A MESSENGER To Mercury Reveals A Strange Little Planet

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.

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