Graphite and faults: the secret of Mercury's contraction

Mercury, the smallest planet in the Solar System, has been contracting for billions of years. The gradual cooling of its interior in fact causes a slow reduction in the planet’s volume, with consequent deformation of the surface. Among the most evident structures are the so-called lobate scarps, ridges associated with large faults along which contraction has occurred: they can extend for hundreds of kilometres and reach up to 2,000 metres in height.

A new study, entitled Graphite lubricates Mercury’s global contraction and published in Nature Communications, explains the mechanisms underlying these deformations in greater detail. The research, carried out by researchers from the University of Padua, the Istituto Nazionale di Astrofisica (INAF-OAPD) and the University of Milan-Bicocca, shows that Mercury’s contraction may have been favoured by the presence, in the crust, of a particular “lubricant”: graphite, the same material used in pencil leads.

The analysis integrated the study of the topography of Mercury’s surface and a series of laboratory friction experiments, conducted at the University of Padua using rocks with characteristics analogous to those making up the planet’s crust.

The results indicate that graphite-rich layers, dating back to the very earliest phases of Mercury’s history, may have continued to influence its tectonics up to the present day. The study also suggests that the planet’s overall contraction may have been much greater than previously formulated estimates.

“The results suggest that, without graphite, some of the imposing fault systems observed on Mercury today could not exist in the form in which we see them,” explains first author Natalia A. Vergara Sassarini, affiliated with the University of Padua at the beginning of the research and now at the Istituto Nazionale di Astrofisica (INAF-OAPD). The study was coordinated by Matteo Massironi, lecturer in the Department of Geosciences at the University of Padua, who also emphasises “how the very first moments of a planet’s formation can determine the shape of its surface billions of years later”.

According to Telemaco Tesei, from the Department of Geosciences at the University of Padua, who was responsible for the mechanical experiments, “systems of large faults can be controlled by thin films of graphite” capable of influencing deformation in the crust of the entire planet, making Mercury “a unique world in the Solar System”.

The identified mechanism is particularly significant because Mercury lacks the conditions found on Earth. “Faults on Earth are often ‘lubricated’ by the presence of fluids that facilitate their movement,” observes Andrea Bistacchi, from the Department of Earth and Environmental Sciences at the University of Milan-Bicocca and co-author of the study. On Mercury, however, analogous fluids are not available because of the planet’s extreme proximity to the Sun and its low gravity. For this reason, the researcher stresses, it was necessary to identify “an alternative mechanism to explain the weakness of Mercurian faults”.

The discovery is also of particular relevance in view of the arrival of the ESA and JAXA BepiColombo mission in Mercury’s orbit, scheduled for November. According to Matteo Massironi, co-author of the article and Project Scientist of the SIMBIO-SYS instrument, the new results lay the groundwork for verifying with greater precision the geometry of the large faults and the possible presence of graphite in the surrounding areas.

SIMBIO-SYS will in fact make it possible to reconstruct these ridges in three dimensions, observe Mercury’s surface in great detail and simultaneously study its mineralogical composition. The new observations may therefore help to verify directly the role played by graphite in the planet’s geological evolution.