Mercury's crust contains far more sulphur than the crust of the Earth or the moon, and its magnetic field is lopsided toward the north, a NASA spacecraft has discovered.

"What we are finding is that in many cases, a lot of the original ideas about Mercury are just plain wrong," said Larry Nittler, one of four scientists affiliated with the Messenger who spoke at a NASA news conference Thursday.

The researchers presented some of the new images and scientific results gathered by Messenger since it became the first spacecraft to enter orbit around the closest planet to the sun on March 17.

Mercury is the only rocky planet in our solar system besides Earth that has a magnetic field, and one of Messenger's surprising findings so far is that Mercury's magnetic field is not a miniature version of Earth's, said Sean Solomon, principal investigator for the mission at the Carnegie Institution of Washington.

"The magnetic field in the northern hemisphere is stronger and different from that in the southern hemisphere," he added.

Mercury's magnetic equator is north of the planet's geographic equator by roughly a fifth of the planet's radius. That means its south pole is far more exposed to charged particles than its north pole, and may help explain the presence of the planet's exosphere — a tail of elements such as sodium that are kicked off the surface by charged particles from space.

Messenger took this high-resolution image, colour of Mercury's Degas crater, shown beside an image from Mariner 10 (left) for comparison. Rock that melted from the heat of the impact, then cooled and shrank, formed the cracks observed in the crater, which is 52 kilometres across. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Messenger has also found that Mercury's crust contains less aluminum and more silicon than the Earth, and 10 times as much sulphur than either the Earth or the moon.

"Mercury most likely formed from building blocks that were fundamentally chemically different from those that formed the Earth and moon originally," Nittler said.

He added that Mercury's high sulphur content could help illuminate the nature of volcanic activity on Mercury, since explosive volcanoes are closely linked to sulphur-containing gases on Earth.

Mercury ice hypothesis tested

One of Messenger's goals is to test a hypothesis proposed 20 years ago about why there appears to be water ice on a planet so close to the sun. Scientists first proposed 20 years ago that bright spots on Mercury's poles, seen by radar telescopes on Earth, might be water ice trapped in craters that kept them permanently in shadow, beyond the sun's reach.

Solomon reported that Messenger has now mapped the topography of much of Mercury's surface. The mapping shows that the parts of some craters in permanent shadow coincide with the location of the deposits that are thought to be water ice.

"The first scientific test of that hypothesis using Messenger data from orbit has passed with flying colours," Solomon said.

Move the slider to switch between a radar image of bright spots believed to be water ice on Mercury's surface and a new image taken by the Messenger spacecraft, showing that the bright spots coincide with craters in permanent shadow. National Astronomy and Ionosphere Center, Arecibo Observatory and NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington