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Mercury is a planet of extremes. The innermost planet of the Solar System, Mercury is subject to

strong temperature fluctuations as it slowly rotates, its solar day twice as long as its year. At the equator, Mercury’s airless surface reaches daytime highs of 700 K (ref. 1). At the poles, the Sun remains half hidden below the horizon year-round, and polar impact craters cast permanent shadows that harbour deposits of water ice and other frozen volatiles2. Mercury is extremely dense owing to a large iron core, which is estimated to be 2,020 km in radius3, leaving only about 420 km for mantle and crust. And Mercury has also been shrinking. Writing in Nature Geoscience, Byrne et al.4 report that Mercury’s faulted and wrinkled surface, as imaged by the MESSENGER spacecraft, accommodates far more surface contraction than previously thought.

That Mercury’s surface area has decreased over geologic time is well known. Mariner 10, the only previous mission to Mercury, flew past the planet three times between 1974 and 1975, and imaged 45% of the surface. Among the ubiquitous impact craters and scattered smooth plains, subsequently determined to be volcanic in origin5, Mariner 10 imaged lobate scarps — sinuous surface features that seem to be caused by thrust faulting (Fig. 1) — scattered across the imaged surface at all stratigraphic levels1,6.

According to this structural interpretation, crustal rocks on one side of a scarp have been pushed up and over those on the other side, shortening the crust in the process. Similar to thrust faults on Earth, the deformation is thought to have occurred along fault planes that dip shallowly at about 30 degrees and extend tens of kilometres deep into Mercury’s crust6,7. Given that the vertical offset of the land surface across the lobate scarps can reach up to 3 km, the horizontal displacements are correspondingly larger. And, given that these scarps can extend laterally for hundreds of kilometres, the inference is that Mercury has lost quite a bit of surface area over its

history. Mariner 10 scientists estimated that the shortening across the observed lobate scarps was equivalent to a loss of 1 to 2 km in global radius6.

This estimate of global contraction clashes, however, with thermal evolution models for Mercury. All planets and satellites, including the Earth8, must ultimately cool and shrink over time, unless their internal engines are somehow renewed, for example by tidal heating. Mercury is again an extreme case because of its enormous iron core. A portion of the core must be liquid and convecting in order to explain Mercury’s dipole magnetic field3. The cooling of Mercury should not only lead to the simple thermal contraction of the entire planet owing to the temperature change, but to the gradual freezing of the molten iron in its core. Notably, the phase transition from liquid to solid iron will reduce Mercury’s overall volume much further. As such, estimates of Mercury’s shrinking based on numerical thermal

evolution models of the planet’s interior yield a substantial loss in volume, equivalent to 5 to 10 km of contraction radially over 4 billion years9. This is much larger than what is consistent with the lobate scarps observed by Mariner 10.

Byrne et al.4 took advantage of a global imaging campaign by the MESSENGER spacecraft that is currently in orbit around Mercury, and carefully mapped compressional structures across the planet’s surface, including the 55% that was missed by Mariner 10. They find that lobate scarps are nearly everywhere, and are more or less randomly arranged. These characteristics are consistent with a planet that is cooling and contracting evenly in all directions, as opposed to alternative hypotheses from Mariner 10 days — such as a despinning planet. In the latter case, the structures would show preferred orientations, because Mercury’s shape would have changed as it slowed down from an initially faster spin rate due to solar tides7.

PLANETARY SCIENCE

Shrinking wrinkling MercuryAs Mercury’s interior cools and its massive iron core freezes, its surface feels the squeeze. A comprehensive global census of compressional deformation features indicates that Mercury has shrunk by at least 5 km in radius over the past 4 billion years.

William B. McKinnon

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Figure 1 | Mercury’s surface contraction. a, MESSENGER image of Carnegie Rupes, a lobate scarp on Mercury. b, A stereo-derived digital elevation model of the scene. Cross-cutting and offsetting a large, 100-km-wide crater, the vertical offset exceeds 2 km, which implies a horizontal shortening to the southwest of about 3.5 km. Thousands of similar structures have been mapped by Byrne et al.4 across the surface of Mercury, implying a loss of at least 0.4% of the planet’s surface area over geologic time. The contraction has been attributed to the solidification of the planet’s large iron core.

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Byrne and colleagues4 also find that the dominant compressional landform on Mercury’s volcanic plains is not the lobate scarp, but the wrinkle ridge. Wrinkle ridges are well known on the Moon, Mars, Venus and Earth, and are formed by smaller hidden thrust faults and surface folding of layered terrain7. Although relatively modest in elevation and displacement compared to the lobate scarps, there are a lot more wrinkle ridges per unit area in the regions where they are found. Together, the lobate scarps and wrinkle ridges accommodate a substantial amount of surface area loss that is equivalent to a reduction in Mercury’s radius of 5 to 7 km. Taking into account observational biases of the MESSENGER images — for example, east–west-trending structures at lower latitudes are difficult to identify — the total shrinkage of Mercury over the past 4 billion years could be even greater.

The presence of vast volcanic plains in Mercury’s north is, however, seemingly inconsistent with such strong contraction. A reduction in the planet’s radius of the magnitude proposed by Byrne et al.4 should lead to strong compression in Mercury’s lithosphere, squeezing volcanic conduits shut and preventing the eruption of buoyant

magma. So, perhaps the lavas erupted sufficiently long ago that contraction and compression were not yet important. Or perhaps residual extensional stresses from an even earlier time, when Mercury’s spin was slowing, offset the compressive stresses at the poles7. Stresses from despinning could have had the required effect, but they would have been symmetric about the equator, and a southern equivalent to Mercury’s northern volcanic plains is not seen.

The survey of compressional structures by Byrne et al.4 suggests that Mercury is 7 km smaller today than it was after its crust solidified. The findings provide a global framework for investigations into Mercury’s surface and interior evolution. It is fascinating to recall that a nineteenth-century notion for the origin of the Earth’s mountains also invoked a cooling, shrinking globe. The foremost geologist of the time, Sir Charles Lyell, was, however, quite critical of what he termed the ‘the secular refrigeration of the entire planet’ as the cause10. Lyell instead argued that volcanic action, and the ‘reiteration of ordinary earthquakes’ driven ultimately by the escape of the Earth’s internal heat, would suffice. The shrinking Earth hypothesis is, of course, long obsolete, and in fact was abandoned

even before the advent of modern plate tectonics. We now know that the Earth’s lithosphere is broken into plates, and the lateral motions of these plates give rise to mountain chains. But Mercury’s lithosphere forms a single shell, and thus Mercury provides an example of what may really happen to a planet that is shrinking. ❐

William B. McKinnon is in the Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in Saint Louis, Saint Louis, Missouri 63130, USA. e-mail: mckinnon@wustl.edu

References1. Strom, R. J. Mercury: The Elusive Planet (Cambridge Univ.

Press, 1987).2. Neumann, G. A. et al. Science 339, 296–300 (2013).3. Hauck, S. A. et al. J. Geophys. Res. Planets 118, 1204–1220 (2013).4. Byrne, P. K. et al. Nature Geosci. http://dx.doi.org/10.1038/

ngeo2097 (2014).5. Denevi, B. W. et al. J. Geophys. Res. Planets 118, 891–907 (2013).6. Strom, R. G., Trask, J. J. & Guest, J. E. J. Geophys. Res.

80, 2478–2507 (1975).7. Watters, T. R. & Nimmo, F. in Planetary Tectonics (eds Watters, T. R.

& Schultz, R. A.) 15–80 (Cambridge Univ. Press, 2010).8. Solomon, S. C. Earth Planet. Sci. Lett. 83, 153–158 (1987).9. Hauck, S. A. et al. Earth Planet. Sci. Lett. 222, 713–728 (2004).10. Lyell, C. Principles of Geology, 11th Edition (D. Appleton and

Co., 1872).

Published online: 16 March 2014

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