By Jonathan Amos
Science reporter, BBC News
A double impact basin may be the site of some of the most recent volcanism
Mercury is even more of an "iron planet" than scientists had previously supposed.
Richer concentrations of iron and titanium have been seen on Mercury's surface by Nasa's Messenger probe.
Previous Earth and spacecraft-based observations had detected only very low amounts of iron in the silicate minerals covering the innermost world.
Because of its immense density, scientists have already assumed much of Mercury's interior contains iron.
Messenger sees the surface iron bound up in oxides with titanium.
The mission's principal investigator, Sean Solomon, said the new observations would keep theoreticians busy.
Closest planet to the Sun; smallest in Solar System
Visited by Mariner 10 in 1970s; by Messenger currently
Diameter: 4,880km, about one-third the size of Earth
Second densest planet in Solar System; 5.3x that of water
Caloris basin is largest known feature (1,300km in diameter)
Possibility of water-ice in permanently shadowed craters
Huge iron core takes up more than 60% of the planet's mass
Surface temperatures swing between 425C and -180C
Has an extremely thin atmosphere (exosphere)
Only inner planet besides Earth with global magnetic field
"The iron is in a form that we don't normally encounter in other planetary situations and so it's going be a volley back to our geochemists and petrologists to come up with a scenario that's consistent with everything we are measuring now at Mercury," he told reporters.
Theories on how the planet formed would also have to take the information into account, he added.
Some of these propose that Mercury is predominantly the remnant core of a body which lost its outer layers in a mighty collision early in its history.
The new data was returned on Messenger's third and final flyby of the planet in September.
The pass, just 228km from the surface, was intended as a brake manoeuvre, using the planet's gravity to help slow the spacecraft enough to enable it to enter into orbit in 2011.
The spacecraft acquired only about half the data it was expected to because of a power "hiccup" just before closest approach.
Nonetheless, Messenger's cameras and instruments collected many high-resolution and colour images, unveiling another 6% of the planet's surface never before seen up-close.
Messenger has now viewed about 98% of the surface at various resolutions.
New features observed in the pass include a region with a bright area surrounding an irregular depression, suspected to be volcanic in origin.
Messenger is on course to enter into orbit in March 2011
It also spied a very young double-ring impact basin approximately 290-km across.
"However, to a planetary geologist, 'young' is a billion years or so. But compared to most of the basins on Mercury, those are three billion years older that. So in a relative sense it is very geologically young," explained Brett Denevi, a member of the probe's imaging team from Arizona State University in Tempe.
The low numbers of superposed impact craters and marked differences in colour across the basin suggest that the smooth area within the innermost ring may be the site of some of the most recent volcanism on Mercury, she added.
Messenger also made new measurements of Mercury's "atmosphere", the extremely tenuous cloud of atoms which is lifted off the surface by solar activity and micro-meteorite impacts.