Mercury has challenged astronomers since the invention of the telescope. It is always too close to the Sun and too low along the horizon for good telescopic observation. Mercury never strays far from the twilight and haze and it is too close to the Sun for the Hubble Space Telescope to safely observe. The dawn of the Space Age lent hope that a spacecraft could be sent to observe Mercury up close, but getting there would prove to be tough.
The Mariner planetary program was highly successful during the 1960s with the first flybys of Venus and Mars and the successful insertion of Mariner 9 into orbit around Mars in 1971. There was an excellent opportunity during late 1973 to send a spacecraft to Mercury with minimal fuel and travel time using Venus for a gravity assist. Venus would slow the spacecraft down enough so it would not go zipping by Mercury so fast that images and data would be worthless. Mariner 10 was ready in time and successfully launched on November 3, 1973, but the success nearly ended there. Mariner 10 suffered a serious problem with its antennae only a few weeks after launch, but it is still speculated how the problem miraculously resolved itself only a few weeks before the Mercury encounter.
Mariner 10 flew past Venus on February 5, 1974 and made its historic flyby of Mercury on March 29, 1974. Mercury looked like the Moon on approach, but turned out to be far different. Mercury had craters, rays, hills, basins, and plains, but it was the numerous scarps that made it so different. These indicated that Mercury had cooled and shrank creating thrust faults, some up to a mile high and hundreds of miles long, which are the scarps seen everywhere. Mercury was also found to have a huge iron core estimated to be up to 70% of its diameter, and a decent magnetic field. Mariner 10 flew past Mercury again in September 1974 and March 1975 before running out of fuel. In spite of the three close flybys, the orbital mechanics of Mercury and Mariner 10 were not optimal, which meant that only 45% of Mercury was imaged. The most obvious follow-up mission was to send a spacecraft that could go into orbit around Mercury to completely map and study it. Mercury averages only 36 million miles from our gravitationally massive Sun. No rocket at the time was powerful enough to do the job since the spacecraft had to be weighted down with heavy fuel necessary to slow the spacecraft down enough so it could be captured by Mercury's lesser gravity and allow orbital insertion. Thus, it would be another 33 years before Mercury would be seen up close again.
Technology improved through the years with the miniaturization of components that made it possible to build lighter spacecraft that used less fuel. Advances were also made in rocket propulsion. All of this made it possible to design a Mercury orbiter, and in July 1999 MESSENGER was born. MESSENGER, short for MErcury, Surface, Space Environment, GEochemistry, and Ranging was launched on August 3, 2004. It used the gravity assists of Earth once, Venus twice, and Mercury three times to slow it down enough to finally be captured by Mercury's gravity and entered into orbit on March 17, 2011. MESSENGER bristled with equipment to thoroughly image Mercury at high resolution and analyze its environment including its magnetic field and surface composition. The most critical component was a giant sunshade that protected the delicate instruments from the fierce heat of the Sun. Mercury was successfully mapped in full during the first year of the mission. MESSENGER orbited Mercury once approximately every 12 hours during the first year and came as close as 125 miles from its surface. It found that the iron core makes up nearly 80% of the diameter of the planet, even more than what the Mariner 10 data suggested, and there were other great discoveries. Deep irregular pits with bright deposits around them appeared to be vents of volcanoes that erupted not long ago and may be venting gases to this very day. Thousands of bright areas dotted the floors and hills within many craters, which are now known as hollows. These are where volatile material, possibly sulfur vaporized by the heat, created small, irregular depressions.
The biggest mystery that MESSENGER tried to solve was if there was water ice in the permanently shadowed regions near the poles. Radar from Earth indicated the presence of bright, highly reflective material in patches where the Sun never shines with constant temperatures as cold as -370 degrees F, which is unusual on a world that gets as hot as 845 degrees F. Mercury orbits the Sun nearly perfectly upright with a tilt of only 0.01 degrees of its axis. This would allow deep craters near the poles to be permanently shadowed for over a billion years creating extraordinarily cold pockets, pockets as cold as Pluto. The water ice could have come from comets and water-rich asteroids that impacted the surface. Water vapor evaporates rapidly in the heat and migrates toward the poles eventually freezing out into these cold pockets as frost and ice. It took nearly a year of observation and analysis before NASA finally announced in November 2012 that the ice was real, and even more interesting. Scientists used MESSENGER's neutron spectrometer to analyze the hydrogen abundance, a significant component of water molecules, and found it abundant, which is consistent with the existence of water ice. The data indicated that most of the ice is buried beneath several inches of a low-hydrogen material. The laser altimeter showed that most polar deposits are dark at near-infrared wavelengths, but some polar deposits nearest the poles are much brighter than average. Scientists matched the topographic maps with these images and found that water ice is thermally stable at the surface in the areas of bright reflectance, but only if buried under something less volatile, which means it is stable at higher temperatures. Finding this dark covering was unexpected especially since it was too dark to be surface material. The darkness matched the organic-rich compounds in comets and some meteorites. Closest to the poles, where temperatures are lowest, is where the water ice is pure and exposed to space. It is bright, which means there is an ongoing process that delivers water to the Polar Regions faster than it can be buried by impact debris and darkened by ultraviolet radiation.
NASA's mantra is to "follow the water" in search for extraterrestrial life. Some might think that Mercury could be a potential habitat for biology, but this is not possible because Mercury lacks an appreciable atmosphere where water could remain stable as a liquid. Water can only exist in a solid form as ice or as a vapor since ice quickly sublimates if exposed to heat at low pressures. There is no evidence that water ever flowed or ponded on Mercury. Some of the ice could be over a billion years old, but there could be remixing with newer ice making it difficult to use ice as a tool to determine Mercury's evolution, or solar activity.
MESSENGER settled the debate over the existence of water ice. The next mission to Mercury, BepiColombo, is a joint mission between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA). It actually consists of two orbiters that launch as one and was to lift off in 2009 for arrival in 2014. It suffered serious technical delays and cost overruns that canceled its lander; it is now scheduled for launch on July 9, 2016 and will arrive at Mercury on January 1, 2024. Housing numerous instruments that are capable of further analyzing the icy deposits, it will take over where MESSENGER left off. It is important to keep in mind that Mercury and the Moon are quite different. Mercury has more water, and it is in a much purer form. The only way to properly analyze the polar ice is to land or use an impactor paired with a low-flying orbiter carrying an aerogel container to collect icy particles for direct analysis. The message is clear that Mercury is no longer a fuzzy, unknown world, but rather a world in sharp focus ripe for serious exploration.