The researchers studied images of a gash in the polar cap that reveals fine, banded layers underneath. The images look straight down on the surface and don't indicate depth, Mustard says. So the Mars Orbiter Laser Altimeter was employed to generate simultaneous measurements to better understand how deep the layers go.
Mustard and his colleagues determined the thickness and brightness of each layer, down to 820 feet (250 meters). They assumed that darker layers are composed of more dust and less ice and were most likely formed in warmer years when more of the ice was evaporated. This allowed a calculation of an average deposition rate of .02 inches (.05 centimeters) per year for the topmost section, making the entire 1.6-mile-deep (2.5-kilometer) cap about 5 million years old.
"Which makes it extremely young," Mustard says. "We always knew the top of the ice cap was extremely young, but we didnt know if that continued down through the whole thing."
Young is a relative term, of course.
"On Earth you can only go back a few hundred thousand years just because the ice circulates via glaciers rapidly enough so that nothing older than that would be around," says Alan Howard, a geologist from the University of Virginia who was not involved in the research but wrote an analysis of it for Nature.
But Mustard and his partners were not done. Laskar, the lead author of the paper, is according to Mustard "the worlds expert in calculating orbits." He and others have used models to determine how much sunlight reaches Mars by factoring in the shape of its orbit. A highly eccentric, or elliptical orbit around the Sun means Mars experiences more extremes in distance and thus temperature, while a circular orbit means steadier average sunlight.
They also considered the tilt of the Red Planet. It can vary from 15 to 45 degrees, largely due to its lack of a significant moon. If Earth did not have a moon, its tilt would vary from its 23.5-degree tilt by far more than the few degrees it experiences now.
Laskar, Mustard and Levrard compared the calculated amount of sunlight Mars has received over the last million years to the values they extracted from the ice cap layers and found a correlation, implying that the orbital models are correct and the amount of light received by the planet is a primary determining factor of Martian climate.
"By assuming that the research pans out in the end, that the story seems consistent, it really gives us an insight into what actually is controlling the variations in Mars climate," says Howard.
The scientists only examined one cut in the polar ice but hope to expand the study to others exposed parts of the northern cap. Also, Mustard says, "theres this entire southern cap that is unexplored for these same purposes. We naturally said if we can do it in the north we should be able to do it in the south, and if they have opposite signals then were good, weve got it down. But the southern cap is quite different."
The layers in the southern cap tend to be "chunkier" and less finely detailed. Furthermore, the south has several impact craters larger than 980 feet (300 meters) across, while the north has nearly none. "That tells us it [the southern ice cap] is older. Some people think its up to maybe 10 million years old," Mustard says.
Observations also indicate a difference in height between the poles, Mustard adds. The southern cap is at 5 miles (8 kilometers) of elevation while the northern cap is at -1.2 miles (-2 kilometers).
"Some peoplehave hypothesized that that has created a kind of south-to-north transfer of things, like water, to concentrate in the north," Mustard says. "Thats another puzzle we dont quite understand and we would certainly like to pursue this and see what the southern cap can tell us."
While that mystery is yet unresolved, the new findings may help shed light on other water-related questions.
Understanding the Martian climate helps scientists better understand what is occurring on the global level. It may even help them better plan future missions.
"Now we can kind of think about how we can go about exploring and making the kind of measurements and do an even better job of understanding how the Martian climate system evolved," Mustard says. "Where would you go to make measurements, say, when the climate gets a lot warmer? Is there water vapor everywhere? How does it behave under that force?"
Another mystery has been imaged by the Mars Global Surveyor, halfway between the poles and the equator in the form of gullies, or valleys that seemed to have been carved by water flowing from the poles, something unlikely given the planets cold temperatures. But if the climate history of Mars is accurate, and it in fact did experience times of extreme sunlight, most of the deposits may actually have been evaporated off the polar areas, Howard says. "Then theres the question of where this water goes."
The amount of water stored in the northern cap is enormous.
"It would be the equivalent of a few meters of water if it was spread out over the whole planet," Howard says. In addition, past work, including some by Mustard, has detected a near-subsurface reservoir of water ice, and others hypothesize that water may be stored even deeper down.
"Theres more than enough water thats being moved around the planet into these different reservoirs to perhaps give us greater confidence that these gullies are indeed young and could be formed by water," Mustard says.
And water, of course, implies life.
One of the major goals of most scientists studying Mars today is to answer the question of whether life ever existed there. "Its one of the great mysteries of the universe and Mars is a great place to go and answer that question because it once had water in a liquid form running on the surface," Mustard says. "Were not looking for little green men. Id be happy with a green sludge."
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