China’s Zhurong Mars rover has found evidence of a dramatic change in Mars’ climate 400,000 years ago, in the form of dark ridges above bright dunes rippling across the sands of Utopia Planitia, which the rover is exploring.
Scientists led by Li Chunlai from the National Astronomical Observatory of the Chinese Academy of Sciences used the rover’s instruments, along with high-resolution observations of China Tianwen-1 Mars orbiter, to take a closer look at the large sand dunes near where Zhurong disembarked in May 2021.
The crescent shape of the dunes has been eroded over hundreds of thousands of years, with long dark ridges, called transverse aeolian ridges (TARs), forming on top of the dune fields, but appearing at a different angle than that of the dunes blown by the wind. TARs have been observed everywhere Mars at low to mid-latitudes, but global atmospheric circulation models that describe the direction of winds on the Red Planet have been unable to explain how the features might have formed — until now.
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Zhurong’s investigation of the dunes found that their crescent-shaped bodies are made of brighter material beneath the darker material that forms the TARs. From orbit, Tianwen-1 observed 2,262 bright dunes on Mars, and based on the number of craters that impacted the dunes, the research team estimates that they formed between 2.1 million and 400,000 years ago. This means that the dark TARs must have formed above them within the last 400,000 years.
These dates coincide with the beginning and end of the last great ice age on Mars. The fact that the TARs formed at a different angle to the dunes implies that the wind direction in the lower mid-latitudes must have changed with the end of the ice age.
The ice age began and ended due to changes in the rotation angle of Mars, caused by Milankovitch cycles. These cycles involve periodic wandering of a planet’s axis of rotation relative to the plane of its orbit, caused by the combined effects of gravity from the sun, Jupiter and other planets, as well as the shape and precession of the planet’s orbit.
Both Earth and Mars experience these cycles, which correspond to climate change. In the case of Mars, its angle of rotation (referred to as its obliquity) varied between 15 degrees and 35 degrees between 2.1 million and 400,000 years ago, wreaking havoc with its climate. Mars’ obliquity is about 25 degrees today.
Surprisingly, it is an ice age on Mars not quite like on Earth. Typically, Martian ice ages see warmer temperatures at the poles and movement of water vapor and dust to mid-latitudes, where they settle. During the last ice age, this water and dust formed a meter-thick layer that still remains below the surface in selected places below 60 degrees latitude and almost everywhere above 60 degrees.
The current geological era on Mars is known as the Amazonian Epoch, which began sometime between 3.55 and 1.88 billion years ago and is defined by the number of impacts during that time.
“Understanding the Amazonian climate is essential to explaining the current Martian landscape, volatile matter reservoirs and atmospheric state, and to relating these current observations and active processes to patterns of ancient Mars climate,” said Li . said in a statement. “Observations of Mars’ current climate can help refine physical models of Martian climate and landscape evolution, and even shape new paradigms.”
Meanwhile, the Zhurong rover went into hibernation during Mars’ long northern winter. It has yet to be reactivatedand his fate remains uncertain.
The findings were published July 5 in the journal Nature.
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