MOSCOW, December 8, Vladislav Strekopytov. Chinese scientists reported that the Zhuzhong rover, which worked on Mars in 2021-2022, discovered polygons of unknown origin at a depth of 35 meters. About what these structures are and how they formed — in the material .
The Second on Mars
«Zhurong» (in another transcription — «Zhurong»), as part of the Chinese interplanetary mission «Tianwen-1», landed on the Utopia Planitia in the eastern part of the northern hemisphere of Mars on May 14, 2021. China became the second country after the United States to successfully deliver a research rover to the Red Planet.
“Zhuzhong” worked not for three months, as expected, but until May 20, 2022, after which communication with it was completely interrupted. During this time, the device traveled about two kilometers along the bottom of the Utopia Plain. This is the largest impact crater, with a diameter of over three thousand kilometers, not only on Mars, but in the entire solar system. The structure is also interesting because here the crust is very thin and the mantle comes closest to the surface.
All along the route, the rover carried out radar surveys using the RoPeR (Rover Penetrating Radar) georadar installed on board. This device performs high-resolution sounding in two ranges of electromagnetic waves to a depth of up to 100 meters. For comparison: the American Perseverance rover, which arrived on Mars three months before Zhuzhong and is still operating in the neighboring crater of Isis Planitia, is capable of “seeing through” the depths of only ten meters.
In addition, Zhuzhong carried out magnetometric mapping of the Martian crust to identify local anomalies in the magnetic field, measured climatic parameters (temperature, pressure, wind speed), studied the mineral composition of the regolith using spectroscopy methods, and searched for water. In total, the rover transmitted 940 gigabytes of various data to Earth.
Discoveries of the Chinese Mars rover
After analyzing the information received, Chinese scientists came to the conclusion that groundwater under the Utopia Planitia remained active until the Amazonian (recent) period of the geological history of the Red Planet, which was previously considered cold and dry. In sedimentary deposits of Amazonian age, Zhuzhong instruments identified hydrated sulfates, hydrosilica, ferric oxides and chlorides. These minerals require liquid salt water to form.
According to the researchers, the hydrated minerals were formed during a period of volcanic activity, when hot magma, rising to the surface, melted underground ice and saturated the resulting solution with salts. Coming to the surface, it evaporated, and minerals precipitated from it.
The fact that there are huge layers of subsurface ice under the Utopia Planitia was established back in 2016 using the SHARAD radar of NASA's Mars Reconnaissance Orbiter. Reserves are estimated at more than 14 thousand cubic kilometers, which is comparable to the volume of water in Lake Superior, the largest and deepest of the Great North American Lakes.
The RoPeR ground penetrating radar also showed that the rocks under the Utopia Plain have pronounced layering, which means that they were deposited in an aquatic environment. According to scientists, in the late Hesperia — early Amazonian times (3.5-3.2 billion years ago), the crater was periodically filled with water, which led to layering.
Three billion years ago, the era of liquid water on Mars ended, and the layered rocks were buried under a layer of regolith consisting of dust and small debris. At the same time, scientists do not rule out that highly mineralized solutions still remain at depth in pore traps.
Witnesses of the climate revolution
Recently, Chinese scientists reported another discovery: at a depth of about 35 meters from the surface, the Zhuzhong ground penetrating radar discovered wedge-shaped polygonal structures up to several tens of meters in diameter. Along a 1.9-kilometer path, the device recorded 16 such polygons, suggesting their widespread distribution throughout the Utopia Plain.
Researchers believe that the mysterious underground structures formed immediately at the end or after the end of the “wet” era. These were originally surface landforms that sank to depth as a result of later geological processes.
According to the model proposed by the authors, when the water drained from the crater, the clayey sediment at its bottom dried out and cracked. Moisture penetrated into the cracks: from the depths — as a result of the rise of groundwater and the diffusion of steam formed during the sublimation of pore ice, and from the surface — in the form of snow. When frozen, the water and soil that filled the cracks worked like wedges.
As a result of the cyclical process of freezing and thawing, which lasted millions of years, a peculiar small-hilly relief was formed. Around 3.2-2.9 billion years ago, conditions on the surface of Mars changed dramatically: the climate became drier and colder. The polygonal relief was eroded, and its remains were found under a layer of regolith.
“The structure of the rocks above and below the conventional boundary, located at a depth of 35 meters, is very different,” the authors of the article write. “This indicates a noticeable transformation of the thermal regime and water activity. Obviously, at this time a climatic revolution occurred in the low and middle latitudes of Mars.” .
Modern cellular structures
Similar polygonal structures are still widespread on the surface of Mars today. They appear every year in the spring in the polar regions of the planet. They were first noticed in 2006 in images from the HiRISE (High Resolution Imaging Science Experiment) camera placed on board the Mars Reconnaissance Orbiter spacecraft.
Researchers from the University of Arizona, after analyzing images from the HiRISE camera, concluded that the honeycomb-like structures are the result of seasonal changes in water and carbon dioxide. Water and dry ice frozen into the regolith split it into polygons, along the boundaries of which solid bedrock protrudes to the surface. And in the central parts of the “honeycomb” loose sandy or dusty deposits accumulate. When temperatures rise in the spring, subsurface ice immediately changes from solid to gas, bypassing the liquid phase.
Scientists believe that the famous polygonal dunes that the HiRISE camera previously recorded on the flat bottom of many Martian craters are formed in approximately the same way. Winds blow away dust and grains of sand, and the ice that fills the cracks forms polygonal ridges.
In detailed images, a network of smaller polygons is visible within large-scale polygons. In shape, they resemble drying cracks at the bottom of dry lakes.
Polygonal relief structures are also common on Earth, in regions of permafrost development . As on Mars, they are associated with seasonal freeze-thaw cycles, causing the formation of erosive forms of cracking.