SPECIALSECTION photometrically corrected to normalized re- REPORT flectance at a standard geometry (30° incidence angle, 0° emission angle) and map-projected. From the 11-color data, principal component anal- Geology of the Caloris Basin, Mercury: ysis and spectral ratios were used to highlight the most important trends in the data (17). The first A View from MESSENGER principal component dominantly represents bright- ness variations, whereas the second principal com- Scott L. Murchie,1 Thomas R. Watters,2 Mark S. Robinson,3 James W. Head,4 Robert G. Strom,5 ponent (PC2) isolates the dominant color variation: Clark R. Chapman,6 Sean C. Solomon,7 William E. McClintock,8 Louise M. Prockter,1 the slope of the spectral continuum. Several higher Deborah L. Domingue,1 David T. Blewett1 components isolate fresh craters; a simple color ratio combines these and highlights all fresh The Caloris basin, the youngest known large impact basin on Mercury, is revealed in MESSENGER craters. PC2 and a 480-nm/1000-nm color ratio images to be modified by volcanism and deformation in a manner distinct from that of lunar thus represent the major observed spectral var- impact basins. The morphology and spatial distribution of basin materials themselves closely match iations (Fig. 1B). lunar counterparts. Evidence for a volcanic origin of the basin's interior plains includes embayed The images show that basin exterior materials, craters on the basin floor and diffuse deposits surrounding rimless depressions interpreted to be including the Caloris Montes, Nervo, and von of pyroclastic origin. Unlike lunar maria, the volcanic plains in Caloris are higher in albedo than Eyck Formations, all share similar color proper- surrounding basin materials and lack spectral evidence for ferrous iron-bearing silicates. Tectonic ties, which continue with little variation azimuth- landforms, contractional wrinkle ridges and extensional troughs, have distributions and age ally around the basin from the part imaged by relations different from their counterparts in and around lunar basins, indicating a different Mariner 10 (Fig. 1). They have a normalized re- stress history. flectance of ~0.085 at 560 nm, a red lunarlike spectral continuum, and an absence of light- he Caloris basin, the youngest large im- terior (Fig. 1A). An outlying darker annulus colored smooth plains that occur as patches in the on July 9, 2008 pact basin known on Mercury, was seen consists of rolling ejecta deposits (the Odin surrounding highlands (17, 18). Smooth plains Tin its entirely during the first encounter Formation), which grade into radially lineated within the Odin Formation lack distinctive color of Mercury by the MESSENGER spacecraft in plains and overlapping secondary craters in clus- properties. In contrast, Caloris interior plains are January2008(1). Caloris provides important in- ters, thought to be distal sculpted ejecta (the van about 10% higher in normalized reflectance and formation for understanding Mercury’s geology Eyck Formation). These formations are compa- have a redder spectral continuum. The north- because it exposes layering of the planet's crust, rable to counterparts in lunar basin materials, such western half of the interior plains is both slightly and it contains tectonic and volcanic features that as the Montes Rook and Hevelius Formations redder and slightly higher in albedo than the are well-preserved as compared with those of that surround the Orientale basin (7). Two types southeastern half. The interior plains exhibit cra- older basins more modified by subsequent im- of smooth plains were recognized in association ters with diameters of several tens of kilometers www.sciencemag.org pact cratering. Imaging coverage of the basin with Caloris. Exterior to the basin, annular smooth whose interiors and ejecta have a lower albedo interior and ejecta was therefore a focus of the plains east of Caloris exhibit pervasive wrinkle and less red color than the basin exterior and re- encounter sequence. Here we make use of color ridges. The interior of the basin contains plains semble darker terrain exterior to the basin (red and high-resolution monochrome images to re- proposed on the basis of Mariner 10 images to circles, Fig. 1A). The northwestern part of the construct the geological evolution of the basin have originated either as volcanic flows (2, 8) basin interior exhibits craters with similar dark and to assess the origin and distribution of smooth or impact melt (9). In the former case, the in- rims but light floors resembling the interior plains plains material interior to the basin, the composi- terior plains would be equivalent to lavas form- (blue circles, Fig. 1A). The margin of the basin tional stratification of Mercury’s upper crust, ing Mare Orientale, and in the latter to the contains diffuse, 30- to 100-km-diameter patches Downloaded from and the large-scale deformational history of the Maunder Formation, which is thought to be im- of very red material, typically ~40% higher in region. pact melt. The interior plains exhibit wrinkle normalized reflectance than the basin exterior Over 30 years ago, Mariner 10 imaged the ridges and younger, cross-cutting extensional (red patches in Fig. 1B, circled in white). Fresh eastern part of Caloris (2–5), and the basin has troughs (10–12). Wrinkle ridges, thought to have impact craters with comparably elevated albedo subsequently been studied with ground-based formed by a combination of thrust faulting and are spectrally distinctive from the red patches, radar (6). Major units forming the basin include a folding (13–15), occur near the eastern basin with a less red spectral continuum than other ma- rim of concentric massifs with intermontane plains margin and are both concentric and radial to the terials (bluer color in Fig. 1B) as is also typical of (the Caloris Montes and Nervo Formations), basin, a pattern common in mare basalt-filled fresh lunar craters (19). which form an annulus varying in width (up to lunar basins. The troughs are graben formed by Higher-resolution views of the western annu- 250 km) surrounding the light-colored basin in- extensional stresses and have linear and sinuous lus (Fig. 1, C to E) show smooth to rolling plains segments that form giant polygons (10, 11, 15). surrounding the edge of the basin, which grade 1Johns Hopkins University Applied Physics Laboratory, Laurel, Before MESSENGER, it was not known if or into radially lineated equivalents of the van Eyck MD 20723, USA. 2Center for Earth and Planetary Studies, Na- how far the wrinkle ridges and graben extended Formation, particularly in the northwest (white tional Air and Space Museum, Smithsonian Institution, Wash- westward into the then-unimaged portion of the arrow, Fig. 1C). The northwestern part of the 3 ington, DC 20015, USA. Department of Geological Sciences, basin, or how consistent their spatial and age re- basin interior also exhibits interior plains embay- Arizona State University, Tempe, AZ 85251, USA. 4Department of Geological Sciences, Brown University, Providence, RI 02906, lations were across the basin. ing knobs of the basin rim as well as a partly USA. 5Lunar and Planetary Laboratory, University of Arizona, Imaging by the Mercury Dual Imaging preserved inner ring, whose eastern portion was Tucson,AZ85721,USA.6Southwest Research Institute, 1050 System (MDIS) (16) during MESSENGER’sfirst suggested in Mariner 10 images but not identified 7 WalnutStreet,Boulder,CO80302,USA. Department of Mercury flyby was optimized for coverage of definitively (2) (red arrows, Fig. 1C). The western Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20650, USA. 8Laboratory for Atmospheric Caloris, with a narrow-angle camera mosaic at equivalent of the Caloris Montes Formation (red and Space Physics, University of Colorado, Boulder, CO 200 to 300 m/pixel, and a wide-angle camera 11- arrows, Fig. 1D) is an inward-facing scarp broken 80303, USA. color mosaic at 2.4 km/pixel. Both data sets were into clusters of knobs, locally higher in normalized www.sciencemag.org SCIENCE VOL 321 4 JULY 2008 73 MESSENGER Fig. 1. Overview of the Caloris basin. All images are mosaics in equirect- angular projection, and north is up. (A) En- hanced (but still subtle) color image map show- ing 1000-, 750-, and 480-nm images in the red, green, and blue image planes. Yellow boxes are insets shown at higher resolution in (C), (D), and (E); red circles indicate super- posed dark craters; blue circles indicate embayed dark craters; and the black arrow indicates the central crater Apollo- dorus of the radial gra- ben complex. (B)Same view after data trans- formation. PC2, which captures variations be- on July 9, 2008 tween light plains and darker terrain, is shown in the green image plane and inverted in the red plane. The ratio of nor- malized reflectances at 480 nm/1000 nm, which highlights fresh impact ejecta, is shown in the blue plane. Small red spots are extremely red and elevated in albedo; those in arrows). (D) High-resolution view of inset d in (A), showing massifs at the inner www.sciencemag.org white circles are centered on small rimless depressions. Thicker circles indicate edge of the western continuation of the Caloris Montes Formation (red arrows). (E) featuresshowninFig.3.(C) High-resolution view of inset c in (A), showing radially High-resolution view of inset e in (A), showing examples of wrinkle ridges (red lineated ejecta (white arrow) and massifs embayed by the interior plains (red arrows) and troughs (white arrows) in the southwestern interior plains. reflectance. Fitting the entirety of Caloris Montes and interior color and albedo properties con- The second line of evidence for volcanism is yields a main basin ring diameter of 1550 km, as sistent with the crater wall but distinct from sur- the presence of diffuse bright deposits concen- compared with 1340 km estimated from Mariner rounding plains, suggesting that an underlying trated along the margin of the basin (Fig.