Mercury, the tiny rocky planet closest to the sun, may have a lot common with Earth, but a spacecraft sent to the least-explored planet in the system is providing surprising data that has revolutionized the way scientists think about the innermost planet.

New close-up images and data provided by NASA's Messenger -- MErcury Surface, Space ENvironment, GEochemistry and Ranging -- spacecraft have revealed an unforeseen class of landforms on the planet closest to the sun.

The Messenger spacecraft, the first to achieve orbit around Mercury, has found the planet has a lot of unfathomable lava flows that envelop its northern polar region with no other Earth-type volcanoes in view, the U.S. space agency says.

Mercury's exterior portion have slopes just like the hills and valleys on Earth, but those of the smallest planet in the solar system are described as hollows to distinguish them from craters.

While the planet is described as having common features like the Earth's surface, close-up images taken by the NASA space vessel prove otherwise.

We have viewed the polar regions clearly for the first time, we have built up global coverage with our images and other data sets, we are mapping the elemental composition of Mercury's surface, we are conducting a continuous inventory of the planet's neutral and ionized exosphere, and we are sorting out the geometry of Mercury's magnetic field and magnetosphere, said Messenger's principal investigator, Sean Solomon of the Carnegie Institution of Washington.

For decades, scientists had puzzled over whether Mercury had volcanic deposits on its surface. Messenger's three flybys answered that question in the affirmative, but the global distribution of volcanic materials was not well understood.

The new findings confirm that the planet's North Polar Region is surrounded with huge expanse of volcanic plains, which cover more than 6 per cent of the planet's total surface.

According to the reports, the deposits are typical of flood lavas and huge volumes of solidified molten rock similar to those found in the northwestern United States.

The findings from Messenger were published in a special series of articles in the Journal of Science.

The volcanic deposits are thick. Analysis of the size of buried 'ghost' craters in these deposits shows that the lavas are locally as thick as two kilometers (or 1.2 miles), said James Head of Brown University, the lead author of one of the Science reports. If you imagine standing at the base of the Washington Monument, the top of the lavas would be something like 12 Washington Monuments above you.

According to Head, the deposits appear typical of flood lavas, huge volumes of solidified molten rock similar to those found in the Columbia River Basalt Group, which at one point covered 150,000 square kilometers (58,000 square miles) in the northwestern United States.

Those on Mercury appear to have poured out from long, linear vents and covered the surrounding areas, flooding them to great depths and burying their source vents, Head said.

The research also uncovered vents or openings measuring up to 16 miles (25 kilometers) spread across what appear to be the source of large volumes of very hot lava, which in turn were responsible for carving valleys and creating teardrop-shaped ridges in the underlying terrain.

These amazing landforms and deposits may be related to the types of unusual compositions, similar to terrestrial rocks called komatiites, being seen by other instruments and reported in this same issue of Science, Head stated. What's more, such lavas may have been typical of an early period in Earth's history, one for which only spotty evidence remains today.

Earlier images of Mercury's surface, collected by the Mariner 10 and Messenger flybys, showed very bright blue-colored floors and central mountain peaks of some impact craters. But because higher-resolution images were lacking, these bright crater deposits remained unexplored.

The new closeup images of these craters, provided by Messenger's orbital mission, have revealed that bright areas are composed of small, shallow, irregularly shaped depressions that are often found in clusters. In order to distinguish these features from other types of pits seen on Mercury, scientists have used the term hollows, said David Blewett, a staff scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and lead author of one of the Science reports, said.

Analysis of the images and estimates of the rate at which the hollows may be growing led to the conclusion that they could be actively forming today, Blewett said.

As Messenger continues to orbit Mercury, the imaging team is building up a global catalog of these volcanic deposits and is working with other instrument teams to construct a history of the planet's volcanism.