The January flyby of MESSENGER spacecraft by Mercury’s surface revealed enough aspects in the past and present of our Solar System’s innermost planet to last us for decades, if we look back at the unsettled theories that Mariner 10 created 33 years ago.

There are several similarities between our planet and Mercury, as there are differences, but the aspect that connects them the most is the core that powers the planet’s magnetic field, which has been a reason for unending discussions among scientists.

While some believed it to have cooled a long time ago, Mariner and MESSENGER’s flybys discovered that Mercury is surrounded by a magnetic field. This led to the contradictory discussions on whether this field is a relic or is actively generated today.

Mercury has the largest core compared with any other planet in our Solar System, strangely enough, considering it is a very small planet. Furthermore, its magnetic field is considered to be a miniature version of Earth’s field, although scientists need more measurements to establish what the nature of the field is.

Another aspect that makes Mercury so unique and puzzling is the ionization in the “atmosphere” or exosphere. The planet is believed to be too small for gravity to support any atmosphere, and so the exosphere (which is normally the uppermost layer of the atmosphere) is the place where all the interactions between the planet’s surface, magnetosphere and solar winds take place.

Mercury’s surface is strikingly similar to that of the Moon, reflecting 6 percent of the light it receives from the Sun. The surface is covered by a thin layer of minerals, and consists mostly of flat plains, steep cliffs and deep craters, much like those on the Moon.

Volcanic activity seems to have played an important role in Mercury’s past. Scientists found evidence of volcanic vents on the margins of the planets largest basin, Caloris, which led them to the conclusion that after the basin formed (probably from an impact with an asteroid), volcanic activity filled it with lava, giving it the aspect it has today.

The surface geological structure of Mercury, which includes lobate scarps, reflects a contraction tendency of the entire planet, several times greater than previously thought. MESSENGER observations revealed that the tendency continues to date, and supports the idea of a cooling core, which completely changed the evolution of the surface of the planet.

Scientists expect to establish a sequence of geological events in the past of Mercury once all flybys are completed. MESSENGER is scheduled for two more flybys in October 2008 and September 2009, and will go into orbit around the planet in 2011, when it is expected to determine the variation levels of contraction across Mercury’s surface.