Gale crater on Mars was once a lake fed by rivers and groundwater. Hurowitz et al. analyzed 3.5 years of data from the Curiosity rover’s exploration of Gale crater to determine the chemical conditions in the ancient lake. Close to the surface, there were plenty of oxidizing agents and rocks formed from large, dense grains, whereas the deeper layers had more reducing agents and were formed from finer material. This redox stratification led to very different environments in different layers, which provides evidence for Martian climate change. The results will aid our understanding of where and when Mars was once habitable.
Structured Abstract
INTRODUCTION The primary goal of NASA’s Curiosity rover mission is to explore and quantitatively assess a local region on Mars’ surface as a potential habitat for past or present life. A necessary component of that assessment involves an investigation of the surface chemical conditions and paleoclimate of ancient Mars. Gale crater was selected as the landing site for Curiosity; it hosts a ~5-km-tall mountain of layered sedimentary rock. The rocks of Mount Sharp preserve a long-duration record of martian environmental conditions. Geological reconstructions from Curiosity rover data have revealed an ancient, habitable lake environment that was sustained for tens of thousands to tens of millions of years by rivers draining into the crater.
RATIONALE We seek to constrain the chemical environment within the lake in Gale crater, as well as short- and long-term climate variations in and around Gale crater. We focus on fine-grained sedimentary rocks that carry information about sediment provenance, the environment of deposition, the conversion of sediment to rock during burial (i.e., lithification), and the chemical conditions of later modification (i.e., diagenesis). These were investigated during the first 1300 martian solar days (sols) of rover operations in Gale crater using bulk geochemical and mineralogical analysis techniques, combined with high-resolution color imagery at a variety of scales.
RESULTS Two mudstone units have been recognized, both deposited in lakes: the Sheepbed member of the Yellowknife Bay formation, an older set of strata defining the base of the stratigraphic section; and the Murray formation, of relatively younger age and positioned higher in the stratigraphic section. The chemical index of alteration (CIA) paleoclimate proxy increases by up to ~10 to 20 CIA units (expressed in %) from the Sheepbed member to the Murray formation. On the basis of mineralogy, geochemistry, textural properties, and stratigraphic relationships, the Murray formation can be subdivided into two sedimentary associations, or facies: the hematite-phyllosilicate (HP) facies and the magnetite-silica (MS) facies. The HP facies is characterized by abundant Fe3+ oxides accompanied by phyllosilicates, as well as indications of Mn oxidation and trace metal concentration. These properties are consistent with deposition in an oxidizing environment. The MS facies is recognized by a near-complete absence of pure Fe3+ minerals, and high concentrations of silica accompanied by magnetite, consistent with deposition in an anoxic environment. Both facies were affected by a saline overprint after burial and lithification.
In an analysis published on Friday in '''', scientists announced data collected from the  show that Mars's  was once a chemically stratified lake, meaning the aquatic environment differed markedly between deep and shallow water. According to the report, "all of the physical, chemical, and energetic conditions necessary to establish a habitable environment were present on Mars between ~3.8 billion and 3.1 billion years ago."
Composite self-portrait of the Curiosity Rover, taken in 2012
Analysis shows the chemical index of alteration fluctuated over time in a way that suggested the lake varied between hot, wet periods and colder, drier periods.  The shallow water would have been rich in oxidants brought in from the atmosphere and groundwater, and the deeper water would have been oxidant poor.  , , ,  and , indicating chemistry suitable for life, have also been found there in a variety of compounds.

Lead author Joel Hurowitz of  explained, "These were very different, co-existing environments in the same lake... This type of oxidant stratification is a common feature of  on Earth, and now we've found it on Mars. The diversity of environments in this Martian lake would have provided multiple opportunities for different types of microbes to survive."

The scientists also evaluated the fineness of the , meaning the sizes of individual particles of dirt, dust and sand.  Curiosity found larger grains near the edges of the lake where sediments from incoming rivers and streams would have fed it, and smaller ones in what would have been the deep lakebed, which is consistent with particles dropping out of the water as the current slows down. "We could tell something was going on," Hurowitz said in a statement. "What was causing iron minerals to be one flavor in one part of the lake and another flavor in another part of the lake? We had an 'Aha!' moment when we realized that the mineral information and the bedding-thickness information mapped perfectly onto each other in a way you would expect from a stratified lake with a chemical boundary between shallow water and deeper water."