Mercury
Overview
Mercury is the closest known planet to the Sun, and it is the smallest planet in the solar system. Mercury is the hottest planet due to its solar proximity, but it spins so slowly on its axis and lacks an atmosphere that the night side of the planet drops several hundred degrees below the day side.
Mercury's surface is covered with craters, including the giant Caloris Basin. Mercury posessess a magnetic field as well as a large iron core, making it an anomaly that planetary scientists are still trying to understand.
Until a few years ago, our only data about Mercury came from Earth-based observations and the 1970s flyby of the Mariner 10 spacecraft. In March 2011, the MESSENGER craft went into orbit and has been returning data that we hope will answer many of the outstanding questions.
Observing from Earth
 |
| The main image shows the disk of the Sun during the November 8, 2006 Mercury transit. The gray box shows the view of the time-lapse insets at the top of the montage. The larger black circle is a Sunspot. Mercury is the small dot that starts near the bottom of the slice. Photographs were taken 1 minute apart. Photographs were taken and processed by Stuart Robbins. |
Being the closest planet to the Sun as well as the smallest planet, Mercury is very difficult to observe. To see it, you generally can only observe it very close to sunset or to sunrise (depending upon which side of the Sun it appears to be on). One helpful aspect is that, because Mercury is so close to the Sun as well as to Earth, it is fairly bright (generally -2.5 to -2.0 magnitude), and so it can easily be spotted in a telescope during a bright twilight.
Mercury, being an inferior planet (meaning that it lies inside Earth's orbit) displays phases, just like Venus and our moon. Mercury's phases coincide with where it is relative to the Sun and Earth. When Mercury is between Earth and the Sun (known as an inferior conjunction), it will display a new phase, being completely dark. When Mercuy is behind the Sun (known as opposition, or a superior conjunction), it will display a full phase, being completely lit. When Mercury is at its greatest elongation (the farthest away from the Sun it can get as seen from Earth), it will be in a half phase. It is generally difficult to actually resolve its disk and see the phases, however, because Mercury is generally ~10 arcsec in diameter as seen from Earth.
Finally, again being an inferior planet, Mercury can transit the Sun as seen from Earth. It takes over 5 hrs for Mercury's tiny disk to transit the Sun, but it can be easily photographed depsite being ~0.5% the Sun's diameter. The image at the right shows one such transit in its early stage, taken on November 8, 2006. The next transit will occur on May 9, 2016.
 |
| Geologic history of Mercury, identifying the five major epochs identified in Mercury's history. Ages are approximate. |
Global History
Mercury is believed to be geologically dead at the present time, except for those processes required to create a magnetic field. The surface of the planet is believed to be mostly ancient, dating back at least 3 billion years, much like Earth's moon. One way to talk about Mercury is in terms of geologic ages - much like the rest of the terrestrial bodies. These are shown in the figure to the right.
These boundaries are not well established in time, unlike Earth's, because they are based upon relative dating techniques as opposed to absolute dating methods. As with Mars and Venus, Mercury's epoch ages are based upon the lunar cratering record (which has been absolutely dated from Apollo sample returns). Very briefly, this works under the assumption that the older a surface, the longer it has had to accumulate craters and so the more craters will be present. We can calibrate the cratering rate from the moon and extrapolate it to other bodies in order to estimate how many craters of a certain size should be present for a surface to be a certain age.
Geologic ages on Mercury are named after major craters (like our moon). The first is simply the Pre-Tolstojan, which covers the planet's formation ~4.5 Gya (billion years ago) until ~4.0 Gya. This covers the period from the formation of the planet, the very earliest surface, and the planet's differentiation from a homogenous mixture into a core, mantle, and crust. This time also includes most of the early Heavy Bombardment of asteroids, when most of the craters seen throughout the solar system were formed. This is likely when Mercury's intercrater plains formed.
Next is the Tolstojan period, named after the crater Tolstoy. It covers ~4.0-3.9 Gya, a fairly narrow 100 million-year period. During the Heavy Bombardment mentioned above, the crater Tolstoy formed, marking the beginning of the Tolstojan epoch. During this period, the intercrater plains continued to form.
The next geologic era is Calorian, named after Mercury's largest crater, Caloris Basin. It covers ~3.9 until 3.5-3.0 Gya (the boundary with Mansurian is not well-determined). The Caloris Basin, a large multi-ring basin on the planet, formed towards the end of the Heavy Bombardment. This basin modified the surface of the entire planet (discussed below). The Calorian period marks the time when most of the smooth plains formed on the planet, likely due to the last thrusts of volcanism before the planet cooled and solidified too much to permit volcanism.
Fourth is the Mansurian era, named after the crater Mansura. It lasts from 3.5-3.0 Gya (the boundary with Calorian is not well-determined) to ~1.0 Gya. This epoch represents light cratering and a continued decrease in planetary activity and increase in cooling. Relatively fresh craters formed during this period, but they do not have rays (because the rays have eroded since formation).
Finally is the Kuiperian age, named after the crater Kuiper. This is the youngest surface age of the planet, lasting from the end of the Mansurian ~1.0 Gya to the present day. This represents a mostly geologically dead Mercury with light cratering, the craters still posessing their rays.
Surface
a
 |
| A pie chart illustrating the compositional breakdown of Mercury's atmosphere. Data is from NASA's planetary factsheet. |
Atmosphere
In the conventional sense that Earth, Venus, or even Mars has an atmosphere, Mercury does not. It does, however, have a very tenuous envelope of gas that surrounds it. The pressure this gas exerts on the surface of the planet is only 10-15 bars. To think about this in everyday life, dropping a feather on your hand would exert a pressure of about 10-3 bars, or 1 trillion times more than Mercury's atmosphere.
The actual fractional values shown in the pie chart are not well constrained. After more data becomes available from MESSENGER, better values should be available. It should be noted that besides these five molecules shown in the chart, additional species (such as water, nitrogen, magnesium, silicon, carbon, and calcium) are present, just at much lower abundances.
It should be noted that the atomic species present - sodium, helium, oxygen, and potassium, for example - are generally ionized due to interactions between the elements and the solar wind - the ionized plasma that streams from the Sun. The atmosphere is generated due to the space weathering itself, where charged solar particles interact with Mercury's surface, knock atoms and molecules off the surface, and excite them in the near-surface environment, becoming part of the atmosphere (Zurbuchen et al., 2008). Material is lost from the atmosphere in much the same way - once it becomes excited and leaves Mercury's surface, it will generally continue to stream away from the surface, leaving the planet's gravity (McClintock et al., 2008).
Magnetosphere
One very surprising feature of Mercury that Mariner 10 discovered was that the planet posesses an active magnetic field. This is surprising for two main reasons. First, Mercury is the smallest planet in the solar system, and it is thought that the larger the planet, the easier it is for a magnetic dynamo to be established in its core. The second reason is that it is also thought that a planet must have a fairly rapid spin rate (the length of its day), too, in order to generate a magnetic dynamo. Mercury's day is 55 Earth days, which is much slower than it was thought was needed in order to create the magnetic field. It also made Mercury one of only two of the four terrestrial planets (rocky planets) to have a magnetic field.
Based upon the latest data from the first MESSENGER flyby of Mercury, on January 14, 2008 (reported in Anderson et al., 2008; and Slavin et al. 2008), the field strength is 230-290 nT·RM3, where RM is Mercury's radius (and nT is the unit nanoTeslas). This is roughly 10% Earth's field strength. It also agrees very well with the results from Mariner 10, which had a larger uncertainty and found a field between 170-350 nT·RM3. The magntic dipole is tilted 5-12° from its rotation axis (this is not rare - most planetary magnetospheres are tilted relative to the planet's rotation axis).
 |
| This figure shows the planet Mercury as the sphere in the center and its magnetic field lines as blue curves radiating from it. Overlaid is data from the MESSENGER craft flyby that took place on January 14, 2008. It points out four main features of the magnetosphere - the magnetopause - the location where outward magnetic pressure from the planet's magnetosphere is counterbalanced by the solar wind - and the two bow shocks - where magnetized particles pile up due to the magnetopause. One can think of a magnetopause as the physical bow of a ship, where it comes into contact with the water, and the bow shock as the actual water that piles up in front of it. Figure is from 30/01/2008 NASA Press Conference. |
Besides a magnetometer (MAG being the instrument abbreviation) that yields data like that described above, MESSENGER also carries an energetic particle spectrometer (EPS), fast imaging plasma spectrometer (FIPS), and x-ray spectrometer (XRS) that were all used to analyze the types of particles in the magnetosphere and to better understand its characteristics. Using these, Slavin et al. (2008) show that Mercury's magnetosphere is filled with ionized sodium (Na+) derived from its exosphere (upper atmosphere) that is consistent with previous models for its magnetosphere. They conclude from this and other results that the magnetosphere seems to be "immersed in a cloud of cometlike planetary ions."
Selected Features and Characteristics
Orbial Characteristics: Mercury's orbit is one of the most intriguing of the planets for several reasons. First, it has the largest eccentricity of the planets, 0.206, which means that its closest approach to the Sun is nearly 35% closer than its farthest point. It also has the highest inclination, being 7° tilted relative to the plane in which Earth orbits.
A second orbital peculiarity is that, originally, it was believed Mercury's day was precisely as long as it's year, about 88 Earth days. Mariner 10 refined this to approximately 59 Earth days, meaning that there are exactly 2 Mercurian years per 3 Mercurian days. It is likely that Mercury's original rotation rate was much faster (possibly up to an 8 hr-long day (Burns, 1976)), but that solar tides caused it to spin down. This would have raised the interior's temperature and caused fractures on the planet's surface, which we do see evidence for (Strom, 1979). A likely side-effect of the tidal locking is that Mercury's axial tilt relative to its orbit is very small, 0.01°, making it nearly perpendicular.
Besides these, however, Mercury presented a problem to classical physicists for around 150 years. The actual position of the planet could be explained to very high precision from all gravitational considerations, such as the tiny tugging on the planet from the other planets, but there was a small, unexplained precession of Mercury's perihelion (the position where it was farthest from the Sun would move around in its orbit). This was discovered by French mathematician Le Verrier.
Le Verrier had used a similar technique to hypotheisize the presence of the planet Neptune due to perturbations of Uranus' orbit, and he tried the same thing - the existence of an unknown planet called "Vulcan" in order to explain the precession problem. However, none was found.
This precession effect was very small, approximately 5600 arcsec (1.556°) per century. The vast majority - 5025.6 arcsec (89.7%) is caused by Earth's precession on its own axis. The second major effect comes from the gravity of other planets, 531.4 arcsec (10.0%). A very tiny fraction - 0.0254 arcsec - is due to the Sun not being a perfect sphere. Remaining is about 43 arcsec, or 0.3% of the total effect.
Finally, Albert Einstein showed in 1916 that his General Theory of Relativity perfectly explains the remaining perturbation as the curvature of space itself. This "simple" explanation of Mercury's precession was one of the motivators for acceptance of his Relativity.
Naming Conventions
The name "Mercury" in English is named after the Roman messenger god, Mercury, also known by the Greek name Hermes. He was also known as the god of travelers, trade, profi, and commerce. The name is related to the Latin word "merx," meaning "merchandise." The planet was named for Mercury because Mercury was known as the swiftest of the gods, oweing to the winged sandals he wore. The name is fitting because Mercury, being the closest known planet to the Sun, travels the fastest in its orbit and has the shortest year.
According to the USGS Gazetteer of Planetary Nomenclature, any new features are named by the following:
- Craters: Dead artists, musicians, painters, or authors.
- Dorsa: Dead scientists who have contributed to the study of Mercury.
- Fossae: Significant works of architecture.
- Montes: Caloris, from the Latin word for "hot."
- Planitiae: Names for Mercury (either as a planet or god) in various languages.
- Rupes: Ships of discovery of scientific expeditions.
- Valles: Radio telescope facilities.
Past, Present, and Future Missions
The following is a list of the missions that have finished, are currently in operation, or are planned to be lanuched to explore Mercury. A brief summary is displayed, but you can click on the name of the mission to be taken to a page detailing the mission.
Mariner 10 ~ 1975-1976 ~ Mariner 10 flewby Mercury three times in 1975 and 1976. Before the first flyby, astronomers did not know the length of Mercury's day. Unfortunately, it was discovered that Mercury's day was such that during the second flyby, the following year, the spacecraft imaged nearly the same section of the planet, providing maps of only ~45% of the surface.
MESSENGER ~ 2004-present ~ MESSENGER (MErcury Surface, Space EnviroNment, GEochemistry and Ranging) became the first craft since Mariner 10 to flyby Mercury in 2008. After two subsequent flybys - October 2008 and September 2009 - it will insert itself into orbit around Mercury in March 2011 and have a nominal mission of 1 Earth year after that.
BepiColombo ~ 2013?-? ~ This is still in the planning stages. It is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). It has repeatedly been scaled back and delayed due to budgetary constraints. It is currently planned to have three orbiting components that will reach Mercury in 2019 and have a 1 Earth year nominal life after that.
Data for the Planets
The following table presents selected information from NASA's planetary factsheet.
| Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune | |
|---|---|---|---|---|---|---|---|---|
| Perihelion (106 km) | 46.00 | 107.5 | 147.09 | 206.62 | 740.52 | 1352.55 | 2741.30 | 4444.45 |
| Mean Orbital Distance (106 km) | 57.91 | 108.2 | 149.60 | 227.92 | 778.57 | 1433.53 | 2872.46 | 4495.06 |
| Aphelion (106 km) | 69.82 | 108.9 | 152.10 | 249.23 | 816.62 | 1514.50 | 3003.62 | 4545.67 |
| Average Orbital Velocity (km/s) | 47.87 | 35 | 29.78 | 24.13 | 13.07 | 9.69 | 6.81 | 5.43 |
| Orbital Inclination (from Earth's orbit) | 7.00° | 3.4° | 0.0° | 1.850° | 1.304° | 2.485° | 0.772° | 1.769° |
| Orbital Eccentricity | 0.2056 | 0.007 | 0.0167 | 0.0935 | 0.0489 | 0.0565 | 0.0457 | 0.0113 |
| Equatorial Radius (km) | 2439.7 | 6051.8 | 6378.1 | 3397 | 71,492 | 60,268 | 25,559 | 24,764 |
| Polar Radius (km) | 2439.7 | 6051.8 | 6356.8 | 3375 | 66,854 | 54,364 | 24,973 | 24,341 |
| Volume (1010 km3) | 6.083 | 92.843 | 108.321 | 16.318 | 143,128 | 82,713 | 6833 | 6254 |
| Ellipticity (variation from sphere) | 0.0000 | 0.000 | 0.00335 | 0.00648 | 0.06487 | 0.09796 | 0.02293 | 0.01708 |
| Axial Tilt (from Earth's geographic North) | 0.01° | 177.4° | 23.45° | 25.19° | 3.13° | 26.73° | 97.77° | 28.32° |
| Mass (1024 kg) | 0.3302 | 4.87 | 5.9736 | 0.64185 | 1898.6 | 568.46 | 86.832 | 102.43 |
| Density (water=1) | 5.427 | 5.243 | 5.515 | 3.933 | 1.326 | 0.687 | 1.27 | 1.638 |
| Escape Velocity (km/s) | 4.3 | 10.36 | 11.19 | 5.03 | 59.5 | 35.5 | 21.3 | 23.5 |
| Gravity (m/s2) | 3.70 | 8.802 | 9.78 | 3.716 | 23.1 | 9 | 8.7 | 11 |
| Surface Pressure (bars) | ≈10-15 | 92 | 1.014 | 0.000636 | N/A | N/A | N/A | N/A |
| Total Mass of Atmosphere (kg) | <1000 | 4.8·1020 | 5.1·1018 | 2.5·1016 | N/A | N/A | N/A | N/A |
| Sidereal Rotation Period (hours) | 1407.6 | -5832.5 | 23.9345 | 24.6229 | 9.9250 | 10.656 | -17.24 | 16.11 |
| Length of Day (hours) | 4222.6 | 2802 | 24 | 24.6597 | 9.9259 | 10.656 | 17.24 | 16.11 |
| Tropical Orbital Period (days) | 87.968 | 224.7 | 365.256 | 686.980 | 4330.595 | 10,746.94 | 30,588.740 | 59,799.9 |
| Bond Albedo | 0.119 | 0.750 | 0.306 | 0.250 | 0.343 | 0.342 | 0.300 | 0.290 |
| Visual Geometric Albedo | 0.106 | 0.65 | 0.367 | 0.150 | 0.52 | 0.47 | 0.51 | 0.41 |
| Visual Magnitude | -0.42 | -4.40 | -3.86 | -1.52 | -9.40 | -8.88 | -7.19 | -6.87 |
| Solar Irradiance (W/m2) | 9126.6 | 2613.9 | 1367.6 | 589.2 | 50.50 | 14.90 | 3.71 | 1.51 |
| Black-Body Temperature (K) | 442.5 | 231.7 | 254.3 | 210.1 | 110.0 | 81.1 | 58.2 | 46.6 |
| Average Surface Temperature (Celsius) | 167° | 464° | 15° | -65° | -110° | -140° | -195° | -200° |
| Number of Moons | ||||||||
| Rings? | No | No | No | No | Yes | Yes | Yes | Yes |
| Global Magnetic Field Strength (Gs) / Tilt | 0.0033 / 169° | No Field | 0.3076 / 11.4° | No Field | 4.28 / 9.6° | 0.210 / <1° | 0.228 / 58.6° | 0.142 / 46.9° |
| Discoverer | Unknown | Unknown | Unknown | Unknown | Unknown | Unknown | William Herschel | Johann Gottfried Galle |
| Discovery Date | Prehistory | Prehistory | Prehistory | Prehistory | Prehistory | Prehistory | March 13, 1781 | September 23, 1846 |