Mercury’s mantle goes with the flow


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.

Model of the crustal thickness of Mercury. Gravity and topographic data obtained by NASA’s MESSENGER spacecraft are used to estimate the current thickness of Mercury’s crust, with shades of red and white indicating thick crust and shades of blue indicating thin crust. The orthographic projection is centered on longitude 120 ° E and the Caloris basin is at the top right. (Smithsonian Institution)

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?

Prominent linear cluster of fault scarps on Mercury. The cluster consists of lobed escarpments and high relief ridges (~ 35ºN, 30ºW) which include Victoria Rupes, Endeavor Rupes and Antoniadi Dorsa, extending 1,500 km (arrows pointing left). This cluster of fault scarps lies on the eastern edge of a zone of thick crust on Mercury. Another prominent lobed escarpment, Carnegie Rupes (arrows pointing up), lies along the southern edge of the thick crust region. The color-coded topography shows lower elevations in shades of blue and higher elevations in shades of red. (NASA / Johns Hopkins University Applied Physics Laboratory / Washington Carnegie Institution / Smithsonian Institution)

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.

Illustration of Mercury showing a new pattern of crustal thickness with a cross section showing a pattern of the inner structure of the planet. Rind thickness is shown in shades of red and white indicating a thick rind and shades of blue indicating a thin rind. The interior of Mercury is dominated by its inner solid core and outer liquid core. The downward flow of the mantle in the mantle of Mercury causes thickening of the crust and also generates a contraction (the highest black arrows in the inset diagram) within the thickening of the crust. It is the contraction that influences the location of the overlap fault scarps. (Smithsonian Institution)


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