RESEARCH ARTICLES shaped subset truncates the subset to its left. In addition, the cross-strata exhibit a small angle of climb from left to right. Truncation surfaces Two Years at Meridiani Planum: and backfilling of subjacent sets are well ex- pressed in the left-center part of the upper unit. Individual troughs are 3 to 4 cm wide. This Results from the Opportunity Rover particular type of cross-stratification can be further described as scalloped cross-lamination, S. W. Squyres,1 A. H. Knoll,2 R. E. Arvidson,3 B. C. Clark,4 J. P. Grotzinger,5 B. L. Jolliff,3 6 6 1 7 8 9 which can form either by migration of a single S. M. McLennan, N. Tosca, J. F. Bell III, W. M. Calvin, W. H. Farrand, T. D. Glotch, bedform during fluctuating flow or by migra- M. P. Golombek,9 K. E. Herkenhoff,10 J. R. Johnson,10 G. Klingelho¨fer,11 12 9 tion of superimposed bedforms in steady flow H. Y. McSween, A. S. Yen (11). We cannot distinguish between these two possibilities because of the lack of a comple- The Mars Exploration Rover Opportunity has spent more than 2 years exploring Meridiani Planum, mentary cut oriented parallel to the bedding traveling È8 kilometers and detecting features that reveal ancient environmental conditions. These plane. include well-developed festoon (trough) cross-lamination formed in flowing liquid water, strata A mosaic of Microscopic Imager (MI) (12) with smaller and more abundant hematite-rich concretions than those seen previously, possible images was acquired to provide improved reso- relict ‘‘hopper crystals’’ that might reflect the formation of halite, thick weathering rinds on rock lution of the Overgaard trough cross-lamination surfaces, resistant fracture fills, and networks of polygonal fractures likely caused by dehydration of (Fig. 1B). The subgrain-scale resolution of this sulfate salts. Chemical variations with depth show that the siliciclastic fraction of outcrop rock has image provides additional supporting detail, undergone substantial chemical alteration from a precursor basaltic composition. Observations including clear truncation of laminae that re- from microscopic to orbital scales indicate that ancient Meridiani once had abundant acidic inforces the Pancam observations. Stratal trun- groundwater, arid and oxidizing surface conditions, and occasional liquid flow on the surface. cations of this sort are the result of primary discontinuities in sedimentation and cannot be he Mars Exploration Rover Opportuni- Erebus. Despite the generally homogeneous ap- artifacts produced by the intersection of bedding ty landed at Meridiani Planum on 24 pearance of Meridiani Planum from orbit, a with topography. Moreover, fine-scale topography on October 12, 2015 TJanuary 2004. During its 90-sol (mar- number of new features and phenomena have derived from stereoscopic MI imaging reveals a tian solar day) nominal mission (1), Opportu- been observed at these locations at rover scale. nearly flat surface for the portion of the outcrop nity explored Eagle crater and the surrounding Here, we present geologic results obtained along that contains the trough cross-lamination. Because plains, studying laminated sulfate-rich sand- the traverse, interpretations of textural and there is no systematic relationship of bedding to stones that contain abundant hematite-rich geochemical observations at Endurance crater, local slopes, this observation confirms that the spherules (2). Opportunity then spent nearly a and a refined model for the formation of the cross-lamination is a primary attribute of the rock. year exploring Endurance crater, investigating sedimentary rocks at Meridiani Planum. Cross-lamination of this type and at this È7 m of exposed stratigraphic section in a unit Depositional processes. Rocks of the Burns scale is known to form subaqueously but is not dubbed the Burns formation (3). Observations formation are finely stratified sandstones. By known to develop in other types of flows, in- www.sciencemag.org there revealed chemical and textural changes itself, this observation could be explained in cluding eolian and volcanic base surges (13–15). with depth (4, 5) and a stratigraphic section terms of eolian or subaqueous sedimentation, We therefore discount suggestions that the dominated by eolian dune and sand sheet accumulation by explosive volcanism, or em- Meridiani sandstones might have been formed facies but with evidence for subaqueous dep- placement as impact ejecta. On the basis of as a part of volcanic (16)orimpact(17)base osition in the uppermost half meter (6). Since sedimentary structures and facies associations surges. Furthermore, the overall stratigraphy and leaving Endurance, Opportunity has traveled at Eagle (2) and Endurance (6)craters,we the succession of facies observed at Endurance nearly 5 km to the south, assessing horizontal identified eolian and subaqueous sedimentation crater are inconsistent with base surge deposits and vertical variations in geology over more as the most probable deposition mechanisms. (6). The latter are characterized by flow decel- Downloaded from regional scales. Major landmarks along the route It is well known that fine- to medium- eration sequences similar to subaqueous turbi- (fig. S1) include Vostok crater, about 1.5 km grained sand, when subjected to shear stresses dites (15), and these have not been observed in south of Endurance, and Erebus crater, about created by shallow, subaqueous flows with mod- the bedrock at Meridiani. Recent observations 4 km south of Endurance. Pausing during the erate current velocities, spontaneously forms thus confirm the earlier interpretation of sub- traverse, Opportunity also spent considerable highly sinuous-crested ripples with amplitudes aqueous transport and suggest that water flow time at the Olympia outcrop, just north of of up to a few cm (7, 8). In cross sections ori- was more extensive across the ancient Meridiani ented transverse to flow, exposed laminae have surface than was evident from observations at 1Department of Astronomy, Space Sciences Building, Cornell a trough-shaped or ‘‘festoon’’ geometry (9). At Eagle and Endurance craters alone. University, Ithaca, NY 14853, USA. 2Botanical Museum, the Olympia outcrop, Panoramic Camera Microscopic images provide additional in- 3 Harvard University, Cambridge, MA 02138, USA. Department (Pancam) (10) images of the rock Overgaard formation for testing depositional models. All of Earth and Planetary Sciences, Washington University, St. Louis, MO 63031, USA. 4Lockheed Martin Corporation, show a planar-laminated unit overlain by the best outcrops imaged to date by Opportunity consist Littleton, CO 80127, USA. 5Division of Geological and example of cm-scale trough cross-lamination of medium-size sand grains that are rounded Planetary Sciences, California Institute of Technology (Caltech), found by Opportunity to date (Fig. 1A). These and well sorted. For example, Cobble Hill, a 6 Pasadena, CA 91125, USA. Department of Geosciences, State trough cross-laminae scour down into the target within the interpreted eolian sand sheet University of New York, Stony Brook, NY 11794, USA. 7Geological Science, University of Nevada Reno, Reno, NV underlying planar laminated unit from left to facies in Endurance crater, has a mean grain 89557, USA. 8Space Science Institute, Boulder, CO 80301, right. The trough cross-lamination forms a bed size of 450 mm, with a standard deviation of USA. 9Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, set with at least 3 to 4 cm of apparent thickness. 170 mm(N 0 200) (Fig. 2A). Walker (18)and 10 USA. U.S. Geological Survey, Flagstaff, AZ 86001, USA. Several superimposed sets show basal scouring Sparks (19) compiled grain-size distribution 11Institut fu¨r Anorganische und Analytische Chemie, Johannes Gutenberg Universita¨t, 55099 Mainz, Germany. 12Department and backfilling by concave- to occasionally data for more than 300 pyroclastic flow de- of Earth and Planetary Sciences, University of Tennessee, convex-upward cross-laminae, which show a posits on Earth. The median grain size in Knoxville, TN 37996, USA. cross-cutting relation in which each trough- Meridiani outcrops falls within the range of www.sciencemag.org SCIENCE VOL 313 8 SEPTEMBER 2006 1403 RESEARCH ARTICLES medians recorded for terrestrial samples, but grains falls well within the range of mean, This characteristic persists south of Endur- the sorting is distinctly different (20). In con- median, and sorting values for terrestrial eolian ance crater. Textures in Overgaard, and also in trast, the size distribution of Meridiani sand sands (21). Strawberry, another outcrop near the rim of Erebus crater, again indicate only well-sorted, medium-grained sandstones. Grain-size distri- butions in sandstones near Erebus approximate those in Eagle and Endurance outcrops (Fig. 2C). Thus, outcrop-level sedimentary structures and fine-scale textures of Meridiani rocks both support the interpretation that they are eolian deposits, reworked locally by surface water. Geochemistry and mineralogy. After using Opportunity’s Rock Abrasion Tool (RAT) (22) to expose fresh rock surfaces, we collected Al- pha Particle X-Ray Spectrometer (APXS) (23) data at Endurance crater that revealed system- atic stratigraphic variations in chemical compo- sition (4). From bottom to top of the measured stratigraphic section, Burns formation sand- stones show decreasing amounts of Si, Al, Na, and K and increasing amounts of Mg and S. This trend is consistent with variations in the ra- tio of siliciclastic components to