Swampy sand dunes and gushing rivers once covered the Martian surface, according to findings from two different robots on the Red Planet.
Chinese explorer Zhurong has discovered signs that frost may have bound the mounds together 400,000 years ago. And farther west, NASA’s Perseverance rover found signs that a swift and powerful waterway forced its way into Jezero Crater, spewing water at breakneck speed.
The crusty features of the dunes, first detected in 2021, are described in a recent study published in . science advance, Zhurong, which is slated to land on Mars in May 2021, is currently inactive after failing to wake up after an expected period of hibernation, due to dust build-up on its solar panels.
The river that Perseverance observed in another area appears to be the most powerful ever found on Mars, more than 20 meters deep in some places, based on the height of rock formations scientists believe preserved sandbars .
According to Jani Radebaugh, a researcher at Brigham Young University in Utah (USA) who was not involved in either of the two missions, both discoveries “highlight that there is a lot to do in putting things on the surface of other planets.” importance.” “We learn something new every time.”
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When China landed its Xuerong rover on Utopia Planasia, the Mars scientific community had some questions about the choice of landing site. Although observations from space had led to theories that the region may have been flooded or even an ocean, no water-rich minerals have been detected.
Zhurong was quick to prove himself. The rover identified indicators of water near the surface almost immediately, and researchers were quick to report the presence of hydrated minerals in the area. Tests of the rover’s ground-penetrating radar also pointed to flash floods in the region some 3 billion years ago.
Now, Zhurong has found more evidence of water on the Martian surface, and from more recent times. Sand dunes near the rover have developed a crust that probably formed when water interacted with minerals. That water could come from frost that formed on the dunes in the past, or it could have fallen as snow hundreds of thousands of years ago, when the planet’s tilt allowed snow to fall in the region. If frost or ice is mixed with salts to lower its melting point, it may melt due to temperature changes on Mars.
The crust has polygonal features whose cracks suggest they repeatedly contracted and expanded over time, “like mud cracks,” says Radbaugh, who studies the sand dunes. “Having these types of shrinking and expanding features suggests that there have been relatively recent or modern or ongoing wetting and drying in these dune regions.” The rover’s weather observations suggest that water vapor may still be causing frost near the landing area today.
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However, whether the water turned into a liquid or not is not known to anyone. According to Ralph Milliken, a planetary scientist at Brown University in the United States) and a member of NASA’s Mars Curiosity mission, the dust on Mars is rich in minerals that can absorb water vapor from the air. If that material covers sand dunes, changes in humidity throughout the season can cause the dust to absorb water vapor and re-release it without liquefying.
But Radbaugh suspects that liquid water may be needed to propel and expand cracks in landforms. “It doesn’t take much,” he says. “You need it to happen over and over again.”
According to Radbaugh, similar crust and polygonal features have been observed elsewhere on Mars, but never in dunes.
“These are potential things that are forming in many different places on Mars,” Milliken says. “This may be a process that may have occurred over a large part of the planet in the recent geological past.”
The crust also appears to be responsible for binding the dunes together. Martian dunes in other areas show signs of recent movement, but the dunes discovered by Zhurong are frozen in time.
Xiaoguang Qin, a planetary scientist at the Chinese Academy of Sciences in Beijing who led the new research, said the rover provided a new explanation for “why the dune has stopped moving”.
Zhurong’s team used the surrounding craters to estimate the age of the frozen dunes at between 0.4 and 1.5 million years, a blink of an eye in geologic time. But not everyone is ready to accept such a recent date.
“For young age,” says Brown’s geologist Jack Mustard, “I’m overconfident.” He points out that crater dating has a wide margin of error.
Even without the craters, Radbaugh and Milliken suspect that the mounds are relatively young. Given enough time, wind erosion will erode the crust and allow the dunes to move again.
“They’re certainly much younger features than any rock type detected strongly today or by Curiosity in recent years,” Milliken says.
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While Zhurong investigates the repeated soaking of the dunes, Perseverance unearths the remains of a mighty stream.
After Jezero Crater was formed, scientists believe that water from a nearby canyon network flowed into the site to form a deep lake billions of years ago, when water was still flowing along the surface. Percy is investigating an area where the water entered the lake, searching for clues as to how the liquid survived on the surface of a now dry, parched planet. Did the water seep in slowly over millions of years or did it fill up in a single explosive eruption?
Images taken by Perseverance in February and March provide evidence of at least one fast and furious flow. Huge boulders, brought down by the river water, fell down in a series of curved bands, like arched rows of cobblestones laid on a river bed. The shape of the stones gives an idea of ​​the brute force that the water once displayed as it fell into the ancient crater lake.
“If you have boulders the size of a meter, they probably won’t move with just an inch of water,” Milliken says.
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According to Katherine Stack Morgan, deputy project scientist for NASA’s Perseverance Project, the current was likely strongest where the river met the lake, so it makes sense that the largest rocks fell there. As the river joined the lake in the basin, it slowed down, leaving smaller, finer particles further away.
The area, recently named Sprinkle Haven after a beach in a British national park, has been of interest to geologists for more than 15 years. Its rock bands may be the remains of a common fluvial element: sandbanks. These structures form when downstream material piles up at the edges or in the center.
These preserved shoals can tell a lot about the development of the river over time. If the waterway is impassable, then the banks proceed with a change of banks. The fast current pushes the bars down over time, leaving an imprint of the various paths cut by the water.
One of the most impressive examples revealed in the new images is the Pinstand, a massive formation 20 meters tall. 400 m further into the basin than Schinkel Haven, Pinestand may represent a vast deposit of river sand and rock. The six-storey structure would have been completely submerged.
Perseverance collected a sample from Scribble Haven one day and returned it to Earth for study.
However, evidence of fast-flowing water may not be a good sign for those hoping to find life at the site. “These types of systems are not good for preserving evidence of biological material,” Mustard says.
However, the region has provided new information about the scale and dynamics of ancient Martian rivers.
“Jezero is unique in that we have well-preserved evidence of sediment formation as these rivers migrate,” says Stack Morgan. “There are other places where we’ve had systems like this, but I can’t remember a shining example like Jezero.”
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