The researchers analyzed data from a high-resolution camera onboard NASA's Mars Reconnaissance Orbiter. The images, from a cratered area called Arabia Terra in Mars's northern hemisphere, show rocky layered mounds on the floors of the craters, ranging from a few meters to tens of meters high. Using stereo images taken from different perspectives, the researchers were able to reconstruct a 3-D topographical map of the mounds.
They found that the layers in one particular crater called Becquerel, worn away by years of erosion, were grouped in repeated patterns of 10 that suggest cyclical climate changes in the planet's distant past.
"What we found is that the layers are remarkably rhythmic," says researcher Kevin Lewis, a graduate student at the California Institute of Technology. "That tells you that it's not a random process or something cataclysmic, like volcanic activity [that caused them]; instead it's something forced by the climate cycle."
The pattern of the layers suggests that the climate cycle may have been due to the changing tilt of Mars's axis, Lewis explains. Like Earth, Mars wobbles back and forth on its axis. Earth wobbles by only a few degrees over a 41,000-year cycle, but Mars tilts back and forth by tens of degrees over about 100,000-year cycle. In addition, the wobble itself changes about every 1 million years, which would fit with the pattern the researchers found of the 100,000-year layers repeating in bundles of 10.
On Earth, these astronomically driven climate cycles caused the ice ages -- as the changing tilt of the planet brought different parts of the Earth closer to the sun -- and they can be seen in the geologic record.
Similarly, it makes sense that such climate cycles would be recorded in Mars's sedimentary rocks, says researcher Oded Aharonson, a professor at CalTech and a coauthor of the study: "Atmospheric density could increase or decrease, changing the ability of the atmosphere to lift and deposit sediments," Aharonson explains. "Water can become more or less stable, and if it becomes more stable at the surface it might help stick sediments together."
However, researchers caution that the layered rocks aren't definitive proof of ancient climate cycles. Taylor Perron, a planetary scientist at the Massachusetts Institute of Technology who was not involved with this study, says that while that is possible, it is also possible that the layers were caused by other, unrelated events. Many different things can cause rhythmic layers, he explains -- for examples, tides cause rhythmic bedding in river deltas on Earth.
In order to more definitively show that the layers in Arabia Terra were caused by orbital variations, scientists would need to know the age of the rocks. While the researchers hypothesize that the layers were laid down billions of years ago over a period of about 10 million years, it's impossible to know for sure.
"The difference between what we see on Mars and what we see on Earth is that we don't have any dates on Mars," Perron says. On Earth, rocks can be accurately dated using radiometric dating, but that's not possible for rocks on Mars.
"It's entirely possible that they're seeing orbital variations here," Perron says, "but the case is far from closed."