WHILE THE SUN is without doubt the most inhospitable place in the Solar System, the inner planets Mercury and Venus are certainly not places you would want to head to for your summer holidays. Travelling around the Solar System on a journey like yours is a huge undertaking, but fortunately the gravity induced by the rotation of your living quarters means that you can at least enjoy a few home comforts.
Baths, for example, can be enjoyed in the normal way because water will act in a way you are used to. Where astronauts are living in zero gravity environments, such as on the International Space Station, personal hygiene tasks are less than easy because water does not flow. If you were to take a sponge that has been immersed in water and squeeze it in zero gravity the water would squeeze out of the sponge but just cling to the outside until you stopped squeezing, at which point it would be reabsorbed. Similarly, if you were to pour water out of a container it would just form into spherical blobs that would float around. Despite these limitations it has always been essential to take personal hygiene in space seriously because the closed and cramped living conditions encourage germs and bugs to spread. To take showers in space, astronauts in zero G have to climb into a cylinder that is effectively a big plastic sleeve which stops the water from floating away. Water is then sprayed on to them from a nozzle, and once finished, a vacuum is used to suck up the water from their skin. Early shuttle astronauts had even less luxury as showers were never installed; instead they used wet wipes to clean their bodies. Because washing facilities were nonexistent, their clothes were disposable and would be thrown away after about a week’s worth of wear. Thankfully baths and showers are easy enough to enjoy as you head along on your journey.
Owing to their close proximity to the Sun, Mercury and Venus are drenched in heat and intense solar radiation, making conditions pretty grim. Mercury is the nearest of the planets to the Sun and because of this it is only ever visible in the sky from Earth in the morning or evening twilight. Venus is the second planet out, but, for reasons we will look at later, it is hotter than Mercury. According to Kepler’s laws of planetary motion, the closer a planet is to the Sun the faster it will travel, and in the case of Mercury it rattles around completing one orbit of the Sun in just 88 Earth days, while Venus takes a little over 224 days.
Mercury and Venus are often referred to as the inferior planets since they orbit the Sun closer than Earth, but they are not the only inner planets, which comprise Earth and Mars too. The four planets share similar properties: they are all rocky bodies composed mainly of silicates in the crust and mantle, and iron and nickel in the cores, and their features differ widely from those of the outer gas giant planets. For starters, they lack significant quantities of moons. Mars has two, Earth has one and Mercury and Venus have none, compared to the outer planets, each of which has anything from just over ten to in excess of fifty. The inner planets also lack a ring system, and one of the most fundamental differences is the extent of the atmosphere that surrounds them. The outer planets are almost entirely made of gas, hence their collective name, their gas atmospheres contributing well over 90% of their composition. To understand the nature and cause of this difference we need to look back to the origins of the Solar System. When it formed over 4.5 billion years ago, it grew out of a vast cloud primarily made of hydrogen, but there were other elements present. As we saw in the previous chapter, the Sun formed out of the collapsing cloud and started to kick out incredible amounts of energy which pushed against the surrounding gas cloud, forcing the lighter chemicals to the outer reaches of the Solar System. It was out of this much lighter gas that the gas giants grew while the heavier elements, which could resist the pressure from the Sun, remained in the inner Solar System and formed the rocky planets Mercury, Venus, Earth and Mars.
Getting to Mercury and Venus will be relatively simple as unlike the outer planets you will not be fighting against the immense pull of gravity from the Sun trying to hold you back. One of the first spacecraft to travel to the two innermost planets was Mariner 10, which was launched in November 1973. Just three months later, in February 1974, it passed Venus at a distance of 5,768 kilometres, and Mercury a month later at a distance of just 703 kilometres. On its way to the two hostile worlds Mariner had to make a number of mid-course corrections, and to ensure the right ones were made it used the stars to navigate.
Bright stars like Canopus are often used by unmanned spacecraft for navigation purposes. This is usually done with a device known as a star tracker which uses any number of stars from fifty-seven or so of the brightest candidates. Not only do they help with identifying location, they are of equal importance in helping to determine orientation. You may well know your position in the Solar System but it is of the utmost importance to be facing in the right direction before making course corrections, and without being able to see your destination this can be difficult. Newton’s laws of motion are key here, in particular the third law, which states that every action has an equal and opposite reaction. Imagine you are facing forward, in the direction of travel, and you fire the thruster on the left or port side of the craft; the path of the spacecraft will adjust slightly to the right. But if the spacecraft is facing backwards, looking in the direction it has just come, then firing the left thruster will result in its path being adjusted to the left. There are an almost endless number of variables when it comes to making course corrections; getting them right means understanding exactly where the spacecraft is, understanding exactly in which direction it is facing, and deciding exactly how much thrust is required to make the correction.
The journey to Mercury, the first of the inner planets you’re going to visit, takes just a couple of months. There was a time when Mercury was the second smallest planet in the Solar System, beaten only by Pluto whose diameter is about 2,519 kilometres smaller. Things changed in 2006 when the International Astronomical Union finally decided to define what a planet actually was. Until then there was no definition. Without any hard and fast rules that objects had to adhere to in order to be considered a planet, our Solar System had kept on growing, from nine planets, to ten, then eleven with the discovery of rocky bodies like Quaoar and Sedna. The definition of a planet was finally agreed and passed at a conference of the IAU on 24 August 2006 and it meant that poor Pluto got demoted to the status of a mere minor planet.
There are three criteria an object must satisfy in order to be classed as a planet, and the first is that it must be a celestial body in orbit around the Sun. It must also have sufficient mass for gravity to assume hydrostatic equilibrium – in other words, it must be nearly spherical in shape. Both Mercury and Pluto meet these requirements, but it is the third criterion that strikes a blow for the status of the latter: it must be gravitationally dominant and have cleared the neighbourhood around its orbit of objects of comparable size. Pluto has not cleared its orbit and become dominant as it is accompanied by other rocky bodies. We will come back to all of this in more detail when you are in the depths of the Solar System, but for now, and until any other discoveries change the landscape of the Solar System, Mercury is the smallest of all the major planets.
As well as being the smallest planet in the Solar System, Mercury is also the fastest: as we have seen, it completes one full orbit of the Sun in just 88 Earth days. As it travels around the Sun, it rotates on its axis very slowly and in a way that is unique in the Solar System. When viewed from a distant star, it rotates only three times for every two orbits it completes, but when viewed from the Sun which rotates as Mercury orbits, it appears to make one rotation on its axis for every two orbits. That means that if you lived on Mercury you would see a sunrise and a sunset once every two years.
There is an even more bizarre effect that can be observed from some parts of Mercury: the Sun can be seen to rise, slow down, stop, and then head back in the direction it just came from before setting over the horizon it just rose from. This happens just before Mercury’s closest approach to the Sun when its angular orbital velocity matches and then exceeds its angular rotational velocity and, a few days after closest approach, the Sun resumes its normal motion. The Sun does generally move slowly across the sky when this happens, but who could resist the chance to pop down to the surface to see it for oneself? The place to go would be the point on the equator where the Sun is overhead and at its nearest point, the perihelion. From there the spectacle could be enjoyed, but it would take about sixteen Earth days to watch the Sun slowly pass overhead, then stop, move back in the other direction and pass overhead again before stopping for a second time and resuming its usual motion, and passing overhead for a third time.
Something that will become really obvious to you once you’ve approached Venus for a good look is how different Mercury appears. Venus is enveloped entirely by a thick dense cloud that restricts the view of the surface. Mercury, on the other hand, seems to have no atmosphere at all so its surface can be seen in all its glorious detail. There is actually a very tenuous atmosphere known as an exosphere that loosely clings to the planet, but conditions on Mercury are not conducive to much more than that. The pull of gravity is not strong enough, given the high temperature and the constant onslaught of pressure from the Sun, for any significant atmosphere to evolve. The thin atmosphere that is there is made up of hydrogen, helium, oxygen, silicon and a number of other chemicals.
The conditions mean that atoms of gas can easily escape into space but they are constantly replenished from a variety of different sources. The hydrogen and helium atoms are thought to originate from the solar wind which has been trapped by the magnetic field, although the radioactive decay of atoms in the surface is also responsible for some of the helium. There is a wonderful process known as ‘sputtering’ where incoming energetic particles, usually ions or micrometeoroids, strike the surface and cause it to eject atoms, typically oxygen atoms. These then exist in the exosphere either as individual oxygen atoms or when combined with hydrogen atoms as water vapour. We know quite a lot about the Mercurian exosphere thanks to the Messenger spacecraft which arrived in orbit during 2011. Since that time it has carefully analysed the planet’s chemical composition and studied the surface geology, the magnetic field and even probed the secrets of the core.
The lack of any decent atmosphere has led to Mercury experiencing the most extreme temperatures of any of the planets in the Solar System. On the Sun-facing hemisphere, the temperatures soar to 427 degrees, but on the night-time face they plummet to minus 173 degrees. On Venus, the dense atmosphere helps to equalize the temperatures across the globe, but on Mercury, heat quickly dissipates out into space. The only areas on Mercury to experience a fairly constant temperature are the polar regions which tend to stay at around minus 90 degrees due largely to the small tilt of the planet. The tilt of Mercury and indeed all planets is measured with respect to the plane of the Solar System known as the ecliptic, and it is along this plane that the planets orbit. The Earth, you will recall, is tilted over by just a little more than 23 degrees, which as we saw is what causes the seasons. The tilt of Mercury is a third of a degree, making it to all intents and purposes upright in its orbit. For that reason, the poles are largely unchanging in their presentation to the Sun, leading to a constant temperature. The lack of axial tilt also means that Mercury does not experience seasonal changes as many of the planets do. What it does have, however, is the most eccentric orbit of all the planets, so that at its nearest to the Sun, at perihelion, it is 46 million kilometres away, and at its most distant (aphelion) it is 69.8 million kilometres away.
Even though there are extreme changes in heat from day to night it looks very likely that ice exists on Mercury. There are craters in the polar regions that never receive sunlight at their bases and temperatures here remain well below zero. Radar has been used to bounce signals off these craters which reveal something highly reflective, which is very likely to be ice. It seems to be a familiar story that water is present in its solid state around the Solar System as it has been detected on the Moon, on Mercury, on Mars and on a couple of satellites of the gas giant planets. This makes longer-term human exploration and even habitation possible, at least in theory, because not only could water locked up in ice provide sustenance, but as we saw earlier the hydrogen and helium could be extracted to create fuel.
As you will notice, Mercury has an appearance very similar to that of the Moon. Thousands of craters are clearly visible, although vast lava plains seem much less obvious on Mercury. The lack of any appreciable atmosphere has allowed the planet’s surface to be pummelled by countless impacts from tiny pieces of space rock. The craters within which the ice has been detected were created in exactly the same way as the craters we see on the Moon, by meteoric impact. Careful studies of the surface of Mercury have shown that there were two periods of heavy bombardment in its history, one shortly after its formation just over 4.5 billion years ago and another which seems to have come to an end about 3.9 billion years ago. For a planet that has had a history of meteoric impacts it is no surprise that craters are present in many different states, from those that appear quite fresh and new to others that are clearly very old and have been subsequently deformed by further impacts. The ejecta that is thrown out from the impact site does not cover as large an area as you saw around some craters on the Moon, chiefly because Mercury has a stronger gravitational pull which restricts the distance ejecta can travel.
The largest crater on Mercury is known as the Caloris Basin and measures 1,550 kilometres across, making it one of the largest impact craters in the Solar System. The impact of the 100-kilometre-diameter object that created it released so much energy that a ring of 2-kilometre-high mountains was forced up out of the crust to create the crater’s edge. Studies of the floor of the crater reveal that it, like large craters on the Moon, is composed of solidified lava, which indicates that the impact likely cracked the crust to allow molten lava from below the surface to seep up and fill the basin. The relative absence of smaller impact craters across the floor suggests that this is a geologically young feature, perhaps having been created towards the end of the heavy bombardment era around 3.9 billion years ago. Spectroscopic studies have revealed that there seems to be a high concentration of sodium in the region around the Caloris Basin, suggesting that some of the cracks and fissures in the basin’s floor may be facilitating the escape of gas from inside the planet which is then added to the tenuous atmosphere.
The naming of surface features on Mercury is a somewhat complex process. By decree of the International Astronomical Union, features including craters must be named after people who have made a substantial contribution in their field. New craters must be named after an artist who has been famous for more than fifty years and deceased for more than three years. Ridge features that stretch across the planet are named after scientists who have dedicated time to studying Mercury, and surface depressions commemorate high-achieving architects. Mountains get their name from various words for ‘hot’, for example Caloris Montes (Latin for ‘mountains of heat’), while steep slopes or cliffs that occur from geographical faults are named after ships that have been involved in scientific exploration.
Aside from the numerous craters that pepper the surface of Mercury, you will see vast plains stretching across the landscape. The lack of significant cratering on them tells us that they are some of the youngest surfaces on the planet, but their origin is still unclear. They may be the result of large impact craters but equally could be volcanic in origin. The Caloris Basin is the only great plain that seems to bear definite signs of an impact origin, such as the fractures and ridges on the basin floor. The plains of Mercury generally resemble the lunar maria although they are much less prominent because they share a similar reflectivity to the surrounding surface, unlike the lunar maria which are less reflective and seem darker in appearance.
There is one other feature on Mercury which seems to be as common in the Solar System as craters – they have been seen on the Moon, Mars and Venus too. They generally appear as cliffs or some kind of fault but are now thought to be folds in the surface material and are known as ‘rupes’. Their origin hails back to the evolution of the planet which, over many millions of years, has slowly cooled and contracted. With this contraction, the crust has had to fold and deform, leaving us with these rupes. However, until 2014 there was a problem: although the rupes suggested shrinkage there was no other direct evidence for Mercury actually having got smaller. Detailed observations by the Messenger space probe, which took high-resolution images of the entire surface, enabled accurate measurements to be made of the many surface features. More importantly, the Sun was in a different position since the images were taken during the Mariner 10 mission; a different level of illumination meant that new features could be studied to build up a much more accurate profile of the surface. Messenger discovered new features that were the result of tectonic activity, like the scarps which are caused by faults in the crust that have pierced through the surface of the planet to altitudes of up to 3 kilometres, and the wonderfully named ‘wrinkle ridges’ that are smaller than the scarps but caused by the same tectonic activity. When compared to the rest of the Solar System planets, the rupes, scarps and ridges on Mercury seem to be particularly prominent, but when taken together the strong suggestion is that the planet has shrunk as it cooled in much the same way that the skin of an apple wrinkles as the fruit dries up. Studies indicate that the crust is on average 250 kilometres thick and that the planet has shrunk by around 14 kilometres since it formed over 4.5 billion years ago.
From the observations of Mariner 10 and Messenger, combined with Earth-based observation, it is possible to determine the density of Mercury by first working out its volume and its mass. Once we know its volume, which is easy enough to measure, we can determine its mass by studying its gravitational influence on objects in orbit around or flying close by it. This is relatively straightforward with planets like Jupiter, Saturn and even Earth which have moons in orbit around them, but Mercury has no such moon. Until the advent of space flight the only way we could estimate the mass of Mercury and indeed Venus, which also has no moon, was to measure the effect their gravity had on neighbouring planets. The changes in the orbits of other planets due to the gravity of Mercury are tiny so getting an accurate estimate of its mass was very difficult. But once space probes like Mariner 10 were able to pay a visit it became possible to measure how much the gravity of Mercury was tugging on the probe, thus revealing its mass. Dividing the mass by the volume tells us the density of the planet, and for Mercury this is 5.43 grams per cubic centimetre, making it the second densest planet in the Solar System, only slightly less dense than Earth. The high density of Earth can easily be attributed to the force of gravity compressing the core, but because Mercury is so much smaller, this cannot be the case. The only likely explanation is that Mercury must have a larger core that is rich in the heavier element iron.
Careful observation by spacecraft as they flew past has revealed clues about the planet’s internal structure, and given such a high density it is believed that Mercury is broken up into the usual three zones. Below the crust is the mantle, estimated to be around 600 kilometres thick and made up of rocky silicates like the mantle of the Earth. The core, however, is much more interesting. Unlike Earth’s, which is 17% of the volume of the planet, Mercury’s core is a much more significant 42% of its body and is likely to be mostly molten.
There are two possible explanations why Mercury has such a high iron content in the core and the first looks back to the violent formation of the Solar System. Before planets existed, large chunks of rock known as planetesimals were commonplace and impacts took place between them, forming planets. It was one such impact on Earth that may have led to the formation of the Moon. The young Mercury is thought to have suffered a similar impact, and when it did, most of the crust and mantle is likely to have been ejected into space leaving behind the iron-rich core with substantially reduced outer layers. There are alternative theories, for example that the intense pressure from the Sun could have simply pushed away the lighter silicate rocks as Mercury was forming, or that the young Sun was significantly hotter before it stabilized, causing the rocky surface to vaporize and get blown away by the onslaught of the solar wind.
The orbit of Mercury is highly elliptical, which means the planet is subjected to high tidal forces that keep the liquid core circulating. This movement is necessary to maintain the dynamo effect which is thought to drive the planet’s global magnetic field. Studies by visiting spacecraft such as Messenger show that the magnetic field seems to be generally stable with the occasional vortex-like structure. These magnetic tornados are thought to form when the solar wind brings with it magnetic field lines from the Sun that connect up to Mercury’s field and combine to produce a funnel. They are generally large in size, measuring upwards of 500 kilometres, and when they form, they expose the surface of the planet to the full force of the solar wind. The gaps in the magnetic field are not the only way the Sun can affect the surface of the planet but the magnetic field itself is strong enough that it can trap solar plasma, which can lead to weathering of surface features. It is also strong enough to have developed its own magnetosphere, which is a region of space surrounding Mercury where magnetic particles are controlled by the planet’s magnetic field, as opposed to any other magnetic field – from the Sun for example – except where magnetic vortices form.
Swinging past Mercury on the way to Venus is a reminder of just how difficult it is to get a spacecraft to land on the planet, or enter into an orbit around it. The challenges stem from flying so close to the Sun, which exerts an immense pull of gravity. A good way of visualizing why this is a problem is to see the Sun as a large bowling ball sitting in the centre of a huge rubber sheet. It is easy to imagine that the presence of the ball will cause the rubber sheet to deform as it tries to support the weight. The dent it produces represents its gravitational pull, and as you ‘roll’ towards it, you fall deeper into it, making it harder to slow yourself down, just like running downhill. To get a spacecraft on to the surface of Mercury, or Venus for that matter, means flying into the ‘gravitational well’ of the Sun. That, without any effort on your part, makes you go faster, and the faster you go, the harder it is to slow down. In fact there are only a couple of ways to do it. One is to use aerobraking in the planet’s atmosphere. This is fine for somewhere like Venus with its thick atmosphere, but Mercury is a different challenge. The only way to get a spacecraft in orbit around Mercury is to use rocket power to act against the acceleration from the force of gravity to slow the spacecraft and allow it to get captured by Mercury gently, rather than crash into it. Surprisingly perhaps, it actually takes more fuel to land on Mercury than it takes to get out of the Solar System.
Fortunately for us, we can use the RSU to drop down on to the surface and take a look around. With the Sun ‘breathing’ down our planetary neck it appears hugely dominating in the sky, almost three times larger than it appears from Earth and about seven times brighter – which, though it sounds a lot, isn’t a particularly noticeable increase in brightness. The sky is still black, even at midday, because there is no atmosphere to scatter the light, and stars can be seen peppered against the blackness of space. In fact except for the fact that the Sun appears so much larger in the sky, you would be hard pushed to recognize any difference between standing on the Moon and standing on Mercury. You have landed in a region just to the north of the equator. As you survey the horizon you notice there is a lack of atmospheric extinction (in this sense referring to the extinction of light as it travels through the atmosphere) which, as on the Moon, makes it very difficult to determine distances to various features. On the northern horizon you can make out the shallow rim of Caloris Basin. As you scan around, you notice the craters are quite deep, and that is because of the lack of an atmosphere which on other terrestrial planets acts to slow down incoming lumps of rock, lessening the impact. Glancing down at your feet, you get your first proper look at the Mercurian regolith, which somewhat resembles what you saw on the Moon but with subtle differences. Its grain size seems to be finer than the regolith on the Moon, making it more powdery, so it sticks more readily to your boots. The reason for this is not fully understood but it is likely that the position of Mercury, further away from the asteroid belt and nearer to the Sun, could be the cause. Its distance from the asteroid belt suggests it may be subjected to more meteoroids of cometary origin which tend to have a higher velocity than those originating from the asteroid belt. Since they hold more energy, their impacts are more destructive, pounding the surface more on impact.
You cautiously make your way over to the rim of a nearby crater, moving in what can only be described as bounding leaps due to the lower gravity which makes this by far the most efficient way of moving around. Peering over the edge of the crater wall, you are surprised to see the floor buried deep in shadow. The crater is deep but the sides aren’t steep, so after switching on the lamps attached to your helmet you rather cumbersomely make your way over the rim and down the slope. At the bottom of the crater the temperature is much colder than up on the surface, although of course your space suit is keeping your body at a comfortable temperature. Generally only in craters near the polar region does the temperature stay low enough for water ice to form; deep craters like this, closer to equatorial latitudes, tend to receive sunlight for a reasonable portion of their time which inhibits the formation of ice. Scanning around the crater floor, you follow the beam of your helmet light which happen upon the central peak of the crater, but it is very difficult to judge just how far away it is.
As you make your way back up the slope and out of the crater, the lower gravity and the finer regolith makes for slow progress, but eventually you are back up on the surface and before long back in the comfort and safety of the Kaldi, ready to head to your next planetary encounter.
At their closest, Mercury and Venus are just over 37 million kilometres apart, so the next leg of your journey will only take a few months and will be fairly uneventful.
On the approach to Venus, the second of the inner planets, the first thing you’ll notice is its dazzling brightness. When viewed from Earth in the morning or evening twilight sky it appears as a bright star, outshining all others. But close observation, both from Earth and now as you make your approach, is rather uninspiring because hardly any features are visible. Like all objects in the Solar System except the Sun, Venus and the other planets are visible only because they reflect sunlight. Venus is a planet that is entirely covered in cloud from its north pole to its south pole and those clouds have a high albedo, which is to say they are highly reflective. This accounts for the planet’s brightness – it’s light bouncing off the cloud – and it also accounts for the fact that we can see nothing of the surface detail of the planet when we look at it.
Aside from the cloud, one of the less well understood atmospheric features of Venus is its ashen light. Because Venus is nearer to the Sun than the Earth it displays almost a full cycle of phases just like the Moon, sometimes appearing as a thin crescent and at other times almost full. The ashen light has been seen by only a few observers and appears as a subtle glow coming from the night-time hemisphere of the planet during some of the darker phases. Before the development of powerful telescopes there were some wonderful theories about its origin, one suggesting that Venusians were burning vegetation. A much more rational explanation is that it is similar to Earthshine seen on the night-time hemisphere of the Moon. Earthshine is the result of sunlight reflecting off the Earth and illuminating the darker part of the Moon; a similar process may be the cause of the ashen light. Another less popular idea says it might be the effect of several flashes of lightning happening in close succession, causing the atmosphere momentarily to glow, but a lack of radio emissions suggests this is not the case.
The thick, cloud-filled atmosphere of Venus makes visual observation of the planet’s surface impossible. The only way to get a glimpse of what it is like is either to land or to use radar to pierce through the cloud. Radar is a technology that has many applications in space exploration but surface mapping is one of the most successful. The term radar is an acronym for ‘radio detection and range’ and it exploits the ability of solid objects to reflect radio waves, as demonstrated by Heinrich Hertz in 1886. Just eighteen years later, in 1904, Christian Hülsmeyer was granted a patent for the first radar device, the ‘telemobiloscope’, which bounced radio signals off distant unseen objects like ships in fog and revealed their direction and distance. It works because we know how fast radio waves travel (the same as all wavelengths of the electromagnetic spectrum, at 300,000 kilometres per second), and by timing how long it takes for the waves to be bounced back from an object we can determine distance.
This technology has been put to great use to pinpoint the location of ships and aircraft but is equally important for mapping the surface of planets like Venus. Fortunately, radio waves will travel straight through the dense clouds and get reflected off the unseen surface, enabling us to build up a full topographical representation of it. Magellan used radar to map the surface of Venus and did so by sitting in a polar orbit, which means it orbited the planet from pole to pole. It was a fairly stable orbit so after each imaging run the planet would have rotated a little underneath, presenting a slightly different face. After completing thousands of orbits, each taking just over three hours, 95% of the surface had been mapped to a high resolution revealing many new features never seen before.
The Venusian atmosphere that Magellan peered through is composed almost entirely of carbon dioxide with small amounts of nitrogen. The high levels of carbon dioxide and the presence of thick clouds of sulphur dioxide are responsible for the intense temperatures felt on the surface, which were recorded by the Russian Venera probe: they reach 460 degrees – nearly five times hotter than boiling water. It is thought that at some point in the planet’s history it was a much more hospitable place with an atmosphere more conducive to the existence of liquid water on the surface. However, over millions of years, evaporation of the water and volcanic activity have been responsible for the breakdown of the carbon cycle. On Earth, carbon is constantly being transferred between the atmosphere, animal and plant life, the oceans and the rocks. The process that allows carbon to get locked away in rocks is its interaction with water, either dissolving directly into the oceans at the surface and then being converted into organic carbon by living organisms – this ultimately finds its way to the floor of the ocean as creatures die and their carbon-rich skeletons become fossilized – or falling in rain, which causes weathering and allows some of the carbon to be absorbed by the rock. However it gets into the rock, it stays locked up for millions of years, or until one of the inhabitants of the planet decides to burn it as a fossil fuel. The lack of bodies of water on Venus means that the transfer of carbon from the atmosphere into the water and therefore rocks ceases, leaving it stuck in the atmosphere, and because carbon dioxide has the ability to absorb heat, it slowly warms up, leading to the greenhouse effect.
When the warmth from the Sun finally reaches the surface of Venus, Earth or any other planet with an atmosphere, it actually travels through the atmosphere without much interaction before arriving at the surface and slowly heating it. The surface then re-radiates energy at a slightly different wavelength which slowly warms the atmosphere. This is the driving force behind weather systems, where the warm air rises, creating areas of low pressure at the surface. The gentle warming of the atmosphere is checked on Earth because heat can slowly dissipate off into space, but this is not the case on Venus where the carbon-dioxide-rich atmosphere traps heat, preventing it from escaping, so the temperature rises unchecked. It is because of this runaway greenhouse effect that Venus is the hottest planet in the Solar System, hotter even than Mercury, which is many millions of kilometres closer to the Sun.
The dense atmosphere not only acts to cause an increase in temperature, but the low-level winds it generates tend to equalize night-time and daytime temperatures, so the surface is said to be isothermal. In other words, there is barely any diurnal variation. Even when temperatures at the poles are compared against temperatures around the equator there is little change. This is in stark contrast to Mercury where the daytime face roasts in the sunlight but plummets into frigidity by night. The only temperature variations experienced on Venus are seen with a change in altitude.
Temperature variations are the driving force behind wind, and with low temperatures at altitude and high temperatures at the surface there is a high temperature gradient between them. A high temperature gradient like this is partly to blame for the extremely high wind speeds seen at altitude, and on Venus they are getting stronger. When the Venus Express orbiter studied them it found that the average wind speeds at the top of the atmosphere, about 70 kilometres above the surface, were 290 kilometres per hour in 2006, but six years later they had increased to over 400 kilometres per hour. At this speed it takes a little under four days for the winds to blow around the planet, which is in stark contrast to the sedate rotation speed of the planet, which takes 243 Earth days to complete one revolution. A revolution in this case is retrograde or clockwise, just as it is on Uranus, whereas all other planets rotate anti-clockwise. The winds still confuse planetary meteorologists as they try to get to grips with Venus’s extreme conditions.
The wind speeds can be measured by studying the movement of features in the clouds in the upper atmosphere. The majority of the carbon dioxide is concentrated in the lower levels of the atmosphere but above it are the thick clouds of sulphur dioxide that reveal the high wind speeds and reflect nearly 90% of the incoming sunlight. The combination of sunlight and sulphur dioxide is a deadly one for any living organisms that might find themselves on the planet’s surface. Some of the incoming light gets absorbed by the carbon dioxide, sulphur dioxide and water vapour and triggers a chemical reaction. In particular, the ultraviolet portion of the sunlight causes carbon dioxide (one carbon atom with two oxygen atoms) to break down into carbon monoxide (one carbon atom with one oxygen atom) and one oxygen atom. The oxygen atoms that this process produces react with the sulphur dioxide to produce sulphur trioxide which, when combined with the trace levels of water vapour in the Venusian atmosphere, creates sulphuric acid.
The temperature in the upper levels of the atmosphere where this process takes place means that the sulphuric acid exists as a liquid and forms thick clouds along with the sulphur dioxide. The clouds, which tend to exist in a layer between 40 and 70 kilometres in altitude, produce sulphuric acid rain which then descends into the warmer lower layers. As the acid rain falls, it warms and releases the water vapour it is mixed with, making the acid more concentrated. By the time it reaches the surface, much of the sulphuric acid has dissociated into sulphur trioxide and water, both in their gaseous state. The sulphur trioxide dissociates further into sulphur dioxide and atomic oxygen which then combines with carbon monoxide to form carbon dioxide. The carbon dioxide tends to remain in the lower levels of the atmosphere but the sulphur dioxide and water rise through convection back to the upper layers of the atmosphere where the process begins again. The thick sulphur dioxide clouds are responsible for blocking most of the sunlight. Given that so much of the incoming light gets reflected back out into space, the surface of the planet is remarkably dark, even in daylight.
The first insight into the surface conditions on Venus came with the landing of the Venera probes in the 1970s. They returned the first images of the surface of this hostile world. On their descent, they measured that the sulphur dioxide clouds were about 40 kilometres thick but also detected hydrochloric acid and hydrofluoric acid, both of which are nasty chemicals. After finally touching down they revealed a world pretty much unfit for human exploration. Fortunately, of course, you can use the Reality Suspension Unit to explore the surface of Venus in complete safety.
Down on the surface it is dark, almost like constant twilight, and as a result of the thick clouds above you can’t see any stars. Being able to stand and take in the scenery is itself a complete impossibility. The dense atmosphere above is exerting a pressure on you that is ninety times stronger than the pressure exerted by the Earth’s atmosphere at the surface. That may not sound too much but it is the same as being a kilometre under water, and that immense pressure is enough to crush a human being. If that doesn’t sound horrific already, then remember the surface temperature, which is around 460 degrees. So without the RSU you would be subjected not only to crushing pressures but also extreme heat. There is a bright side though, and that is that the extreme surface temperatures mean that the vicious sulphuric acid rain does not actually reach the surface, instead evaporating at an altitude of around 20 kilometres.
In addition to these harsh conditions, moving around on the surface is difficult, if not impossible and dangerous. The winds at the surface of Venus are moderately slow, rarely reaching anything quicker than a metre per second; compare that to the fastest wind speed on Earth of 113 metres per second, recorded during a tropical cyclone. Regardless of the sedate wind, the density of the atmosphere means it exerts quite a force on any object on the surface, which makes walking difficult. It also manages to lift and transport surface particles so it’s like walking around in a sand storm, but instead of sand, stones and pebbles could be the projectiles.
Looking around at the surface reveals a world sculpted by volcanic activity, and it is apparent from the data gathered by Magellan that Venus has many more volcanoes than Earth. This is not because Venus is more volcanically active but its surface is much older than the surface of Earth. The crust on Venus is estimated to be about 600 million years old while the Earth’s crust is only 100 million years old. The constant movement of the Earth’s tectonic plates also means its surface is refreshed and renewed so any old volcanoes are likely to be slowly erased from view. Venus doesn’t have plate tectonics so its surface is scarred by ancient volcanoes. Interestingly, volcanic activity on Venus seems to drive thunderstorm activity whereas on Earth we are used to storms being driven by rainfall instead. We have already seen that the only rainfall on Venus is of sulphuric acid but the extreme surface temperatures preclude the rain from reaching the lower levels of the atmosphere, so instead it is thought that Venusian storms are driven by ash particles that are being expelled during eruptions. The detection of lightning and thunderstorms on Venus suggests that some of the volcanoes are still active, and measurements of atmospheric sulphur dioxide also support the idea of recent eruptions because the quantities seem to have reduced between 1978 and 1986, which can be explained by a recent volcanic eruption sending plumes of the chemical high up into the atmosphere.
Like all the rocky objects in the Solar System, Venus displays thousands of craters, and the majority of them are still in excellent condition. This suggests that there has been minimal erosion of surface detail. More interestingly, it implies that the surface underwent some kind of global resurfacing event around 600 million years ago, but the lack of plate movement means the mantle is less able to redistribute and dissipate heat so instead it builds up until it reaches a critical level. In a global event that lasted perhaps up to 100 million years, the entire crust weakened and yielded to the mantle, in effect recycling itself.
We know that the surface is an inhospitable place because the various spacecraft that have landed there have demonstrated it. All of them were destroyed after just a few hours in the hostile environment. Spacecraft that orbited Venus were able to build up a gravitational map of the planet, allowing us to probe its inner structure. Common sense alone can help us to get a feel for the internal composition of the planet, which is similar in many ways to the Earth. Not only are they very similar in size, with Earth just 642 kilometres larger, but they have a similar density and mass. They also formed in roughly the same part of the Solar System, so it is a reasonable assumption that their internal structures resemble each other too.
The crust of Venus is similar to Earth’s but with a uniform thickness. The crust of Earth varies from 10 kilometres thick for oceanic crust to around 50 kilometres for continental crust whereas the Venusian crust is believed to measure a fairly consistent 50 kilometres in depth, although the lack of moving plates suggests it may be even thicker than this. It also hints that the convection currents inside the mantle are less vigorous than those of Earth, whose plates are moved around by them. The mantle is thought to make up the majority of the bulk of Venus and mostly to consist of solid rock. Even though the mantle may be solid it acts like a viscous liquid – it can actually move, very slowly, over time. Convective currents within the planet are one of the primary ways in which heat is transferred from deep within to the surface. The heat is generated from the decay of radioactive elements in a process very similar to that on Earth. Even the temperature of the core is thought to be broadly the same, perhaps a little cooler, because a temperature that was much hotter would mean the mantle material would be less viscous, leading to more active convection and more geological evidence on the surface. Interestingly, the surface topography of Venus may be a direct result of the movement of the mantle. Where there are areas of higher than average surface levels there may be convective upwelling below, deforming the crust.
It is not only gravitational effects that allow us to probe inside a planet: we can tell a lot about the core of Venus by studying its magnetic field. The landers that visited the planet studied it extensively. One of the earliest, Venera 4, discovered it was much weaker than Earth’s and that it was not generated by an internal dynamo like the Earth’s. Instead it is the result of an interaction between the solar wind and the upper levels of the Venusian atmosphere, in particular the ionosphere. The different origin of the magnetic field tells us quite a bit about the core of the planet. For a dynamo to exist there needs to be three things: rotation at the core, a conducting liquid and convection. The Venusian core is likely to be made of the same chemicals as ours, which means it is composed of iron and nickel so it will also be conductive. If it is conductive and there is a very good chance it is rotating then the one ingredient missing is convection. For there to be an adequate level of convection there must be a liquid outer core like the Earth’s, so no convection suggests that there cannot be a liquid core. The driving force behind such convection is usually that the bottom of the outer core is a lot hotter than the top and the high temperature difference drives the currents. The lack of a dynamo may alternatively mean that there is a liquid outer core but the lack of moving plates leaves no way for heat to escape. This may mean that the temperature at the top of the outer core is higher than it should be, thus lowering the temperature difference and reducing the convection currents. Until we can return to Venus with seismometers to probe the inner structure we will never really know the answer.
It’s time to leave, because you have a seventeen-month leg ahead of you during which you will perform another orbit of the Sun, then slowly wind up speed by completing a gravitational fly-by of Venus again before taking in a final view of one of the most beautiful sights the Solar System has to offer, planet Earth. This will increase your velocity relative to the Sun by 21,000 kilometres per hour, and having attained that speed you can head out towards the outer planets, with Mars next on the itinerary.