Less than one month after MESSENGER conducted its second flyby past our Solar System’s innermost planet, NASA revealed the first details on the probe’s journey above Mercury’s surface. In addition to conducting a series of scientific experiments, the spacecraft also returned a large number of images, covering 30 percent of a never-before-seen part of Mercury.

On October 6, at 4:40 a.m. EDT, MESSENGER successfully completed its second flyby past Mercury, and continued its journey in space; in March 2011, the spacecraft is scheduled to position itself in Mercury’s orbit. In September 2009, MESSENGER will execute a third and last Mercury flyby before becoming the first Mercury orbital mission in the history of space exploration.

The images captured earlier this month revealed a new region the size of South America, and left investigators with only 5-10% of uncharted territory left to explore. Out of what they got so far, scientists described Mercury as very different from Moon and Mars, in the sense that it lacks hemispheric-scale geologic differences.

“On the Moon, dark volcanic plains are concentrated on the near side and are nearly absent from the far side,” explained MESSENGER co-investigator Mark Robinson of Arizona State University. “On Mars, the southern hemisphere consists of older, cratered highlands, whereas the northern hemisphere consists of younger lowlands. Mercury’s surface is more homogeneously ancient and heavily cratered, with large extents of younger volcanic plains lying within and between giant impact basins.”

The images returned revealed traces of volcanism covering a large part of the planet’s western hemisphere, which indicate that the volcanic activity here may have been even more intense than on the Moon. Maria Zuber of the Massachusetts Institute of Technology suggested that although the planet’s volcanism is likely to have occurred in its early history, juts like on other small celestial bodies such as the Moon, it may have continued for a longer period of time here.

The images captured by MESSENGER drew attention on the alternation of empty and filled craters at the surface of the planet, but one particular crater in the western hemisphere intrigued scientists. As Maria Zuber explained, the solidified lava inside the crater was enough to fill the Baltimore – Washington D.C. metropolitan area in a layer 12 times the height of the Washington Monument.

“That’s an awful lot of volcanic material in one place for such a little planet,” Zuber said in a press conference held on Wednesday. This discovery points to intense activities inside the planet that eventually melted and produced such an impressive amount of lava.

On MESSENGER’s first flyby earlier this year, the scientists gathered data which suggested that Mercury’s smooth plains mapped by Mariner 10 more than three decades ago were shaped by volcanic activity.

In 1972, Apollo 16’s mission on the Moon gave birth to a series of hypothesis on the origins of Mercury’s plains. By comparison, some scientists suggested that the formations on Mercury were the result of the material ejected by large impacts, which later formed smooth plains. Others believed that volcanic activity was responsible for shaping the surface of the Planet; however, Mariner 10’s images failed to provide any information supporting this last theory, and Mercury continued to be an enigma for scientists.

“When we had information on Mercury only from Mariner 10, there was a fair amount of ambiguity about whether or not volcanism was even an important process on the planet,” Zuber said. However now, “we have a better understanding that volcanism is quite an important process, and we’ve even beginning to quantify that in an important way.”