Climate Model For Earth Also Describes Changes On Mars

Their study appears in the Sept. 26 issue of Nature, and suggests that a climate change theory for Earth can also be applied to Mars and possibly to other Earth-like planets.

"The orbital theory of climate change has been successful in explaining changes in the Earth's climate, and we have used cores of the Greenland and Antarctic ice caps to reconstruct past climates and atmospheres on Earth," said author Jack Mustard, associate professor of geological sciences at Brown University. "This means that we can now use the Mars caps in a similar way."

The study also produced "a much better constraint on the time required to form the layers on the polar cap of Mars," he said. "This has importance in understanding the Martian climate, and also the water cycles and history. We have so little information on the rates of change on Mars, but this gives us a solid marker."

Besides Mustard, the other two authors of the study are Jacques Laskar and Benjamin Levrard of the Astronomie et Systèmes Dynamiques in Paris.

The trio used orbital calculations and rotational parameters of Mars, new high-resolution images of its north pole terrain, and high-resolution topography data to correlate exposed layers of ice and dust with changes in climate, particularly the sum of solar radiation reaching the terrain. Their techniques mirrored those used for orbital-based climate studies on Earth.

Changes in the ratio of dust and ice over time are visible in the variations of brightness seen in the layers of polar deposits on Mars. First noticed in the earliest Mars missions, the layers were thought to be related to changes in climate possibly linked with the evolution of Mars' orbit such as the tilt of the axis and deviations in circularity. But the image resolution of past data was insufficient to resolve such key details.

High-resolution images from the Mars Global Surveyor allowed Mustard to resolve the fine detail and analyze the patterns. "After correcting the observations for topography, we produced a measure of the brightness of the layers as a function of depth," he said. "We assume that depth equals time and thus can peer into the past. We then compared this record with the predicted amount of sunlight received at the pole over the last 10 million years, which varies with orbital evolution involving tilt and circularity."

The researchers' goal was to determine whether the sunlight record at the north polar cap of Mars correlated with the brightness-depth profile, and over what time frame. If the record correlated, that would solidify their hypothesis that the layers are due to climate changes related to orbit and allow the researchers to determine the formation rate of the layers.

"Indeed we find an excellent correlation and show that the 350-meter thick package of layers formed within the last 1 million years or so," Mustard said. For the most recent 250-meter thick deposit of the north ice polar cap, the researchers found an average deposition rate of 0.05 cm/yr. "For the first time, we showed that the orbital theory of climate change has a record in the polar deposits on Mars."

In orbit, Earth tilts about 23-25 degrees. In contrast, Mars tilts as little as 15 degrees and as much as 40 degrees, which is enough to redistribute moisture from polar caps to equatorial regions, Mustard said. In the 1970s, research by geologists at Brown first showed an orbital effect on climate change for Earth.

"This study further ties Mars and Earth as similar planets and strengthens the foundations to compare their climates and orbital evolutions," Mustard said. "Showing that a planet with as strange an orbit as Mars has climate changes recorded in its surface means we have a tool in this orbital driver to explore what happens on the surface of planets with differences and similarities to Earth. And as we learn more about deposits on Mars we can compare what we find with what we know about Earth."

The Centre Nationale de la Recherche Scientifique Programme Nationale de Planétologie and NASA's Solar System Exploration programs supported the research.