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Mars, Like Earth, Has Cyclical Ice Ages, Study Says

Brian Handwerk
for National Geographic News
September 14, 2007
 
Mars has gone through 40 ice ages during the past five million years that regularly send the planet's permanent ice sheets cascading toward the equator, then melting backward, a new theory suggests.

The climate changes are likely driven by cyclical fluctuations in the planet's orbit that alter the amount of sunlight that falls on the planet's surface, says astronomer Norbert Schörghofer of the University of Hawaii at Manoa.

Understanding the sun's exact role in the Martian ice ages could help solve longstanding puzzles about the red planet.

It could also help scientists better understand Earth's complex climatic systems, which are also affected by orbital variations.

The new theory appears in this week's issue of the journal Nature.

Mystery of the Ice

In recent years extensive amounts of ice have been discovered below the surface of Mars. Much of the ice mysteriously survives far from the planet's poles. (Related photo: "'Frozen Sea' Seen on Mars [February 23, 2005].)

Schörghofer suggests that this ice is newer than previously believed.

"Earlier theories have tried to explain this ice with snowfall that would have happened some five million years ago [but struggle] to explain how that ice could have stayed there," Schörghofer said.

"I'm saying it didn't stay. It went away and then came back many, many times."

According to Schörghofer, much of Mars's ice is formed by vapor diffusion—the seeping of gas directly into underground pockets during cold periods.

"The water cycle on Mars is very different than what we see on Earth," said Joshua Bandfield, a research specialist at Arizona State University's School of Earth and Space Exploration who was not involved in the study.

(Related: "Mars's Ice Patchy, Water Cycle Quite Active, Study Reveals" [May 2, 2007].)

"Water vapor in the air basically diffused into the subsurface through pores in the soil and filled up that pore space with ice. It goes away by the same process—it heats up and goes from solid to gas and diffuses back out of the rock."

Schörghofer's theory may therefore reveal a very different recent history for the red planet than once thought.

"The surprise is how frequent Mars's ice ages are and how young some of the ice is," Schörghofer said. "[Some may be] only half a million years old, which is not very old for Mars because most of the surface is billions of years old."

"Wobbly" Theory

Mars, like all planets, experiences small "wobbles" in its axis as it orbits the sun.

Such variations change the amount of sunlight falling on a planet's surface, which can cause major climate shifts, Schörghofer said.

Earth's wobbles, known as Milankovitch cycles, occur in 20,000- and 100,000-year periods and are thought to impact the waxing and waning of the planet's ice ages.

According to Schörghofer, this means both planets have an ice record that tracks the activity of the sun.

"Mars's ice cores could tell us not only what the climate was on Mars but also changes in the sun, the same solar history that Earth was exposed to," he said.

"Understanding ice ages on Mars is an important step toward understanding the ice record on Mars," he added. "Samples of the Martian ice record could be used to interpret Earth's climate record.

In some ways, the Martian climate is easier to understand and model than Earth's.

Because Mars has no large moon to stabilize its orbit, the planet has much larger wobbles and thus greater climate shifts than Earth.

Mars also has more consistent weather and no oceans.

"Planets are sort of these huge lab experiments that were set up 4.5 billion years ago," Arizona State University's Bandfield said. "[Mars is an opportunity] to test how the physics work on a simpler system and get a better handle on how to apply it to a more complex system."

A first examination of the theory will arrive with the Phoenix lander, already en route to Mars for a 2008 landing. The craft will try to sample Martian ground ice, and the results could make or break Schörghofer's theory. (Related photo gallery: "Phoenix Lander's Search for Mars Water" [August 3, 2007].)

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