spacetoday.net: Spacecraft confirms Mercury is shrinking

Posted: Fri, Jul 4, 2008, 11:15 AM ET (1515 GMT)
Mercury craters seen by MESSENGER (JHUAPL)

Data collected by a NASA spacecraft as it flew past the innermost planet, Mercury, in January has confirmed that the planet is shrinking as it core cools, scientists reported Thursday. In a series of papers published in the journal Nature, scientists said that images of the planet performed by the MESSENGER spacecraft as it flew past the planet in January spotted features called lobate scarps, huge cliffs created in the planet's crust as it contracts. That contraction is caused because the planet's iron core is cooling; that core also drives a magnetic field whose existence was first discovered by the Mariner 10 spacecraft in the mid-1970s but confirmed and better analyzed by MESSENGER. Spacecraft observations also led scientists to conclude that the planet's plains were formed by volcanic activity early in its history, rather than as the result of giant impacts. An analysis of the planet's exosphere turned up evidence that the planet may have a source of water, either hidden in shadowed craters near the poles or from comet and asteroid impacts. MESSENGER will fly past the planet again in October and in 2009 before entering orbit around the planet in 2011.