Download Text File
Chapter 2:  Mercury

The most comprehensive portrait ever of this scorched little planet.  Double sunrises, craters, cracks and - incongruously - maybe polar ice.


From a flattened disk of gas and dust, the birth of the Sun and planets - outwards from the rocky inner planets, past the outer gas giants, to Pluto and the icy debris at the edge of the Solar System.

From Earth, Mercury is visible only in the evening twilight sky - because of the planet's tight orbit around the Sun.  Mercury has a large, iron-rich core.

The first and, so far, the only space mission to Mercury - Mariner 10.  Mariner's pictures of the planet's surface.  Numerous craters, faults, lava plains, rugged highlands and a huge, multi-ringed basin.

Oddities:  Mercury's day is twice as long as its year.  From Mercury's surface, the Sun appears to grow and shrink in size.  Why there are double sunrises.  And, with such great extremes of temperature, could there be ice at Mercury's poles?  Despite continued research, more than half of Mercury remains unseen by rototic probes.


Building the Planets

*  The formation of the Solar System from a flattened, slowly rotating disc of gas and dust.

*  A tour of the planetary system, travelling out into the icy debris of the Kuiper Belt and Oort Cloud.

*  How the inner planets formed from the heaviest elements.

Closest to the Sun

*  Why Mercury is consistently difficult to see, being observable with the unaided eye only when low in the sky in the twilight of dusk or dawn.

*  The orbits of Mercury and Venus as viewed from Earth, plus transits of Mercury across the solar face.

*  Mercury as the densest planet, with a large iron-rich core.

Craters, Cracks and Basins

*  Mariner 10, the first space mission to Mercury, launched in November 1973.

*  Mariner 10's images reveal Mercury's surface detail for the first time.

*  Depiction of the giant impact which gouged a huge, multi-ringed basin on Mercury four billion years ago.

*  Mercury's surface features, including craters, faults, lava plains and mountainous highlands.

Weird Effects

*  Illustrating how Mercury's rotation causes the planet to spin one-and-a-half times during each orbit of the Sun, resulting in Mercury's day being twice as long as its year.

*  Presenting Mercury's elongated, oval shaped orbit.  How this causes the Sun to grow and shrink in the sky.

*  Explaination of Mercury's double sunrise.  When orbiting closest to the Sun, the planet's orbital speed increases, then slows again.  The result is the Sun's apparent rising, setting, then rising again.

Temperature Extremes

*  Extremes of temperature on Mercury.

*  The surprising discovery that ice may be present at both poles on Mercury - in deep craters permanently shielded from the heat of the Sun.

*  Mercury is only partially studied - its proximity to the Sun ruling out the possibility of close scrutiny with large telescopes.

*  For this reason, more than half of Mercury remains to be photographed, awaiting a second space mission to the planet.


Formation of the Planets

Most astronomers believe the Sun and the planets originated at the same time, from a cloud of interstellar material.  The cloud collapsed, forming a slowly spinning disc of gas and dust, the hot centre of which became the Sun.  As the centre contracted, the internal temperature rose to a point where nuclear fusion reactions occured, and the young Sun began to shine.

Simultaneously, the outer limits of the cloud cooled, the material beginning to solidify.  Small grains became pebbles and boulder-sized crags, which collided and coalesced - a process called accretion - forming larger objects known as planetesimals.  These planetesimals eventually accreted to form the planets of the Solar System.

The outer gas giant planets, orbiting within the cooler regions of the system, retained vast envelopes of the lightest gases hydrogen and helium, surrounding a rocky core.  The inner terrestrial planets, with higher temperatures and lower gravities, were unable to retain their gases, but became rich in minerals.  For this reason, the four inner planets, Mercury, Venus, Earth and Mars, have far higher average densities than the four gas giants, Jupiter, Saturn, Uranus and Neptune.

The Density of Mercury

Density is the amount of mass for every unit of volume of an object, ie - mass divided by the volume.  Whereas we can weigh and measure objects on Earth, we have to estimate the amount of space a planet occupies (its volume) and the amount of matter it contains (its mass).  Scientists do this by analysing the gravitational pull of a planet on nearby objects and by using data obtained by space probes.  The average density of a planet is obtained by dividing its mass by its volume.

Mercury has considerable mass for it size.  Although only slightly bigger than Earth's Moon, Mercury's mass is four times greater than the Moon's.  Mercury's density (5.42 grams per cubic centimetre), therefore, is comparable with that of Earth (5.52 grams per cubic centimetre).  This is due to Mercury's large iron core which accounts for 65 to 70% of the planet's mass, explaining both the high density and Mariner 10's detection of a significant magnetic field.  Mercury is the most iron rich planet in the Solar System.

Mercury's Double Sunrise

Due to Mercury's elongated orbit, its distance from the Sun fluctuates between 46 million and 70 million kilometres.  When Mercury is closest to the Sun, the Sun appears one-and-a-half times as large as when Mercury is farthest away.  The elongated orbit also means that Mercury travels one-and-a-half times faster when closest to the Sun, than when farthest away.

As Mercury orbits the Sun, its orbital motion causes the Sun to move from west to east across the sky.  At the same time, its axial rotation causes the Sun to move from east to west across the sky.  Normally, Mercury's east-to-west motion is greater than the west-to-east motion, so the Sun appears to move slowly from east to west - the time between sunrise and sunset being 88 days.  When Mercury is closest to the Sun, however, and Mercury's orbital velocity is greatest, the increased west-to-east motion of the Sun in the sky exceeds the east-to-west movement caused by the axial rotation.  The Sun, therefore, appears to slow down, stop, reverse direction for a while, stop again, then continue its east to west trek across the sky.  Viewed from a particular location on Mercury, the result is a double sunrise - or double sunset.

Links for Further Information:

A detailed site covering the Mariner 10 mission to Mercury.

A good site for images of Mercury.

A comprehensive site about Mercury, offering information at three different levels - Beginner - Intermediate - Advanced.

An informative site featuring general topics of interest about the planet Mercury.

A selection of thumbnail images from Mariner 10's mission to Mercury and Venus, with higher resolution images available.

Questions and Activities for the Curious:

1.  The Sun constitutes 99.86 per cent of all the matter in the Solar System.  Imagine you have been given a box of 100 apples.  Devise a demonstration that could be used to illustrate this point.

2.  If the diameter of Mercury is 4,878 kilometres, and that of the Earth is 12, 756 kilometres, find the volume of both Mercury and the Earth.  How many times could a globe the size of Mercury fit inside the Earth?

3.   When are the best times to observe Mercury with the unaided eye from Earth, and why is this so?

4.  Describe the different kinds of surface features found on the planet Mercury.

5.  Explain why Mercury's day, the interval between one sunrise and the next, is exactly twice as long as the length of its year - the time taken to complete an orbit of the Sun.

6.  The distance of Mercury from the Sun varies between 46 million kilometres (at its closest) and 70 million kilometres (at its farthest).  Make two sketches that show the relative sizes of the Sun as viewed from Mercury at these times.

7.  The temperature on the surface of Mercury can rise as high as 430 degrees Celsius.  If you had four metal pots, made of tin, lead, copper, and iron, research which ones would melt on the surface of Mercury.

8.   Mercury is the closest planet to the Sun, yet some astronomers believe there is water ice at the poles of the planet.  Discuss how this might be so.