It's been nearly six months since the Messenger spacecraft began orbiting Mercury. In that time the innermost planet has circled the Sun twice and twirled around three times — long enough for NASA's robotic emissary to get a really good look at the entire globe at close range.
Not surprisingly, then, a tidal wave of new results have come from the eight instruments aboard, and much of Science magazine's September 30th issue showcases all that's new with the innermost planet. I can't do justice to each investigation here, so I hope the science team will forgive me for focusing on the compositional findings — from which a remarkable story is emerging.
You'll recall that, as of mid-June, it was already becoming clear that Mercury's surface is unlike that of its planetary siblings near the Sun. Its exterior bears only slight geochemical resemblance to the outer layer of Mars and to the seafloor crust on Earth and none at all to the Moon's low-density, metal-poor crust. Instead, the combination of a high magnesium-to-silicon ratio and an abundance of the "volatile" rock-forming elements sodium, potassium, and sulfur make Mercury unique among the terrestrial worlds.
Of special interest to geochemists are the abundances of naturally radioactive isotopes in the crust — specifically potassium (K), thorium (Th), and uranium (U) — which give off gamma rays that the spacecraft detects as it swoops overhead. According to Patrick Peplowski (Applied Physics Laboratory), who heads the team analyzing results from Messenger's gamma-ray spectrometer, the ratio of potassium to thorium in Mercury's surface rocks is some 15 times that of the Moon. Likewise, the X-ray spectrometer has found at least 10 times more sulfur than occurs in the silicate rocks of either the Earth or Moon. Yet, what's remarkable for a planet whose metallic core takes up more than three-fourths of its diameter and half its volume, the surface rock contain very little iron.
Ever since Mercury was first visited by a spacecraft, back in the 1970s, scientists have wondered how this cannonball-hearted planet came to be. Finally, after nearly four decades of collective head-scratching, Messenger's compositional results are starting to reveal what really happened — or at least what didn't happen — when Mercury came together 4½ billion years ago.
"The exciting thing about our observation of volatiles on the surface of Mercury is that it rules out most theories for the planet's formation," Peplowski explained during a press briefing on Thursday.
For example, in order to end up with such a plus-size core, some researchers had speculated either that the planet's outer layers were vaporized by an early bout of intense sunlight or that its crust ended up in smithereens after one or more giant impacts. But either of these scenarios would have left young Mercury boiling hot, driving away virtually all of the easy-to-vaporize elements that Messenger now finds so abundantly.
Instead, cosmic chemists are now thinking that Mercury didn't suffer some bizarre accident in its infancy but instead formed from planetary building blocks rich in volatile elements and metals like iron and magnesium. It all seems to fit (particularly highly diagnostic K/Th and Th/U ratios) if the planet had accumulated from a little-known type of meteorite known as enstatite chondrites. Gather enough of these in one place, then heat them until all the iron sinks to form a core, and you'd be left with a surface layer much like what the spacecraft is finding.
This story is holding up for now, but we've yet to hear from one of key Messenger team: the dynamicists who are using spacecraft tracking data to deduce how mass is distributed inside the planet. I hope those results emerge soon — but in the meantime I'll sit back and enjoy all the wild and wonderful views that Messenger beaming back to Earth daily.