Jason Rosenberg, ’18

It is one of the strange ironies of science that the second-closest planet to Earth, Mercury, is among the solar system’s most mysterious. That is, it was until the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, or MESSENGER for short, was launched in 2004. Since then, MESSENGER has completely reshaped our understanding of Mercury, filling in large gaps of knowledge, settling long-controversial issues, and raising new questions in the process. This spring, the MESSENGER mission comes to an end as its fuel supply runs out, but the data it has collected during its more than a decade-long odyssey in space will continue to captivate researchers for years to come.

The Elusive Planet

            Mercury may be relatively close to the Earth, but it is also close to the sun. This location has been historically problematic for astronomers. For Earth-based observations of Mercury, the planet’s proximity to the sun leads to short viewing windows, generally around sunset and sunrise (MESSENGER). At these times, since the planet is low in the sky, one must look through a large portion of the Earth’s atmosphere to see it, blurring observations. Even space-based telescopes have trouble viewing Mercury because the brightness of the Sun can damage the delicate apparatuses.

Thus, sending spacecraft close to Mercury is one of the best means of acquiring detailed images of and information regarding the planet. Here again, however, astronomers and scientists encounter enormous problems (MESSENGER). Near Mercury, a spacecraft must be able to function under extreme conditions, including high radiation levels and temperatures. Moreover, it is difficult to maintain a trajectory—let alone an orbit—close to the sun due to the sun’s large gravitational strength at those distances. For these reasons and more, Mercury has often been dubbed “the elusive planet.”

                                                            Of Mass and Magnets                  

These observational difficulties have long shrouded Mercury in mystery, and consequently there are many unknowns regarding the planet. The MESSENGER mission team early on categorized the questions we have about Mercury into six main topics (Solomon et al., 2001). First, it has been observed that Mercury has a very high density, suggesting an iron core that accounts for roughly 75% of the planet’s radius. For comparison, the Earth’s core is 55% of its radius (Williams). Multiple hypotheses have been developed as to what in Mercury’s formation led to this large and dense core, but until the MESSENGER mission there was no way of ruling out any of the leading theories. In addition, much of Mercury’s surface has not been imaged in detail, and the parts that have been imaged have led to scientific controversies. Foremost among these is the extent to which Mercury’s surface has been shaped by volcanic processes. The MESSENGER mission also noted that very little is known about Mercury’s magnetic field, including the field’s origins. Fourth, while the size of Mercury’s core has been roughly determined, its structure has not. While some scientists contend that the core is solid, others believe it has a molten element. The last two unknowns regarding Mercury that the MESSENGER team noted concern the composition of the surfaces of the planet’s poles and the composition of the atmosphere. Poised to answer many difficult and important questions about Mercury, the MESSENGER mission was ready to transform our understanding of the planet.

A Mission is Born

Prior to MESSENGER’s launch, a large portion of scientists’ understanding of Mercury came from flybys by NASA’s Mariner 10 spacecraft in the mid-1970s (Solomon et al., 2010). In the late 1990s, many scientists felt that it was necessary to revisit Mercury for a longer duration and with enhanced scientific equipment in order to answer some of the aforementioned mysteries. Hence, NASA created the MESSENGER mission, a nodding reference to the occupation of the Roman god Mercury for whom the planet was named. The Johns Hopkins Applied Physics Laboratory then built the spacecraft, and in 2004 MESSENGER was launched into space (MESSENGER). The actual spacecraft is a small-scale bundle of scientific apparatuses and propellant tanks contained within a sunshade that protects MESSENGER from the radiation and temperature of the sun. It also possesses two solar panels to power the electronics inside. In 2008, after several flybys of Venus, the spacecraft reached Mercury and went into orbit around the planet in 2011. Since then, it has made over 4,000 orbits of Mercury, acquiring a wealth of new data.

Many Questions Answered, Many Remain

One important consequence of the MESSENGER mission is that we now have a complete and detailed map of Mercury’s surface (MESSENGER). These new images have revealed surprising information about the planet. They have, for instance, conclusively revealed that volcanism is a major presence on Mercury’s surface (Head et al., 2008). Many features, such as volcanic vents and craters filled in with new lava, indicate this volcanic activity. Additionally, the images show curious depressions inside impact craters, known as hollows (Blewett et al., 2011). Scientists believe these are caused by materials below Mercury’s surface vaporizing through various processes, leaving behind depressions in the planet’s surface (Blewett et al., 2011). The abundance of hollows observed indicates that Mercury contains a large amount of volatile substances. The exact composition of these volatile substances will be a subject of future research.

The composition of the atmosphere was also studied, “confirm[ing] the presence of sulfur, sodium, potassium and chlorine,” according to Nori Laslo, the MESSENGER Payload Operations Manager and former Mission Planner for the Chandra X-ray Observatory at the Harvard-Smithsonian Center for Astrophysics. Laslo added, “These concentrations were much higher than anyone would have guessed. This finding has big implications for how Mercury formed and, by extension, how all of the terrestrial planets formed.”

One of the most important findings solves a 20-year-old question as to what the reflective material is on Mercury’s poles that was originally observed by radar in 1991 (Solomon et al., 2001). “The unusual material at Mercury’s poles is water ice, an exciting discovery,” said Laslo. Mercury, the closest planet to the Sun, is a surprising planet on which to find ice. The reason this ice can exist is that Mercury’s axis is only slightly tilted, and therefore certain areas in the polar regions are always in the shade (Lawrence et al., 2012). The ice was first observed indirectly by analyzing the flux of neutrons from Mercury’s polar regions, but in October 2014, optical images were acquired of this ice (Chabot et al., 2014). It now remains to be seen where the ice found on Mercury originally came from.

Data also indicate that Mercury’s iron core is larger than previously thought, now estimated as making up 85% of the planet’s radius, and that it contains both molten and solid iron (Hand). Finally, MESSENGER has revealed that Mercury’s magnetic field is not aligned around the planet’s center, as Earth’s is, but rather is shifted toward the north pole (Anderson, 2011).

What’s Next?

MESSENGER’s mission is coming to an end as the spacecraft’s fuel runs out, but its data will take years to analyze. The MESSENGER mission marks a pivotal moment in the history of our understanding of Mercury and in our understanding of all terrestrial planets, but as Laslo remarked, “regarding questions that remain, there are still many.” Further analysis of MESSENGER’s data may solve some of these questions, but it will not be able to answer them all. A second mission to attempt to repeat MESSENGER’s success, therefore, may lie in the not-too-distant future.