NASA's first close-up images of Mercury in more than 30 years provided scientists with the first glimpse of a side of the planet never before seen.

The Messenger space probe soared within 200 kilometres of the planet's surface on Monday, the first time a probe has visited Mercury since Mariner 10 mapped less than half the planet's surface during visits that began in March 1974.

One of the highest and longest cliffs seen on Mercury curves from the top centre down across the right side of this image.
(NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

The first images of the previously unseen side of the planet, released on Tuesday and Wednesday, were consistent with what scientists already knew; Mercury is a rocky planet pock-marked with impact craters.

But the new images provided far greater resolution than those images taken by Mariner 10 and also revealed new topographical formations and craters never before seen.

One close-up image shows one of the highest and longest cliffs curving across the planet's surface and casting a deep shadow. Messenger scientists said the cliff resulted from forces in Mercury's crust thrusting terrain from one side up and over terrain on the opposite side.

Another image shows a previously unseen crater with distinctive rays of ejected material extending from the crater's centre, looking like a can of light grey paint splashed against a dark gray surface.

The Messenger spacecraft flew by the tiny planet on Monday as part of a complicated flight pattern that will see two more flybys before it finally enters orbit around the planet in 2011.

The images and data collected from the flyby will be presented in full on Jan. 30.

Sean Solomon of the Carnegie Institution of Washington and Messenger's principal investigator said he hopes Messenger can help fill in details about a planet astronomers know very little about.

Among the unique features of Mercury are its density and the presence of a magnetosphere, features that led scientists to believe the planet is made up mostly of iron metals and may have a molten core.

The small planet's proximity to the sun, slow rotation and thin atmosphere create a huge difference in surface temperatures between the side facing the sun and the dark side, with temperatures getting as hot as 467 C and as cold as –170 C.

The Messenger team hopes the mission, which NASA said cost $446 million US, can shed light on planetary and solar system formation and better its understanding of how magnetic fields and planetary cores work.