Mercury is the smallest of the planets, and despite being known from antiquity, due to its proximity to the Sun, it has proven hard to study. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is designed to fill in many of the gaps in our knowledge of our small planetary cousin. Two new papers from the MESSENGER team respectively highlight the surface and internal structure of Mercury, from detailed observations of the northern hemisphere.

By mapping the fluctuations in Mercury's gravitational field, the researchers measured the variations in mass, indicating how the planet's interior structure differs from place to place. In addition, the spacecraft probed the forces shaping the planet's surface using laser altitude measurements as it flew over the surface. These results show Mercury to be a recently active world with an interior markedly different from the other terrestrial planets—Venus, Earth, and Mars—as well as from Earth's Moon. Specifically, the team found evidence for a solid shell just outside the planet's core, a feature found on no other world.

The surface of a formerly active planet

While it is impossible to observe the long history of Mercury or probe its interior directly, many features allow scientists to study its structure and evolution. The topography (surface features) of a planet is shaped by internal forces and weathering—which in the case of Mercury is primarily from meteor impacts, since it lacks an atmosphere. Tectonic activity, any volcanism, and cracks in the surface as the crust changes size over time all provide clues about a planet's detailed history.

Mercury has a lot less variation in its topography than Mars or the Moon: its mountains are relatively lower, its basins and ravines are proportionally shallower. However, that doesn't mean Mercury's surface is smooth or uninteresting. Using laser ranging, MESSENGER found a region of uplift, similar to plateaus on Earth, where part of the Caloris basin was pushed up by tectonic forces above the elevation of its rim. Additionally, some lowlands show signs of flooding by very high-temperature lava during the planet's early history, which erased some of craters.

While none of these features appear to be consistent with recent volcanic or tectonic activity, it's not all confined to antiquity. From what we know of the history of our Solar System, meteor bombardment was much more frequent during early times, so the relative number of craters indicates the age of a particular surface feature. (On Earth, many craters are erased by weather and water, but on Mercury and the Moon, a crater can stick around for billions of years untouched.) The Caloris uplift and at least some of the lava flows postdated the end of the heavy bombardment period, showing that Mercury continued to evolve later than was previously thought.

A strangely layered interior

Real planets are not precisely spherical, and the terrestrial planets in particular have complex interior structure. Variations in density and composition inside change how Mercury rotates and give rise to fluctuations in its gravitational field, which can be measured by MESSENGER. Regions of higher mass concentration are more attractive, making the probe speed up slightly. (This technique is also used by GRACE and the GRAIL spacecraft currently in orbit around the Moon.) In this way, MESSENGER made a map of the thickness of Mercury's crust over the northern hemisphere.

Additionally, the research team determined how deeper structures—the core and mantle—affect the planet's rotation. Just as a solid sphere rotates differently than a hollow spherical shell, different layers in a terrestrial planet's interior determine how the planet rotates. Mercury has a solid crust floating on top of a plastic silicate mantle (the source of tectonic activity on Earth), and a core that may resemble Earth's in having a molten outer layer surrounding a solid inner core. (The inner core is still conjectural for Mercury, based on the available data.)

The MESSENGER team also found a surprise: there appears to be another solid layer outside the molten core, probably composed of iron and silicon. This type of internal shell doesn't appear to be present in any other planet in our Solar System (and of course data isn't good enough for exoplanets yet to say either way).

An altogether strange world

Ultimately the interior and exterior of Mercury are not independent: the inner processes shape what we see on the surface. The new data from MESSENGER shows Mercury to be a strange world, similar to the other terrestrial planets in some respects, yet having features not present on any of the others. As MESSENGER is the first probe to study Mercury in this much detail (Mariner 10 only mapped about 40 percent of the planet's surface and never went into orbit), astronomers are learning more about its history and structure than was previously possible.

That Mercury was tectonically and volcanically active into relatively late periods of time reveals how it evolved over its nearly 5 billion year history. The possible presence of the mysterious solid shell outside the core also is a hint about the planet's history. Further data as MESSENGER continues to orbit should help answer some of the questions raised by these findings.

Sciencexpress, 2012. DOIs: 10.1126/science.1218809, 10.1126/science.1218805 (About DOIs).