The two Earth-sized planets in our solar system have taken wildly different evolutionary routes. The surface of the Earth became a temperate utopia for a liquid water and a myriad of life. But while similar in both size and mass, the surface of the neighboring Venus is hot enough to melt lead.
These differences are the key to understanding the possible outcomes for a rocky planet after it forms out of the dusty disk around a young star. Knowledge of the rocky options is needed to identify the surface environments of extrasolar planets from the limited data we can gleam through our telescopes, and to unpick the properties needed to form a habitable planet. It is a task considered so important that three new Venus missions were approved by NASA and ESA in the last month.
One such difference between the Earth and Venus is the type of planet surface or, more precisely, the structure of the planet lithosphere that comprises of the crust and uppermost part of the mantle.
The Earth’s lithosphere is broken into mobile chunks that can subduct beneath one another, bunch up to form mountain rages, or pull part. This motion is known as plate tectonics, and it allows material to be cycled between our surface and the hidden mantle deep below our feet. It is a geological process that replenishes nutrients, cools the planet interior, and also forms part of the Earth’s carbon cycle that adjusts the levels of carbon dioxide in our atmosphere to keep our environment temperate. Without this cycling ability, the Earth would not have been able to stay habitable over such a long period.
By contrast, the lithosphere of Venus does not form plates. This prevents carbon from being drawn into the mantle, and any nutrients below the surface are unreachable. Indeed, the surface of Venus has long been thought to be a single piece of immobile, stagnant lid, with no connection at all with the planet interior.
Not only does the lack of geological processes throttle Venus’s environment, the seemingly complete immobility of the lithosphere was extremely annoying.
With no link at all between Venus’s surface and interior, we have no clues as to what might have prevented Venus developing an Earth-like lithosphere and geological cycles. We are stuck trying to understand the progression of an illness from one healthy body and a skeleton. To estimate the chances of an Earth-sized planet being healthy, we really need an example of a sick one.
But new research led by Paul Byrne from North Caroline State University and published in the Proceedings of the National Academy of Sciences has suggested the situation is not as bone-dead as it has seemed.
While Venus’s top seemed to be a stagnant shell, the surface still shows heavy deformation. This deformation has been long recognized, but thought to either be regions of stretching or folding produced by events such as volcanoes or cratering. Venus’s surface appeared to be like a punctured and stretched rubber sheet.
However, fresh examination of the Venusian surface in radar images captured from NASA’s Magellan Venus orbiter, revealed regions over the planet that also appeared to have shifted side-to-side. This lateral motion does not imply a puncture, but rather discrete blocks that are shuffling around. While the Earth plates can move over and under one another, it appears Venus may have lithospheric blocks that jostle in a fashion similar to pack ice.
Computer models run by the team suggest that a “pack ice” motion could be driven by movement in Venus’s mantle. The motion of the Earth’s tectonic plates is also driven by mantle movement and while Venus’s interior motion is highly sluggish in comparison, it is the first evidence that Venus’s surface is connected to a mobile planet interior.
Moreover, the lateral jostling of blocks seems to have occurred recently. It could therefore be that “pack ice tectonics” is active on Venus today, painting a more dynamic picture of the planet than has previously been assumed.
In an intriguing connection with the Earth, the motion of the Venus blocks might resemble the behavior of Earth’s surface during the early Archaean period, when our planet was much younger.
“The thickness of a planet’s lithosphere depends mainly upon how hot it is, both in the interior and on the surface,” Byrne says in a press release by NC State. “Heat flow from the young Earth’s interior was up to three times greater than it is now, so its lithosphere may have been similar to what we see on Venus today: not thick enough to form plates that subduct, but thick enough to have fragmented into blocks that pushed, pulled, and jostled.”
Venus’s current mode of tectonics could therefore shed light on the development of our habitable world, as it developed a plate tectonic system that could ultimately keep the surface temperate.
The fact Venus did not follow the Earth into a system of plate tectonics could be due to the planet’s closer location to the sun preventing the lithosphere from thickening sufficiently to snap into plates. Alternatively, Venus may be transitioning back from a more Earth-like plate regime that was once possible on a cooler and potentially habitable Venus before another mechanism sent the planet horribly wrong.
For habitability, Venus remains a very ill planet. But the pack ice tectonics is evidence that a mobile planet surface and interior did develop and may continue to this day. This offers clues as to how healthy geological cycling can develop on a rocky planet.
A deeper exploration will be possible with new data from the upcoming missions EnVision led by ESA and VERITAS led by NASA, that will substantially enhance our view of these Venusian blocks, and hopefully allow a picture to be built of how a planet can sustain a habitable environment.