Mars is currently a dry planet, but has water in the form of polar ice, trace atmospheric gases, and an unknown amount of sub-surface ground water bound in minerals and ice. However the red planet wasn’t always so arid.
Since the landing of NASA’s Perseverance rover three years ago, a picture of its watery past has been emerging via an international research collaboration that includes NASA and ESA scientists. Multiple teams are finding that based on hydrated magnesium sulfate (similar to Epsom salts) and dehydrated calcium sulphate identified in rock samples, the planet may have once had streams, rivers, and lakes, and hydrothermal systems.
A recent paper led by Sandra Siljeström at the RISE Research Institutes of Sweden in Stockholm, published in the Journal of Geophysical Research, Planets, describes the results of rover analyses in a 3.8 billion-year-old region called the Jezero crater, where there is evidence of a delta and ancient lake.
“Now, Mars is a dry planet,” says Siljeström. But with signs of stream beds and other watery traces, the question is, she says, “where is the water now?”
Seeking answers, the scientists gleaned geological clues from hydrated compounds of one of the most common elements on Mars: sulphur.
“Mars is a very sulphur-rich planet,” explains Siljeström. So in every sample analysed remotely by the rover, “we find sulphur and sulphates,” she says.
“Sulphates can become hydrated so they can have different amounts of water attached to them in many different states,” says Siljeström. Water trapped within sulphate minerals record the history of how and when they formed. The study, involving Siljeström, University of Cincinnati scientist Andrew Czaja and a team of over 50 others, was an effort to measure the waters trapped in sulphates and other forms.
As Siljeström explains, the rover has several instruments that measure surface chemistry. One uses X rays to infer sample geochemistry. Another uses a laser. The team combined data from these two instruments: the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemistry (SHERLOC) and Planetary Instrument for X-ray Lithochemistry (PIXL).
PIXL provides elemental data that allows identification of the sulfate as magnesium sulphate.
The Raman, she explains, indicates how many molecules of water are attached to the magnesium sulphate minerals. “You illuminate the sample with the laser and then you get some light back.” The instrument allows interpretation of how many water molecules there are, based on the wavelength of the light that returns.
The team reports, based on the rover’s chemical analysis, that the sulfate minerals of the Jezero crater floor were deposited from salty water. The team describes this ancient water source as a sulphate-rich fluid of moderate pH that probably formed these minerals at low temperature, likely over multiple episodes.
The hope is that these Jezero mineral samples held in hermetically sealed titanium tubes will eventually be returned, via future missions, to allow laboratory analysis on Earth. That will allow methods like stable isotope analysis, revealing more about Mars’s geological evolution and hydrology.
Postdoctoral researcher Nicolas Randazzo at the University of Alberta, a geochemist co-author on the study, notes that “sulfur is a wonderful mineral from multiple different perspectives,” revealing things about water and climate during the time of deposition.
The mutual consistency of SHERLOC and PIXL data and interpretations supports confidence in the identification of hydrated magnesium (and calcium) sulfates in these rocks, says Michigan State University geoscientist and Mars minerology expert Michael Velbel who was not involved in the study. He adds that “Retrieving these well-characterized samples from Mars will enable even more detailed and sensitive studies of these minerals in labs on Earth, revealing much more about the ancient environment for possible life in Jezero crater.”
Indeed, notes Randazzo, “Once we bring those [samples] back, we can start doing stable isotope work, which can tell us more information about things like thermometry [temperature regimes],” he says, and perform other tests not possible with current rover technology.
“Mars is not habitable anymore,” Randazzo says. But around the same time that life was getting launched on Earth, 3.8 – 4.1 billion years ago, Mars was a warm wet planet. That’s when the Jezero delta was being deposited, he explains. “Life formed on Earth around that time period, and perhaps life on Mars could have formed around that same time.”
“Sulfate is also really good for preserving organisms,” and organic molecules, says Randazzo, so it remains to be seen if Martian life will be revealed. “There is still a lot to learn.”
As for what it’s like to work with data from another planet? “It’s kind of like a dream come true,” says Siljeström. “It’s honestly surreal,” agrees Randazzo. The quest to understand the history of water on a dry planet is thirsty work, but for finding clues, Mars geochemistry rocks.