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limited by the resolution of the timing electronics. REPORT The radial accuracy of ~100 m is limited by uncer- tainties in the trajectory associated with errors in the ephemerides of MESSENGER and Mercury. Laser Altimeter Observations from The profile spans ~ 20% of the circumference of the planet and shows a 5.2-km dynamic range MESSENGER’s First Mercury Flyby of topography and 930-m root-mean-square (RMS) roughness (Fig. 1). The radius of Mercury Maria T. Zuber,1* David E. Smith,2 Sean C. Solomon,3 Roger J. Phillips,4 Stanton J. Peale,5 apparently decreases by 1.4 km along the equator James W. Head III,6 Steven A. Hauck II,7 Ralph L. McNutt Jr.,8 Jürgen Oberst,9 from ~10° to 90° E, corresponding to a 0.02° GregoryA.Neumann,2 Frank G. Lemoine,2 Xiaoli Sun,2 Olivier Barnouin-Jha,8 John K. Harmon10 downward slope to the east. This long-wavelength surface tilt begins 30° west of the previously esti- A 3200-kilometers-long profile of Mercury by the Mercury Laser Altimeter on the MESSENGER mated COF/COM offset (6) and was not sampled spacecraft spans ~20% of the near-equatorial region of the planet. Topography along the profile is in Earth-based radar altimetry (4). Such a long- characterized by a 5.2-kilometer dynamic range and 930-meter root-mean-square roughness. At wavelength slope, if a fundamental feature of the long wavelengths, topography slopes eastward by 0.02°, implying a variation of equatorial shape equatorial shape of the planet, might arise from that is at least partially compensated. Sampled craters on Mercury are shallower than their crustal thickness or crustal density variations, global- counterparts on the Moon, at least in part the result of Mercury’s higher gravity. Crater floors vary scale mantle density variations, or topography along in roughness and slope, implying complex modification over a range of length scales. the planet’s core-mantle boundary, which for Mercury is ~600 km beneath the surface. opography is a fundamental measurement that was in darkness during the flyby, and within The slope can be interpreted in the context of to characterize quantitatively the surfaces the hemisphere not imaged by Mariner 10. Con- an ellipsoidal planetary shape (10). If we suppose Tof solid planetary bodies at length scales sequently, there are no optical images of the re- that the difference in principal moments of in- ranging from the long-wavelength shape to such gion in which altimetry was collected, so we used ertia, B – A, is entirely a result of an ellipsoidal local and regional processes as impact cratering, an Arecibo radar image (8) to correlate the profile distribution of surface mass with density rs and volcanism, and faulting. During the first flyby with surface features. The MLA began ranging with semi-axes a>b>c,then ’ of Mercury by the MESSENGER spacecraft on ~1 min before the spacecraft s closest approach ÀÁ 4 4prsabc 8prsR 14 January 2008 (1), the Mercury Laser Altimeter and continued for ~10 min. Usable returns were B − A ¼ a2 − b2 ≈ ðÞða − b 1Þ (MLA) (2, 3) successfully ranged to the planet’s received up to an altitude of 1500 km, which was 15 15 surface, providing the first altimetric observations larger than the expected maximum of 1200 km from which we may write of the planet from a spacecraft. (2). As the spacecraft velocity and range from B − A C C 〈r〉 a − b ¼ 5 R m ð Þ Previous measurements of the shape and to- Mercury changed during the flyby, the size of 2 2 2 Cm C MR rs pography of Mercury had been derived from laser spots on the surface varied from 23 to 134 m Earth-based radar ranging (4, 5) constrained by and the shot spacing varied from 888 to 725 m (9). where A 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139–4307, USA. 2Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA. 3Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA. 4Southwest Research Institute, Boulder, CO 80302, USA. 5Department of Physics, University of California, Santa Barbara, CA 93106, USA. 6Department of Geological Sciences, Brown University, Providence,RI02912,USA.7Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, USA. 8Johns Hopkins University Applied Physics 9 Fig. 1. (Top) MLA profile (vertical exaggeration 105:1). (Bottom) Arecibo radar image [adapted Laboratory, Laurel, MD 20723, USA. Institute of Planetary 8 Research, German Aerospace Center (DLR), Berlin, D-12489 from ( )] with MLA profile location (white line) superposed. Arrows at top indicate locations of Germany. 10National Astronomy and Ionosphere Center, craters in Table 1 interpreted from detailed analysis of MLA profile points. The locations of several Arecibo Observatory, Arecibo 00612, Puerto Rico. of the major craters are indicated by arrows on the radar image. The two-ringed circular structure *To whom correspondence should be addressed. E-mail: in the Arecibo image at ~55 to 60°E is represented in part by a deep depression in the altimetry, [email protected] but north-south radar ambiguities may be contributing to the structure in the image. www.sciencemag.org SCIENCE VOL 321 4 JULY 2008 77 MESSENGER Table 1. Crater apparent slopes, RMS roughness, and pulse widths. Apparent Emission Apparent RMS Spot Longitude floor Range angle† floor slope‡ roughness Pulse width (ns) roughness§ (oE) diameter* (km) (deg) (deg) (m) (m) (km) 21.1–22.4 55.3 415 61 0.298 89.8 5.0 ± 2.1 0.3 ± 0.7 33.7–33.9 8.5 293 49 –0.451 11.8 12.4 ± 1.8 1.8 ± 3.4 35–35.3 12.8 289 48 1.26 24.3 14.6 ± 10.8 2.3 ± 2.4 38.5–38.9 17 285 44 –0.571 46.3 22.5 ± 18.4 3.8 ± 3.9 42.9–43.2 12.8 293 40 0.252 20.3 12.0 ± 10.6 1.5 ± 2.1 44.5–44.6 4.3 298 38 –0.573 7.9 19.6 ± 20.7 2.9 ± 4.0 46–47.3|| 55.4 313 36 0.177 66.5 19.3 ± 14.5 2.8 ± 2.7 48.8–50.6|| 76.7 336 34 –0.0784 31.9 20.9 ± 15.3 3.0 ± 2.7 52.3–52.4 4.3 365 31 0.117 5.7 18.0 ± 16.4 2.4 ± 2.9 56.8–59.1 98 429 26 0.0852 96.5 20.6 ± 16.7 2.7 ± 2.8 81–82.3 55.4 1020 1.5 –0.0008 110 6.3 ± 2.1 0.2 ± 0.3 *Length of MLA-crossing chord. †Emission angle is the angle between the range vector and surface normal. ‡Positive slopes are defined to be downward to the west. §Roughness from pulse width corrected for spacecraft range and emission angle. ||Craters shown in Fig. 2. Fig. 2. Close-up of two cra- 2 are complex and do not operate uniformly over ters showing contrast in floor different length scales. Potential sources of mod- roughness and tilt. The ver- ification include anelastic relaxation, volcanic re- tical exaggeration is 30:1. 0 surfacing, tectonic subsidence, wall slumping, and ejecta emplacement from younger nearby im- pacts, and the variability implies that a combina- Elevation, km -2 50 km tion of these processes operated on the profiled 44 46 48 50 52 54 craters. Longitude,oE R, and 78 4 JULY 2008 VOL 321 SCIENCE www.sciencemag.org SPECIALSECTION 14. R. J. Pike, in Mercury, F. Vilas, C. R. Chapman, returned pulse is passed through three matched filters 20. The MESSENGER project is supported by the NASA M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, AZ, after the detector amplifier, and subsequently time-to- Discovery Program under contracts NAS5-97271 1988), pp. 165–274. digital converters are employed to measure the leading- to Johns Hopkins University Applied Physics Laboratory 15. J.-L. Margot, D. B. Campbell, R. F. Jurgens, M. A. Slade, and trailing-edge times of the pulse. MLA measures pulse and NASW-00002 to the Carnegie J. Geophys. Res. 104, 11875 (1999). widths between 6 and 1000 ns, corresponding to an RMS Institution of Washington. We acknowledge important 16. R. J. Pike, Proc. Lunar Planet. Sci. Conf. 11, 2159 (1980). variation in range to the surface within each laser spot of contributions from the MLA instrument and 17. Surface roughness and small-scale tilts, and off-nadir 0.4 to 64 m (2, 19). MESSENGER spacecraft teams. ranging, which occurred at angles of up to 70° during 18. These apparent slopes are likely minimum values. the flyby, combine to spread MLA’s reflected pulse. To 19. J. B. Abshire, X. Sun, R. S. Afzal, Appl. Opt. 39, 2449 14 April 2008; accepted 3 June 2008 maximize the probability of detecting spread returns, the (2000). 10.1126/science.1159086 REPORT as gravitational resonances swept through the belt during giant-planet migration ~4 billion years ago (Ga) (7, 8). Population 2 reflects im- Mercury Cratering Record Viewed pacts of near-Earth asteroids (NEAs), mainly derived by the size-dependent Yarkovsky ef- ’ fect that causes smaller main-belt asteroids to from MESSENGER s First Flyby preferentially enter resonances and be placed into planet-crossing orbits (9, 10). Heavily cra- Robert G. Strom,1* Clark R. Chapman,2 William J. Merline,2 Sean C. Solomon,3 James W. Head III4 tered regions of Mercury have a SFD similar to that of the highlands of the Moon and Mars Morphologies and size-frequency distributions of impact craters on Mercury imaged during (Fig. 1A). However, on Mercury and Mars, there MESSENGER’s first flyby elucidate the planet’s geological history. Plains interior to the Caloris is a dearth of craters with D < 40 km relative to basin displaying color and albedo contrasts have comparable crater densities and therefore similar the Moon (11). On Mercury, smaller craters ages. Smooth plains exterior to Caloris exhibit a crater density ~40% less than on interior plains were apparently removed by the formation of and are thus volcanic and not Caloris impact ejecta. The size distribution of smooth-plains craters “intercrater plains” (12) during the LHB (differ- matches that of lunar craters postdating the Late Heavy Bombardment, implying that the plains ent processes erased smaller Martian craters). formed no earlier than 3.8 billion years ago (Ga). At diameters less than or equal to 8 to 10 The shapes of the SFDs for Mercury, Mars, and kilometers, secondary impact craters on Mercury are more abundant than primaries; this transition the Moon for D = 40 to 150 km (a range not diameter is much larger than that on the Moon or Mars. A low density of craters on the peak-ring affected by intercrater plains and with good basin Raditladi implies that it may be younger than 1 Ga. statistics) match each other better if shifted some- what in diameter, apparently because heliocentri- ercury has been struck by asteroids and cratering and of volcanic versus impact-ejecta cally orbiting NEAs collide with Mercury at higher comets since it formed, resulting not modes of plains formation. During its first fly- velocities and Mars at lower velocities as com- Monly in primary impact craters of all by of Mercury, the MESSENGER spacecraft pared with the Moon (fig. S4). sizes but also in secondary craters made by re- imaged portions of a crater-scarred landscape MESSENGER data show that the north- impact of ejecta from the primary craters. Such never before seen at close range. These images western half of the Caloris floor has a slightly secondaries typically have morphologies differ- show the entire 1550-km-diameter Caloris im- lower albedo and different color than the south- ent from the pristine shapes of primary craters, pact basin (3) and a broad surrounding annulus eastern half (Fig. 1C) (3). Could these two and many secondary craters form clusters and of smooth plains. They also reveal diverse cra- plains regions have been formed by volcanic chains. Geological processes such as faulting, vol- tered terrains, some nearly saturated with large episodes at widely different times? SFDs (for canism, downslope motion, and continued crater- craters but others very sparsely cratered. Here D > 10 km, craters unlikely to be secondaries) ing all degrade crater shapes, eventually erasing we report preliminary analyses of crater mor- for these regions (Fig. 1B) (as well as for an them by erosion or covering. The statistics of cra- phology and SFD measurements from several east/west division) show no significant differ- ter sizes, shapes, and spatial relations—especially selected regions, based chiefly on images from ences, so their ages are comparable within 10 their size-frequency distributions (SFDs) (1)— the narrow-angle camera (NAC) of the Mercury to 20%. Until we can measure crater densities provide information (including relative ages) about Dual Imaging System (4). on the Caloris rims and ejecta, we cannot de- the processes that formed and reshaped the cra- Observations of the Moon and Mars have termine if the flooding of the floor was con- tered landscapes. shown that craters in the inner solar system have temporaneous with the impact (e.g., by impact Images of Mercury by Mariner 10 from 1974 two SFD components (5, 6). Terrains with a high melt) or occurred later; the uncertainties none- to 1975 and subsequent studies of other planetary density of large craters have a complex differen- theless permit a lengthy period for emplace- surfaces have raised issues that the MESSENGER tial SFD that approximately follows a power law ment of successive volcanic flows, particularly mission to Mercury can address (2), including the with a slope of –2 for crater diameter D =~2to if they occurred after the end of the LHB when relative importance of secondary versus primary 50 km. This “Population 1” was formed primar- the cratering rate was low. ily during the Late Heavy Bombardment (LHB), Since the Apollo-era discovery that the 1Lunar and Planetary Laboratory, University of Arizona, characterized by large impact basins such as the Cayley plains on the Moon were basin impact Tucson, AZ 85721, USA. 2Southwest Research Institute, Caloris basin on Mercury, only part of which was ejecta rather than of volcanic origin, a major issue 1050 Walnut Street, Suite 300, Boulder, CO 80302, USA. seen by Mariner 10. Younger surfaces have an in planetary geology has concerned the relative 3 Department of Terrestrial Magnetism, Carnegie Institution of SFD with a slope of –3(“Population 2”). (Figure importance of volcanism in plains formation. The Washington, 5241 Broad Branch Road, NW, Washington, DC 20015, USA. 4Department of Geological Sciences, Brown Uni- S1 summarizes these two SFDs.) Both impac- crater density on some of the darker exterior versity, Providence, RI 02912, USA. tor populations were probably derived origi- smooth plains that form an annulus around *To whom correspondence should be addressed. E-mail: nally from the asteroid belt (6). Population 1 may Caloris is about 40% lower than that on plains [email protected] have resulted from size-independent ejection inside Caloris (Fig. 2A), which is consistent with www.sciencemag.org SCIENCE VOL 321 4 JULY 2008 79