Los Alamos, N.M., June 1, 2017 - A long-lasting lake on ancient Mars provided stable environmental conditions that differed significantly from one part of the lake to another, according to a comprehensive look at findings from the first three-and-a-half years of NASA's Curiosity rover mission. While previous work had revealed the presence of a lake more than three billion years ago in Mars' Gale Crater, this study defines the lake's chemical conditions and uses Curiosity's powerful payload to determine that the lake was stratified.
Stratified bodies of water exhibit sharp chemical or physical differences between deep water and shallow water. In Gale's lake, the shallow water was richer in oxidants than deeper water was.
"We're learning that in parts of the lake and at certain times, the water carried more oxygen," said Roger Wiens, a planetary scientist at Los Alamos National Laboratory and co-author of the study, published today in the journal Science. "This matters because it affects what minerals are deposited in the sediments, and also because oxygen is important for life. But we have to remember that at the time of Gale Lake, life on our planet had not yet adapted to using oxygen--photosynthesis had not yet been invented. Instead, the oxidation state of certain elements like manganese or iron may have been more important for life, if it ever existed on Mars. These oxidation states would be controlled by the dissolved oxygen content of the water."
"These were very different, co-existing environments in the same lake," said Joel Hurowitz of Stony Brook University, lead author of the report. "This type of oxidant stratification is a common feature of lakes 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."
Whether Mars has ever hosted any life is still unknown, but seeking signs of life on any planet, whether Earth, Mars or more-distant icy worlds, begins with reconstruction of the environment to determine if it was capable of supporting life. NASA is using Curiosity to explore habitable environments on the ancient surface of Mars.
Over more than 1,700 sols (martian days, which are 24 hours, 39 minutes long), Curiosity has traveled more than 16 km from the bottom of Gale crater part way up Mount Sharp near the center of the crater. Los Alamos National Laboratory developed the laser-shooting Chemistry and Camera (ChemCam) instrument that sits atop Curiosity in conjunction with the French space agency. Los Alamos' work on discovery-driven instruments like ChemCam stems from the Laboratory's experience building and operating more than 500 spacecraft instruments for national security. Scientists are using all the data collected by ChemCam and other on-board instruments to put together a more complete picture of the geological history of Mars.
###
About Los Alamos National Laboratory
Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, a team composed of Bechtel National, the University of California, BWXT Government Group and URS, an AECOM company, for the Department of Energy's National Nuclear Security Administration.
Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction and solving problems related to energy, environment, infrastructure, health and global security concerns.
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."