In the Earth's interior, rock starts doing that at roughly 25 km (15.5 mi) beneath its surface, its brittle-ductile transition (BDT) point. Above it is the "lithosphere", including the crust and uppermost mantle, and below it is the "asthenosphere", including most of the mantle.
Paul Byrne and his colleagues have considered where the BDT might be in super-Earth exoplanets, using 300 megapascals (3 kilobar) pressure to get the maximum depth and that pressure with a temperature gradient of 25 K/km to get the minimum depth. This is roughly consistent with experiments on deformation of presumably typical rocks. This research, presented at the 50th Lunar and Planetary Science Conference (2019), finds much thinner lithospheres for some super-Earths, around 3 to 5 km.
Here in the Solar System, Venus likely has a similarly thin lithosphere, and the Earth likely had a similarly thin lithosphere early in its history. Venus does not have very elevated topography, and the early Earth likely also did not have very elevated topography.
A super-Earth planet close to its star or with a very thick and hot atmosphere or with lots of heat flux from its interior is a planet that may have no lithosphere, thus being a "toffee planet".
Testing this hypothesis will be very challenging. I've found Simulations of Light Curves from Earth-like Exoplanets - Planetary Habitability Laboratory @ UPR Arecibo, but that does albedo features, not topography, and some large albedo features are not associated with prominent topography, albedo features like the Moon's maria -- big flood basalts. The Earth's continents are both topographic and albedo features, with their surfaces averaging about 4.5 km above the ocean floors.