Sedimentary rocks at Meridiani Planum: Origin, diagenesis, and implications for life on Mars

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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.

Introduction

The scientific objectives of the Mars Exploration Rover (MER) mission are to explore two sites on the martian surface and to determine whether environmental conditions there ever were suitable for life. The rovers Spirit and Opportunity have spent more than a year and a half on the surface of Mars, exploring their landing sites at Gusev crater and Meridiani Planum, respectively. One of the principal discoveries of the MER mission is ancient sedimentary rocks, exposed in craters and along fissures in the Meridiani plain. These beds have provided the first opportunity to investigate the sedimentology, stratigraphy, and geochemistry of sedimentary rocks at the outcrop scale on another planet. In this paper and others in this issue, we describe and interpret the sedimentary rocks discovered by Opportunity, expanding on initial results presented earlier [1], [2]. Our central finding is that the beds exposed at Meridiani preserve a rich record of past aqueous processes on Mars, including both subsurface and surface water. Conditions there may have been suitable for some forms of life, at least transiently, although the Meridiani environment also would have presented some substantial challenges to biology.

The MER rovers are solar-powered, six-wheeled robotic vehicles capable of executing long traverses across the martian surface. Each rover carries a copy of the Athena science payload [3]. The key elements of the payload are:

  • Pancam (Panoramic camera), a high-resolution, multispectral, stereo imaging system [4].

  • Mini-TES(Miniature Thermal Emission Spectrometer), an infrared spectrometer that performs remote sensing over a wavelength range from 5–29 μm [5].

  • Microscopic Imager, a camera that provides close-up imaging of an area 3×3 cm in size with a resolution of 30 μm/pixel [6].

  • APXS (Alpha Particle X-Ray Spectrometer), an in situ instrument that determines the abundances of major and some minor elements [7].

  • Mössbauer Spectrometer, an in situ instrument that identifies and determines the relative abundances of Fe-bearing phases [8].

  • RAT (Rock Abrasion Tool), a tool that can brush or abrade a rock surface, exposing subsurface materials for the other instruments to investigate [9].

Opportunity landed on January 24, 2004, coming to a stop in a small impact feature, about 20 m in diameter, that we named Eagle crater [1]. This landing location was fortuitous, as the walls of the crater exposed outcrops of layered rock, tens of cm thick, that were readily accessible to the rover (Fig. 1). Opportunity spent the first 57 martian days, or “sols,” of its mission exploring within Eagle crater, focusing primarily on the outcrop. Many of the fundamental discoveries from which our interpretation of Meridiani sedimentary rocks has emerged were made within this crater.

After exiting Eagle crater, Opportunity drove approximately 800 m eastward across flat, nearly featureless plains to another much larger crater that we named Endurance (Fig. 2). Endurance crater is about 150 m in diameter and 20 m deep, and its walls expose a substantially greater thickness of layered rock than Eagle crater does. We used the rover initially to survey Endurance crater from points along its rim, obtaining Pancam images and Mini-TES spectra of the interior from several locations. At the same time, MER project engineers conducted extensive tests to determine whether Opportunity could safely descend and ascend the steep walls within the crater. After determining that the rover could operate on rocky slopes as steep as ∼30°, we made the decision to send Opportunity into Endurance crater.

Opportunity entered Endurance on Sol 134 of its mission. Over a period of several months, we drove the rover down a steep slope at a location on the crater wall dubbed “Karatepe West,” grinding with the RAT to expose layered sedimentary rock at eleven locations. The result was the first stratigraphic section ever measured on another planet.

After reaching the base of the accessible section at Karatepe West, we explored near the bottom of the crater for a time, and then began a long and arduous ascent to a feature high on the southern wall of the crater that we named “Burns Cliff,” after the late Roger Burns of MIT, who predicted some of the key geochemical and mineralogical features discovered by Opportunity. Opportunity reached the base of Burns Cliff on Sol 276 and, while perched at an angle that eventually reached 32°, began to conduct remote and in situ sensing of cliff materials. The investigations within Endurance crater, particularly at Karatepe West and Burns Cliff, provided rich additional details regarding past environmental conditions at Meridiani Planum. Opportunity exited Endurance crater on Sol 315 and proceeded southward in search of additional outcrops. The rover's traverse, from landing to the egress from Endurance crater, is shown in Fig. 2.

In the sections below, we provide an overview of Opportunity's key results at Meridiani, including the depositional origin of the rocks there, their subsequent diagenesis, and the implications for paleoenvironmental conditions, astrobiology, and future exploration. Other papers in this volume develop these themes at length, providing details of observation and interpretation, as well as discussions of relevant physical and chemical analogs on Earth.

Section snippets

Sedimentology and stratigraphy

Early observations by Opportunity at Eagle crater enabled some noteworthy sedimentological findings [2], but the observations were frustratingly difficult to put into context because of the small amount of stratigraphic section exposed. This paucity of section motivated the traverse to Endurance crater, which was amply rewarded with the observations made at Karatepe West. In this issue, Grotzinger et al. [10] discuss sedimentological and stratigraphic observations at both locations in detail,

Diagenesis

As detailed by McLennan et al. in this issue [11] the rocks at Meridiani underwent variable and in some cases substantial diagenetic modification after their initial deposition. Most conspicuous among diagenetic phases are the small concretions [2], informally referred to as “blueberries,” that are found in all outcrops observed at Meridiani to date. Blueberries play a major role in the development of an integrated understanding of Meridiani geology, not least because a surface veneer of these

Ancient environmental conditions at Meridiani Planum

All of the outcrop rocks examined to date at Meridiani Planum are sandstones. The sand grains that form them are dominated by a mixture of fine-grained altered siliciclastic phases and sulfate salts. This is true even for the lower unit of the Burns formation, which was not studied in situ but which has the same sulfate-rich Mini-TES spectral properties as the rest of the sequence. So we interpret the entire Burns formation as owing its origin to the reworking of mixed sulfate-silicate

Astrobiological implications

The Athena science payload carried by Opportunity was designed to search for geologic evidence of paleoenvironments that might have permitted life [3], not to search for evidence of life itself. Of the several physical and chemical biosignatures found in ancient terrestrial rocks, only macroscopic bedding features formed by the interaction of microbial communities with physical sedimentary processes might have been detected in Pancam images. They have not been observed. Nonetheless, as explored

Implications for future exploration

The MER mission has carried out the most far-reaching exploration of the surface of another planet in history. At Meridiani, Opportunity has discovered compelling evidence for an ancient aqueous environment that may have been suitable for some primitive forms of life. But to put the accomplishments of the rovers in perspective, they have traveled (as of July, 2005) a total distance of about ten kilometers on a geologically diverse planet that has as much surface area as all the landmasses of

Acknowledgment

At the time of this writing (late July, 2005), Opportunity is in its 537rd sol on the martian surface, in excellent health and still exploring Meridiani Planum. Its performance and longevity are testimony to the efforts of a talented and dedicated army of engineers and scientists who made the MER mission possible. We owe them our deepest appreciation. We also thank in particular the other contributors to this special issue of EPSL, whose insights we have summarized here.

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