Mercury

The series continues, hopefully this one won't be as long as the previous installment. From here on we will be heading outwards from the Sun, starting with Mercury and continuing from there.


An image of Mercury taken by NASA's MESSENGER probe. Note the prominent rays emanating from the various craters

Rotation and Orbit

Mercury, being the innermost planet, has a faster and shorter orbital path meaning that it overtakes the Earth every 116 days. It is best seen from the Earth 22 days before or after these occasions, where it is seen furthest from the Sun in the sky. This situation is described as maximum elongation. Even then it is difficult to spot, because the two bodies are never more than 28 degrees apart. Mercury has an orbital eccentricity of 0.206, the greatest of all the terrestrial planets, and when maximum elongation coincides with Mercury's perihelion it's angular seperation from the Sun is only 18 degrees.


Image/diagram charting Mercury's elongation in October 2008

The rotation period of Mercury is exactly two-thirds of it's orbital period, meaning that ir rotates three times during the course of two complete orbits. The effect of this 3:2 ratio as seen from Mercury's surfaces means that the time from sunrise to sunset is exactly one Mercury year (88 Earth days). Mercury's day is twice as long as its year!


A: The rotational period of the planet Mercury is exactly two-thirds of its orbital period. Hence, on every second passage of the planet through the pericentre, Mercury presents the same face to the Sun. B: The dynamical stability of this unusual resonant lock, or spin–orbit coupling, can be understood by plotting, at equal intervals of time, the position of the Sun in a reference frame that is centred on Mercury and rotates with the solid body of the planet. The angle described by the long axis of the planet and the direction of pericentre oscillates like a pendulum and follows the damped-pendulum equation.

The reason for this exact relationship between orbital period and rotational period must be related to the huge tidal influence of the Sun on such a nearby planet. Mercury was probably spinning far more rapidly soon after it had formed, but within half a billion years tidal forces would have slowed down the rate to its present value. However, it is a mystery as to why Mercury's rotational period was not slowed down further to match its orbital period (88 Earth-days) in which case Mercury would always keep the same face towards the Sun, much like our Moon does to Earth. Similar tidal forces have had the same effect on the rotational periods of most satellites of the giant planets.

Missions to Mercury

The first spacecraft to visit Mercury was NASA's Mariner 10 probe, which made three fly-bys of the planet in 1975-75, imaging 45% of the surface. NASA launched a second probe in 2004 named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging), which flew past twice in 2008 and will fly-by once more on September 29, 2009 before inserting itself into Mercury's orbit on March 18, 2011 for a year-long mission. The European Space Agency (ESA) plans to follow up with a more ambition mission called BepiColombo in 2012, which after a four year flight will deploy a sophisticated orbiter to obtain detailed images and a smaller independant orbiter operated by the Japanese Space Agency (JAXA) to study Mercury's magnetic field.


From left to right: The Mariner 10 and MESSENGER probes. Bottom: proposed BepiColombo probe.

Temperature Extremes and Polar Ice

Because Mercury is so close to the Sun, has a long rotational period, and has the most eccentric orbit of the terrestrial planets, its surface is subject to great extremes of temperature. When Mercury is at perihelion, the surface temperature at the point where the Sun is directly overhead reaches a maximum of about 470 degrees Centigrade. Even when Mercury is at aphelion the noontime temperature is about 250 degrees Centigrade. Conversely, towards the end of Mercury's long night the temperature drops to as low as -190 degrees Centigrade. Furthermore, Mercury's axis of rotation is hardly tilted relative to its orbit so the the floors of craters near its poles never seen the sun at all, and the local temperature is permanently cold. Polarised radar reflections from these craters indicate the presence of ice, either at the surface or more likely dispersed within the shall surface layer of fragmented debris known as regolith that covers all solid bodies lacking an atmosphere thick enough to act as a shield against meteorite impacts. How the water to form this ice has been preserved is a mystery. It is highly unlikely that any water can have survived on Mercury during the time of giant impacts when the planet was taking shape - it is far more reasonable to assume that much of the polar ice is inherited from comets that have collided with Mercury during the 4.5 billion years since its formation.


Radar image taken by the Arecibo radio telescope shows possible deposits of water ice (bright areas)

The Interior

In the absence of any natural satellite, precise tracking of the Mariner 10 probe's trajectory as it passed Mercury enabled the planet's mass to be determined more accurately than previously possible. This showed that Mercury is nearly as dense as the Earth. This is remarkable in so small a planet, because the Earth's high density is attributable to the compression of its interior resulting from it's large mass. Mercury's high density despite its small size suggests that it has an iron-rich core making up 70% of the planet's mass and 40% of its volume. This means that the outer edge of the core is at a comparitively shallow depth, only about a quarter of the way between the surface and the centre. Relative to its total size, Mercury's core is much bigger than that of any other terrestrial planet.


A cross-section of Mercury showing its core

Current models for the condensation of the solar nebula cannot account for the preponderance of iron within Mercury. It is hypothesised by planetary scientists that Mercury once had a much thicker rocky mantle that was blasted away by a giant impactor, which unlike the collision that formed the Moon did not coalesce into a satellite.


A simulation showing the giant impact hypothesis in action

Mercury has a relatively strong magnetic field, about 1.1% as strong as the Earth's. While the core of a planet this size would be expected to have solidified long ago, tidal forces due to Mercury's eccentric orbit could serve to keep at least the outer layers of the core liquid. Another theory has it that Mercury's magnetism is a "remnant" that has been "frozen in" to the core since solidification, just as a toy magnet retains a magnetic field that has previously been imposed upon it.


Mercury's magnetic field

The Atmosphere

Mercury's gravity is too slight to hold on to a gaseous envelope, so its atmosphere is tenuous in the extreme. All the types of atoms found in the atmosphere are light enough to escape into space, and what has been detected must be a steady-state mixture that is continually replenished. Presumably these elements are supplied continually by micrometeorites or are liberated from the surface under the influence of sunlight or micrometeorite impact.


Proportions of elements found in Mercury's tenuous atmosphere


A diagram showing Mercury's magnetosphere, atmosphere and internal composition

The Surface

Images taken of Mercury by the Mariner 10 and MESSENGER probes show a surface that is deceptively Moon-like, though in fact the most densely cratered regions of Mercury bear fewer impact scars than the equivalent areas on the Moon, suggesting that overall Mercury has a younger but still very ancient surface. The largest impact feature seen on Mercury is the Caloris Basin (pictured below) and is 1340km across. This is desribed as a multi-ringed impact basin, because it consists of a series of concentric fractures, and is comparable in size with several similar features on the Moon. The object responsible for creating the Caloris Basin was probably an asteroid about 150km in diameter. It must have struck Mercury about 3.85 billion years ago, to judge from the number of smaller and younger craters that have formed on top of it.


The Caloris Basin

Much of the terrain in the upper left of the above image shows the scars of ejecta that was flung out for a thousand or more kilometres by the impact, but the effects of the Caloris impact were truly global in extent. The landscape on the part of the globe lying exactly opposite to the impact was severely disrupted by seismic shockwaves that travelled right through the planet's interior and surface waves that travelled around the globe and converged on this most distant point.


Diagram illustrating the Caloris impact

Global Contraction

Apart from impact cratering, which must be continuing to the present day, the youngest event identified in Mercury's global record is the formation of a number of sinuous features known as lobate scarps. These range 20 to 500 kilometres in length, and up to 2km in height. They are regarded as unmistakable signs of compression of Mercury's lithosphere, indicating where the edge of a tract of lithosphere has been thrust over an adjacent tract. Summing the deformation indicated by all the observed lobate scarps indicates a reduction in Mercury's radius of between 1 and 2 kilometres. This could have been caused either contraction of Mercury's mantle as it cooled, or by solidification of a previously liquid part of the core.


An image of lobate scarps (the line going diagonally from top left to bottom right) captured by the MESSENGER probe as it flew past Mercury. Also visible in the upper right is the Sullivan multi-ringed impact basin, approximately 135km in diameter

This deformation of the surface appears to have been the "last gasp" for Mercury's geology. Judging from the number of younger craters that are superimposed upon the scarps, it must have happened several billion years ago. Mercury is too small a planet to have retained enough primordial heat, or to generate enough heat through radioactive decay, to have geologically active since then. However, many other planetary bodies, among them Venus, the subject of the next installment in this series, show abundant signs of geological activity continuing into more recent times and have substantial atmospheres with complexities of climate and weather that rival those of our own planet.