Sequences of cyclic sedimentary rock layers exposed in an unnamed crater (located at 8N, 353E) in Arabia Terra, Mars.
Credit: Topography: Caltech; HiRISE Images: NASA/JPL/Univ. of Arizona
New images of rock layers at Mars' surface have given scientists evidence of climate swings on ancient Mars that were driven by the wobbling of the red planet's axis ? the same mechanism that causes Earth's ice age cycles.
The stereo topographic maps of rock outcrops within four craters of Mars' Arabia Terra region were obtained by the high-resolution camera onboard NASA's Mars Reconnaissance Orbiter, which is still circling the planet.
The team looked at the layering of deposits of sedimentary rocks in the outcrops to see if they could discern any patterns. The layers were a few meters (yards) to tens of meters thick. At Becquerel crater, the researchers found an alternating pattern of layers within layers that suggests that each one formed over a period of about 100,000 years as a result of cyclical climate changes.
These cycles are a result of the changing degree of tilt of Mars' orbital axis, the imaginary line running between a planet's two poles around which it rotates, said study leader Kevin Lewis, a graduate student at Caltech.
"Due to the scale of the layers, small variations in Mars's orbit are the best candidate for the implied climate changes," Lewis said. "These are the very same changes that have been shown to set the pacing of ice ages on the Earth and can also lead to cyclic layering of sediments."
The study is detailed in the Dec. 5 issue of the journal Science.
The tilt of Earth on its axis varies between 22.1 and 24.5 degrees over a 41,000-year period. That seemingly small variation leads to large changes in the amount of sunlight reaching the polar regions of Earth ? when less sunlight reaches the poles, more ice can accumulate there, leading to ice ages that can last thousands of years.
Mars' tilt has more variation than Earth's, wobbling by tens of degrees over a 100,000-year cycle, which can produce even more dramatic changes in climate. When Mars' axial tilt, or obliquity, has been low, the poles have been the coldest places on the planet, which has resulted in atmospheric changes that can impact how material is deposited.
And when the poles are colder, water and carbon dioxide in the atmosphere migrate pole-ward where they are locked up as ice. When the obliquity is higher though, the poles get more sunlight and the water and carbon dioxide migrate way.
"If you move carbon dioxide away from the poles, the atmospheric pressure would increase, which may cause a difference in the ability of winds to transport and deposit sand," said study team member Oded Aharonson, also of Caltech. This could change the rate at which material is deposited on the Martian surface.
The changing tilt would also affect the stability of any surface water, which would alter the potential for sand grains to stick together and cement into rock layers.
In addition to the apparent 100,000-year cycle of layering, every 10 layers in the craters were bundled together into larger units that correspond to a longer climate cycle of about one million years, the scientists found. In Becquerel crater, the 10-layer pattern is repeated at least 10 times.
The one-million-year cycle corresponds to a known pattern of change in Mars's obliquity caused by the dynamics of the solar system.
Many geologic processes, in addition to changes in atmospheric density and surface water stability, could change with Mars' wobbling, so researchers can't tie the sediment layering patterns to any particular geologic processes. But, "a strength of the paper is that we can draw conclusions without having to specify the precise depositional process," Aharonson said.
"This study gives us a hint of how the ancient climate of Mars operated, and shows a much more predictable and regular environment than you would guess from other geologic features that indicate catastrophic floods, volcanic eruptions and impact events," Lewis said. "More work will be required to understand the full extent of the information contained within these natural geologic archives."
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