Mercury, The Not-So-Shrunken Planet
The MESSENGER spacecraft was the first to orbit the innermost planet after a series of flybys. The images returned during the orbital phase and from three flybys revealed a remarkable landscape – one broken by large thrust fault scarps.
The first evidence of these fault scarps was detected in images from the flybys of Mariner 10 in the mid-1970’s. However, the full scale and number of the fault scarps didn’t become clear until MESSENGER imaged the entire surface of Mercury. These fault scarps were one of my primary interests. I had spent a lot of time before the MESSENGER mission looking at the fault scarps in the hemisphere imaged by Mariner 10, so I had a pretty good idea what to look for in MESSENGER images. However, in the Mariner 10 hemisphere – in regions where I knew they were present – some prominent fault scarps weren’t evident in the MESSENGER orbital images. This was because during the first solar day of the orbital mission, the images that had been acquired were not optimum for detecting landforms. The optimum are high-incidence angle images – those taken when the sun is low on Mercury’s horizon and shadows are cast. After pointing this out, a campaign to obtain optimized images was initiated and resulted in high-incidence angle image coverage of the surface illuminated when the sun was in the east and the west. Two near-global image mosaics facilitated the identification of the fault scarps everywhere on the planet.
MESSENGER confirmed that the population of large fault scarps was evidence Mercury had experienced global contraction as the planet’s interior cooled. This caused the crust to contract, forming thrust faults and scarps hundreds of kilometers long and many with over a kilometer or more of relief. One of the key questions that could be addressed by identifying all the fault scarps is the amount of contraction Mercury experienced since the end of the period of heavy bombardment, about 4 billion years ago, before which no record would be preserved.
The amount Mercury has shrunken as expressed by the population of fault scarps has been a matter of debate. Some investigators estimated the decrease in the planet’s diameter to be as much as 14 km or more – what I describe as a “super-contracted” Mercury. In this study, using the high-incidence angle MESSENGER images and topographic data, only landforms with clear evidence of contractional deformation were mapped. I estimate the amount of contraction to be 2 to 4 km at most.
The loss of heat from the interior is a driving force on rocky planets. On a multi-plate planet like Earth, most of the major faults are located along the interacting plate margins. Mercury, by contract, is a one-plate planet and could be considered the archetype of how one-plate planets express the loss of interior heat. On a one-plate planet, the interior heat loss results in contraction of the entire plate and the formation of a broadly distributed array of fault scarps as we see on Mercury. Conventional wisdom has it that the smaller the body, the more quickly it loses its interior heat and becomes geologically inactive. The relatively small amount of global contraction I estimate for Mercury tells a very different story. It indicates an evolutionary path for small rocky planets where interior heat is retained and slow cooling results in less overall contraction. Slow cooling may drive very recent and even current tectonic activity on Mercury.