The Chinese Mars rover Zhurong has found evidence of a dramatic change in Martian climate 400,000 years ago in the form of dark ridges overlying light dunes that stretch across the sands of Utopia Planitia 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 in conjunction with high-resolution observations from China Tianwen-1 Mars orbiter to take a closer look at the large sand dunes near Zhurong landed in May 2021.
The crescent shape of the dunes has been eroded over the course of hundreds of thousands of years, with long dark ridges called transverse aeolian ridges (TARs) forming on the dune fields, but apparently running at a different angle than the windblown dunes. TARs were observed everywhere Mars in the lower middle latitudes, but global atmospheric circulation models that describe the direction of winds on the red planet have not been able to explain how the structures might have formed.
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Zhurong’s study of the dunes revealed that their crescent-shaped bodies are composed of lighter material underneath the darker material that makes up the TARs. From orbit, Tianwen-1 observed 2,262 bright dunes across Mars, and based on the number of craters that hit the tops of the dunes, the research team estimates they formed between 2.1 million and 400,000 years ago. This means that the dark TARs must have formed on them within the last 400,000 years.
These dates coincide with the beginning and end of the last great ice age on Mars. That the TARs formed at a different angle to the dunes means that the wind direction at 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 a periodic wander of a planet’s axis of rotation relative to the plane of its orbit, caused by the combined effects of gravity The sun, Jupiter and the other planets, and the shape and precession of the planet’s orbit.
Both Earth and Mars experience these cycles, which correspond to climatic changes. In the case of Mars, its rotation angle (also called obliquity) varied between 15 and 35 degrees between 2.1 million and 400,000 years ago, which had a negative impact on the climate. Today the tilt of Mars is about 25 degrees.
An ice age on Mars is somewhat surprising not quite the same as on Earth. Typically, Martian glaciations result in warmer temperatures at the poles and movement of water vapor and dust toward the mid-latitudes, where they are deposited. During the last ice age, this water and dust formed a meter-thick layer that still remains below the surface in select locations below the 60th parallel and almost everywhere above the 60th parallel.
The current geologic epoch on Mars is known as the Amazon Epoch, sometimes beginning 3.55 to 1.88 billion years ago and defined by the number of impacts during that period.
“Understanding the Amazonian climate is important to explain the current Martian landscape, volatile matter reservoirs and atmospheric state, and to relate these current observations and active processes to models of ancient Martian climate,” Li said said in a statement. “Observations of the current Martian climate can help refine physical models of Martian climate and landscape evolution, and even develop new paradigms.”
During Mars’ long northern winter, the Zhurong rover went into hibernation. It has yet to be reactivatedand his fate remains uncertain.
The results were published in the journal on July 5 Nature.