The US space probe MESSENGER's second fly-by of the planet Mercury in October 2008 revealed the solar system's smallest planet to be far more active than previously thought, four studies said Thursday.

The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) probe's cameras took more than 1,200 images of the surface, including details of a mammoth well-preserved 692-kilometer (430-mile) impact basin that shows signs of a volcanic past.

The so-called Rembrandt basin is the first such geological feature observed on Mercury where the ground is well exposed and not covered by a thick layer of volcanic ash, like most of the planet's other features.

"This basin formed about 3.9 billion years ago, near the end of the period of heavy bombardment of the inner solar system," said Thomas Watters from the Smithsonian Institution in Washington, a lead author of one of the studies, all of which are published in the May 1 edition of the journal Science.

"This second Mercury flyby provided a number of new findings," said Sean Solomon, the probe's principal investigator from the Washington-based Carnegie Institution.

"One of the biggest surprises was how strongly the dynamics of the planet's magnetic field-solar wind interaction changed from what we saw during the first Mercury flyby in January 2008.

"The discovery of a large and unusually well preserved impact basin shows concentrated volcanic and deformational activity."

Using revolutionary image-capturing technology and a laser altimeter to survey the ground, MESSENGER revealed like never before 30 percent of the mysterious planet.

In a grand feat of engineering, the probe soared past the innermost planet's equator at an altitude of 201 kilometers (125 miles) at a speed of 23,818 kilometers per hour (14,800 miles per hour).

Combined with data from the first flyby and from Mariner 10, which made three passes in 1974 and 1975, the latest coverage means scientists have now seen about 95 percent of the planet.

The probe is on course to make its third flyby on September 29 this year.

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 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.