Astronomy; The Planet Mercury - Facts and Photos
Credit To Nasa
All photos on this page are credited to Nasa.
My thanks to Nasa - without these spectacular images, this webpage would not have been possible.
This is the first of a series of pages in which I will look at the major objects which make up the solar system - the local group of planets and moons, asteroids and comets which orbit our star, the Sun. In this page I look at the planet which is closest of all to the Sun - the planet Mercury.
Of all the planets, Mercury has been among the most neglected, as it seems superifically at least to be among the less interesting. At first glance it really resembles nothing more closely than our own Moon - a barren rocky world strewn with craters and little else. But there is a bit more to Mercury than meets the eye.
And the planet has recently become the focus of much attention as a Nasa probe entered orbit around Mercury - a mission which, no doubt, will add much to our sparse knowledge of this world.
VISIBILITY OF MERCURY IN THE NIGHT SKY
It may come as a surprise to many, but Mercury - largely unnoticed by the majority of casual observers of the night sky - is potentially easily visible to the naked eye. I say 'potentially' because its maximum brightness is comparable to the very brightest objects in the sky (the scale which is employed by astronomers to measure this is called 'apparent magnitude' and Mercury's apparent magnitude varies from +5.7 (barely visible in the clearest of skies) to -2.3 (the sixth brightest object after the Sun, Moon, Venus, Jupiter and Mars).
However, because Mercury is the closest of all planets to the Sun, it never strays more than 28 degrees from the Sun in our line of sight, and this means that it rises and sets on the horizon more or less with the Sun, and is usually lost in the star's overwhelming glare. Even if visible at dawn or dusk, (when the Sun is diminished in brilliance), the planet is so low on the horizon that heavy atmospheric turbulence impedes clear vision.
When viewed through a telescope, Mercury can often appear as a crescent. This is because we often see the planet to one side of the Sun; we therefore see only the sunlit side of Mercury, just as we often only see the sunlit side of our Moon resulting in its familiar crescent shape.
THE HISTORY OF OUR KNOWLEDGE ABOUT MERCURY
Despite the difficulty of viewing a planet so close to the Sun, Mercury is one of five planets (and Earth) which were known to the ancients. Indeed, it was known at least as long ago as 3000 BC. But for many centuries the planet was believed to be two distinct bodies - one which appeared in the hours around dawn, and another which appeared in the hours around dusk. The early Greeks had separate names for these 'two' planets - Apollo and Hermes respectively. Later Greek scientists however knew full well that Apollo and Hermes were, in fact, just one planet (and some even figured that it probably revolved around the Sun, and not the Earth (a theory which was only proven to be fact in 1639 by Italian astronomer Giovanni Zupus).
Hermes was the Greek Messenger of the Gods, and his Roman equivelant was Mercury, and it is this name by which we now know the closest planet to the Sun. (The Gods Hermes and Mercury were swift-footed, and the planet Mercury has the most rapid motion of all planets across the sky and around the Sun, and this is probably the reason for the Roman association between the God and the planet).
THE CURRENT MISSION TO MERCURY
The ongoing Messenger mission to Mercury has meant that the planet has received much more attention in recent years than previously. Our knowledge of the planet will be greatly increased as a result of this mission, and about 75,000 images including many improved colour photos should be received from the surface, superceding some of the older black and white pictures on this page.
Eventually I will either revise this page, or compile a second page detailing the new information, though it will be some months before all of the information transmitted from the Messenger mission is fully analysed and interpreted.
MODERN EXPLORATION OF MERCURY
Although it was first observed with a telescope by Galileo, this planet is not easily viewed with ground-based instruments, and apart from the phases of the planet as described above, little else of interest could be discerned about the surface features of the planet until the 1960s.
(At this point can I offer the hopefully needless warning about viewing Mercury through binoculars or telescopes? The proximity of the planet to the Sun makes such viewing a potentially disastrous exercise without proper filtration of the light. Catch the Sun in the field of view, and blindness may be the result).
There have been just two space missions to investigate this small world. In 1974-5 the Mariner 10 space probe made the first visit to Mercury. 40-45% of the planet was mapped during three fly-bys, revealing much data about the surface features.
Another 30 years passed before America commenced its second big mission to Mercury. The orbiting probe Messenger launched on 3rd August 2004. It reached Mercury in 2008 and over the next two years made three photographic fly-bys. Messenger finally assumed an orbit around the planet in March 2011, and began to intensively map the surface, taking more than 100,000 images. Data has also been collected by the probe about the interior structure and magnetic field and much else.
BASIC FACTS AND FIGURES
DISTANCE FROM SUN - This varies from about 46 million kilometres to 70 million kilometres. The average distance is 58 million kilometres (36 million miles). Mercury has the most elliptical orbit of any planet.
DIAMETER - 4878 kilometres (3000 miles). Mercury is the smallest of all the 8 currently recognised planets. The surface area is about 14% as big as the Earth, but Mercury is rather larger than our Moon.
GRAVITY - Gravity is about 40% as strong as it is on Earth.
LENGTH OF YEAR - 88 Earth days.
LENGTH OF SIDERIAL DAY (Period of rotation) - 59 Earth days.
LENGTH OF SOLAR DAY (Sunrise to Sunrise) - 176 Earth days.
THE MOVEMENTS OF MERCURY WITHIN THE SOLAR SYSTEM
Mercury rotates on its own axis, with the smallest axial tilt of any planet, in about 59 Earth days, This is a rotational period (also known as a 'siderial' day) slower than that of any other planet except Venus. Revolution around the Sun takes just 88 Earth days which is the shortest year of any planet. It is also a highly eccentric revolution; at times Mercury is 70 million km (43 million miles) distant from the Sun, but at its closest approach is a mere 46 million km (29 million miles) distant. At its closest approach the Sun is about 3 times bigger in the Mercurian sky than it is here on Earth.
The relationship between the period of rotation and the period of revolution is quite interesting. Simple mathematics shows that a 59 Earth day rotation on its axis, and a revolution of 88 Earth days means that Mercury rotates exactly 3 times in 2 of its years. The consequence of this is that although a rotation or 'siderial day' is 59 Earth days in length, the actual interval between one sunrise and the next is closer to 176 Earth days, and the same face is directed towards the Sun's heat, (or shielded from it on the dark side) for 88 Earth days.
(For a diagrammatic representation of this complex rotation/revolution and siderial day/actual day relationship, follow the Mercuryenchantedlearning link)
THE SURFACE FEATURES OF MERCURY
This planet is much more similar in appearance to our own Moon than it is to any of the other planets in the Solar System. Like the Moon, Mercury is a dark world reflecting just 6% of the light it receives. Also like the Moon the geography of the surface comprises large cliffs and escarpments, great flat plains, and huge numbers of craters. The cliffs and escarpments, some of which are hundreds of kilometres in length, and as high as 2000m, may result from contraction of the planet's surface crust, and from the upheavals caused by meteor impact. The origin of the comparatively featureless plains is unknown, but it is possible that they may result from ancient volcanic activity. There are even more craters on Mercury than on the Moon, and in the areas so far imaged, they are more evenly distributed. Mercury has virtually no atmosphere (see below); as a result, small meteors do not burn up as they do when they approach the Earth, and therefore far more will hit the surface creating impact craters. A lack of atmosphere also means a lack of wind or water erosion, so it is possible for craters to survive for millions - even billions - of years intact.
Probably the most famous feature on Mercury is the huge Caloris Basin, bigger than the State of Texas. It's origin is unknown; there has been speculation that It may have been formed by a massive meteor impact early in the history of the Solar System, though volcanic lava flows also seem to be a possibility. If it is indeed a meteor impact crater, then it is the largest of its kind known in the Solar System.
One of the most surprising revelations of recent years is the possible detection of water ice on Mercury. Initially it was Earth-based radar in the 1990s which made this discovery, but recent images from Messenger appear to confirm it. This water ice, if it does exist, is not of course to be found in regions exposed to the intensely hot sunlght, but may be present in craters near the North and South Poles where the floor beneath the crater rim is permanently shielded from the Sun. If ice exists here, it probably originates from cometary or meteor collisions. Another possibility is that water vapour buried deep inside the interior has escaped and frozen at the surface.
At the time of writing this is pretty much all that is known about the surface of Mercury, although the current Messenger mission is already imaging parts of the planet never seen before. Much more information will be forthcoming from Nasa's planetary scientists in due course.
THE GEOLOGY OF MERCURY
The geology of Mars is not particularly well understood, but it is known to be the second most dense planet or moon in the Solar System (second only to our own planet Earth). Although smaller than Ganymede and Titan (the great moons of Jupiter and Saturn), Mercury is more than twice as massive, or heavy. The reason for this great density and mass is that Mercury possesses a substantial iron core. Indeed Mercury's core is proportionately much larger than that of the Earth, and has for some time been believed to exceed half the diameter of the planet. Recent evidence from Messenger suggests the core may amount to substantially more than this - an extraordinary 85% of the planet.
Mercury has a small but significant magnetic field, about 1% as powerful as Earth's magnetic field. For a magnetic field to exist around a planet, convection currents emanating in the core, and a rapid rotation of the planet on its axis, are normally required. In the case of Mercury, planetary rotation is very slow, and would be expected to have a negligible effect. Therefore, convection currents are assumed to be responsible for the magnetosphere. However, convection requires fluidity, and it had long been assumed that the small size of Mercury would have led to the core cooling and solidifing long ago. The presence of even a weak magnetosphere indicates that the core is at least partially molten, though scientists are unsure how such a small planet could have retained a molten core over billions of years.
By contrast to the massive core, the rocky outer mantle and particulate silica crust of Mercury seems to be relatively thin at only 500 to 600 kilometres (300-370 miles) thick. (The Earth's mantle and crust is more than 3000 kilometres thick, and our Moon is composed almost entirely of this rocky matter). It remains to be discovered why Mercury's core is so thick, its crust is so thin, and its geological make-up is so different to the other inner planets of the Solar System.
The role of volcanism on Mercury is also unclear. As already indicated, ancient volcanic lava flows may be responsible for some of the surface features of the planet, and data from Mariner 10 even suggests that recent volcanism may have taken place, but this is far from being proven. The presence of so many intact craters on Mercury somewhat contradicts this, as intact craters are evidence of a lack of disruptive geological activity.
THE ATMOSPHERE AND CLIMATE OF MERCURY
Mercury is the closest planet to the Sun, and the the sunlight here is about 7 times as intense as it is on Earth. Unsurprisingly this planet is extremely hot on the Solar side (albeit not quite so hot as Venus which suffers from a runaway greenhouse effect). However, the very peculiar relationship between the planet's revolution around the Sun and it's rotation on its own axis, means that one side of the planet is shielded from the Sun's heat for a full year at a time, and this results in Mercury having the greatest temperature range in the Solar System, from minus 183 °C (-297°F) to plus 427 °C (800 °F).
There is effectively no atmosphere on Mercury. There is, however, an extremely thin 'exosphere', which consists only of atomic particles of various elements blasted from the surface dust by the Solar wind or by striking meteors. In the extreme temperatures of Mercury, these particles quickly escape the planet's low gravitational pull, but then the processes which liberate these particles, also regularly replace them.
SUMMARY OF INTERESTING FACTS ABOUT MERCURY
Mercury has the longest period of daylight of any planet in the Solar System.
Mercury has the most elliptical (least circular) of all planetary orbits.
Mercury has proportionately, the largest iron core of any planet.
Mercury and Venus are the only planets without attendant moons.
Mercury has the most extreme temperature range in the Solar System.
Mercury - like all the planets of the Solar System - is blessed with so many unique features. However it seems, without doubt, a barren world. It is also a planet which lacks a moon of its own, so it travels through space alone - or at least it did until March 2011 when the Messenger orbiter became a temporary, artificial attendant. Mercury does not feature on the Wonders of the Solar System discussion linked to this page as there are worlds with more obviously diverse atmospheres and geologies than exist here. However, there is no such thing as a dull world, and the current Messenger mission will undoubtably reveal some extraordinary new facts about this enigmatic planet of eccentric orbits, mysterious internal geology, and extreme temperatures.
For up to date information about the Messenger mission, you may find it useful to go to the official Nasa page, by following the highlighted link.
IF YOU WISH TO LEARN MORE ABOUT MERCURY ...
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