Could weird ridges on Venus tell us about even weirder ridges on Miranda?
Miranda may be one of the weirdest moons in the solar system. It is the innermost large moon Uranus and roughly the size of Saturn’s moon Enceladus (Radius ~ 235 km or 146 miles), famous for its water ice plumes and a suspected subsurface liquid water ocean. Miranda also has three regions of concentric ridges on its surface, called coronae (singular: corona). These regions are each about 200-300 km (124-186 miles) across. Two of the coronae appear to straddle Miranda’s equator while one is at Miranda’s south pole. In addition to their bizarre appearance, the coronae of Miranda also suggest that Miranda experienced a recent heating episode to allow for the formation of young surface features, since based on its size, Miranda should be a heavily cratered ice ball dead as Earth’s Moon.
Geologic formations similar to the coronae occur on only one other planetary body in the solar system. The identity of this body makes the coronae even weirder—Venus.
Venus is a very different world than Miranda. Whereas Miranda is tiny, about the size the U.S. state of Arizona, Venus roughly the same size as Earth. Also, whereas Miranda has a frozen crust with surface temperatures of a frigid -170 °C (-274 °F), Venus has a balmy year round temperature of 465 °C (870 °F) due to a runaway greenhouse effect. Nonetheless, Venus is the planet with the closest analogue to the coronae of Miranda. The coronae of Venus, like on Miranda, consist of concentric ridges. Many of them are more circular than on Miranda but they also have a variety of polygonal shapes. The Coronae on Venus also vary more in size, ranging from 100-700 km across (62-435 miles)
If coronae occur on both one of the hottest rocky planets and on one of the coldest icy moons, what could they be? There are many competing explanations for the formation of the coronae, but the main hypotheses for corona formation on Miranda and Venus both involve upwelling of hot material from within the mantle creating deformation at the surface. Currently it is unclear what coronae are and so it is also unclear what coronae tell us about the geologic history of Miranda and Venus. One intriguing possibility though is that coronae are caused by a process similar to plate subduction.
On Earth, subduction happens because Earth’s crust is divided into plates that move along the upper mantle. When two plates of ocean crust collide or a plate of oceanic crust collides with a plate of continental crust, the plate with higher density crust will be thrust beneath the plate with lower density (and more buoyant) crust.
On Earth plate tectonics is how carbon gets recycled by being locked away in sedimentary rocks on the ocean floor that get subducted rather than building up in the atmosphere as carbon dioxide. The lack of plate tectonics on Venus is one of the reasons why Venus is so hot, as carbon dioxide traps heat.
This is why Recent studies looking at experimental and observational evidence suggesting that the concentric ridges making up the coronae of Venus could be undergoing a process similar to plate subduction are so interesting. To be clear, this is not true plate subduction since it is not part of a global process of moving tectonic plates like on Earth. Nonetheless, it does result in a scenario which is analogous to plate tectonics at least on a regional scale. It could even be evidence of an emerging plate tectonics on Venus, though this is pure speculation at this point.
Could this also be happening with the coronae on Miranda? Upwelling of material requires convection, where hot material in the mantle moves upwards to release heat before descending again as it gets colder and denser. This process is occurring in Earth’s mantle and may be happening in Venus’s mantle as well. Could it be happening in the ice shell Miranda? This would require Miranda to have a fairly thick crust to allow for convection of ice, but it is not impossible.
What is the significance of Miranda having a subduction-like mechanism behind its surface geology? For one thing, a plate subduction-like process on Miranda would allow for transfer of material between the surface and its possible subsurface liquid water ocean.
The presence of subsurface liquid water oceans within icy moons is significant because of the importance of liquid water as an ingredient for life. A major limitation to life on these ocean worlds, however, is a lack of nutrients. If your ocean is encased under an ice sheet, it is going to be difficult to give get nutrients, like phosphorus, to the subsurface ocean if they are not already being produced there. To be fair though, not all icy moons are expected to have a missing phosphorus problem.
Unless there is a mechanism to get nutrients into the subsurface ocean, ocean-bearing icy moons may only be able to have a sparse population of simple microbes at most, which means no alien squids unfortunately. A subduction-like mechanism in Miranda’s surface geology would mean material is being actively transferred from the surface to the subsurface ocean (assuming it exists), allowing nutrients to accumulate. In this case, nutrients not produced in the ocean could be deposited onto the surface as meteorites, or through solar wind, and transported to the ocean via plate subduction.
Currently we cannot send a mission to Miranda to place seismometers on its surface a probe its interior, but we do have images of the surface geology from the Voyager 2 spacecraft which can be compared to the radar maps made from Magellan spacecraft data on Venus. One way to test the hypothesis that the coronae on Miranda and Venus are analogous features related to plate subduction is to see if they are really so similar in morphology.
One way of testing this is to use a machine learning algorithm that identifies coronae on Venus and systematically compares them to the three coronae on Miranda. If the ML positively confirms the coronae on Miranda as being coronae based on similarities to the Venus coronae, this would be model support for them being the same type of feature.
While confirmation of this hypothesis wouldn’t mean that Miranda has complex life, let alone life, it would be significant if compelling reasons (not found so far) are revealed to suspect life exists on Miranda since life would have a way to get access to vital nutrients otherwise scarce or absent in the subsurface ocean.
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