Nearly all Mercury's surface will be imaged in stereo to determine the planet's global topography and landforms.

08 July 2008

(After flybys in 2008 and 2009, orbiter will circle planet for a year in 2011)

Washington -- The planet closest to the sun has an active magnetic field, plains formed by volcanoes and evidence of water ice in the protected shadows of some of its craters, according to scientists who have analyzed data from the most recent flyby of Mercury.

The brief January 2008 visit by NASA’s Mercury Surface, Space Environment, Geochemistry and Ranging (Messenger) spacecraft was the first since 1975, when NASA’s Mariner 10 completed three flybys of the planet to measure its environment, atmosphere and surface.

Two more flybys are scheduled for October 2008 and September 2009. Then Messenger will orbit Mercury for a year beginning in 2011.

“Our flyby was short by planetary exploration standards, but it returned a rich load of data,” Messenger principal investigator Sean Solomon of the Carnegie Institution-Washington said at a July 3 briefing.

“We had 55 hours to do imaging prior to and after closest approach,” he added. “We were inside the magnetosphere [the area around a planet dominated by a magnetic field] no more than 30 minutes and less than 10 minutes within close range. After five months of analysis we’ve got some fascinating new results and some of those have resolved debates that go back to the time of Mariner 10.”

ANSWERED QUESTIONS

One debate involved Mercury’s magnetic field, discovered by Mariner 10. Until 1974, when Mariner began its Mercury flybys, Earth was the only other terrestrial planet known to have a global magnetic field. (The terrestrial planets are those of the inner solar system: Mercury, Venus, Earth and Mars.)

“The debate set up by that discovery is whether Mercury’s magnetic field is produced like on Earth by a dynamo in the fluid outer core of the planet, or is a frozen field like that of the planet Mars,” Solomon said.

Results from Messenger pointed to an active source for Mercury’s magnetic field -- a dynamo stirred by motions in Mercury’s fluid outer core. The results also verified a prediction that arose from the analysis of Mariner 10 images.

“If on Mercury the dynamo is similarly stirred by inner core growth as a consequence of core cooling,” Solomon said, “a prediction of that inner core growth -- because Mercury’s core is at least 60 percent of the planet’s mass and at least 75 percent of the planet’s radius -- is that the planet should shrink. It should contract because solid iron is denser than molten iron.”

Messenger verified the prediction through images of huge cliff-like wrinkles on the planet’s surface called scarps that formed as the planet shrunk.

Scientists also took their first look at the chemical composition of the planet's surface, and Messenger probed the composition of Mercury's thin atmosphere, sampled charged particles called ions near the planet, and showed new links between Mariner 10 observations and materials on Mercury's surface.

Radar observations of Mercury's North Pole, a region not mapped by Mariner 10, show evidence of water ice in the shadows of some craters.

“A unique characteristic of Mercury is that the surface geology preserves the history of the cooling of Mercury’s core and the growth of the solid inner core, the power source for the planet’s magnetic field,” Solomon said. “We have no other example in the solar system where that record is clearly preserved in the geology.”

MISSING CHAPTERS

Some of Messenger’s images showed evidence of volcanic vents along the margins of Caloris basin, one of the solar system’s biggest and youngest impact basins. The science team reconstructed the basin’s geologic history by combining Mariner 10 and Messenger data.

The basin formed from an impact by an asteroid or comet during a period of heavy bombardment in the first billion years of solar system history, said James Head, professor of geological sciences at Brown University in Rhode Island. A period of volcanic activity followed, producing lava flows that filled the basin’s interior.

Finding volcanic vents around Caloris confirmed for planetary scientists that the smooth plains on Mercury were caused by erupting lava.

“Messenger has given us a brand new view of volcanism on Mercury,” Head said. “It’s provided insight into the formation of surface plains and given new life to what many thought might be a rather dead planet.”

Learning more about Mercury will help scientists better understand the rest of the inner solar system’s planets, satellites and small bodies.

“On Earth, because of plate tectonics and erosion, those first chapters of Earth history are missing,” Head said. “That’s why we study the moon, Mercury and Mars. Mercury is filling a major gap in our understanding because here we have a planetary body that has a huge iron core, and it’s Earth-like in that sense, but on the other hand it seems to have a surface that’s more like the moon and the southern highlands of Mars, for example. What makes it different?”

“Every planet is a laboratory for studying planetary-scale processes,” Solomon said. “We know that the physics and to a large extent the chemistry of what governs these processes is shared among the planets.

“To say that we understand those processes demands that we test the generality of our model on planets where those conditions are quite different,” he said. “Mercury allows us to generalize that understanding. It’s just one of our siblings in this small family of inner planets that have a common parentage.”

More information about Messenger’s Mercury mission is available at the NASA Web site.