The innermost planet of the solar system, Mercury, revealed its mysteries 33 years after the first flyby performed by Mariner 10 in 1974-1975. In January 2008, the MESSENGER spacecraft sent never-before-seen images of Mercury’s surface, expanding the data scientists have had at their disposal until now.

For most of us, Mercury is one of those planets that allow the naked eye to watch its spectacle in the morning and evening twilight. For scientists, there are still a lot of unanswered questions about the planet that looks somewhat similar to the Moon.

The images sent by MESSENGER on January 14 covered an area of the planet that Mariner 10 was unable to map, and revealed details about the planet’s surface and dynamism. The journal Science published a total of 11 reports in its July 4th issue on the Mercury flyby.

MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft’s flyby was just the first of a series of events, which include two more flybys on October 6, 2008 and September 29, 2009. On March 18, 2011, MESSENGER is scheduled to go into orbit around the planet.

The data gathered earlier this year revealed that Mercury is a dynamic system, and that the planet’s system of volcanoes contributed to the formation of the smooth plains mapped by Mariner 10 three decades ago.

In 1972, Apollo 16’s mission on the Moon gave birth to a series of hypothesis on the origins of Mercury’s plains. By comparison, some scientists suggested that the formations on Mercury were the result of the material ejected by large impacts, which later formed smooth plains. Others believed that volcanic activity was responsible for shaping the surface of the Planet; however, Mariner 10’s images failed to provide any information supporting this last theory, and Mercury continued to be an enigma for scientists.

MESSENGER was able to gather information not only on Mercury’s volcanic activity, but also on Mariner’s initial discovery of a magnetic field, powered by a core similar to that of our planet. The hot, liquid-iron core was believed to have cooled a long-time ago, which means it shouldn't be able to generate magnetism anymore.

However, MESSENGER’s measurements indicated otherwise. As Brian Anderson of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, MD, explained, the fact that the magnetic field is dipolar and that they didn’t find any signature shorter-wavelenght anomalies that would indicate a magnetic crust means we are looking at a modern dynamism, which excludes the idea of a frozen core. The next flyby is expected to provide even more data on Mercury’s puzzling magnetic field.

Mercury’s core is believed to be the largest one compared to other terrestrial planets, and scientists were surprised to discover the dynamic interaction it produces between the interior of the planet, the surface, the exosphere and the magnetosphere.

Thanks to the interactions between the planet’s surface and the magnetosphere, MESSENGER had the chance to take a first glance at the surface composition of Mercury. The high levels of ionization in the atmosphere are a proof of these interactions with the planet’s surface; the “Mercury plasma nebula” is considered to be far richer and much more complex than the Io plasma torus in the Jupiter system, as Thomas Zurbuchen of the University of Michigan pointed out.

Mercury is surrounded by a complex system, and scientists have been more than surprised to discover that the planet is actually far more dynamic than previously though. They are now looking forward to the next flyby, which is expected to return satisfying results as well.

The geological history of Mercury is another element that scientists were able to reconstruct by combining data from both Mariner and MESSENGER. They managed to establish that the Caloris basin was possibly formed from an impact with an asteroid. On the margins of the basin, they found evidence of volcanic vents, which led them to believe that volcanic activity in the past may have been the source of lava flows that filled the basin interior.

The tectonic landforms on Mercury, also captured by MESSENGER, which gave birth to huge cliffs atop of the faults, are a sign that the planet’s cooling core was a major factor in the evolution of the surface, leading to the contraction of the entire planet. The phenomenon proved to be much more intense than previously thought.

MESSENGER’s 2011 mission is expected to determine the variation levels of contraction across Mercury’s surface, which should also help scientists establish a sequence of geological events in the planet’s history.

After MESSENGER, another mission is scheduled to study Mercury, as a result of the collaboration between the European Space Agency and the Japan Aerospace Exploration Agency. The mission is expected to launch two spacecrafts in 2013, one to map the planet and another one to examine its magnetosphere.