Mercury's newfound 'great valley' formed as planet cooled, buckled
The rocky planet contracted as it cooled – and without plate tectonics, something had to give. Belinda Smith reports.
Planetary scientists discovered a new large valley on Mercury that may be the first evidence that the planet's outer shell crinkled as it contracted.
Thomas Watters form the Smithsonian Institution and colleagues from the US, Germany and Russia examined images taken by NASA's now-dead Messenger spacecraft and found a 400-kilometre-wide valley extending more than 1,000 kilometres across Mercury's southern hemisphere.
This feature, they reason, could have only arisen as the planet's top layers buckled.
They published their observations in Geophysical Research Letters.
On Earth, our crust and upper mantle – collectively called the lithosphere – can shift around, thanks to tectonic plates.
But Mercury doesn't have such features. Instead, its lithosphere can be thought of as one giant plate that encapsulates the entire world.
This means that as the planet's interior cooled and contracted, the overlying lithosphere crumpled.
But evidence for this has been lacking – until now.
When Messenger whizzed around the little rocky planet during three flybys before settling into orbit in 2011, it snapped images of a 750-kilometre-wide basin which was dubbed Rembrandt basin.
While not the biggest basin on the planet, it's the second-largest well-preserved basin, and one of the youngest impact basins on Mercury.
Rembrandt basin is cut by large scarps – also revealed during Messenger's flybys – hundreds of kilometres long.
Two of these scarps extend out of Rembrandt basin. They're fault scarps, produced along a fault line as one side slips up relative to the other. The scarp pair form the newly discovered "Great Valley".
As the planet contracted, Watters and crew write, the fault scarps bounding the valley became so large they became towering cliffs.
Meanwhile, the valley floor slumped below the terrain surrounding the cliffs, suggesting they too were shifted by the shrinking planet.
Even though you might expect lithospheric buckling on a one-plate planet that is contracting, it is still a surprise when you find that it’s formed a great valley that includes the largest fault scarp and one of the largest impact basins on Mercury," Watters says.