Standing On The Shore Of An Ancient Martian Lake Thinking About Microbes

Skeleton of an ancient lake — This evenly layered mudstone rock photographed by NASA’s Curiosity Mars rover records sedimentation in a lake, where plumes of river sediment settled out of the water column and onto the lake floor. The view spans about 5 feet (1.5 meters) across in the foreground. Credit: NASA/JPL-Caltech/MSSS

Three billion years ago you could have stood along the shore of a lake in Mars’ Gale Crater and watched a sunset reflected in its waters. The lake lasted a long time and provided a stable environment that may have fostered the growth of two very different types of microbes, according to a recent NASA study.

Curiosity’s primary goal when it landed inside Gale Crater in 2012 was to determine whether Mars has ever offered environmental conditions favorable for microbial life. In its first year, on the crater floor at “Yellowknife Bay,” the rover found evidence of ancient freshwater river and lake environments with all the main chemical ingredients for life and a possible energy source for life in the form of nitrogen, sulfates and sulfides (compounds made of sulfur joined with oxygen).

This illustration show the possible extent of an ancient lake inside Gale Crater. Curiosity confirmed the existence of a lake there billions of years ago after examining mudstones in the crater’s Yellowknife Bay area. The lake’s extent was estimated by mapping ancient lake and stream deposits and assuming that water flowed from the crater rim into the basin (arrows). The water would have pooled in the trough created between the crater rim (left) and Mt. Sharp (center right). The area’s history likely included the coming and going of multiple lakes of different sizes as climate conditions evolved. Credit: NASA/JPL-Caltech/MSSS

Curiosity has since driven to the base of Mount Sharp, a layered mountain inside the crater, and inspected rock layers that grow progressively younger as the rover climbs lower Mount Sharp. A long-lasting lake on ancient Mars provided stable environmental conditions that differed significantly from one part of the lake to another, according to findings from the first 3½ years of NASA’s Curiosity rover mission.

The new study determined that the lake was stratified. Stratified bodies of water exhibit sharp chemical or physical differences between deep water and shallow water. Lakes on Earth also become stratified into warmer and colder layers that turn over every spring. In Gale’s lake, the shallow water was richer in oxidants than deeper water was. An oxidant is an element or compound that likes to grab on to other materials to borrow their electrons. Oxygen is probably the most familiar oxidant and responsible for aiding combustion.

This diagram presents some of the processes and clues related to a long-ago lake on Mars that became stratified, with the shallow water richer in oxidants than deeper water was. Chemical stratification likely occurred in the lake because the water closer to the surface was more exposed to the oxidizing effects of oxygen in the atmosphere and ultraviolet light. Each layer could have supported different kinds of bacteria just as chemically stratified lakes on Earth do. Credit: NASA/JPL-Caltech/Stony Brook University

“These were very different, co-existing environments in the same lake,” said Joel Hurowitz of Stony Brook University, lead author of a report of the findings in the June 2 edition of the journal Science. “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, including those that thrive in oxidant-rich conditions, those that thrive in oxidant-poor conditions, and those that inhabit the interface between those settings.”

Whether Mars has ever hosted any life is still unknown and will have to wait for future missions.

Differences in the physical, chemical and mineral characteristics of several sites on lower Mount Sharp at first presented a puzzle to the rover team until they realized that the data revealed they were dealing with a stratified lake with a chemical boundary between shallow water and deeper water.

Left: An artist’s conception of Mars some 3.5 billion years ago when lakes filled craters and rivers cross-cut the landscape. Some scientists think there may even have been a ocean at the time. Right: Mars as it is today — cold, dry and windswept. The cause? Early on, Mars lost its planetary-wide magnetic field that protected the atmosphere from being gradually stripped away by the solar wind. As the air thinned, temperatures and pressures dropped and a warmer climate evolved into a cold-desert one. Credit NASA, GSFC; NASA, JPL, MSSS

In addition to revealing new information about chemical conditions within the lake, the report by Hurowitz and 22 co-authors also documents changes in the climate of ancient Mars. Similar to how scientists study ice cores to detect changes in climate and in the composition of the atmosphere, the team compared differences in the chemical composition of layers of mud-rich sedimentary rock that were deposited in quiet waters in the lake. While the lake was present in Gale, climate conditions changed from colder and drier to warmer and wetter within the broader context of a much warmer and wetter Mars in the distant past.

In mid-2017, Curiosity is continuing to reach higher and younger layers of Mount Sharp to study how the ancient lake environment evolved to a drier environment more like modern Mars. I’d give anything to share some of Earth’s warming climate and humid summer weather with Mars right now, but all must run its course.