Visions of the Cosmos

Planetary Science

The Solar System

Mercury - Companion to the Inferno

Introduction

            The great blazing star shines directly overhead above a hot spot on the equator of Mercury.  It is noon and mid-summer.  The Sun hangs like a huge incandescent grapefruit in a jet-black sky.  It is over three times its apparent size on Earth and a massive stream of radiation emanates from the great swollen orb of the star as it rains down a fierce and terrible heat that raises the temperature of the rocks to over four hundred degrees Celsius.  Midday on Mercury during the 'summer season' is almost as hot as Venus and there is not even a greenhouse effect on this airless little world.

 Up until 1965 it was believed that Mercury always turned one face towards the Sun so that there was a permanent day-side and a permanent night-side.  In that year R.B.Dyce and G.H.Pettengill bounced radar off the surface of Mercury using the enormous dish of the Arecibo Radio Telescope in Puerto Rico (see illustration ).   They discovered that it takes 87.97 Earth days to orbit the Sun and 58.65 days to make one turn on its axis.  This means that one year on Mercury is exactly one and a half times its rotation period.   This relationship is called a 3.2 spin-orbit coupling.   On Mercury there are three days in every two years.

The Arecibo Radio Telescope

New color picture of the Arecibo dish (98195 bytes)

The Aracebo Radio Telescope is one of the largest in the world and is situated in Puerto Rico.  Anyone seeing it for the first time is astounded at the enormous size of the reflecting surface of the radio mirror.   The huge "dish" is 305 m (1000 feet) in diameter, 167 feet deep, and covers an area of about twenty acres. The surface is made of almost 40,000 perforated aluminium panels, each measuring about 3 feet by 6 feet.  

Acknowledgements Arecibo Telescope Puerto Rico / Cornell University /National Science Foundation

The orbit of Mercury is highly elliptical.   It is 45,900,000 kilometers from the Sun at its closest.  This is called its PERIHELION.   At its furthest it lies 69,700,000 million kilometers from the Sun.   This is called its APHELION.   This produces decidedly different climatic conditions and the apparent size of the Sun's disc varies greatly during the Mercurial year.  This is very readily appreciated by studying the diagram shown in the illustration that compares the apparent sizes of the Sun's disc from Earth and from Mercury at perihelion and aphelion.  (The difference between perihelion and aphelion for the Earth is very small and there is no appreciable difference during the year).

 

        Unlike the Earth and Mars the axis of Mercury is almost perpendicular to the plane of its orbit.  Thus there are no seasonal effects produced  by an axial tilt.  However very large differences in its distance from the Sun during its highly elliptical orbit cause considerable seasonal temperature variations at the hottest points (called hot poles) reach about 427 degrees Celsius at perihelion in 'mid-summer and 'only' 277 at aphelion in 'mid-winter'.Like the Earth and all other planets the globe of Mercury is divided into lines of latitude and longitude.   A meridian of zero degrees longitude has been selected that runs through the so-called 'weird terrain'.   A 180° line of longitude passes through the Caloris Basin Region.   There are two spots on the equator, which are known as hot spots.  They are at 0° longitude and 180° longitude.   These are the points on the surface of the planet, which lie directly under the Sun at the moment of its closet approach. 

Midnight anywhere on Mercury is very cold and the temperature drops to around minus 170 degrees below zero Celsius, which is almost as cold as one of the moons of Saturn.

At the north and south poles of Mercury there may be permanently illuminated mountain tops and permanently shadowed crater basins.  There is a  possibility that there are deposits of water ice deep within some of these craters.   There may be places in the craters where the temperature is permanently as low as -213° Celsius, which is only 60 degrees above absolute zero.

Unlike the Earth and Mars, the axis of Mercury is almost perpendicular to the plane of its orbit.   Thus there are no seasonal effects produced by an axial tilt.   However the very large differences in the distance from the Sun during its orbit causes considerable seasonal variations Temperatures at the hottest points (called hot poles) reach about 427° Celsius in mid-summer at perihelion and 'only' 277° Celsius in mid-winter at aphelion.. 

 Like the Earth and all other planets the globe of Mercury is divided into lines of latitude and longitude.   A meridian of zero degrees longitude has been selected that runs through the so-called 'weird terrain'.   A 180° line of longitude passes through the Caloris Basin Region.   There are two spots on the equator, which are known as hot spots.  They are at 0° longitude and 180° longitude.   These are the points on the surface of the planet, which lie directly under the Sun at the moment of its closet approach. 

Midnight anywhere on Mercury is very cold and the temperature drops to around minus 170 degrees below zero Celsius, which is almost as cold as one of the moons of Saturn.

At the north and south poles of Mercury there may be permanently illuminated mountain tops and permanently shadowed crater basins.  There is a strong possibility that there are deposits of water ice deep within some of these craters.   There may be places in the craters where the temperature is permanently as low as -213° Celsius, which is only 60 degrees above absolute zero.

 The Brief History of Mercury and the Mariner 10 Mission

Being far smaller than the Earth and Venus it is doubtful if Mercury ever had a long lasting atmosphere.  Like the Moon its volcanic activity would have been relatively short lived.   Because of the lack of late geological activity Mercury has a very old surface and seems to bear the extensive scars of impact craters acquired during the time of the 'Late Heavy Bombardments' in the early years of the life of the Solar System around 3,900 million years ago.   The craters appear to be similar to those on the Moon but because of the higher gravity of Mercury the ejecta, which were sprayed out around the impact sites, were flung for smaller distances around the craters.  Like the lunar craters very large impact sites are characterised by a series of between one and five mountain rings similar in type to the transient ripples produced when a stone is thrown into a pond.   This is similar to the Mare Orientale on the Moon an illustration of which is shown on the relevant section of this web-site.

 Sometime in its early history Mercury suffered an enormous impact, which produced the Caloris Basin.  The basin is so named because of its proximity to one of the 'hot poles' on the equator at 180º longitude 0o latitude.   The Basin is 1,300 kilometers in diameter.  It is a multi-ringed structure and was formed by an event so violent that the effects are visible over the whole planet.   Inside the basin itself there are tension block faulted depressions known to geologists as GRABENS.   These are formed as a result of readjustments of the crust following the impact. 

An unusual feature of the planet is a large area of depressions and hills known as the 'weird terrain', which is directly antipodal to the Caloris Basin.    Some believed that it was formed by the massive seismic shock waves produced by the impact.

                   South Pole Regions                                                     Caloris Basin                           Ridges and Fractures on floor of the Caloris basin

The photographs of the Caloris Basin were taken when Mariner10 passed over the planet.  Unfortunately only half of the Caloris Basin was visible at the time since the other half was in darkness during the fly-by.  Surrounding the impact sites there are concentric ridges within relatively smooth plains probably produced by renewed volcanic activity which started after the impact. NASA/JPL/Mariner 10

Photos Credit: NASA/JPL/Northwestern University

               Photograph of the weird terrain by NASA Mariner 10/JLP

Weird terrain best describes this hilly, lineated region of Mercury. Scientists note that this area is at the antipodal point to the large Caloris basin. The shock wave produced by the Caloris impact may have been reflected and focused to the antipodal point, thus jumbling the crust and breaking it into a series of complex blocks. The area covered is about 800 km (497 mi) on a side.

Very little was known about Mercury until the Mariner 10 space probe passed above its tortured surface.  On 3 November 1973 the Mariner 10 space probe left its launch pad and headed towards Venus.   It arrived close to Venus on 5 February 1974 where it carried out some useful observations before proceeding towards Mercury.   It passed within 703 kilometers of the surface of Mercury on 29 March 1974.    The space-craft then looped around the Sun to make a second visit possible on 21 September 1974 when it reached a closest distance of 48,069 kilometers.  Mariner returned a third time when it came within 327 kilometers of the surface on 16 March 1975.  349 images of the planet's surface were obtained during the third pass, which produced resolutions down to 140 meters.   The Mariner 10 mission produced some highly successful results.

 

When Mariner 10 passed by Venus on February 5, 1974 at a distance of 5770 km, it gained energy from the collision in what is called a slingshot manoeuvre. This was a particularly favourable manoeuvre, because the project directors discovered that the orbit could be fine tuned to loop around  back to Venus in twice Mercury's orbital period, so that it could loop back to look at Mercury again every second orbit. So instead of one look at Mercury, the Mariner craft got three flybys before its fuel ran out

Picture and description credit to NASA

 

Another point of interest is the fact that geological features called scarps run all over the planet suggesting that it has shrunk around its large metallic core and decreased the diameter of the planet by as much as 4 kilometers.

Next to the Earth, Mercury has the highest density of any of the planets.  This hides one important factor and that relates to what is called the uncompressed densities.   These relate to what the density of a planet would be at zero internal pressure. 

The density of a planet is due to two basic factors.  1) The intrinsic zero pressure density of the bulk planetary material and 2) the  'mass dependant effects' of compression due to the very high pressures present in the deep interiors of the planet.  These effects are much greater for the Earth and Venus than they are for smaller planets such as the Moon, Mercury and Mars.  The table below lists the equatorial radii, the masses, the densities and the 'zero-pressure densities' of Mercury, Venus, Earth and Mars.

 
 Some Physical Properties of the Terrestrial Planets
Planet Equatorial Radius in kilometres Mass in Grams X 10 27 Density in grams per cc  

Zero Pressure  Density

 

Mercury 2,439 0.330 5.43 5..30
Venus 6,051 4.869 5.42 4.00
Earth 6,378 5.974 5.515 4.05
Mars 3,396 0.642 3.93 3.74

As indicated in the above table, Mercury is found to have by far the greatest uncompressed density of any planet.   As such it has been calculated that its core may be extremely large.  It is suggested that the iron core is between 1,800-1,900 kilometers in diameter with an outer shell only about 500-600 kilometers thick.  There have been two theories advanced to account for the relatively large core and small mantle.   One is that very early on in its history Mercury suffered a catastrophic collision that striped away a large amount of mantle material, which was lost into interplanetary space.  The other is that because it was formed so close to the Sun there was less silicate material in that region of the solar nebula to start with and that thus only a small mantle was formed in the first place.   The iron concentration may have been high in the region of space where Mercury was forming

Magnetic Field

An unexpected result of the Mariner observations was the discovery that Mercury has a small magnetic field and it is believed that like the Earth it may have a solid core, which is surrounded by a liquid core producing a similar dynamo effect to that of the Earth. The dynamo model implies furthermore that the outer core of Mercury is still partially molten and probably contains a small fraction (2 - 4%) of sulphur.

 The Future of Scientific Investigations of Mercury

The Mariner 10 mission produced some highly successful results.   However, so far only about a third of the planet has been mapped and in order to make a thorough investigation an orbital explorer satellite of the type used on Mars is required. 

Two future missions to Mercury are planned for this decade – the American Messenger Project sponsored by NASA and the European Space Agencies Bepi Colombo Mission.

The Messenger Mission

The objectives of the mission are to study the surface composition, the geological history, the core and the mantle and the tenuous atmosphere.  It will also hunt for water and other frozen ices that may exist hidden in craters near the poles.

On 3rd August 2004 the Messenger Space Craft was launched aboard a Boeing Delta 11 rocket towards Venus.  The table below shows details of the journey
August 3, 2004 -- MESSENGER Launch
August 2005 -- Earth flyby
October 2006 -- Venus flyby
June 2007 -- Venus flyby
January 2008 -- Mercury flyby
October 2008 -- Mercury flyby
September 2009 -- Mercury flyby
March 2011 -- Yearlong science orbit of Mercury begins
 

The details of the mission as listed above indicates the difficulty of journeying to this small planet and the details of the voyage are summarised in the table below. As the table shows it is necessary to undergo a number of complicated manoervers around three planets before finally inserting the probe into an orbit around Mercury.  Once the probe is in stable orbit it will carry out its task of collecting data for at least a year.  The orbit will be highly elliptical and inclined at 80 degrees.  The probe will orbit once every 12 hours and will be at its closest at 200 km above the surface at 60o North. 

The Bepi Colombo Mission

The Bepi Colombo Mission is one the most elaborate and ambitious undertakings by the European Space Agency (ESA).    It is also a great example of international co-operation in that it is being planned together with the Japanese Space Agency.

In 1994 the European Space Agency began planning a voyage to Mercury. It is most fitting that at the suggestion of Roger Bonnet the then Head of the European Space Agency that the mission should bear the name Bepi-Colombo. The brilliant Italian scientist Giuseppe Colombo suggested the trajectories used in the 1974-75 Messenger Mission to Mercury.  Also it was Colombo who, whist he was at the University of Padua, calculated the relationship between the orbit of the planet around the Sun and its rotation rate (the 3.2 spin coupling).  Prior to that and the results of the observations of the Arecibo Telescope it was thought that Mercury always kept one face towards the Sun similar to the Moon and Earth system.  When Colombo died at the comparatively early age of 64 in 1984 it was a great loss to planetary science.  BepiColombo will provide the most complete exploration yet of Mercury, the innermost planet of our Solar System.

The Mission will consist of two orbiting spacecraft - The Mercury Planetary Orbiter (MPO) will map the planet, while the Mercury Magnetospheric Orbiter (MMO) will investigate its magnetosphere.
several launch methods have been extensively studied. In the selected scenario, for its journey BepiColombo will cleverly use the gravity of the Moon, Earth, Venus and Mercury itself in combination with the thrust provided by solar-electric propulsion (SEP). During the voyage to Mercury, the two orbiters and the electric propulsion unit will form one single composite spacecraft. Both orbiters will be launched together on a single Soyuz-Fregat rocket from ESA's Spaceport in Kourou, French Guiana.  During the voyage to Mercury the two orbiters and the electric propulsion unit form one single composite spacecraft.

Like the two American missions it is anticipated that the journey will be a long one.  The launch date is planned for August 2013 and the time of arrival will be in the late summer of 2019, six years after launch

The journey from Earth to Mercury is also special in that the spacecraft must brake against the Sun's gravity, which increases with proximity to the Sun, rather than accelerate away from it, as it is the case with journeys to the outer Solar System.

On approaching Mercury, each spacecraft will use the planet's gravity and conventional rocket engine to get into the two different polar orbits. Observations from orbit will go on for one Earth year.

Most of ESA's previous interplanetary missions have been to relatively cold parts of the Solar System. Bepi-Colombo will be the Agency's first experience of sending a planetary probe to very 'hot' regions.

The journey from Earth to Mercury is also special in that the spacecraft must brake against the Sun's gravity, which increases with proximity to the Sun, rather than accelerate away from it, as it is the case with journeys to the outer Solar System.

When approaching Mercury, the side of Bepi-Colombo facing the Sun will have to withstand incredibly high temperatures, while that facing away from the Sun incredibly cold temperatures, all at the same time.

The day side of Mercury is very hot while the night side is very cold. Bepi-Colombo will attempt to find out if there is water ice in permanently shadowed areas (craters in polar regions).

Colombo explaining the celestial mechanics of  Mercury                                                                                      Mercury Bepi Colombo Planetary Orbiter

ESA's mission to Mercury is named BepiColombo in honour of the late space pioneer Professor Giuseppe ‘Bepi’ Colombo.

ESA's mission to Mercury is named BepiColombo in honour of the late space pioneer Professor Giuseppe ‘Bepi’ Colombo.

Giuseppe Colombo was born in 1920 in Padua, Italy, where he attended primary and secondary schools. After graduating from the University of Pisa in Mathematics in 1944, he returned to Padua where he worked as Assistant and then Associate Professor of Theoretical Mechanics at the University of Padua.

 1955 he became Full Professor of Applied Mechanics at the Faculty of Engineering. In his career he lectured on Mechanical Vibrations and Celestial Mechanics, as well as Space Vehicles and Rockets during his last years.

In 1970, he was invited to NASA’s Jet Propulsion Laboratory (JPL) to participate in a conference on NASA's Mariner 10 Venus/Mercury mission. Early in that year he had noted that the period of the spacecraft's orbit, after it flew past Mercury, would be very close to twice the rotational period of the planet itself. He suggested that a second encounter with Mercury could be achieved.

Almost everything known until now about the planet Mercury comes from these orbits of Mariner 10 in 1974-75, which were inspired by Colombo's calculations.
 

To commemorate this great scientist, ESA created a 'Colombo fellowship' in 1985, to be granted to European scientists working in related astronautical fields. An asteroid discovered by the Sormano Astronomical Observatory in Italy, asteroid number 10387, was also named in his honour.

 

Aims of the Bepi Colombo Mission

Of all of the planets, we know the least about Mercury. However, Mercury's position as the innermost member of the Solar System makes it of key significance in our attempts to understand the formation of the terrestrial planets including our own Earth.  Bepi-Colombo, the ESA cornerstone mission to Mercury, aims to find answers to fundamental questions about this fascinating planet, such as:

 

 The Search for Water

The possibility that water ice may be present in permanently shadowed craters near the poles is potentially important for the study of surface processes.  A major discovery was made by radar observations in 1992. A search for water ice deposits will be conducted with a neutron mass spectrometer

.The Exosphere

Mercury has no stable atmosphere; the gaseous environment of the planet is best described as an exosphere, i.e. a medium so rarefied that its neutral constituents never collide. The existence of five elements O, H, Ne, Na and K has been established by Mariner 10 and by ground-based observations. Other elements, contributed by the regolith and possible ices near the poles, may be detected using UV spectroscopic observations of the limb. Production mechanisms include solar photo- and ion sputtering, impact vapourisation by in-falling micrometeorites. The study of the exosphere will therefore provide another clue as to the chemical composition of the surface

 

The Solar System