Earth and Planetary Science Letters 240 (2005) 1–10 www.elsevier.com/locate/epsl Sedimentary rocks at Meridiani Planum: Origin, diagenesis, and implications for life on Mars Steven W. Squyres a,*, Andrew H. Knoll b a Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA b Botanical Museum, Harvard University, Cambridge MA 02138, USA Accepted 22 September 2005 Available online 25 October 2005 Editor: A.N. Halliday Abstract The MER rover Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks, exposed in craters and along fissures in Meridiani Planum, are sandstones formed via the erosion and re-deposition of fine grained siliciclastics and evaporites derived from the chemical weathering of olivine basalts by acidic waters. A stratigraphic section more than seven meters thick measured in Endurance crater is dominated by eolian dune and sand sheet facies; the uppermost half meter, however, exhibits festoon cross lamination at a length scale that indicates subaqueous deposition, likely in a playa-like interdune setting. Silicates and sulfate minerals dominate outcrop geochemistry, but hematite and Fe3D3 (another ferric iron phase) make up as much as 11% of the rocks by weight. Jarosite in the outcrop matrix indicates precipitation at low pH. Cements, hematitic concretions, and crystal molds attest to a complex history of early diagenesis, mediated by ambient ground waters. The depositional and early diagenetic paleoenvironment at Meridiani was arid, acidic, and oxidizing, a characterization that places strong constraints on astrobiologial inference. D 2005 Published by Elsevier B.V. Keywords: Mars; Meridiani; sedimentology; astrobiology 1. Introduction tary rocks, exposed in craters and along fissures in the Meridiani plain. These beds have provided the first The scientific objectives of the Mars Exploration opportunity to investigate the sedimentology, stratig- Rover (MER) mission are to explore two sites on the raphy, and geochemistry of sedimentary rocks at the martian surface and to determine whether environ- outcrop scale on another planet. In this paper and mental conditions there ever were suitable for life. others in this issue, we describe and interpret the The rovers Spirit and Opportunity have spent more sedimentary rocks discovered by Opportunity, expand- than a year and a half on the surface of Mars, ing on initial results presented earlier [1,2]. Our cen- exploring their landing sites at Gusev crater and tral finding is that the beds exposed at Meridiani Meridiani Planum, respectively. One of the principal preserve a rich record of past aqueous processes on discoveries of the MER mission is ancient sedimen- Mars, including both subsurface and surface water. Conditions there may have been suitable for some * Corresponding author. Tel.: +1 607 255 3508; fax: +1 607 255 forms of life, at least transiently, although the Mer- 5907. idiani environment also would have presented some E-mail address: [email protected] (S.W. Squyres). substantial challenges to biology. 0012-821X/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.epsl.2005.09.038 2 S.W. Squyres, A.H. Knoll / Earth and Planetary Science Letters 240 (2005) 1–10 The MER rovers are solar-powered, six-wheeled exploring within Eagle crater, focusing primarily on the robotic vehicles capable of executing long traverses outcrop. Many of the fundamental discoveries from across the martian surface. Each rover carries a copy which our interpretation of Meridiani sedimentary of the Athena science payload [3]. The key elements of rocks has emerged were made within this crater. the payload are: After exiting Eagle crater, Opportunity drove ap- proximately 800 m eastward across flat, nearly feature- ! Pancam (Panoramic camera), a high-resolution, less plains to another much larger crater that we named multispectral, stereo imaging system [4]. Endurance (Fig. 2). Endurance crater is about 150 m in ! Mini-TES(Miniature Thermal Emission Spectrome- diameter and 20 m deep, and its walls expose a sub- ter), an infrared spectrometer that performs remote stantially greater thickness of layered rock than Eagle sensing over a wavelength range from 5–29 Am [5]. crater does. We used the rover initially to survey En- ! Microscopic Imager, a camera that provides close- durance crater from points along its rim, obtaining up imaging of an area 3Â3 cm in size with a Pancam images and Mini-TES spectra of the interior resolution of 30 Am/pixel [6]. from several locations. At the same time, MER project ! APXS (Alpha Particle X-Ray Spectrometer), an in engineers conducted extensive tests to determine situ instrument that determines the abundances of whether Opportunity could safely descend and ascend major and some minor elements [7]. the steep walls within the crater. After determining that ! Mo¨ssbauer Spectrometer, an in situ instrument that the rover could operate on rocky slopes as steep as identifies and determines the relative abundances of ~308, we made the decision to send Opportunity into Fe-bearing phases [8]. Endurance crater. ! RAT (Rock Abrasion Tool), a tool that can brush or Opportunity entered Endurance on Sol 134 of its abrade a rock surface, exposing subsurface materials mission. Over a period of several months, we drove for the other instruments to investigate [9]. the rover down a steep slope at a location on the crater wall dubbed bKaratepe West,Q grinding with the RAT to Opportunity landed on January 24, 2004, coming to expose layered sedimentary rock at eleven locations. a stop in a small impact feature, about 20 m in diameter, The result was the first stratigraphic section ever mea- that we named Eagle crater [1]. This landing location sured on another planet. was fortuitous, as the walls of the crater exposed out- After reaching the base of the accessible section at crops of layered rock, tens of cm thick, that were Karatepe West, we explored near the bottom of the readily accessible to the rover (Fig. 1). Opportunity crater for a time, and then began a long and arduous spent the first 57 martian days, or bsols,Q of its mission ascent to a feature high on the southern wall of the Fig. 1. Composite Pancam image of sedimentary rocks exposed in blocks along the wall of Eagle crater, Meridiani Planum, Mars. Note bedding, small scale festoon cross lamination, and the hematititc concretions dubbed bblueberries.Q About 35 cm of section is visible in this image. S.W. Squyres, A.H. Knoll / Earth and Planetary Science Letters 240 (2005) 1–10 3 Fig. 2. The traverse of the Opportunity rover, from landing through Sol 324. The upper panel shows the traverse in map view, on a base image obtained using the spacecraftTs Descent Image Motion Estimation System (DIMES) camera during the descent to the martian surface. Landmarks indicated include Eagle and Endurance craters, Fram crater, and the Anatolia fracture system [1]. The dark spot just above the letter bSQ in bSol 319 to 320Q is the shadow of the vehicleTs parachute. The lower panel shows the traverse in oblique view, on a base image obtained by the Pancam camera from a point (indicated with a green dot in the upper panel) on the southeast rim of Endurance crater. Major landmarks in and around Endurance crater are indicated. Rover localization and map generation by R. Li, Ohio State University. crater that we named bBurns Cliff,Q after the late Roger tion, as well as discussions of relevant physical and Burns of MIT, who predicted some of the key geo- chemical analogs on Earth. chemical and mineralogical features discovered by Op- portunity. Opportunity reached the base of Burns Cliff 2. Origin on Sol 276 and, while perched at an angle that eventu- ally reached 328, began to conduct remote and in situ 2.1. Sedimentology and stratigraphy sensing of cliff materials. The investigations within Endurance crater, particularly at Karatepe West and Early observations by Opportunity at Eagle crater Burns Cliff, provided rich additional details regarding enabled some noteworthy sedimentological findings past environmental conditions at Meridiani Planum. [2], but the observations were frustratingly difficult to Opportunity exited Endurance crater on Sol 315 and put into context because of the small amount of strati- proceeded southward in search of additional outcrops. graphic section exposed. This paucity of section moti- The rover’s traverse, from landing to the egress from vated the traverse to Endurance crater, which was Endurance crater, is shown in Fig. 2. amply rewarded with the observations made at Kara- In the sections below, we provide an overview of tepe West. In this issue, Grotzinger et al. [10] discuss Opportunity’s key results at Meridiani, including the sedimentological and stratigraphic observations at both depositional origin of the rocks there, their subsequent locations in detail, and infer from them a succession of diagenesis, and the implications for paleoenvironmen- changing environmental conditions in this part of Mer- tal conditions, astrobiology, and future exploration. idiani Planum. Other papers in this volume develop these themes at Grotzinger et al. [10] divide the Burns formation into length, providing details of observation and interpreta- distinct lower, middle, and upper units. The total thick- 4 S.W. Squyres, A.H. Knoll / Earth and Planetary Science Letters 240 (2005) 1–10 ness of the three units exposed at Endurance crater is at [10], is interpreted at both locations to result from ripple least 7 m. All three units are observed or inferred to be migration induced by surface flow of liquid water over sandstones, formed from sand grains that are rich in a sand. variety of sulfate salts (see below). Taken together, these observations point to an origin The lower unit of the Burns formation was not of the explored portion of the Burns formation as a sampled directly at Karatepe West, but was observed bwetting upwardQ succession from dry eolian dunes to by Pancam and Mini-TES at the eastern end of Burns wet interdune deposits.