Posted: 09 December, 2008
Climate cycles persisting for millions of years have left their mark in thick stacks of sedimentary rock and are found to be caused by the regular variation in the planet’s tilt, or obliquity, a phenomenon already known to control the Earth’s dip in and out of ice ages.
Using NASA’s Mars Reconnaissance Orbiter HiRISE camera, scientists at Caltech identified and measured layered rock outcrops within four craters in the planet's Arabia Terra region. Although the layering in different outcrops occurs at scales ranging from a few metres to tens of metres, at each location the layers all have similar thicknesses and exhibit similar features. At Becquerel crater, the scientists propose that each layer was formed over a period of about 100,000 years and that these layers were produced by the same cyclical climate changes.
Rhythmic bedding in sedimentary rocks reflects changes in Mars’s tilt. This view covers an area 1.15 kilometres wide, with individual layers averaging 3.6 metres thick. Sand appears blue and sedimentary rocks appear pink. Image: NASA/JPL-Caltech/University of Arizona.
"Each layer has weathered into a stair step in the topography where material that's more resistant to erosion lies on top of material that's less resistant to erosion," says Kevin Lewis of the California Institute of Technology, lead author of a report on the study published in the 5 December edition of the journal Science. "Due to the scale of the layers, small variations in Mars's orbit are the best candidate for the implied climate changes. 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."
Furthermore, at Becquerel, every 10 layers are bundled together into larger units, which were laid down over an approximately one million year period and repeated at least 10 times. This one million year cycle corresponds to a known pattern of change in Mars's obliquity of tens of degrees.
The tilt of Earth on its axis varies between 22.1 and 24.5 degrees over a 41,000-year period and is responsible for seasonal variation in climate. The portion of the Earth that is tipped toward the Sun receives more sunlight hours during a day, which gradually changes throughout the year. During phases of lower obliquity, polar regions are less subject to seasonal variations, leading to periods of glaciation. Because Mars’s tilt varies so dramatically in comparison, there is even more dramatic variation in climate. When the obliquity is low, the poles are the coldest places on the planet, while the Sun is located near the equator all the time.
Scientists suspect that this could cause volatiles in the atmosphere, like water and carbon dioxide, to migrate poleward, where they would be locked up as ice. When the obliquity is higher, the poles get relatively more sunlight, and those materials would migrate away. "That affects the volatiles budget," says Professor Oded Aharonson. "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."
This oblique view of periodic layering in Martian sedimentary layers was derived from three-dimensional modeling based on HiRISE images. The vertical dimension is exaggerated by a factor of two. It shows the regularity in repetition of layers with approximately the same thickness. Image:NASA/JPL-Caltech/University of Arizona.
This is just one effect that could change the rate of deposition of layers. Another effect of the changing tilt would be a change in the stability of surface water, which alters the ability of sand grains to stick together and cement in order to form the rock layers. "The whole climate system would be different," comments Aharonson.
"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," says Lewis. "More work will be required to understand the full extent of the information contained within these natural geologic archives."
The 10 layer pattern of Mars' wobble lasts about 1.2 million years. If the 10-layer bundles in Becquerel crater are indeed signatures of that cycle, the 10 or more bundles stacked on each other record about 12 million years when environmental conditions affecting sedimentation were generally steady except for effects of the changing tilt.
"One of the fun things about this project for me is that we were able to use techniques on Mars that are the bread and butter of studies of stratigraphy on Earth," says Aharonson. "We substituted a high-resolution camera in orbit around Mars and stereo processing for a geologist's Brunton Compass and mapboard, and were able to derive the same quantitative information on the same scale. This enabled conclusions that have qualitative meaning similar to those we chase on Earth."
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