Barr explains moon structure dichotomies

Thursday, Dr. Amy Barr from the Southwest Research Institute unraveled the mysteries of the origins of Ganymede and Callisto to a crowd of space enthusiasts and geophysicists at 2011’s first Heiland lecture.

From an external standpoint, Ganymede and Callisto, which are both composed primarily of ice and rock, should be fairly similar, yet from an internal standpoint, they differ greatly. Ganymede’s structure is similar to that of Earth. It has layers that compare to Earth’s core, mantle, and crust, composed of ice and rock rather than rock alone. Callisto, on the other hand, is fairly well mixed, with no distinct layering or density differences. Barr’s research seeks to explain the dichotomy between these two bodies, which she described as “a branching of an ancestral line into two equal diverging branches.”

Currently the Nice model of solar system formation appears to best explain Barr’s problem, tying a heavy bombardment period in solar system history with gravitational interactions between large planets such as Jupiter and Saturn. While the model does a good job of explaining certain observed features such as large impacts on the Moon and other bodies that all date to around the same time, the explanation has been criticized for its simplicity. However, Barr’s research efforts appeared to validate the model. “I started out trying to disprove the Nice model,” stated Barr, but her efforts “just ended up proving it more.”

Barr’s model for the dichotomy between the two moons assumes that, given a certain amount of impacts on a mixed ice and rock body, there will be different paths that a moon’s development can follow. This works because the model assumes that a hyper-velocity comet impact acts as a subsurface nuclear impact; the comet will dig into the ground somewhat before exploding and melting the ice. When the ice melts, all of the rock material in the area sinks down to the bottom of the comet impact crater, where it begins to lump into a much larger mass. These larger blobs of rocks in turn sink down into the core of the planet or moon, outputting energy as they drop down their gravitational potential gradient. If the above process happens often, enough heat will be generated to melt the whole planet, resulting in a differentiated body like Ganymede. If not, a more homogeneous body like Callisto will result.

According to Barr’s model, Callisto received less impacts than Ganymede due to Ganymede’s proximity to Jupiter, which puts it in the way of larger, faster-moving objects. “For every single gram of material that hit [Earth’s] Moon, forty times that much hit Callisto and eighty times as much hit Ganymede,” explained Barr. “Since we have an idea of how much hit the Moon through the Nice model, we can get an idea of how much hit the two other moons.” Barr admitted that, at first, her data and theory did not fit the Nice model. However, creative thinking and equation manipulation largely rectified the issue. “While I haven’t disproved the Nice model, I have put an upper limit on the amount of material involved,” Barr noted. She is hopeful that, through observing other satellites in the solar system, an estimate can be made of what the solar system looked like at its inception.

That said, if other moons fail to conform to Barr’s model, the model may be invalidated. The lecturer’s response to the concern was to question the validity of such conflicting data, “[A]re we being fooled [with wrong data] or is all of our research wrong? Do you cut the piece to make it fit or do you throw the puzzle out?”