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Planetary News: Mars (2005)

Scientists Solve Mystery of Mars' Off-Center South Pole

May 31, 2005

For more than 150 years, astronomers and sky watchers have noticed that Mars' south pole is off center, and Mariner 4 confirmed it in the mid-1960s with the first close-range images of the Red Planet. But why the cap is offset from its geographical pole has remained an enigma all these years -- until two summers ago when a group of planetary observers and theoreticians decided to take on the challenge at the first annual Mars Polar Atmospheric Interactions Workshop, held in Santa Fe, New Mexico.

Now, with publication of their research earlier this month* the mystery is officially solved -- and not so surprisingly Mars' dynamic topography is at the heart of it.

"We found that the offset is a result of two Martian regional climates, which are on either side of the south pole," said the paper's lead author, Anthony Colaprete, a space scientist from NASA Ames Research Center. Weather generated by the two Martian regional climates east and west of the pole create conditions that cause the Red Planet's southern polar ice to freeze out into a cap, the center of which lies about 93 miles [150 kilometers] from the actual, geographical south pole, the team reported.

Intriguingly, the team -- which included Colaprete, Jeffrey Barnes of Oregon State University, Corvallis; Robert Haberle, also of NASA Ames, Jeffery Hollingsworth of San Jose State University Foundation, NASA Ames, and Hugh Kieffer and Timothy Titus, both from the U.S. Geological Survey -- discovered that the location of two huge craters, Argyre and Hellas, in the southern hemisphere of Mars is what causes the two distinct climates. "The two craters' unique landscapes create winds that establish a low pressure region over the permanent ice cap in the western hemisphere," Colaprete explained.

Just like on Earth, low-pressure weather systems signal cold, stormy weather and snow. "On Mars, the craters anchor the low pressure system that dominates the southern polar ice cap, and keep it in one location," he added. The low-pressure system results in white fluffy snow, which appears as a very bright region over the ice cap.

By contrast, the scientists found that 'black CO2 ice' forms in the eastern hemisphere, where Martian skies are relatively clear and warm. "The eastern hemisphere of the south pole region gets very little snow, and clear ice forms over the Martian soil there," Colaprete said. Black ice forms when the planet's surface is cooling, but the atmosphere is relatively warm, and he pointed out, "a similar process occurs on Earth when black ice forms over highways."

The way they discovered all this and sleuthed the craters to be the perpetrators was through observations and climate modeling. In a nutshell, they input all the observational data they had available into a computer simulation system called the Mars General Circulation Model (MGCM) at Ames. "It's a similar system to the ones meteorologists use on Earth to make weather and climate predictions," Colaprete explained in an interview with The Planetary Society.

The simulations in the Mars General Circulation Model are all built from first principle physics -- our understanding of how the atmosphere works, how fluids behave, how sunshine warms the surface, etc. One significant difference between Earth model simulations and this Martian model, Colaprete point out, is that Earth models are able to input wind speeds and other aspects of the conditions to initialize model runs. "We don't have that luxury, because we don't have the observations," he said. "So we can only rely on first principle physics, good old Newtonian physics and fluid mechanics, and what we know to be true."

No worries there. The physics, for the most part, is exactly the same. The boundary conditions, however, are a different story -- "they are what make Mars a very different place than Earth," Colaprete said. In fact, while there are similarities between Earth and Mars, the boundary conditions in many respects are radically different. Mars, for example, gets far less sunlight than Earth, and its orbit is much more eccentric. "Other important differences are that there is no ocean on Mars, and the atmosphere is extremely thin. And another most important boundary condition difference is the topography of Mars -- it is unlike anything in the solar system in terms of extremes. You have impact basins that are 14 kilometers deep and mountains 12 kilometers high. There is nothing like that comparable on Earth, unless, possibly, you drained the oceans."

Once Colaprete had input all of the first principle physics, the known boundary conditions differences and all the other known observational data into the program, he began running the simulations. "The beauty of being a climate modeler is you get to play God, at least in your little part of a simulated universe," he said. In the standard scenario -- "a standard run where we try to match all the observations we can with first principle physics" -- [the modeling system] reproduces the observations [we have] nicely," Colaprete said. "In that standard state, the system revealed a very asymmetric climate in the south, where one side of the pole -- the western hemisphere -- was very cold and stormy, and the other, eastern side was warm and clear. We saw that very distinctly in our model and we saw that in the observations very distinctly." So, the simulation system was rendering exactly the conditions they knew existed on Mars.

To figure out what was causing those two distinct climates, Colaprete began to eliminate various parts of the topography. "I got rid of the Tharsis rise and wiped down the mountains, just flattened them out, and ran that simulation -- the western hemisphere was still stormy and cold, and the eastern side was still clear and warm? "It changed a little, but just a smidgen," he recalled.

Given the Martian topography, the team members had suspected the craters might be responsible in some way, so Colaprete went back to the simulation model and got rid of one of the impact basins. "Immediately, I saw a big difference, and then I got rid of the other one, and the asymmetry disappeared," he recounted. The cap moved into a nearly perfectly symmetric position, and "the climates predicted by the models became symmetric," he said. The team was then able to hypothesize that the two climates are controlling the deposition of ice and the type of ice formation to be offset, hence the observed polar cap to be offset.

The result, once he 'erased' the craters in the simulation, was "robust," Colaprete said. "That was nice. Quite often when you work with dynamics in a model like this, the changes are very subtle, and it's very hard to pull out the pieces that are really doing the work. Here it was easy. We got rid of the craters and it was black and white -- there it was. It was a very robust signature, and a very robust signature in the observations as well."

What this means of course is that Mars' southern cap could be engineered, theoretically, back onto its geographical center. "If we got rid of the two impact basins -- Argyre Basin [497 miles/800 kilometers diameter] and Hellas Impact Basin [1,118 miles/1800 kilometers diameter] -- by, say, filling those in, which would be quite a feat, the southern cap would become symmetric and the permanent CO2 cap, which is now offset would move directly onto the geographical pole," Colaprete said. "The black ice would probably disappear all together, although slab ice would still form, you would get a much more uniform snow cover so you just would never really see it."

Solving an age-old mystery was satisfying, of course, for all the team members, Colaprete said. It was a mystery that did go back more than a century. Interestingly, it was Percival Lowell in the mid-1890s offered up "the very first good observations" of the south pole cap in his book, Mars, Colaprete pointed out. "Lowell's got some beautiful plates, drawings that he and [Andrew Ellicott] Douglas did. Plate II shows their mapping of the South Polar cap for that southern spring, and shows how it receded and they actually show very accurately where the perennial cap is. That was not the first time the cap had been observed, of course, but Lowell's work was the first good continuous mapping of it," he said.

Although Lowell was wrong about his observations of the signs of intelligent Martians, "[w]hat is amazing are Lowell's descriptions in that book," Colaprete continued. "He describes a vast black sea that forms just adjacent to the perennial cap, which is the black ice, and he interpreted it as a water sea and low albedo of an open ocean or an open sea. It made very good sense. He had a receding cap and it was forming liquid water, and then the liquid water disappears, evaporates. So it was Lowell who really mapped out for the very first time, with any real precision and continuity in time the recession of the south polar cap, the formation of the cryptic region, which is the black ice, and the position and recession of the perennial cap." Lowell, no doubt, would have marveled at the solution, despite the fact his Martians weren't there.

Beyond the reward of accomplishment in solving the mystery of Mars south pole, Colaprete said there was also reward in how it all came together. "To be able to pull all the data together and accomplish something like this from a workshop was very rewarding," he reflected. "When we were finished, I felt that this is the way science is supposed to be done."