The mystery of Mercury’s core

The MESSENGER spacecraft recently completed its study of the planet Mercury which it orbited for one Earth-year.

According to MESSENGER Principal Investigator Sean Solomon of the Carnegie Institution, the spacecraft’s mission has “completely altered” current theories about the solar system’s innermost planet. 

Indeed, scientists have discovered that Mercury’s core – already suspected of occupying a greater fraction of the planet’s interior than Earth, Venus, or Mars – is even larger than anticipated.

In addition, elevation ranges on Mercury are much smaller than on Mars or the Moon, with evidence pointing to large-scale topographical changes since the earliest phases of the planet’s geological history.

Both discoveries were facilitated by MESSENGER’s radio tracking, which allowed scientists to develop the first precise model of Mercury’s gravity field which. When combined with topographic data and the planet’s spin state, the model helps shed light on the planet’s internal structure, the thickness of its crust, the size and state of its core, along with its tectonic and thermal history.

As noted above, Mercury’s core occupies a large fraction of the planet, approximately 85% of the planetary radius, even larger than previous estimates. Because of the planet’s small size, at one time many scientists hypothesized the interior cooled to the point that the core would be solid. 

However, subtle dynamical motions measured from Earth-based radar, combined with MESSENGER’s newly measured parameters of the gravity field and the characteristics of Mercury’s internal magnetic field that signify an active core dynamo, indicate that the planet’s core is at least partially liquid.

Essentially, Mercury’s core is different from any other planetary core in the Solar System. For example, Earth has a metallic, liquid outer core sitting above a solid inner core – while Mercury appears to boast 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.

MESSENGER also managed to generate the first-ever precise topographic model of the planet’s northern hemisphere by characterizing slopes and surface roughness over a range of spatial scales. From an eccentric, near-polar orbit, the MESSENGER’s Mercury Laser Altimeter (MLA) illuminates surface areas as wide as 15 to 100 meters (50 -325 feet), spaced about 400 meters apart (1,300 feet).

Interestingly enough, the spread in elevations is significantly smaller than those of Mars or the Moon. The most prominent feature is an extensive area of lowlands at high northern latitudes that hosts the volcanic northern plains. Within this lowland region is a broad topographic rise that formed after the volcanic plains were emplaced.

At mid-latitudes, the interior plains of the Caloris impact basin – 1,550 kilometers (960 miles) in diameter – have been modified so that part of the basin floor now stands higher than the rim. The elevated portion appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. These features imply that large-scale changes to Mercury’s topography occurred after the era of impact basin formation and large-scale emplacement of volcanic plains had ended.