A TALE OF TWO SPACECRAFT: THE EXPLORATION OF MERCURY

Mercury is the smallest of the inner planets, but its uncompressed bulk density and implied core fraction are the highest. Mercury’s orbit is the most eccentric of the planets, and it is the only known solar system object in a 3:2 spin:orbit resonance. For more than three decades, much of what we knew about Mercury came from the three encounters by Mariner 10 in 1974–75. Mariner 10 imaged 45% of Mercury’s surface at about 1 km/pixel average resolution, discovered Mercury’s internal magnetic field, documented that H and He are present in the planet’s tenuous exosphere, and made the first exploration of Mercury’s magnetosphere and solar wind environment. Ground-based astronomers thereafter reported Na, K, and Ca in Mercury’s exosphere, the presence of deposits in the floors of polar craters having radar characteristics best matched by water ice, and strong evidence from the planet’s forced libration amplitude that Mercury has a fluid outer core. Spacecraft exploration of Mercury resumed, nearly 33 years after the last Mariner 10 flyby, with the MESSENGER flybys of 2008-2009. The MESSENGER spacecraft, successfully inserted into orbit about Mercury in March of this year, has been collecting orbital observations nearly continuously for more than the six-month duration of a Mercury solar day. MESSENGER’s first chemical remote sensing measurements of Mercury’s surface indicate that the planet’s bulk silicate fraction differs from those of the other inner planets and is richer in volatile constituents than predicted by most planetary formation models. Global image mosaics and targeted high-resolution images (to resolutions of 10 m/pixel) reveal that Mercury experienced globally extensive volcanism, including widespread examples of pyroclastic deposits. The tectonic history of Mercury, although dominated by near-global contractional deformation as first seen by Mariner 10, is much more complex than first appreciated, with numerous examples of extensional deformation tied to impact crater and basin modification. Mercury’s magnetic field is dominantly dipolar, but the field is axially symmetric and equatorially asymmetric, a geometry that poses challenges to dynamo models for field generation. Mercury’s unusually dynamic solar wind interaction serves both to replenish the exosphere and space weather the surface.