NASA's Curiosity rover has discovered that an ancient lake on Mars had a chemical stratification similar to that of lakes on Earth—a potential prerequisite for the presence of life.
Previous exploration by the rover revealed the presence of a lake more than 3 billion years ago in Mars's Gale Crater. A new study further defines the chemical conditions that existed in the lake, showing that the lake was stratified—with the shallower water richer in oxidants than the deeper water.
That sort of chemical stratification is common in lakes on Earth, where the surface water absorbs oxygen from the air. Oxidants (such as oxygen) remove electrons from other atoms—an energy transfer known as oxidation-reduction that is fundamental to the basic functions of life.
"This is a defining moment for the mission. Given our payload, this is everything we hoped to accomplish in terms of understanding an ancient habitable environment," says John Grotzinger, the Fletcher Jones Professor of Geology and Ted and Ginger Jenkins Leadership Chair of the Division of Geological and Planetary Sciences at Caltech. Grotzinger was the project scientist for the Curiosity mission from 2007 to 2015.
A report of the findings will be published on June 2 in the journal Science.
"It's getting difficult to ignore that this seems very Earth-like," Grotzinger says.
If signs of life are never discovered on Mars, these additional similarities between Earth and Mars will help scientists discover what key ingredient or ingredients caused life to flourish on Earth while Mars remained empty and dead.
Whether life ever existed on Mars is still unknown. NASA landed Curiosity inside Gale Crater in 2012 with the primary goal of determining whether Mars has ever offered environmental conditions favorable for microbial life. Studies leading up to and including this one show that this lake on Mars had all of the right ingredients and the right chemical reactions were taking place.
"There was energy to be had if there were organisms to take advantage of it," says Woody Fischer, professor of geobiology at Caltech and a co-author of the Science paper.
In addition to revealing new information about chemical conditions within the lake, the report also documents subtle fluctuations in the climate of ancient Mars.
Curiosity compared differences in the chemical composition of layers of mud-rich sedimentary rock that were deposited in the lake. These chemical observations indicate that during the time when the lake was present in Gale Crater 3.5 billion years ago, climate conditions changed from colder and drier to warmer and wetter. Such short-term fluctuations in climate took place within a much longer, million- to billion-year climate evolution—from the ancient warmer and wetter conditions that supported lakes to today's cold, arid Mars.
"These results give us unprecedented detail in answering questions about ancient environmental conditions on Mars," said Curiosity project scientist Ashwin Vasavada of NASA's Jet Propulsion Laboratory.
Now, 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. The mission is run by JPL, which is managed by Caltech for NASA.