Astronomy Beginner’s Guide: Mercury

By Duncan Lunan.

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington, Public domain, via Wikimedia Commons

For an adjective derived from Latin and relating to Mercury the grammatically correct usage would be ‘Mercurial’, like ‘Jovial’, ‘Martial’, ‘Saturnine’ and ‘Venereal’, and those are the words used by Jonathon Norton Leonard in “Flight into Space”  (Sidgwick & Jackson, 1953).   But all of them have hangover meanings from the remnants of astrology, so the rest of us use the ungrammatical modern versions when talking about the actual planets.  ‘Venusian’ is a particularly unattractive one and ‘Cytherean’ isn’t much better.  ‘Hermian’ from Greek is one of the few classical adjectives not to have been corrupted by astrological meanings, but it’s sufficiently obscure that most modern writers say ‘Mercurian’ anyway.

Mercury stays close to the Sun and is always in the twilight when seen from Britain;  best plan is to locate it first with binoculars before trying to find it with the telescope.  It can be seen as a disc, but only large telescopes will show markings.  Pluto and Mercury are the only two planets with sufficient orbital inclination to appear occasionally outside the band of Zodiacal constellations.  Mercury’s orbit is also one of the ones with highest eccentricity, and since it’s only 36 million miles from the Sun on average, orbiting in only 88 days, the difference in speed at perihelion  (nearest to the Sun)  is enough to produce relativistic effects on its orbit – showing as a revolution of its long axis around the Sun which at one time was thought to be due to the pull of an inner planet, Vulcan.

From time to time nature plays jokes on astronomers.  The classic example is the rotation of Mercury, which is never easy to observe because it’s small, far away and so near the Sun.   Antoniadi and others had recorded blurred dark markings, and indeed the maria in the impact basins of Mercury are much patchier than those on the Moon.  But sometimes they couldn’t be seen, and at others they were in exactly the same places.  So it was concluded that Mercury had an atmosphere dense enough to support clouds, and that it had a trapped rotation, keeping the same face always to the Sun as the Moon does the Earth.  That in turn raised the possibility of life in a habitable ‘twilight zone’, the origin of the expression given another use by Rod Serling’s TV series.  One particularly intriguing detail in the mediaeval mystery of the Green Children of Woolpit, discussed in my book “Children from the Sky”, was that the children said they came from a ‘land’, or world, of permanent twilight, separated from a country of permanent sunlight by a very broad river.   In the early 1600s Robert Burton knew that such conditions couldn’t be found anywhere on Earth, so he put the incident in the astronomy section of “The Anatomy of Melancholy”, where I discovered it as a student.

By the 1950s it was obvious that Mercury had no atmosphere:  repeated transits of Mercury across the face of the Sun had shown no sign of one, so the changing visibility of features remained a mystery.  It was still thought that the planet had a trapped rotation, keeping the same face to the Sun, which would swell and shrink noticeably in the sky as the planet moved along its elliptical orbit.  ‘Brightside Crossing’, a classic Mercury story of 1956 by Alan E. Nourse, features a record-setting attempt to do that when the Sun is at its closest – but all the while the dark side of the planet would be as cold as Pluto.  The story was reprinted in his collection “Tiger by the Tail”  (Dobson, 1962), and in “The Science Fiction Solar System”, edited by Asimov, Greenberg and Waugh, which also included a story of mine. 

The intensity of sunlight near the orbit of Mercury is often exaggerated, especially in the 2007 blockbuster Sunshine – where the Sun is supposed to be nearly out in any case.   Sunlight at Mercury has about ten times the strength it has here, but in the mid-1970s Mariner 10 made three flybys of Mercury, 176 days apart, using only a light parasol and angled solar panels to cope with that.  

An important part of the Mariner mission was to search, in the ultraviolet, for small satellites of Venus and Mercury.  They weren’t seriously expected due to what Isaac Asimov called ‘the tug-of-war effect’, the same one which makes the orbits of satellites of satellites unstable and causes them to crash into their primaries.  The critical limit is where the Sun’s pull is 1/30 of the planet’s.  As applied to Mercury, he calculated that would be only 2093 km from the centre, within the Roche Limit where tidal forces would pull any sizable moon apart.  (‘Just Mooning Around’ in “Asimov on Astronomy”, Macdonald & Jane’s, 1974.)   Since Mercury’s diameter is 4800 km, it would actually be underground!   A u-v source was spotted, first against the dark side of the planet and then off the edge of it, but that was later identified as the star 32 Crater  (A799-800).  The first source might have been auroral activity, but then it was discovered that there was no significant atmosphere, none over the nightside and only a bowshock in the solar wind on the sunlit face.  Although two more flybys were achieved before the spacecraft’s attitude control gas was exhausted, the same face of Mercury was turned towards the Sun each time, so we never saw what was on the surface at the site of the u-v anomaly.  

The puzzle of Mercury’s appearing and disappearing features was solved in 1965 when radar studies revealed that Mercury rotates on its axis in 59 Earth days.  It turns three times on its axis, with respect to the stars, in two orbital revolutions about the Sun;  from a solar or planetary viewpoint, it makes half a turn in each Hermian year, so there are opposite ‘heat poles’ on the equator taking turns to receive the worst summer heat.  This means that every other year the same ‘heat poles’ of Mercury take turns to face the Sun at its closest.  One of them is occupied by the huge impact feature of Caloris Basin, the Bowl of Heat, and the other by the ‘chaotic terrain’ created where its shockwaves were focussed on the far side of the planet.

But although there’s no frozen atmosphere at Mercury’s equator, in 1991 radar scanning by the Very Large Array in New Mexico found evidence that there is ice at Mercury’s north pole.  It shouldn’t have caused that much surprise because the Scottish astronomer V.A. Firsoff had calculated that ice caps on Mercury were possible  (letter to The Observatory, April 1971).  Patrick Moore wrote, “I admit to being profoundly sceptical.  It does not seem likely that there has ever been water on Mercury, and without water there can be no ice.”  (“New Guide to the Planets”, Sidgwick & Jackson, 1993).  There are areas of permanent shadow near the poles, and radar scanning shows that apparently there is an ice-cap at the north pole.  Presumably the water vapour was released from cometary impacts, and there’s ice in  ‘cold trap’ craters at our own Moon’s poles;  the Messenger orbiter proved that the situation is the same on Mercury, and also found widespread evidence of volcanic activity, including metallic ‘ice’ on craters further south, so some of the water at the poles may have come from within the planet after all.

My friend Jonathon Vos Post came up with some intriguing uses for the ice at the Hermian and lunar poles, which the spaceflight society ASTRA published in our journal Asgard with a cover by Andy Paterson showing a robot ice-sampler on Mercury  (1998).  At the time of the discovery in 1991 I asked Lorna Napier to come up with a cartoon cover for our magazine Spacereport, featuring the galactic figure-skating championships on Mercury.  She drew a six-legged creature on its back with all its skates in the air, while the judges behind are holding up score cards for technical skill and artistic interpretation:  ‘0,0’, ‘0, 0’, ‘0, 0’…

Because Mercury rotates on its axis in two-thirds of its year, it rotates with respect to the Sun once over two of its years.  Nobody had noticed that the markings disappeared on every second presentation.  Otherwise, the rotation would go out of synchronisation when the planet was moving faster at its closest to the sun:  as Mercury approaches perihelion, at the terminator the Sun comes back up where it previously set, even larger than before.  

Even when Mariner 10 was in flight to Mercury in the 1970s, it was being predicted that the surface would be ‘relatively crater-free’ because the planet was so far from the asteroid belt.   Instead it turned out to be heavily cratered, from the Final Bombardment 4200-3900 million years ago in the last phase of the Solar System’s formation – although due to the higher surface gravity, the surface isn’t ‘saturated’ and there are gaps between the craters which may be primal Solar System rock.

We know now that impacts have been a much greater force in the evolution of the Solar System than astronomers used to think.  The Moon is formed from the crust that ripped off a Mars-sized protoplanet in a collision with the proto-Earth;  the Asteroids were formed by the collisions of protoplanets under the influence of Jupiter.  Many of them have huge impact craters, almost big enough to break them up and possibly big enough to have ripped off their crusts by the effect called ‘spalling’.  This may have happened with Phobos, the inner moon of Mars, on the formation of the crater Stickney, and with Mercury on the formation of the huge Caloris impact basin.  

Mariner 10 added to that with the discovery of huge cliffs, hundreds of miles long and thousands of feet high, cutting across the older features on Mercury’s surface.  In “Higher than Everest, an Adventurer’s Guide to the Solar System”, Prof. Paul Hodge imagines an expedition to the Discovery Rupes, one of the largest of these  (Cambridge University Press, 2001).  

The unphotographed hemisphere of Mercury remained the largest unexplored surface in the Solar System.  In “Solar System” by Peter Ryan and Ludek Pesek  (Allen Lane, 1978)  the two quadrants we had seen were captioned ‘Mercury West One’ and ‘Mercury West Two’.   On 3rd August, 2004, the Messenger probe was launched on a six-year mission which brought it to orbit around Mercury and answered many of the questions, while generating new ones.   Since every planet and moon studied so far had one hemisphere markedly different from the other, we were prepared for surprises:  one we’d already had, in 2007, was that Mercury does have a liquid core after all.  That made it possible to account for the formation of the huge scarps:  the planet is shrinking as the core solidifies towards the centre.

The first Messenger views clearly showed that the other hemisphere was much smoother, with more extensive lava plains than the heavily cratered surface we had seen.  There are more examples of scarps, and there are some very strange craters.  There are also several ‘parentless’ crater chains like the ones on Ganymede and Callisto, which are thought to be caused by the break-up and subsequent impact of comets like Shoemaker-Levy 9, which hit Jupiter in 1994.  When the rest of Mercury was photographed by Messenger on 6th October 2008, from the new angle the planet looks just like a peeled orange, with straight lines running from north pole to south.   (Emily Baldwin, ‘Messenger Revisits Mercury’, Astronomy Now, December 2008).  They may be rays of ejecta from craters, but if they’re fault lines or scarps they suggest a major change in the planet’s rotation, probably due to an oblique impact, before it became tidally locked to the Sun in the present 2:1 ratio.

With its high metallic content, active magnetic field, lots of solar power and high temperature gradients, Mercury has a huge industrial potential:  the Mercury chapter of my “Man and the Planets” was titled ‘The Workshop of the Solar System’, and the ice at the poles makes it an even more interesting place.  A planet with a trapped rotation would probably not have a magnetic field, but Mercury’s is active, not ‘frozen’ as was first thought when it was discovered by Mariner 10.  (Venus has none, apparently due to its slow rotation, and Mercury’s is small for the same reason, but it’s there and active, not a residual field as has been thought for most of the last forty years.).  During the North Lanarkshire Astronomy Project, one of our regular presentations to schools was on ‘Comets and Meteorites’, with a display of them lent by the late John Braithwaite.  We had fragments of lunar and Martian meteorites, and on the internet my colleague Bob Graham picked one up which  (according to the label)  wasn’t terrestrial rock, but had formed in the presence of a magnetic field, so it could have been from Mercury.  As a result of the Messenger mission one class of meteorites has now been identified as coming from Mercury, so maybe it really was.

Messenger’s mission ended on 30th April 2015, when it was low on fuel and was deliberately crashed near a crater named ‘Shakespeare’, close to Mercury’s north pole.   Two and a half years later, Europe’s Bepi-Colombo probe  (named after a pioneer of planetary slingshot theory)  was launched, making an Earth slingshot on 9th April 2020 and the first of two Venus flybys on 20th October.   The second will be on August 11th this year, and in October it will make the first of six flybys which will bring it into orbit in 2025 – after which it will separate into two spacecraft for different studies, and the mission story will really start.

Click on this link for more information on Mercury: NASA Mercury

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