The US space probe MESSENGER's third and final fly-by of the planet Mercury in September revealed an almost complete view of the solar system's smallest planet, leaving only the polar regions to be surveyed, NASA said.

Flying at a low altitude, the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) probe's cameras have now mapped some 98 percent of Mercury's surface, and will complete the job after settling into permanent orbit in 2011.

Although the region viewed in September for the first time by spacecraft "was less than 350 miles (563 kilometers) across at the equator, the new images reminded us that Mercury continues to hold surprises," said Sean Solomon, principal investigator from the Washington-based Carnegie Institution.

Using revolutionary image-capturing technology and a laser altimeter to survey the ground, MESSENGER revealed at close-range regions of the mysterious planet like never before.

Among the details collected during the latest fly-by, the probe captured images of large double-ringed impact basin about 180 miles (290 km) across, NASA said this week.

Brett Denevi, a member of the probe's imaging team at Arizona State University, said the find could signify the youngest example of volcanic activity ever found on the planet.

The basin is about one billion years old -- where most similar features are about four times older -- and its inner floor appears to be even younger, said Denevi.

The third survey also revealed an abundance of iron and titanium on the planet's surface, a surprise for the mission because the two previous fly-bys, earlier this year and in late 2008, observed a low concentration of such materials.

"Now we know Mercury's surface has an average iron and titanium abundance that is higher than most of us expected, similar to some lunar mare basalts," said David Lawrence, another member of the MESSENGER research team.

In a grand feat of engineering, the probe has completed almost three-quarters of its 4.9-billion-mile (7.8-billion-kilometer) journey to enter orbit around Mercury.

NASA scientists are combining data from the first two fly-bys and from Mariner 10, which made three passes in 1974 and 1975.

Mercury is the closest of all the planets to the sun, and because of the high-risks of its proximity -- the sun's enormous gravitational pull, and massively high levels of radiation -- it is one of the most mysterious bodies in the solar system, even though it is relatively close to Earth.

The January 2008 visit showed scientists that enormous volcanic eruptions produced many of Mercury's expansive plains, littered with meteor craters, and that its magnetic field appears to be actively generated in a molten iron core.