(PhysOrg.com) -- On March 17, the tiny MESSENGER spacecraft
completed its primary mission to orbit and observe the planet
Mercury for one Earth-year. The bounty of surprises from the
mission has completely altered our understanding of the solar
system's innermost planet. As reported in one of two papers
published today on Science Express, scientists have found that
Mercury's core, already suspected to occupy a greater fraction
of the planet's interior than do the cores of Earth, Venus, or
Mars, is even larger than anticipated. The companion paper shows
that the elevation ranges on Mercury are much smaller than on
Mars or the Moon and documents evidence that there have been
large-scale changes to Mercury's topography since the earliest
phases of the planet's geological history. The mission's many
successes have allowed it to be extended for another year. "The
first year of MESSENGER orbital observations has yielded a
wonderful harvest of results," says MESSENGER Principal
Investigator Sean Solomon, of the Carn