ARTICLES PUBLISHED ONLINE: 16 MARCH 2014 | DOI: 10.1038/NGEO2097 Mercury’s global contraction much greater than earlier estimates Paul K. Byrne1,2*†, Christian Klimczak1, A. M. Celâl Şengör3, Sean C. Solomon1,4, Thomas R. Watters5 and Steven A. Hauck, II6 Mercury, a planet with a lithosphere that forms a single tectonic plate, is replete with tectonic structures interpreted to be the result of planetary cooling and contraction. However, the amount of global contraction inferred from spacecraft images has been far lower than that predicted by models of the thermal evolution of the planet’s interior. Here we present a synthesis of the global contraction of Mercury from orbital observations acquired by the MESSENGER spacecraft. We show that Mercury’s global contraction has been accommodated by a substantially greater number and variety of structures than previously recognized, including long belts of ridges and scarps where the crust has been folded and faulted. The tectonic features on Mercury are consistent with models for large-scale deformation proposed for a globally contracting Earth—now obsolete—that pre-date plate tectonics theory. We find that Mercury has contracted radially by as much as 7 km, well in excess of the 0.8–3 km previously reported from photogeology and resolving the discrepancy with thermal models. Our findings provide a key constraint for studies of Mercury’s thermal history, bulk silicate abundances of heat-producing elements, mantle convection and the structure of its large metallic core. lobal contraction as a result of interior cooling was invoked MESSENGER began orbital operations at Mercury in 2011, thus as an explanation for mountain building and tectonic limiting the accuracy of earlier estimates of the amount of planetary Gdeformation on Earth in the nineteenth century1,2, but contraction accommodated by surface structures. Determining the the idea was abandoned even before the recognition of the extent to which Mercury contracted is key to understanding the horizontal mobility of tectonic plates3, with the realization that planet's thermal, tectonic and volcanic history. Earlier estimates of contraction cannot account for the amount, style and distribution Mercury's radial contraction since the last major episode of global of deformation on the Earth's surface4. Large-scale deformational resurfacing (by impact cratering and/or widespread volcanism10) systems on Earth are localized along plate margins, unlike the quasi- obtained from photogeological studies of tectonic landforms5,8,9,11,12 homogenous distribution of shortening structures predicted for a were in the range 0.8–3 km, substantially less than the ∼5–10 km contracting planet3. predicted by interior thermal history models13–15. However, other worlds in the Solar System do not exhibit plate The key to addressing these outstanding problems is to tectonics today, so the intriguing possibility exists that some of the characterize how contraction is manifest on Mercury through old concepts of contraction theory for global tectonics, long obsolete photogeological mapping of the entire planet, in as detailed a for Earth, may be valid for one-plate planets. Mercury, in particular, manner as current MESSENGER data allow. Here, we present the displays no evidence of plate boundaries that segment its globally results of the most comprehensive survey yet of Mercury's global continuous lithosphere. Yet observations made by the Mariner 10 contraction-induced tectonic features formed since the end of the and MErcury Surface, Space ENvironment, GEochemistry, and late heavy bombardment (LHB) of the inner Solar System. Wereport Ranging (MESSENGER) spacecraft have shown that the innermost the distribution, morphology and likely kinematic development of planet displays myriad landforms—lobate scarps, wrinkle ridges four classes of shortening structure on the planet. In quantifying and high-relief ridges—that have been interpreted as the tectonic the surface expression of global contraction on Mercury, we note result of horizontal shortening of the lithosphere as Mercury conceptual similarities between our observations of large-scale contracted in response to secular cooling of its interior5–9. crustal deformation on Mercury and explanations offered for a Still, important details of Mercury's contraction, such as the globally contracting Earth before the ascendance of plate tectonics timing, duration and spatial concentration of surface deformation, theory. From two complementary methods, we show that the have remained elusive. Until the MESSENGER flybys of Mercury planet experienced much greater contraction than has heretofore in 2008–2009, an entire hemisphere of Mercury had yet to been recognized. Our results resolve a decades-old paradox in our be imaged, so inferences made on the basis of Mariner 10 understanding of Mercury's geological history and provide the basis data could not reliably be generalized globally. Furthermore, for a general framework for investigating global tectonics on other widespread topographic data for the planet were not available until one-plate planets. 1Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA, 2Lunar and Planetary Institute, Universities Space Research Association, Houston, Texas 77058, USA, 3Department of Geology, Faculty of Mines and the Eurasia Institute of Earth Sciences, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey, 4Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA, 5Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, DC 20560, USA, 6Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Cleveland, Ohio 44106, USA. †Present address: Lunar and Planetary Institute, Universities Space Research Association, Houston, Texas 77058, USA. *e-mail: [email protected] NATURE GEOSCIENCE | VOL 7 | APRIL 2014 | www.nature.com/naturegeoscience 301 © 2014 Macmillan Publishers Limited. All rights reserved. ARTICLES NATURE GEOSCIENCE DOI: 10.1038/NGEO2097 a b E B’ ° 0 1 48° E 3 60° N 62° N B A A’ Carnegie Rupes 30 150 km 30 50 250 km 50 A A’ B B’ −0.5 1.0 −0.7 0.5 −0.9 0.0 −0.5 Elevation (km) Elevation (km) 0 20 0 50 Distance (km) Distance (km) Figure 1 | Primary styles of thrust faulting on Mercury. a, A typical wrinkle ridge in the northern volcanic plains20; a topographic profile (along the line A–A’ shown in the top panel) shows its characteristic ridge-like morphology. b, Carnegie Rupes, a lobate scarp in Mercury’s northern hemisphere; in cross-section the structure is a steep, convex escarpment. Note the dierence in vertical relief across the two structures. Profiles are from MDIS stereo digital terrain models49; elevation values here and in subsequent figures are relative to a sphere of 2,440 km radius32. Azimuthal equidistant projections centred at 61.4◦ N, 49.9◦ E(a) and 59.1◦ N, 304.5◦ E(b). Classes of shortening structures In contrast to previous mapping studies that separated wrinkle Early descriptions of contractional deformation on Earth included ridges, lobate scarps and high-relief ridges (for example, ref. 9), two dominant styles of deformation. Large, stable regions (cratons), here most mapped shortening structures are classified by the outlined by mobile belts, are subject to shortening by means of primary terrain type—smooth plains18 or cratered plains (a term deeply rooted thrust faults (termed germanotype faults16) that we use to refer to both the intercrater plains and heavily cratered extend far into their interior. In contrast, the belts themselves are terrain units described from Mariner 10 images19)—in which composed of smaller, more densely grouped alpinotype faults16, they occur. Under this approach, most mapped landforms are some of which may root into the deeper structures. A third, classified as smooth plains structures or cratered plains structures now conceptually abandoned, type of shortening structure on (Supplementary Fig. 1). The remaining structures are either spatially Earth consisted of geosynclines and geanticlines, complementary associated with impact craters and so are termed crater-related long-wavelength fold structures that were hypothesized to deform or border areas of high-standing terrain and are catalogued as the entire lithosphere and were invoked to account for the high-terrain bounding. Long-wavelength undulations are mapped immense thicknesses of shallow-water sedimentary rocks observed as either troughs or crests on the basis of their elevation relative to in mountain belts1,2. surrounding terrain and the directions of tilt of crater floors on their In 1974–1975, Mariner 10 data revealed two primary tectonic flanks (Fig. 2). These structure classes are described in greater detail expressions of horizontal shortening on Mercury5. Wrinkle ridges in the accompanying Supplementary Discussion. are typically broad, low-relief arches—essentially an anticlinal fold above a blind thrust fault—often superposed by a narrow ridge Regional-scale crustal deformation (Fig. 1a). Lobate scarps are characterized by a steeply sloping scarp Shortening-related faulting is not uniformly distributed on Mercury. face and a gently sloping back limb, probably represent a monocline The northern volcanic plains, representing just ∼6% of the planet's or asymmetric hanging-wall anticline atop a blind or surface- surface20, host a disproportionately large number of contractional breaking thrust fault