Why Mercury is a hard orange, not a soft peach

Object: Mercury-like planets
Composition: Large, iron-rich cores
Position in solar system: Closest to their stars

If our solar system's hellishly hot, innermost planet Mercury were an orange, its enormous, iron-rich core would be the juicy, fruity bit, leaving just the thin rind for the crust and mantle. This has puzzled astronomers for decades, as conventional planetary formation models cannot produce such a relatively large core. Earth and the other terrestrial planets, by contrast, have cores more like the pit of a peach – giving them a lower density overall.

Astronomers speculated that Mercury could have suffered a massive impact that stripped away a silicate mantle. Alternatively, its outer layers could have evaporated away from the heat of the sun. But in the last few years, NASA's Messenger probe has found volatile elements like potassium in the planet's crust. If it had suffered either the trauma of an impact or evaporation, those elements should not have survived and persisted – according to most models.

In the meantime, the mystery has only become more pressing. Recent observations of extrasolar planets suggest that Mercury's structure might not be unique: the two smallest exoplanets whose densities are known, Kepler-10b and Corot-7b, are also far denser than expected, suggesting they share Mercury's orange-like structure. And these planets, like Mercury, also sit close to their sun. Now, a new theory may explain the whole coterie in one fell swoop. The culprit? The heat provided by starlight.

Hot side

When gas molecules collide with a hot dust grain, they pick up heat, bouncing off faster than they approached. This gives the grain a little shove. Gerard Wurm of the University of Duisburg-Essen in Germany and colleagues calculated how this photophoretic force would affect dust grains swirling around a star.

Because metallic grains conduct heat, they are evenly heated throughout. As a result, they will be shoved from all sides and so will not move far from the star, the team found. Insulating grains, however, such as less-dense silicates, have a hot, sun-facing side, where departing gas molecules will give a bigger shove than those on the cold side.

Wurm's team says the effect of this over time will be to sort the grains in a nascent solar system, with metals left close to the star, and less dense particles pushed further out. Planets eventually form from these grains, so this process could explain why inner planets like Mercury, Kepler-10b and Corot-7b, are so dense.

"I think it all fits together logically," Wurm says. "You have metal-rich objects closer to the star, because you can't push them. The further you go out in a planetary system, the less metal you have to build planets."

The research will appear in The Astrophysical Journal.

Tower drop

Photophoresis is not a new idea. A century ago, physicists who worked with vacuum chambers worried about it constantly. "This was in every experiment because the [vacuum] pumps were so bad," says Wurm. The force is only significant in imperfect vacuum conditions, where there's a little bit of gas but not too much. As pumps improved, physicists began to leave photophoresis out of their calculations. "It kind of vanished for 100 years," says Wurm

.

some like it hot.

Larry Nittler of the Carnegie Instition of Washington likes the idea of linking photophoresis to the Mercury mystery, but he cautions that Wurm's team's theory on planet formation is not conclusive, and he stresses that the rival mantle-stripping explanation is not dead.

Future work could include carrying out computer simulations of our solar system that take photophoresis into account and comparing the predicted composition with measurements of Mercury taken by the Messenger probe, Nittler suggests.

Wurm is actually planning a cruder simulation – run in the real world, not on a computer. He hopes to drop a sealed capsule containing metals and dust from a 110-meter tower in Bremen, Germany, to simulate the weightlessness of space. He will zap the falling capsule with an infrared laser, and check whether the dust and metals start to separate as he predicts.

In the meantime, you could say it's oranges 1, peaches 0.