As MESSENGER zoomed toward Mercury for its third flyby, it was commanded to rotate in a maneuver that would help it test a surprising result from the second flyby. Just after that rotation maneuver began, there was a "hiccup," in the words of principal investigator Sean Solomon; the spacecraft computer detected something unexpected in the power system, and halted the acquisition of science data. But in the minutes before the shutdown of science systems, MESSENGER gathered just enough data to prove that Mercury's surface is unexpectedly rich in iron and titanium, a finding that will force planetary geologists to rewrite their stories about the formation of the innermost planet.
The data haul from the flyby, though only half the amount that had been planned, included the images necessary to fill the largest remaining gap in MESSENGER's global map of the planet. Those images included the "most spectacular" volcanic vent feature yet seen on the planet, as well as evidence for geologic activity on Mercury as recently as a billion years ago. "After Mariner 10, it was thought that internal volcanic activity on Mercury ended earlier than on any other planet," MESSENGER imaging team member Brett Denevi said. "Now we're realizing that's not the case."
Not Iron Poor After All
Mercury has long presented a conundrum to planetary scientists, MESSENGER participating scientist David Lawrence explained in a press briefing Tuesday. Mercury is known to have the largest metallic core, relative to its diameter, of any of the rocky planets. But, Lawrence said, "many space and Earth based measurements have shown Mercury's surface has a low concentration of iron within silicate minerals. Since silicate minerals typically dominate the bulk composition of planetary bodies, this observation has led to a commonly-held view that Mercury's surface has generally low abundances of iron as well as titanium. As a consequence, it has been a puzzle for studies of Mercury to explain how a planet can have such a large, iron-rich core but have a surface with such a very low iron content."
The dearth of iron-bearing silicate minerals on Mercury is documented using infrared spectroscopy. Before MESSENGER's flybys, there had been no direct test of the elemental composition (as opposed to the mineral composition) of Mercury's surface. As MESSENGER flew past Mercury, it pointed its neutron spectrometer at the surface, providing the first direct test of the exact abundance of iron and titanium in Mercury's rocks.
Lawrence explained how the measurement works. "Traveling throughout the galaxy are particles called galactic cosmic rays, which are very high-velocity protons. All objects in the solar system are constantly bombarded by these cosmic rays. When the cosmic rays hit a planetary surface, the atoms in that surface are blasted apart, producing high-energy neutrons that bounce around [within] the surface and slow down, and then escape the surface. Neutrons that have significantly slowed down from their initial high energy we call 'thermal neutrons.' It turns out that iron and titanium, these elements that we're interested in measuring, are very efficient absorbers of thermal neutrons. As a consequence, we can measure iron and titanium on Mercury's surface by measuring these thermal neutrons, such that if you [detect] a large number of thermal neutrons, this would indicate a low iron and titanium content. Conversely, a low number of thermal neutrons would indicate a high iron and titanium content." The neutron spectrometer data from the first and third Mercury flybys are shown below.
The low amount of neutrons is a perfect fit to models containing high iron and titanium content, similar to that found in the dark basaltic rock of the lunar maria. "It's significantly higher than previously appreciated. That's a pretty exciting result for us," Lawrence said. But it's also "perplexing," because the iron is known not to be in the usual location, silicate minerals. "The iron is probably wrapped up in some other kind of mineral, like titanium oxide." Although the results from MESSENGER's neutron spectrometer mean that planetary scientists no longer have to evoke convoluted ad-hoc explanations for how Mercury could have an enormous iron core while lacking iron in its crust, the puzzle has just been replaced with a different one, as Solomon explained. "The neutron spectrometer results have confirmed that the outer silicate shell of Mercury had more iron than most of us appreciated, because that iron does not appear in silicates. So we have an even greater challenge because the iron is in a form that we don't normally encounter in other planetary situations. It's going to be a volley back to our geochemists and our petrologists to come up with a scenario that's going to be consistent with our observations."
Mercury's Story Is Longer than We Thought
Those observations also include evidence that Mercury has been a volcanically active place for much longer than previously imagined. This much had been learned during the previous two flybys, but the terrain newly imaged during the third flyby held two previously unseen outstanding examples of that volcanic activity.
The first is a feature that had been previously seen as a bright splash on Mercury's surface, but only at relatively low resolution. The high-resolution photos snapped by MESSENGER on approach to this flyby make it clear that the splash is a halo of material, probably volcanic cinders, tossed out of an enormous, steep-sided hole in the ground that is almost certainly a volcanic vent, Denevi said: "It doesn't have a raised rim, it is very steep, and it has this odd shape, all of which are characteristics of a volcanic vent. It's one of the best examples on the planet." At roughly 30 kilometers across, it is one of the largest such features yet spotted by MESSENGER.
The other is a peak-ring basin, an impact crater large enough to have formed a central ring complex in addition to its outer rim. (These basins are more common on Mercury than on the Moon because of Mercury's higher gravity.) Seen for the first time by MESSENGER, the 290-kilometer-diameter basin has no name, but resembles another one discovered during MESSENGER's first flyby and now named Raditladi. Raditladi and this basin are unusual for having extensional faults within their inner basins; most of the faults visible on Mercury formed not from extension but from contraction, as the entire planet contracted while it cooled. But what is even more unusual about this basin is the dearth of impact craters on the smooth inner basin floor, which formed some time after the impact basin. Its relatively unmarked surface indicates that it has not had much time exposed to space to accumulate impact craters, meaning that it must be geologically young, perhaps about a billion years old. Since Mercury has long been thought to have been inactive since 3.8 billion years ago, such a relatively youthful age for a volcanic feature is surprising.
MESSENGER will not enter Mercury orbit until March 2011. Solomon noted that just an hour before Tuesday's press briefing, the countdown clock to orbit insertion ticked past the 500-day mark. But its three flybys have provided plenty of data for scientists to chew on until then. "Studying the planet is like reading a mystery novel by Dorothy Sayers or Agatha Christie," Solomon concluded. "It's like we've read the first four chapters, where Mariner 10 was the first chapter, and each of our flybys was another chapter. We've gained important clues from each of those flybys, but we still have a long way to go to understand the full plot."