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The Planetary Society BlogBy Emily LakdawallaGetting to the core of the matterMay. 4, 2007 | 08:26 PDT | 15:26 UTC
There's news this week that Mercury's core may be molten. How could we just be discovering this now? The fact that Earth's outer core is molten is taken for granted as scientific fact, so it tends to surprise people that we actually don't know very much about what's going on deep inside any of the other terrestrial planets, which include Mars, Venus, Mercury, and the Moon. (Yes, the Moon isn't technically a planet, but it's big enough and rocky enough and well enough studied that most scientists tend to lump the Moon in with the four inner planets. There's no other large, round, rock-and-metal moon like it in the solar system, with the possible exception of Io -- it's much more like the inner planets than it is like any of the other moons.)
The way we know what the inside of Earth is made of is through the science of seismology. Whenever there is a large earthquake or underground nuclear test, vibrational waves from the event radiate out from the source and are measured at seismographic stations around the world. Whenever those waves encounter an interface between one layer within Earth and another of different composition or state, the waves are reflected and refracted, decelerated or accelerated. By cataloging the arrival times of the earthquake waves at all the world's seismographic stations, seismologists can back-calculate the locations and sound speeds of all the layers through which the earthquake waves passed. Because of seismology, we've mapped out Earth's interior extraordinarily well. We not only know the composition, density, temperature, and state of major concentric layers within Earth's interior, we can even map local variations in these qualities, measuring, for example, topography along the boundary between the core and mantle, thousands of kilometers below our feet. Seismologists have even captured data that suggest that Earth's solid inner core, separated from the mantle by the molten outer core, rotates at a different rate from Earth's mantle and crust. The only other terrestrial planet that has ever had a seismic network is the Moon. The Apollo 12, 14, 15, and 16 missions left seismographic stations there, which recorded data from 1969 to 1977. (Apollo 11 also had a seismometer but that one only collected data for three weeks.) But the Moon isn't particularly geologically active and the seismic network wasn't very broad, so while those instruments gave us important and valuable data, it's nothing like what we have for Earth. And for Mars, Venus, and Mercury, there's nothing. So how can we find out what their interiors are made of, much less figure out whether that stuff is molten or not? The most accessible fact about the interiors of the other planets comes from their bulk density. You can readily determine the bulk density of a planet from the speed with which satellites -- real or artificial -- orbit it, or even by how the trajectory of a spacecraft or an asteroid is altered by a close passage by the planet. The bulk density is an important number. For Mercury, the bulk density -- 5.43 grams per cubic centimeter -- informs us that it has the heaviest composition of all of the planets in the solar system; it must have a higher proportion of metals like iron and nickel, relative to the lighter elements like carbon and oxygen, than Earth does. However, that one number only very poorly constrains what the interior is made of, because other factors, like the temperature of the interior and whether it is differentiated or not, strongly influence the density. Mercury must have been hot when it formed, like all the planets, so it's reasonably certain that it has differentiated, and has a more metal-rich core and a rock-rich mantle, just like Earth does; that core very likely takes up a large part of Mercury's interior, because of its high bulk density. But is the core molten or not? Scientists predicted that Mercury's core was not molten, because Mercury is much smaller than Earth, smaller even than Mars, and not much bigger than the Moon. Being so small, it has a large surface area relative to its volume, so it should have radiated away its primordial heat rather quickly, leaving it with a solid interior and a geologically dead exterior. The very heavily cratered surface revealed by the Mariner 10 flybys tends to confirm that prediction; nothing much appears to have happened on Mercury's surface for billions of years.
Enter Stan Peale from the University of California at Santa Barbara. He devised a method to use radio astronomy to probe Mercury's interior. His method hinges on the fact that Mercury is the only planet that is locked into a spin-orbit resonance: it rotates three times for every two times that it orbits the Sun. But because Mercury's orbit is elliptical, it doesn't orbit the Sun at a constant speed; it's faster when it's closer to the Sun, and slower when it's farther away. Therefore, the spin-orbit resonance doesn't work out perfectly; when Mercury is moving more slowly in its orbit, the rotation gets a bit ahead, and when it's moving faster, the rotation lags a bit behind. This motion is called "libration," You can write down a mathematical model telling you by how much Mercury should librate if it is a solid, coherent body. But if it has a molten (or even partially molten) interior layer, then the exterior of the planet -- the mantle and crust -- is physically decoupled from the interior, meaning that when the Sun's gravity tugs on the lumps and bulges in the mantle, the Sun's gravity could rotate the mantle shell of Mercury separately from the inner iron core. There's less mass in the mantle of Mercury than in the whole planet of Mercury, so the Sun's gravity can move a decoupled mantle a greater amount than it can move a solid Mercury. Therefore, if the libration of Mercury is much higher than the mathematical model of a solid Mercury predicts, you have compelling evidence for a molten interior layer. That's exactly what Peale and several coworkers, including first author Jean-Luc Margot of Cornell University, reported in today's issue of the journal Science. Through precise tracking of surface features on Mercury using three of the world's largest radio antennas (Green Bank, Goldstone, and Arecibo), they found Mercury's forced librations to be roughly 36 arcseconds, much more than the 19 to 22 arcseconds predicted for a solid Mercury. They perform various analyses to conclude that they are 95% confident that Mercury has a molten outer core. So it's back to the drawing board for the theoreticians. How can a planet as small as Mercury still have a molten outer core? One thing that helps is for the metal in the core to be doped with something that lowers its melting temperature. The usual suspect is sulfur, and you don't need a huge amount -- a few to 10 percent -- to create a model Mercury that could have a liquid outer core throughout the age of the solar system. But Mercury lies very close to the Sun, where conditions in the early solar system should have been so hot that light elements like sulfur didn't condense readily. So this conclusion hints that Mercury is made of stuff that condensed both near the Sun and somewhat farther away, where sulfur compounds could have formed. It's funny -- just last year, evidence came from the opposite end of the solar system spectrum -- the cold interior of the comet visited by Stardust -- that suggested that the cold stuff found in the outer solar system is made partially from materials that originally condensed in a much hotter location in the inner solar system. Now Mercury appears to be telling the flip side of that story, that a planet in its hot position is made partially from stuff that originally condensed in cooler regions. This paper is just adding to the general buzz about Mercury, a lately ignored planet that will soon, finally, be receiving its first orbiter. I just checked the MESSENGER calendar and was astonished to realize that it's only seven months until the first Mercury flyby! I can't wait. Did you like this post? Send it to a friend or share it at:
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