Shrinking, wrinkling, cooling Mercury

As one of the five planets of our Solar System visible to the naked eye, Mercury has been known since antiquity. However, Mercury remains the least explored of the terrestrial, rocky planets because its proximity to the Sun makes it difficult to observe using optical telescopes. To date, only two spacecraft have visited Mercury: in 1974, NASA’s Mariner 10 spacecraft provided the first glimpse of Mercury’s surface (Fig. 1), imaging about 45% of the surface during its three flybys; while NASA’s MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) spacecraft, which orbited the planet between 2011 and 2015, mapped the entire surface. A third mission is on its way; the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency’s (JAXA) joint mission, BepiColombo, will arrive at Mercury in late 2025.

Figure 1: Mariner 10 orthographic photomosaic of the southern hemisphere of Mercury. The planet has a diameter of 4,879.4 km.

The Mariner 10 images unveiled a surface similar to the Moon’s: barren, dull, grey, and heavily pockmarked by craters. One crucial difference, however, was the identification of extensive scarps and scarp systems (Fig. 2) that snake their way across Mercury’s surface. Attributed to tectonism, (Murray et al., 1974) comparison with those observed on other planetary bodies showed that Mercury’s scarps were far more abundant and grander in scale.

The Mariner 10 images unveiled a surface similar to the Moon’s: barren, dull, grey, and heavily pockmarked by craters

Figure 2: MESSENGER image showing the escarpment Victoria Rupes. This long scarp formed from the cooling and contraction of Mercury. Image is 500 km across, with the crater in the centre measuring approximately 40 km in diameter.

While the Mariner 10 images highlighted the dominance of compressional structures caused by crustal shortening (see Box ‘Compressional structures’), evidence for widespread, recent compressional tectonism was lacking.

To test whether Mercury is experiencing considerable contraction, I worked together with a team at the Open University, UK, University of Nantes, France, and ESA to carry out a global survey of shortening structures using MESSENGER imagery (Man et al., 2023a). We found extensive evidence for widespread, geologically recent contraction, supporting the idea that Mercury continues to shrink, wrinkle, and cool.

BOX | Compressional structures

The Mariner 10 images revealed three morphological types of compressional structure, all of which showed no apparent trends in orientation and were occasionally observed truncated by craters, suggesting that they formed early in the planet’s history.

Lobate scarps

Lobate scarps (Fig. 3A) are asymmetric escarpments with gentle back slopes and steeper fronts (Strom et al., 1975) and are the most commonly observed compressional structure on Mercury. Linear or arcuate in plan view, lobate scarps commonly span tens or hundreds of kilometres in length and up to a couple of kilometres in height. These structures are theorised to represent the surface manifestations of thrusts or reverse faults produced by compressional stresses.

Wrinkle ridges

Wrinkle ridges (Fig. 3B) are linear or sinuous antiforms that can braid and re-join when viewed planimetrically. In cross-section, they can have asymmetric or symmetric profiles and are generally morphologically complex compared to lobate scarps (Plescia & Golombek, 1986). Akin to the Moon, wrinkle ridges on Mercury are typically observed on the surface of geomorphologically smooth plains and are attributed to thrusts that do not break the surface and thrust-related folding. Wrinkle ridges vary in size; the largest named wrinkle ridge, Schiaparelli Dorsum, is 374 km long.

High-relief ridges

High-relief ridges (Fig. 3C) resemble much larger versions of wrinkle ridges, generally spanning hundreds of kilometres in length and up to two kilometres in height. Typically symmetric in profile and bounded by lobate scarps (Massironi & Byrne, 2015). High-relief ridges are commonly observed transitioning into lobate scarps, which suggests that they are a morphological variant and therefore likely another expression of thrusting and folding.

Figure 3: Schematic diagrams of Mercury’s compressional tectonic structures. (A) Lobate scarp; (B) Wrinkle ridge; (C) High-relief ridge. Colour scheme illustrates structures and does not represent Mercury’s geology. The diagrams are at different scales. High-relief ridges are the largest structures and can be comprised of lobate scarps. Wrinkle ridges are commonly the smallest structures out of the three.

A second look

Less than half of Mercury’s surface was imaged during Mariner 10’s three flybys. The abundance of compressional structures implied their presence across the planet’s entire surface, but this could not be confirmed for three decades. The unimaged half of Mercury remained shrouded in mystery until the arrival of MESSENGER, the first spacecraft to orbit the planet and image the entire globe in monochrome and colour. The probe confirmed that tectonic structures are present at all latitudes and longitudes, and are observed cross-cutting all types of geomorphological units and landforms.

While MESSENGER revealed that Mercury’s tectonic structures are globally distributed, they are not uniformly distributed. There are marked concentrations of structures, particularly in expanses of smooth plains in the northern hemisphere, and there appears to be latitudinal trends in the orientation of structures. The most widely accepted explanation for the formation of these compressional structures is global contraction caused by cooling of the planetary interior. However, alternative theories exist (see Box ‘Formation theories’). Regardless of the formation mechanisms, scientists studying Mercury mostly agree that tectonism is likely still ongoing today.

BOX | Formation theories

Four main theories explain the abundance and distribution of compressional structures on Mercury: global contraction, tidal despinning, true polar wander, and mantle convection.

Global contraction

Cooling of the planet’s interior could cause global contraction. Early geochemical processes, such as a phase change from liquid to solid within part of Mercury’s large metallic core, would have resulted in a decrease in the planet’s radius (Strom et al., 1975), and so too would cooling of the lithosphere and contraction of the crust (Solomon, 1976).

Tidal despinning

If Mercury rotated faster earlier in its history (Dombard & Hauck, 2008), the planet would have a more pronounced equatorial bulge as a result of greater centrifugal force. To accommodate this more oblate spheroid shape (similar to a satsuma in terms of aspect ratio), different tectonic structures with preferred orientations would form. Over time, tidal-despinning, where Mercury’s rotation slowed down, would cause the equatorial bulge to relax, with the planet losing oblateness, but the global population of tectonic structures with preferred orientations would remain (Melosh, 1977).

True polar wander

In a similar vein to tidal-despinning, true polar wander can be explained as the reorientation of Mercury, where changes in the planet’s rotation lead to the geographic location of the north and south poles changing, similar to a ball spinning and wobbling on the end of a finger. True polar wander leads to a redistribution of a planet’s mass, which can affect tectonic processes on a global scale. Since a planet’s spin evolves temporally, a variety of tectonic structures will form to accommodate this (Keane & Matsuyama, 2017).

Mantle convection

Current or past mantle convection on Mercury is predicted to be weak given the planet has a thin mantle thickness of around 400 km. Mercury also has a thin lithosphere, so it is possible that stresses caused by mantle convection could have influenced the formation of tectonic structures (King, 2008), however to what degree is not clear. When dense mantle layers sink into less-dense layers, it can produce instabilities that cause complete mantle overturn (Mouser & Dygert, 2023). Upwellings generated by overturn are hypothesised to produce significant stresses at the base of the lithosphere, inducing the propagation of tectonic structures at the surface.

With mapping comes discovery

Analysis of MESSENGER images and data is still ongoing, with the only evidence of recent tectonism previously being the discovery of 39 small (<10 km) pristine scarps in the northern hemisphere of Mercury (Watters et al., 2016) and 14 lobate scarps observed cross-cutting Kuiperian aged (⪅280 million years old) craters (Banks et al., 2015). Until now, conclusive evidence for geologically recent tectonism (within the past few hundreds of millions of years) across Mercury’s globe has been on a very small scale, implying localised contraction. Whilst mapping a previously uncharted region of Mercury, the Neruda quadrangle (Man et al., 2023b), I was captivated by a large lobate scarp system that extended beyond the borders of my mapping area. Whilst tracing this system, I made the serendipitous discovery of not one, but two sets of grabens (depressed blocks of material bordered by roughly parallel normal faults) situated on two different prominent lobate scarps, one of which is shown in figure 4.

Figure 4: Grabens discovered on the scarp Alvin Rupes. Grabens (and neighbouring horsts) are observed on top of the larger compressional structure (left panel). Schematic illustration of graben formation (right). (Original MESSENGER image from NASA/JHUAPL/CIW; Left panel modified from Man et al. (2023a) Nat. Geosci. 16, 856–862; doi.org/10.1038/s41561-023-01281-5 published open access under a CC BY 4.0 license creativecommons.org/licenses/by/4.0).

Grabens are formed by tensional stresses, but these extensional features can form locally from continued strain of a parent structure under compression (Fig. 4). The grabens I discovered were relatively small scale and shallow – tens of kilometres long, tens of metres deep, and generally less than one kilometre wide – and in relatively pristine condition. Given their unspoilt appearance, my colleagues and I soon realised that these structures were likely to be geologically young. Such small structures would not survive impact gardening, where micrometeorite bombardment gradually overturns the top layer of a planet’s surface, or topographic diffusion, where new crater formation erases the signature of other topographic features. Consequently, we set out to undertake a global survey for grabens located atop compressional structures.

I made the serendipitous discovery of not one, but two sets of grabens situated on two different prominent lobate scarps

Recent tectonism

We started by re-mapping all of Mercury’s compressional tectonic structures. We then looked at over 27,000 images with the highest spatial resolution (≤150 m/pixel) obtained by MESSENGER and discovered hundreds of grabens associated with mapped compressional structures (Fig. 5).

Figure 5: Global population of grabens associated with shortening structures. Yellow triangles represent grabens we are confident with, whilst black circles represent probable grabens – structures that fulfil the morphological and positional requirements to be a graben, but are just beyond the resolution to be definitively confirmed. Red lines represent Mercury’s global population of shortening, or compressional, structures (excluding basin-specific structures).
(Original MESSENGER image from NASA/JHUAPL/CIW; Figure originally published in Man et al. (2023a) Nat. Geosci. 16, 856–862; doi.org/10.1038/s41561-023-01281-5 published open access under a CC BY 4.0 license creativecommons.org/licenses/by/4.0).

The presence of so many newly discovered grabens on top of compressional structures provides crucial evidence that tectonism on Mercury is widespread and geologically recent. To augment our interpretation, we measured the lengths and depths of the newly discovered grabens to understand their ages. Continuous impact gardening on the surface of Mercury distributes regolith (rocky, soil-like material) over craters and into depressions like grabens.

We measured the lengths of the shadows cast by the grabens to determine their current depth using trigonometry. Then, using fault displacement-length scaling relationships that are well documented on Earth (Cowie & Scholz, 1992), we estimated the original depth of the graben. By working out the difference between the current and original depth of the graben, hence the amount of regolith that had infilled the graben, we were able to work out how long it would take for a graben to be infilled, using a realistic infilling rate for Mercury. Consequently, we were able to calculate the age of the grabens.

Our results show that grabens are mostly shallow and young, many of them likely having formed in the past few hundreds of millions of years

Our results show that grabens are mostly shallow (tens of metres deep) and young, many of them likely having formed in the past few hundreds of millions of years. With this evidence of widespread young tectonism in the form of grabens atop parent compressional structures, the most plausible cause for continued strain is ongoing global contraction, indicating that Mercury’s interior is still cooling and the planet contracting.

Out of the formation theories discussed, our results support global contraction being the main driver of tectonism at the present day. Alternative mechanisms (tidal despinning, true polar wander, mantle convection and overturn) likely influenced the initial formation, distribution, and orientation of structures early in Mercury’s history. Global contraction is postulated to have reactivated many of the early structures and that is why we see a global population of thrusts faults rather than many different types of faulting. With BepiColombo beginning its science campaign in early 2026, and with improved spatial resolution of images and digital elevation data, I am confident that many more grabens will be resolved and discovered, supporting our theory that recent tectonism on Mercury is not just widespread but global.

Author

DR BENJAMIN MAN

Doctoral Researcher in the Planetary Environments Research Group, School of Physical Sciences, Open University, UK.
ben.man@open.ac.uk
linkedin.com/in/benjamin-man
x.com/AstroBenjamin

Further reading

• Cowie, P.A. & Scholz, C.H. (1992) Displacement-length scaling relationship for faults: data synthesis and discussion. J. Struct. Geol. 14, 1149–1156. doi.org/10.1016/0191-8141(92)90066-6
• Davies, M.E. et al. (1978) Atlas of Mercury. NASA Science and Technical Information Office, 128 pp.
• Dombard, A.J. & Hauck, S.A. (2008) Despinning plus global contraction and the orientation of lobate scarps on Mercury: Predictions for MESSENGER. Icarus 198, 274–276. doi.org/10.1016/j.icarus.2008.06.008
• Grott, M., Breuer, D., & Laneuville, M. (2011) Thermo-chemical evolution and global contraction of mercury. Earth Planet. Sci. Lett. 307, 135–146. doi.org/10.1016/j.epsl.2011.04.040
• Keane, J.T. & Matsuyama, I. (2017) Reorientation Histories of Mercury, Venus, the Moon, and Mars, in: EPSC Abstracts. pp. EPSC2017-415.
• King, S.D. (2008) Pattern of lobate scarps on Mercury’s surface reproduced by a model of mantle convection. Nat. Geosci. 1, 229–232. doi.org/10.1038/ngeo152
• Man, B. et al (2023a) Geological mapping of the Neruda Quadrangle (H13), Mercury: Status Update, in: Lunar and Planetary Science Conference. pp. 1–2.
• Man, B. et al. (2023) Widespread small grabens consistent with recent tectonism on Mercury. Nat. Geosci. 16, 856-862 doi.org/10.1038/s41561-023-01281-5
• Massironi, M. & Byrne, P.K. (2015) High-Relief Ridge, in: Encyclopedia of Planetary Landforms. Springer New York, New York, NY, pp. 932–934. doi.org/10.1007/978-1-4614-3134-3_185
• Melosh, H.J. (1977) Global Tectonics of a Despun Planet. Icarus 31, 221–243. doi.org/10.1016/0019-1035(77)90035-5
• Murray, B.C. et al. (1974) Mariner 10 Pictures of Mercury: First Results. Science 184, 459–461. doi.org/10.1126/science.184.4135.459
• Plescia, J.B. & Golombek, M.P. (1986) Origin of planetary wrinkle ridges based on the study of terrestrial analogs. Geol. Soc. Am. Bull. 97, 1289–1299. doi.org/10.1130/0016-7606(1986)97%3C1289:OOPWRB%3E2.0.CO;2
• Solomon, S.C. (1976) Some aspects of core formation in Mercury. Icarus 28, 509–521. doi.org/10.1016/0019-1035(76)90124-X
• Strom, R.G., Trask, N.J., & Guest, J.E. (1975) Tectonism and volcanism on Mercury. J. Geophys. Res. 80, 2478–2507. doi.org/10.1029/jb080i017p02478
• Tosi, N., Grott, M., Plesa, A.C., & Breuer, D. (2013) Thermochemical evolution of Mercury’s interior. J. Geophys. Res. E Planets 118, 2474–2487. doi.org/10.1002/jgre.20168
• Tosi, N. & Padovan, S (2021) Mercury, Moon, Mars: Surface expressions of mantle convection and interior evolution of stagnant-lid bodies, in: Marquardt, H., Ballmer, M., Cottar, S., Jasper, K. (Eds.), Mantle Convection and Surface Expressions. pp. 455–489. doi.org/10.1002/9781119528609.ch17

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