Boy Mercury shooting through every degree
The B52s, ‘Roam’, (c) 1989 CE
Most people, if they wanted to associate music with the planet Mercury, would probably either think of Freddie Mercury or Gustav Holst’s Planet Suite. Not me of course, because I can’t think of the obvious. It seems that this song has erotic innuendo which totally whooshed over my head, but that still doesn’t exempt it from being associated with Beppicolombo today. So far as I can tell, there’s nothing particularly special about today’s encounter compared to the series of other encounters which the probe will undergo over the next few years, but it’s also true that Beppicolombo is only the third spacecraft ever sent to the planet in question. The first, Mariner 10, flew past in 1974 and erroneously reported the presence of a moon, and I think that was also the one which established that Mercury didn’t simply show one face to the Sun all the time. Certainly this is what was reported in the popular science books and articles I read at the time. It also detected a strong magnetic field, which is apart from Earth the only planet in the inner solar system with one.
MESSENGER was the next probe, whose mission took place in the first half of the 2010s. The problem is that Mercury is difficult to reach because spacecraft have to be moving relatively fast and because it’s near the Sun the gravity of the star will overwhelm that of the planet at a low altitude above the surface, since it’s also the smallest and least massive major planet. This would, incidentally, make the presence of any moons hard to maintain. But Mercury is not just a clone of Cynthia even though the two seem quite similar, and even are to some extent.
Mercury and Earth are the densest planets in this Solar System. They also both have strong magnetic fields. The surface gravity is close to that of Mars, but because it’s physically smaller and a lot hotter it has much more difficulty holding onto an atmosphere, which is extremely thin and consists of what to us would seem like a bizarre array of gases such as calcium, sodium, potassium, hydrogen, atomic oxygen, helium and molecular oxygen along with water vapour. The hydrogen and helium are captured from the solar wind by the magnetic field and I presume the water vapour is from ice in polar craters. Because it has hardly any axial tilt, there are craters near the poles, such as Chao Meng-Fu, which are in permanent darkness at their bottoms, where the ice resides. The metals are forced away from the surface by the Sun and form a tail so many millions of kilometres long along the orbit that they are something like an eighth of the way round before they become undetectable. This feature is shared with Jupiter’s moon Io, which also has a sodium tail. However, it seems a bit of an exaggeration to dignify the sparse atoms and molecules of gas hanging around near the surface as an atmosphere, since they never collide with each other like they would in an ordinary gas, but do the same kind of things as they do on Cynthia, ricocheting off the surface, bouncing up and down and so forth.
So far as I know, Beppicolombo has no colour cameras. It was also going to deposit a lander, which it didn’t do in the end because it would’ve been too expensive. Both of these decisions, if the first is true, strike me as bad PR. Colour photos of Mercury and data, and hopefully images, from the surface would surely be really impressive, and it’s worth doing those just to engage the public, but apparently not.
Just a quick infodump to get all this out of the way. Mercury is intermediate between Cynthia and Mars in size, is the densest planet in the Solar System other than Earth and has a lemon-shaped orbit, which is again the most elliptical of any solar planet known. It rotates once every fifty-seven days with a negligible axial tilt and orbits once every eighty-eight. It isn’t as hot as the solid surface of Venus during the day, at around 400°C, but is the coldest planet in the Solar System at night at -200°C. It has a fairly heavily cratered surface and it can be difficult to distinguish whether a small portion of the surface is Cynthia or it. It was instrumental in corroborating the General Theory of Relativity which predicted that its orbit shifted its angle by 1.2 arcseconds each time, but before Einstein it was thought that there was an even closer planet to the Sun, named Vulcan, which explained this orbital perturbation. There are Mercury-crossing asteroids, including the relatively famous Icarus. Astrologically, Mercury often goes retrograde, meaning that it appears to reverse its direction in the sky, because it’s orbiting inside our orbit and will inevitably dip towards or fall away from the Sun from our perspective. There are even some professional astronomers who have never seen it because it stays so close to the Sun and is smaller and further away than Venus along with reflecting less light. It can, however, be observed in broad daylight with the right telescope if you know where you’re looking, though this would be risky to the eyesight. I think that’s it as far as what I assume “everybody” knows about the planet.
The reason it used to be thought that Mercury always faced the Sun was that it rotates three times for each two of its years and its synodic period (the time taken between successive closest approaches to us) is almost exactly two Mercurian days.
The above map was made by the Greek astronomer Antoniadi in the mid-twentieth century. Although like many such maps it’s pretty inaccurate, it does at least record the presence of Caloris Basin in the southeast as Solitudo Hermæ Trismegisti. Some of the features on Mercury have quite odd names. For instance, there’s a series of cliffs called Pourquoi Pas Rupes and the twentieth longitudinal parallel is called Hun Kal after the Mayan for twenty. Caloris Basin is somewhat similar to the Mare Orientale on Cynthia or Asgard and Valhalla on Callisto, being a vast impact crater, but Mercury doesn’t really have maria like Cynthia.
There should as far as possible be a link between people’s everyday experience and scientific phenomena. This is difficult with Mercury because it’s almost invisible to most people. If you believe in Western astrology, you’re probably aware of Mercury retrograde at least, and Mercury does transit the Sun more often than Venus from where we are. This is where Mercury can be seen to cross the Sun’s disc, meaning that it might be projectable using a telescope. They happen in May or November, and occur much more often than Venus at about once every seven years on average. Sometimes Mercury only passes over the edge of the Sun. The planet is both smaller and further away than Venus when it transits. Venus I have observed doing it, and it gave me a major impression of the sheer size of the Solar System that even the nearest planet, practically Earth-sized, looked that tiny when it was closest to us. Mercury is kind of more like Cynthia orbiting alone. One significant issue with Mercury’s transit compared to that of Venus is whether the black drop effect would be visible. When Venus, with her thick atmosphere, crosses the limb of the Sun, there’s a kind of fuzzy line joining the shadow to the rest of the sky, and this is often attributed to that atmosphere, but in fact Mercury, with no significant atmosphere, exhibits the same effect even when observed from space, thereby eliminating the factor of our own air. Hence it doesn’t seem to be due to a planet having an atmosphere. This is also significant for detecting planets orbiting other stars and whether they have atmospheres. Incidentally, it’s also possible to observe changes in light level caused by transits by observing moonlight, although of course it’s very subtle. There will be a simultaneous transit of the two planets in the year 69163, and before that Mercury will transit the Sun during a partial solar eclipse in 6757.
Meteorites very occasionally reach us from Mercury. One was found in the northwestern Sahara containing chromium diopside crystals, which are green, but may not be Mercurian given known facts about the composition of the surface. Although this is not the meteorite, this is what chromium diopside looks like:
It reminds me of lunar olivine, and like it, is a semi-precious stone. This is an actual chunk of the meteorite itself:
It’s a magnesium calcium silicate, and can become asbestos. There are faults on the surface of the planet, meaning that like Earth and no other planet in the system, it’s tectonically active, which in turn means that if this mineral is indeed from Mercury, it could have been transformed to asbestos on its surface. However, this may not make it intrinsically more hazardous to potential astronauts (and there will be none) than moondust, which is also potentially quite harmful, being jagged and unoxidised until it comes in contact with a terrestrial organism and this rock may not be from Mercury anyway. I would imagine that the extremes of temperature there have considerably weathered the terrain.
The interior is largely taken up by a core rich in iron and the magnetic field may be caused by the same dynamo effect as here, since the Sun’s tidal forces are much stronger there than here, or it may be residual from formation or the result of being directly magnetised by the solar magnetic field. I don’t know if this is true, but I would expect the crust to be higher in heavy elements than here, and for them to be more exposed due to the lack of weather and oxygen, although I would also suppose that their distribution would tend not to be in the form of specific ores due to the lack of liquid water. There are no Van Allen belts because the magnetic field is too weak in comparison with the solar wind. Heat could also be expected to weaken the magnetosphere.
Hun Kal is at 20° because the prime meridian had been decided approximately as the subsolar point at the first perihelion in 1950, and when Mariner 10 got there that location was on the night side. At the time, presumably it had been thought that that point was permanently at noon with the Sun directly overhead.
Caloris Basin is so-called because it’s directly under the Sun at closest approach, and is therefore the hottest area on Mercury. It has about the same area as Mexico, which by scale is similar to the size of Antarctica compared to Earth, and is surrounded by a ring of fairly small mountains. It’s many times the size of Mare Orientale. Around the exact opposite point is an area of so-called “weird terrain”, which is hilly and thought to result from the conduction of seismic vibrations around the planet from the impact into a focal point there. Just as on Earth the type and deflection of quake waves is like an X-ray of the planet, revealing where the solid core is, so does the terrain on the opposite side from Caloris Basin reveal Mercury’s internal structure, since much of it was formed by the shock waves. Superimposed on that are the ejecta splashed up by the impact, which also travelled all round the planet.
Unique to Mercury are the “blue hollows”. Although these are somewhat mysterious they seem to be linked to the evaporation of solid material and resemble craters to a limited extent except for showing none of the usual signs other than being dents in the surface. There’s no rim, central peak or ejecta. They are of course blue, light blue in fact.
The planet seems to have shrunk by seven kilometres since its formation, which has led to ridges appearing on the surface. I wonder if this is to do with substances such as potassium and sodium with low sublimation points being lost to space during the day, which I also think might explain the hollows.
There’s something about craters, though, which I find somehow tedious and deadening. I could go on and on about the craters there at this point but it would probably bore you stiff. And the question there is why? Mountains are not boring after all, are they? This links into my post about whether Cynthia is boring. I suppose the thing about mountains is that you can imagine climbing or exploring them. But a crater such as Arizona Meteor Crater seems very interesting to me, as does the Chicxulub Impact which wiped out the non-avian dinosaurs. Maybe it’s just me. So that concludes this bit of the post.
Only three spacecraft have ever been sent to Mercury. The first of these was in 1974, Mariner 10. For over three decades this was the only source of images of the planet and only just over a third of the surface had been photographed. By a stroke of luck, Caloris Basin was at the terminator at the time, meaning that the weird terrain was also, but this also meant that the full extent of the basin was unknown. It also flew by Venus. Mariner 10 was the first spacecraft to use the gravity of another planet to aid its trajectory and also the first to send back live TV pictures of another planet, although I would expect “live” to be a fairly misleading description of something whose bandwidth was 117.6 kilobaud. This is actually pretty impressive when you consider modems of the turn of the millennium were only half that fast. Because it used the gravity of Venus, it didn’t need to carry much fuel, as that was only needed to make fine course corrections, which it did by attitude adjustment nozzles firing nitrogen along the edges of the two solar panels. It used a sunshade to protect its instruments against the intense radiation at the orbit of Mercury. Like many other space probes it was designed to orient itself using the Sun and the bright star Canopus. It carried a TV camera connected to a Cassegrain telescope, which gave it a long focal length in a short tube, able to image things in ultraviolet as well as visible light. The resolution was a total of 640 000 pixels, which is 800×800 if that’s the aspect ratio decided, each pixel being represented by a byte. There was also a radiometer able to measure temperatures to within 0.5°C, a plasma detector which discovered Mercury’s magnetic field, a magnetometer, a second telescope to detect charged particles which also detected the magnetic field and an airglow spectrometer which was able to detect the glow of sodium in the atmosphere and beyond. This is actually bright enough to be seen by the human eye, so looking into the sky on Mercury an astronaut would perceive a faint orange tinge. Another instrument was able to detect gases by absorption of light.
When I look at something like that, it always makes me think that technology even that long ago was a lot more advanced than we give it credit for. Although it obviously wasn’t using internet protocols, this probe was able to transmit wireless data over millions of kilometres at twice the rate of a dial-up modem two dozen years later, and 800×800 resolution in eight-bit colour, which is what this and many other spacecraft had in conjunction with Mission Control, wasn’t achieved in affordable PCs until the late 1990s either. On the other hand, the processing power of these machines was very limited. Although I can’t track down the details, Mariner 10 cannot possibly have been using a microprocessor to do its stuff and even the Mars rovers only used CPUs which went out of date in about 1980. This isn’t so much a criticism of them as the hardware which exists now. If you can build a spacecraft which goes to Mercury and does all that stuff without even using a microchip, and if later on very modest processors indeed can be used to achieve even more, why are we now using so much more advanced computers to do much less impressive stuff?
We had to wait until the next century for the next probe, MESSENGER. This is named after the messenger of the gods, Mercury, but it actually purports to stand for “MErcury Surface, Space ENvironment, GEochemistry, and Ranging”, clearly a backronym. MESSENGER managed to image the entire globe, as it was designed to go into orbit around it. It detected the first clear images of the blue hollows, which Mariner 10 had only managed to get rather blurry pics of. It imaged the whole of Caloris Basin, measured the concentration of calcium over the planet anddiscovered that the magnetosphere was at twenty degrees to the axis of rotation. It also imaged a “family portrait” of the whole Solar System and was eventually crashed into the planet. It described one of the more recent and to me baffling trajectories which seem to involve a large number of orbits around the Sun while the spacecraft gradually approaches its destination, therefore taking several years to reach it. If you think about it, no destination within our orbit ought to take more than half a year. I’m sure there’s an answer.
Bepicolombo is of course the current mission. It’s joint European-Japanese and I’d expect it to be a lot more sophisticated again, although I’m not sure what that would mean. Like MESSENGER, and presumably all contemporary probes, it’s doing the same kind of weird orbit which takes a very long time to get anywhere. I really want to know what that’s about. It comprises a photographic orbiter and a magnetosphere investigating satellite – two different satellites and is named after the scientist who came up with the slingshot idea for Mariner 10. Since its name is in the title, I’d better go on about it.
It’s flown by Venus twice, the second time on 10th August this year (2021) and has just flown past Mercury for the first time, only seven and a half weeks after Venus. I think this may be the fastest journey between planets ever. It will fly by the planet a further five times, then go into orbit round it on 5th December 2025. It has an ion drive, using xenon. It’s about time really – the concept behind these, which is to ionise atoms and accelerate them out of the engine using linear induction and can theoretically accelerate up to over 160 000 kph, is decades old and even though it has one I have my doubts about whether it’s really using it in earnest or it’s just there to be fancy. It isn’t being used as the main propulsion system and will only be used with a very low thrust, and there are also chemical rockets. One of the instruments was built in Leicester, which makes me happy. It isn’t the first time either.
Mercury is seen as breaking a pattern because for the other terrestrial planets there is a relationship between their density and their size, such that the smaller a planet is, the lighter the materials it’s made from are. This applies to Cynthia as well. However, Mercury is an exception. It’s also particularly close to its (and our) sun and this is a possible clue as to what happened in other solar systems, which also have very close planets. There’s a hypothesis that Mercury was previously a gas giant but has lost all its atmosphere because it fell so close to the Sun, but I think this idea is deprecated now. It also has an unusual orbit, which is also strongly influenced by the other planets in the Solar System. For all these reasons, the European Space Agency and the Japan Aerospace Exploration Agency are quite interested in it. It’s also searching for asteroids inside Earth’s orbit. The bandwidth is slower than Mariner 10’s was, I presume because data compression is better nowadays.
The Mercury Surface Element, which was cancelled, would’ve been a 44 kg disc ninety centimetres across to land 85° from the equator near the terminator, battery-powered due to 40% of the landscape there being likely to be in shadow and it would’ve had a camera. I just think it’s really sad they didn’t do this.
Right: that’s it. This is unfortunately later than I’d hoped as I missed the actual first rendezvous, but it is what it is.
A Box Of Chocolates 