Sediments discovered by NASA’s Curiosity rover in Gale Crater imply that early Mars was wet and cold, according to a recent study that compared Martian samples to similar soils on Earth. The finding could inform whether the Red Planet was once habitable.
Curiosity’s Chemistry and Mineralogy (CheMin) instrument, which uses X-rays to measure the composition of minerals in soil and rock samples on the basis of their crystal structure, has found high concentrations of amorphous materials in Gale Crater—15%–73% by weight, depending on the location.
“This stuff is stable on Mars for billions of years, but on Earth it’s just gone—there’s nothing older than about 50,000 years.”
“These materials suggest that the warmest conditions on Mars probably weren’t all that warm,” said Anthony Feldman, a postdoctoral researcher at the Desert Research Institute in Las Vegas and lead author of the study.
Minerals have a highly ordered crystalline structure, like rows of encyclopedias neatly arranged on a library shelf. Geologists have determined how those structures, and thus different minerals, form, so knowing what minerals are present in rocks from a landscape allows them to piece together that location’s history. Amorphous materials, on the other hand, are disordered, as though the encyclopedias were strewn at random across the shelves and floor, making it difficult to find out how they formed.
Finding such a high level of amorphous materials on Mars was unexpected, in part because such materials are rare on Earth. The ordered crystalline structure of minerals makes them more stable, but the Martian amorphous materials were found in layers deposited a few billion years ago. “This stuff is stable on Mars for billions of years, but on Earth it’s just gone—there’s nothing older than about 50,000 years,” said Kirsten Siebach, a planetary scientist at Rice University in Houston who was not involved in the study. “The abundance of amorphous materials is one of the big mysteries from the Curiosity mission.”
Thinking Outside the Black Box
Amorphous materials can form when magma cools very quickly, when impactors slam into the surface, or through chemical reactions between rocks and water. Curiosity’s Sample Analysis at Mars instrument found relatively high concentrations of water, carbon dioxide, and other volatile compounds in Gale Crater’s amorphous materials, which wouldn’t be present in material formed in the kind of high-temperature environments expected during an impact or in a volcanic setting. That left chemical reactions as the most likely process.
To better understand that process in Gale Crater, the researchers wanted to compare the Curiosity samples with amorphous materials on Earth, which wasn’t an easy task. “Amorphous materials haven’t been studied much in a terrestrial setting,” Feldman said. “They’ve always been a sort of black box—we know they’re there, but we don’t know much about them.”
The team identified sites in California, Nevada, and Newfoundland that have soils with chemistry similar to Gale Crater’s amorphous materials, which have high concentrations of iron and silica but relatively low abundances of aluminum. These terrestrial sites have different climates, allowing the scientists to compare the influence of both temperature and precipitation on the formation of amorphous materials.
The California site was in the Klamath Mountains in the northwestern part of the state, which has cold, wet winters and warm, dry summers. In Nevada, the team sampled a hot desert site near the one-time mining camp of Pickhandle Gulch, close to the California border. And in Newfoundland, they visited Tablelands in Gros Morne National Park, which has a subarctic climate.
The scientists tested the samples using X-ray diffraction spectroscopy, transmission electron microscopy, and other techniques. The analyses revealed that the amorphous materials in Newfoundland, most of which are about 15,000–20,000 years old, were the best match to those in Gale Crater. Tablelands has a mean annual temperature of 3.9°C and receives about 120 centimeters of precipitation each year, suggesting the Martian samples formed from interactions between rocks and water at near-freezing temperatures.
“We’re speculating that iron- and silicon-rich materials formed from surface and groundwater alteration of iron-bearing silicates at low temperatures,” said coauthor Elizabeth Rampe, a planetary scientist at NASA’s Johnson Space Center. The cold then preserved these materials for billions of years.
“Cold makes a lot of sense because low temperatures slow down chemical reactions,” Siebach said. “High temperatures would provide the energy for the materials to crystallize. But if it’s cold, it takes longer.”
A Question of Habitability
“This is all super important for our understanding of habitability and habitable environments on Mars,” Rampe said, including the suitability of a location for the development of life versus sustaining life as conditions change.
“If microbes on Mars evolved to live in these fluids, then, technically, [Gale Crater] was habitable, but maybe not the most comfortable place.”
Early Gale Crater sediments were deposited in water with low salinity and acidity, Rampe said, perhaps providing a reasonably comfortable environment for the formation of life. Over hundreds of millions of years, however, the water became more caustic. “That might not be the best place for life to develop, but there are microbes on Earth that live in those kinds of environments today,” Rampe said. “So if microbes on Mars evolved to live in these fluids, then, technically, [Gale Crater] was habitable, but maybe not the most comfortable place.”
The Perseverance rover, which is trundling across Jezero crater, isn’t equipped with the same X-ray instrument that Curiosity has, so it can’t quantify any possible amorphous materials. It continues to cache samples for possible return to Earth, however, where laboratory analysis would provide more detail on the Martian climate. “I’d love to get my hands on some of those samples,” Feldman said.
—Damond Benningfield, Science Writer
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