A flyby to Mercury this year shows pit craters similar to ones in the East Rift Zone on the Big Island, indicating magma under them, says a University of Hawaii-Manoa researcher on the Messenger space mission team.

“;From studying these craters on Earth, we know they occur because there is magma underneath them, and it's moving and eroding some of the rock, and that leaves a void space, a hole and overlying rock will collapse from time to time,”; UH researcher Jeffrey Gillis-Davis said.

Five were identified in Messenger's January flight and he hopes data from the latest visit Monday will reveal more, he said. Gillis-Davis is studying them to learn why they are found predominantly within impact craters and what they can reveal about magma activity that occurred but never erupted onto the surface.

The two flybys to Mercury this year have produced some surprising results, particularly regarding the planet's composition, Gillis-Davis said.

“;They have pretty much filled in missing data,”; he said.

He said the second flyby, which occurred Monday, coupled with the first one Jan. 14 and a 1974 Mariner 10 visit, “;will provide nearly global cover of Mercury for the first time.”;

The UH scientist, studying Messenger data under a six-year, $580,000 grant from the National Aeronautics and Space Administration, said it's important to characterize molten rock below and above ground to understand the thermal history of a planet.

Mercury's history has largely been a mystery because it's the smallest and densest of terrestrial planets and so close to the sun it was difficult to send a spacecraft there until technology improved. Mariner mapped only about 45 percent of one side of the planet.

Gillis-Davis said Messenger's cameras in January covered about 21 percent of the planet's surface never seen by space probes.

A major finding was that volcanic eruptions produced most of Mercury's smooth plains, he said. The science team had questioned whether the plains areas were created by volcanism or an impact of ejected material from another body.

A magnetometer on the first flyby also revealed that Mercury's magnetic field appears to be actively generated in a molten iron core, he said. Being the smallest terrestrial planet, he added, “;For it to still have enough heat to have a molten iron core is a bit unexpected.”;

Flying just 125 miles above Mercury's surface on Monday, Messenger's cameras took pictures of about 30 percent of the surface never seen before, he said.

Combining all the data for a “;global perspective”; will enable scientists to better understand the distribution of the volcanic plains and the amount of contraction the planet has undergone as it has cooled off, Gillis-Davis said.

The near side of the moon has a lot more volcanism than the far side, he noted, “;so it's important to know if a similar event has occurred on Mercury where one hemisphere or one area is more vulnerable to volcanism.”;

  A third flyby is scheduled in September 2009 and in March 2011 the spacecraft will begin orbiting Mercury.