Editorial: Weird Mercury

Published in The Hindu on October 12, 2011

Our basic understanding of the fundamental aspects of Mercury stands challenged just six months after the Messenger spacecraft settled into orbit around the innermost planet. If the three fly-bys of Mariner 10 in the 1970s and Messenger indicated the strange nature of the planet, the first major release of results from NASA’s spacecraft has confirmed its weirdness. The results published recently in Science are prompting researchers to reconsider some of the fundamental ideas about the nature and history of the planet. Mercury and earth are the only two terrestrial planets that have global magnetic fields generated internally. But the magnetic field of Mercury has some strange characteristics, that is, in comparison with the earth. The magnetic equator is displaced northward of the geographical equator by nearly 500 km. The net result of this displacement is that the surface field at the North Pole is larger than at the South Pole by a factor of nearly 3.5. But the surprising find is that unlike the earth, Mercury shows “no evidence” of electron radiation belts (Van Allen belts) surrounding it. Also, an acceleration phenomenon that produces energetic electrons fails to produce energetic protons. This phenomenon is unique to Mercury, and may be due to the “weakness of the planetary field.”

The next strange feature confirmed is the presence of northern high-latitude smooth plains, produced by the globally extensive flood volcanism that occurred once the heavy bombardment ceased. Flowing out of the cracks some 3.8 billion years ago, the low-viscosity lava with high effusion rates apparently covers six per cent of the total surface area of the planet. Data suggest that Mercury’s mantle must have undergone extensive partial melting at the fag end of heavy bombardment to produce large-scale flood volcanism. Internal heat production may be declining post-planet formation as there has been only isolated volcanic activity after the massive lava flow. This may also explain why naturally occurring radioactive isotopes of potassium, thorium, and uranium are found in abundance on the surface. In fact, contrary to models explaining Mercury’s formation, the evidence collected strongly suggests that the planet was not subjected to extreme heating. For instance, sulphur and potassium, which are moderately volatile, are found in abundance. Similarly, there is plenty of potassium relative to thorium and uranium. The overall composition makes the planet comparable to chondritic meteorites. These baffling observations invest the Messenger mission with great significance, and the remaining six months of its mission life will be keenly followed by scientists round the world.