Secrets of Mercury’s core revealed by Messenger spacecraft

Secrets of Mercury’s core revealed by Messenger spacecraft


The Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft finished its primary mission to orbit and examine the planet Mercury for one Earth-year last Saturday, according to the Carnegie Institution for Science.

“Messenger successfully wrapped up a year-long campaign to perform the first complete reconnaissance of the geochemistry, geophysics, geologic history, atmosphere, magnetosphere, and plasma environment of the solar system’s innermost planet,” the Messenger team noted in a statement released Wednesday.

“Six plus years of cruise operations, capped by a year of nearly flawless orbital operations, with an additional year of scientific return ahead in the harsh environment at 0.3 astronomical units (27,886,766 miles) from the Sun,” said Messenger Mission Systems Engineer Eric Finnegan, of the Johns Hopkins University Applied Physics Laboratory (APL), in a statement.

Mr. Finnegan expressed his amazement at what the mission “achieved with a 1,000 kg satellite, designed, built, and launched in less than four years for a total mission cost of less than $450 million.”

In a statement released Wednesday, astronomers working on the mission revealed stunning details about Mercury’s landscape and core, the latest discovery to shake the astronomy community.

“The first year of Messenger orbital observations has revealed many surprises,” said Messenger Principal Investigator Sean Solomon, of the Carnegie Institution for Science.

“From Mercury’s extraordinarily dynamic magnetosphere and exosphere to the unexpectedly volatile-rich composition of its surface and interior, our inner planetary neighbor is now seen to be very different from what we imagined just a few years ago. The number and diversity of new findings being presented this week to the scientific community in papers and presentations provide a striking measure of how much we have learned to date,” Mr. Solomon added.

Messenger’s Mercury Laser Altimeter (MLA) was able to create the first-ever accurate topographic model of the planet’s northern hemisphere. The topographic model reveals an array of elevations that are much smaller than that of Mars or the Moon, according to Messenger Co-investigator Maria  Zuber, author of one of the studies published in Science Express.

Astronomers also discovered that Mercury’s core is massive for the planet’s size, approximately 85 percent of the planetary radius, far larger than earlier estimates. Scientists also believe that Mercury’s core is liquid to a certain degree. Scientists cite faint dynamical motions and examinations of the magnetic field that imply an active core dynamo as evidence against the theory that Mercury has a solid core.

Scientists know that Mercury is rich in sulfur at the surface and has a massive iron core — so massive, in fact, that they wondered if Mercury had once been a much larger planet whose outer layers were stripped off, perhaps by a major impact.

“Messenger’s observations of the gravity field have let us peer inside Mercury and get the first good look at its largest component — the core,” said Steven Hauck, an associate professor of planetary geodynamics at Case Western Reserve University and a coauthor of one of the studies.

The team noted that the discovery would likely force scientists to rethink how the planet was formed in the early days of the solar system.

“The structure certainly is different from that of Earth, which has a metallic, liquid outer core sitting above a solid inner core. Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deeper liquid core layer, and possibly a solid inner core,” Mr. Huack added.

Astronomers involved with the mission are excited about the second year of the spacecraft’s mission.

“The second year of orbital operations will not be a simple continuation of the primary mission,” said Messenger Project Scientist Ralph McNutt, of the APL.

“Extended mission themes will include more comprehensive measurement of the magnetosphere and exosphere during a period of more active Sun, greater focus on observations at low spacecraft altitudes, and a greater variety of targeted observations,” Mr. McNutt added.

The two studies will both appear in the March 23 issue of the journal Science.