REPORTS 28. K. J. Hon, J. Kauahikaua, R. Denlinger, K. Mackay, D. A. Williams, Eds. (Special Paper 477, Geological Supporting Online Material Geol. Soc. Am. Bull. 106, 351 (1994). Society of America, Boulder, CO, 2011), pp. 1–51. www.sciencemag.org/cgi/content/full/333/6051/1853/DC1 29. L. R. Nittler et al., Science 333, 1847 (2011). Acknowledgments: We thank the MESSENGER team for SOM Text 30. C. I. Fassett et al., Earth Planet. Sci. Lett. 285, 297 development and flight operations. The NASA Discovery Figs. S1 to S3 (2009). Program supports the MESSENGER mission through 31. H. Hiesinger, J. W. Head III, U. Wolf, R. Jaumann, contract NAS5-97271 to The Johns Hopkins University G. Neukum, in Recent Advances and Current Research Applied Physics Laboratory and NASW-00002 to the 1 August 2011; accepted 5 September 2011 Issues in Lunar Stratigraphy, W. A. Ambrose, Carnegie Institution of Washington. 10.1126/science.1211997 with spectral properties such as those of the hol- Hollows on Mercury: MESSENGER lows in Figs. 1D and 2A were also seen in lower- spatial-resolution multispectral images from the Evidence for Geologically Recent Mariner 10 and MESSENGER flybys (3–7). Materials with high reflectance and extremely blue color are global spectral outliers and have Volatile-Related Activity been termed “bright crater-floor deposits” (BCFDs) (5–7). MESSENGER flyby images indicated that 1 1 1 1 2 David T. Blewett, * Nancy L. Chabot, Brett W. Denevi, Carolyn M. Ernst, James W. Head, BCFDs occur in several morphologic types, in- 1 1 3 3 4 Noam R. Izenberg, Scott L. Murchie, Sean C. Solomon, Larry R. Nittler, Timothy J. McCoy, cluding varieties that have lobate outlines and 5,6 2 2 7 8 Zhiyong Xiao, David M. H. Baker, Caleb I. Fassett, Sarah E. Braden, Jürgen Oberst, those on central peaks and peak rings. Apart from 8 8 2 Frank Scholten, Frank Preusker, Debra M. Hurwitz Raditladi, Tyagaraja, and Sander (Figs. 1C and 2, A and B), prominent named examples include High-resolution images of Mercury’s surface from orbit reveal that many bright deposits within the deposits on the floors of the craters Balzac, impact craters exhibit fresh-appearing, irregular, shallow, rimless depressions. The depressions, or de Graft, Kertesz, and Zeami and on the central hollows, range from tens of meters to a few kilometers across, and many have high-reflectance peaks or peak rings of Eminescu and Vivaldi. interiors and halos. The host rocks, which are associated with crater central peaks, peak rings, floors, MESSENGER’s orbital high-resolution and and walls, are interpreted to have been excavated from depth by the crater-forming process. The color imaging reveals that the areas identified as most likely formation mechanisms for the hollows involve recent loss of volatiles through some BCFDs are composed of hollows and etched combination of sublimation, space weathering, outgassing, or pyroclastic volcanism. These features terrain; hence, hollows are widespread across the support the inference that Mercury’s interior contains higher abundances of volatile materials than planet (Fig. 3). Hollows have been found be- predicted by most scenarios for the formation of the solar system’s innermost planet. tween ~66° N and 54° S and across all longitudes covered so far by orbital imaging. Many hollows he MESSENGER spacecraft entered orbit orbit is closest to Mercury. Several of the targeted occur in areas where there are exposures of low- about Mercury on 18 March 2011, after areas (Figs. 1, A and B) show depressions, or hol- reflectance material (LRM) (5, 6), a major global Twhich the Mercury Dual Imaging System lows, that are irregular in shape with generally color-compositional unit thought to have been (MDIS) (1) acquired high-spatial-resolution images. rounded edges. Horizontal dimensions range from originally emplaced at depth (8). Many of the images reveal an unusual landform tens of meters to several kilometers. The hollows Volcanism, explosive outgassing, collapse on Mercury, characterized by irregularly shaped, are shallow and rimless, and many have high- into a subsurface void, and loss of volatile-rich shallow, rimless depressions, commonly in clusters reflectance interiors and diffuse bright halos. The material through sublimation are capable of creat- and in association with high-reflectance materials. interiors are mostly smooth and flat, but some have ing irregularly shaped depressions on planetary Here, we describe this class of landform and its small bumps, hills, or mesas, the tops of which surfaces. Volcanism was an important and wide- distribution and suggest that it indicates recent may be remnants of the original surface. Many spread process on Mercury (9–14). Extrusive volatile-related activity. form clusters, although some are isolated. The and explosive volcanism can produce rimless MESSENGER is engaged in global imaging hollows appear fresh and lack superposed impact depressions in the form of calderas, vents, and of Mercury’s surface at a pixel dimension of craters, implying that they are relatively young. collapse pits. A number of probable pyroclastic ~250 m. As part of this mapping, targeted ob- To date, we have found hollows within im- vents and deposits have been identified on Mercury servations of selected areas are made using the pact craters that span a range of sizes. The exam- (12, 13, 15–17). Also, an intrusive magmatic MDIS, with pixel dimensions of ~10 m for mono- ples in Fig. 1, A and B, are on impact crater process has been proposed as the source for a chrome imaging and 80 m for multispectral im- central peaks or the peak rings of impact basins. class of pit craters on Mercury (18), via collapse ages. Resolution is greatest—more than a factor Additional examples on basin peak rings are after withdrawal of magma from a near-surface of 10 better than with standard mapping—for shown in Fig. 1, C and D. Similar high-reflectance chamber. However, the size and morphology of areas northward of 20° N, where the spacecraft hollows occur on the floors, walls, and rims of the pyroclastic vents and pit craters identified to some medium-sized impact craters (Fig. 1, E date on Mercury differ from those of the hollows. 1The Johns Hopkins University Applied Physics Laboratory, and F). The hollows are found in comparatively Most of the irregular depressions associated with Laurel, MD 20723, USA. 2Department of Geological Sci- fresh (Kuiperian) rayed craters, more degraded pyroclastic deposits are large [several tens of ences, Brown University, Providence, RI 02912, USA. 3De- craters, and basins in a variety of states of erosion. kilometers (16, 17)] relative to the hollows and partment of Terrestrial Magnetism, Carnegie Institution of Craters such as Tyagaraja and Sander (Fig. 2, typically occur as isolated depressions rather than Washington, Washington, DC 20015, USA. 4Smithsonian In- stitution, Washington, DC 20013, USA. 5Lunar and Planetary A and B) exhibit extensive fields of coalescing in clusters. Moreover, the hollows occur on the Laboratory, University of Arizona, Tucson, AZ 85721, USA. bright-interior/bright-halo hollows on their floors, tops and sides of central peak mountains as well 6Faculty of Earth Sciences, China University of Geosciences, lending an etched appearance to the terrain. The as across impact crater walls and rims, which are 7 Wuhan, Hubei, 430074, P. R. China. School of Earth and etched terrain on Tyagaraja’s floor displays some unlikely locations for volcanic eruptions. The rec- Space Exploration, Arizona State University, Tempe, AZ 85251, USA. 8Institute of Planetary Research, German Aerospace of the highest reflectance on the planet (~2.5 ognized pyroclastic deposits are associated with Center, D-12489 Berlin, Germany. times the global average), has a relatively shallow strong positive (“red”) spectral slope across the *To whom correspondence should be addressed. E-mail: (“blue”) spectral slope (2), and lacks clear spec- visible and near-infrared (5, 6, 15–17)incontrast [email protected] tral features in MDIS multispectral data. Surfaces to the “blue” character of the hollows. Thus, if the 1856 30 SEPTEMBER 2011 VOL 333 SCIENCE www.sciencemag.org REPORTS hollows and etched terrain are products of vol- suggested as the origin of the bright, blue patches temperature environment of the surface or shal- canism, it must be a form that is physically and first observed in Mariner 10 images. The location low subsurface, leading to sublimation and subse- compositionally different from that which produced of hollows and etched terrain within impact cra- quent collapse. The example in Fig. 1F is instructive. the recognized pyroclastic vents and deposits. In ters is consistent with the idea that deep-seated The crater is at a high northern latitude. High- some cases, such as Praxiteles and Tyagaraja gases gain access to the surface along impact- reflectance material is present on the upper por- (Figs. 1D and 2A), the blue materials are found induced fractures (3). tion of the northern wall, and several bright-halo together with red materials. The association of hollows with impact cra- hollows are located in a slump in the middle of Episodic minor explosive release of gas with ters and basins indicates a link with material that the wall. Solar heating would be maximal on these no ejected juvenile material, as was proposed for has been brought near or to the surface from south-facing surfaces, suggesting that develop- an unusual lunar feature [Ina (19)], could disrupt depth. For example, the peak rings of the 170-km- ment of the hollows and associated bright material the country rock and maintain a fresh, high- diameter basin in Fig. 1A originated at depths of is correlated with peaks in diurnal temperature. The albedo surface against space weathering by solar ~14to20km(21), whereas the walls and ejecta of steep slopes on the crater wall and slump should wind and micrometeoroid bombardment.