As the smallest of the terrestrial planets in our solar system, Mercury has much to teach us about the evolution of small rocky planets. Unlike Earth, whose tough outer layer, the lithosphere, is divided into a mosaic of plates that move relative to each other, Mercury is a single-plate planet. This view of Mercury was determined from images and data returned by two spacecraft, Mariner 10 and MESSENGER. On Earth, the movement of the plates is driven by the flow of hot, semi-molten rock in the mantle, the layer on which the lithosphere literally floats. The flow of this semi-molten rock is a mechanism by which the Earth cools its interior. There is ample evidence for this mantle-induced process at the edges of terrestrial plates where large faults and volcanic activity are concentrated.
On a single-plate planet, heat loss manifests itself differently. Instead of the movement and interaction of multiple plates, the internal heat loss acts on a single plate, causing it to contract and shrink. The evidence that Mercury contracted comes in the form of a population of mountain fault scarps spread around the world. As the interior of the planet cools, the crust – the rocky upper part of the lithosphere – is forced to contract, forming overlapping fault scarps several hundred miles long and many with more. one kilometer or more of relief. However, the forces of global contraction alone are expected to result in a global set of faults evenly scattered around the planet.
Prior to the flyovers of the MESSENGER spacecraft and the orbit of Mercury, I studied the fault scarps detected in images from the Mariner 10 overflights. Although the full extent of the population of Mercury fault scarps is not Not known, there were indications that some faults were clustered into long, linear clusters. MESSENGER confirmed the existence of long clusters, or belts, of overlapping fault escarpments, some extending for thousands of kilometers. Since these clusters are most likely not the result of an overall contraction, another process causes the formation of faults in the linear belts. Could it be that something happening in Mercury’s mantle is responsible for this?
My colleagues and I have introduced new models of the crustal thickness of Mercury, created using gravity and topographic data obtained by MESSENGER. In our analysis, we find that the fault scarp clusters are found in areas of thick crust. The association between the clusters and the thicker crust may be evidence of a flow in the mantle of Mercury. We suggest that the downward flow of the mantle could thicken and contract the crust of Mercury, helping to form the long fault scarp belts. We are also introducing a new dynamic mantle pressure model that shows positive values, indicating upward flow, and negative values, indicating downward flow. This model shows a correlation between regions with a greater number of fault scarps, or higher contraction stress, and regions of negative mantle dynamic pressure.
Downward mantle flow is a process that has been proposed for intra-plate tectonics on Earth where overlap faults form far from the plate margins. A link between mantle flow and tectonics suggests that an Earth-like process influenced the formation of large faults on Mercury, and may still be.