The Chinese Mars rover Zhurong has found evidence of a dramatic shift in the Martian climate 400,000 years ago, in the form of dark ridges atop bright dunes lapping over the sands of Utopia Planitia, which the rover is exploring.
Scientists led by Li Chunlai of the National Astronomical Observatories of the Chinese Academy of Sciences used the rover’s instruments, coupled with high-resolution observations of China’s Tianwen-1 Mars orbiter, to take a closer look at large sand dunes near Zhurong landed in May 2021.
The crescent shape of the dunes has eroded over hundreds of thousands of years, with long dark ridges called transverse aeolian ridges (TARs) formed on top of the dune fields, but apparently at a different angle from that of the windblown dunes . TARs have been observed everywhere Mars at lower mean latitudes, but global atmospheric circulation models describing the direction of winds on the Red Planet have so far been unable to explain how the features could have formed.
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Zhurong’s study of the dunes found that their crescent-shaped bodies are made of brighter material beneath the darker material that makes up the TARs. From orbit, Tianwen-1 observed 2,262 bright dunes over Mars, and based on the number of craters impacted on the dunes, the research team estimates they formed between 2.1 million and 400,000 years ago. That means the dark TARs must have formed on top of them over the past 400,000 years.
These dates coincide with the beginning and end of the last major ice age on Mars. That the TARs formed at a different angle to the dunes implies that 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 angle at which Mars rotates, caused by Milankovitch cycling. These cycles involve a periodic wandering of a planet’s axis of rotation from the plane of its orbit, caused by the combined effects of the gravity of the sun, Jupiter and the 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 changes. In the case of Mars, the angle of rotation (also called tilt) varied between 15 degrees and 35 degrees between 2.1 million and 400,000 years ago, wreaking havoc on the climate. Today, the inclination of Mars is about 25 degrees.
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Somewhat surprisingly, there is an ice age on Mars not quite the same as on Earth. Typically, Martian glaciations see warmer temperatures at the poles and movement of water vapor and dust to the mid-latitudes, where they are deposited. During the last ice age, this water and dust formed a meter-thick layer that still subsurfaces at selected locations below 60 degrees latitude, and almost everywhere above 60 degrees.
The current geologic epoch on Mars is known as the Amazonian epoch, which sometimes began between 3.55 and 1.88 billion years ago and is determined by the number of impacts during that time.
“Understanding the Amazon climate is essential to explain the current Martian landscape, volatiles reservoirs and atmospheric conditions, and to relate these current observations and active processes to models of the ancient Martian climate,” Li said in a statement. “Observations of the current climate of Mars can help refine physical models of Martian climate and landscape evolution, even shaping 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.