Since the angular separation of Mercury from the Sun is never more than 28 degrees at maximum elongation, visual observation and study of the planet is fraught with technical difficulty. In fact, the rugged, little planet spends more time lost in the Sun’s glare than it does out of it, meaning that much about Mercury remains unknown.
However, the two NASA missions to visit the planet Mercury, namely Mariner 10 and MESSENGER, have yielded much useable information, and while much about the “Swift Planet” remains unknown, many questions have been answered, albeit only partially in some cases. Below are some details of these questions and their answers, and it is our hope that at least some of the information presented here is new to you.
Getting into orbit around Mercury is not easy
Although the mean distance between Earth and Mercury is only 77 million km (48 million miles), a space probe that is required to enter into a stable orbit around the little planet needs to travel at least 91 million km (57 million miles) before it can do so. The issue revolves around the fact that Mercury’s orbital velocity is 48 km/sec (30 mi/sec) as compared to Earth’s, which is only 30km/sec (19 mi/sec), and that to reach Mercury in the first place, the space probe is forced to dip deeply into the Sun’s gravitational well.
The latter problem means that the probe gains a lot of speed, which can only be bled off by orbiting Venus repeatedly to slow it down, since Mercury does not have an atmosphere that is dense enough for a probe to use aero-braking maneuvers as a means to lose velocity. In practice, this means that a space probe bound for a stable orbit around Mercury uses more fuel than would be required for that same probe to escape the solar system completely. As a result, only two space probes have visited Mercury, the first being the Mariner 10 fly-by mission.
The Mariner 10 fly-by mission
The image opposite shows the first view of Mercury taken by a spacecraft, in this case, the Mariner 10 craft that also visited several other planets. Mariner 10 orbited Mercury three times after using the gravity of Venus to slow it down sufficiently, and it came to within 327 km (203 miles) at closest approach.
The image clearly shows Mercury’s heavily cratered surface, as well as some of the scarps that formed when the planet’s crust cracked after the core shrank as it cooled down. This mission also gathered extensive data on the planet’s magnetic field, which turned out to be remarkably similar to Earth’s, albeit a lot weaker. The mission ended on March 24, 1975, when the craft ran out of fuel after its third close approach. Mission controllers shut down the craft but it is still thought to orbit the Sun, and to pass close by Mercury every few months.
…and then the Messenger mission
This image shows the image of Mercury’s surface taken by the MESSENGER probe (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) which left Earth on August 3, 2004, and made one orbit around our planet, and two braking orbits of Venus in October 2006 and June 2007, respectively. MESSENGER then made three fly-by orbits of Mercury to further slow it down (Jan 14, 2008, a second on Oct 6, 2008, and a third on Sept 29, 2009), before entering a stable orbit around the planet on March 18, 2011.
The mission had six objectives, which were to investigate the cause(s) of Mercury’s high density, its geological history and evolution, the nature and extent of its magnetic field, the planet’s internal structure, whether or not it has ice at its poles, and the origin of the planet’s tenuous atmosphere. The mission was also used to map parts of the planet’s surface that escaped mapping during the Mariner 10 mission.
By all accounts, the MESSENGER mission was successful in all respects. The craft’s last maneuver happened on April 24, 2015, and it was allowed to crash onto the planet’s surface after running out of fuel, which happened at 3:26 PM EDT on April 30, 2015, leaving an estimated 16-meter (52 ft) wide crater.
MESSENGER found many volcanic flows
The MESSENGER probe located a total of 51 pyroclastic lava flows on Mercury, with 90% of the flows located inside huge impact craters. Analysis of the flows and the state of degradation of the craters suggest that volcanic activity on Mercury had occurred over very long time scales, and perhaps for much, if not most of the planet’s history.
The image opposite is a colour-enhanced view of the western side of the Sholem Aleichem Crater situated in the planet’s northern hemisphere. Clearly apparent is the brightly haloed volcanic hollows, with the dark staining also possibly suggestive of ash.
Another such volcanic system in the Caloris Basin, located north of the planet’s equator, which consists of at least nine overlapping, or intersecting volcanic vents whose floors are more than one km below their brinks. This indicates that the entire system had spewed lava explosively, before the lava had retreated back down into the mantle, thus creating the system of pits. A color-enhanced image of the feature can be seen at the bottom of the page, and based on MESSENGER data, this particular volcanic system is at least one billion years old.
MESSENGER was five times faster than the Space Shuttles
The MESSENGERS’ average speed during its 6.5 years in space was a respectable 136,000 km/h (84,500 m/ph), which is about five times higher than the highest speeds attained by the Space Shuttles in low Earth orbit. However, at some points during its journey, the MESSENGER probe had reached speeds in excess of 225,000 km/h (140,000 mph), nearly matching the speed record for spacecraft held by NASA’s Helios-2 spacecraft, which was clocked at speeds of 241,000 km/ph (150 000 m/ph) in 1976.
Mercury has ice caps
Although Mercury approaches the Sun to within 47 million km (29 million miles), images from the Arecibo Radio telescope suggested that the areas of high radio reflectivity near Mercury’s North Pole might be frozen water ice. This was confirmed by the MESSENGER probe when images from various sources were superimposed on one another.
In the image opposite, the red areas are in permanent shadow, while the yellow spots are areas of high reflectivity, which MESSENGER’s instruments have confirmed as water ice that coincide exactly with the highly reflective spots detected by the Arecibo instrument.
While the presence of water ice on super-hot Mercury might sound implausible, it must be remembered that the planet’s axis is tilted by less than half a degree, which guarantees that some areas on the planet will always be in shadow.
Mercury has a weird magnetic field
Although Mercury is only marginally larger than Earth’s Moon, it has a large-scale magnetic field that is not typically associated with an object of its size and mass. The only other bodies in the solar system with comparable magnetic fields are Earth, Saturn, Jupiter, and Jupiter’s moon Ganymede. Weirder still is the fact that Mercury’s magnetic field is three times stronger in its northern hemisphere than it is in the southern hemisphere, which to date, is a mystery that remains unresolved.
Mercury is not tidally locked to the Sun
Prior to 1965, it was thought by most investigators that Mercury was tidally locked to the Sun, since they always observed the same face of the planet turned towards us whenever they looked at it. However, improved observational techniques developed during the mid-1960s showed that Mercury is locked into a 3:2 spin/orbital resonance with the Sun, meaning that it rotates twice around its own axis during the time it takes to complete three orbits around the Sun.
In practice, this means that whenever Mercury is best placed for observation it is at, or very nearly at the same point in its orbit around our Sun, which explains why observers always saw the same side of the planet.
Mercury consists mostly of its iron core
Unlike other solid bodies in the solar system whose cores comprise only a small percentage of their diameters, Mercury’s core comprises about 55% of its diameter. The reasons for this are still unclear, but current theories include speculation that a huge impact or a series of impacts over extended time periods had stripped off the planet’s outer layers, or that the planet had formed before the Sun had stabilized its energy output.
If the latter were the case, the planet’s outer layers would have been vaporized as the proto-Sun heated up as it contracted further. The vaporized rock would then have formed a silicate-based “atmosphere”, and been blown away by the Sun’s energetic solar wind.
The MESSENGER mission showed that both of these hypotheses are improbable due to the presence of relatively light elements such as sulfur on the planet’s surface, which leaves the possibility that Mercury did not attract lighter material during its formation from the nebula out of which the rest of the solar system had formed, for reasons that remain unclear.
Mercury’s biggest crater is 1,550 km in diameter
Mercury has some of the biggest craters in the solar system, with the biggest example of a major impact on Mercury being the Caloris Basin, which stretches over 1,525 km.
The image opposite shows a (partial) perspective view, with the highest part of the basin rendered in red, and the lowest parts in blue. The main impact site is visible toward the bottom edge of the frame- note the crater rim that is more than 2,000 meters high.
It is also worth noting that the impact that created the Caloris Basin was so powerful that the shockwaves from the impact traveled right round the planet to create an area of chaotic terrain known as “The Weird Terrain” at a point on the planet’s surface that is exactly antipodal (diametrically opposite) to the impact site. Part of “The Weird Terrain” can be seen in the image above.