CLOSE to the sun lie a pair of sizzling coals. You could be forgiven for thinking these strange worlds were two circles of hell: Mercury, a black and blasted plain, and Venus, a sweltering world beset by rain of pure acid.
But for all the terror of their outward appearance, their insides are remarkably familiar. Along with Earth and Mars, they form the solar system鈥檚 only rocky planets, a stark contrast to the bloated gas giants that make up its outermost worlds. In the language of those hunting for extraterrestrial life, such planets count as potentially habitable. In fact, if we found a Mercury or a Venus in a solar system far from our own, we might even call them Earth-like.
So what caused these seemingly Earth-like planets to become so resolutely, well, not? Was it some accident of composition, or millions of years spent in an inhospitable environment? Getting to the bottom of these questions, and understanding the diversity of rocky planets, is of paramount importance for astronomy. Without clear answers, we aren鈥檛 only ignorant of our own solar system鈥檚 history, but blinkered in our search for life elsewhere.
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Part of the problem is that the innermost planets are incredibly hard to visit. Only two spacecraft have ever made it to Mercury and of the numerous Venus landers, none survived longer than 90 minutes. Painting a more detailed picture will involve a new generation of probes, such as the European Space Agency and Japan Aerospace Exploration Agency鈥檚 BepiColombo, currently on its way to Mercury. While the chances of finding any signs of life are close to zero, the data it sends back will help us look for it elsewhere.
To find out anything about Mercury, though, we first have to get there.
The biggest problem when it comes to planning a trip to Mercury is the sun. When your next-door neighbour is a star 6 million times heavier than you with a gravitational field 80 times stronger, visitors have a tendency to get redirected.
If a spacecraft flies by Mercury too quickly, it will be trapped by the sun鈥檚 powerful gravity and dragged to its doom. But even heading straight for the planet doesn鈥檛 guarantee you will arrive. 鈥淭he second you point your rocket toward Mercury, you are going far too fast to get caught in Mercury鈥檚 gravity,鈥� says Paul Byrne at North Carolina State University. The sun has so much gravity that a direct mission to Mercury will always miss.
We didn鈥檛 know how to get into orbit around Mercury until the mid-1980s, when Chen-Wan Yen at NASA鈥檚 Jet Propulsion Laboratory in California figured it out. A spacecraft has to take the scenic route, with loops around Earth and Venus and multiple swoops past Mercury to slow it down before it can enter orbit.
The first probe to visit was Mariner 10, which flew past, rather than orbiting, just three times in 1974 and 1975. The second was Messenger, which orbited for four years from 2011. 鈥淲e had Mariner 10 go by, and it was craters and all a little bit boring,鈥� says David Rothery at the Open University, UK. 鈥淲e would still wonder if Mercury was boring if we hadn鈥檛 had Messenger there to prove it wasn鈥檛.鈥�
One thing Messenger experienced directly was just how hot the planet gets. The orbiter had to periodically back away from Mercury just to keep its instruments cool 鈥� on the day side, the surface reaches temperatures of up to 430掳C. And because there is barely any atmosphere to hold heat in and spread it around the planet, the temperature drops precipitously on the night side when the sun sets 鈥� down to about -175掳C. Day and night are drawn out affairs, too: with an average of 176 Earth days elapsing between sunrises, one day is longer than the planet鈥檚 year.
These temperature extremes mean life as we know it is almost impossible on Mercury. There are a few sunless craters near the poles that may have a small amount of water ice, but they are probably too cold for liquid water, with little or no oxygen. 鈥淧retty much every place gets very hot and very cold,鈥� says Byrne. 鈥淚t鈥檚 not great if you鈥檙e a bug, let alone anything more complicated.鈥� The low surface pressure and lack of air make it even less hospitable.
More problems come from within. The planet is intensely volcanic, says Lillian Ostrach at the US Geological Survey鈥檚 Astrogeology Science Center in Arizona. 鈥淭he whole northern plains area, which makes up more than 10 per cent of the surface, is solidified lava.鈥� Mercury has signs of what is known as explosive volcanism, a more dramatic type of eruption. Elements that boil at relatively low temperatures, known as volatiles, form gas bubbles in lava, which pop at the surface to spew hot liquid rock in all directions. Most of these eruptions seem to have happened about 3.8 billion years ago, when much of Mercury鈥檚 interior was still molten from the heat of formation.
Volatile nature
In theory, most volatile elements on Mercury鈥檚 surface should have already boiled away due to its proximity to the sun. But they are still there. 鈥淏efore we got there, we had this idea that it should be depleted in volatile elements,鈥� says Kathleen Vander Kaaden at the Lunar and Planetary Institute in Texas. 鈥淎nd then we get there and we realise that there are lots of these volatiles on the surface.鈥�
On a planet with no liquid water and little weather, these volatiles play a key role in shaping Mercury鈥檚 surface features. In addition to powering its volcanoes, they are also responsible for Mercury鈥檚 most un-Earth-like attribute. Instead of an atmosphere, the tiny planet has an exosphere, a thin and tenuous layer of particles floating above the surface. 鈥淚t鈥檚 not so much an atmosphere as a slow evaporation of the planet,鈥� says Valeria Mangano at the National Institute of Astrophysics in Rome, Italy.
The exosphere is mostly hydrogen and oxygen, with some helium, sodium and potassium as well. It probably forms when charged particles from the sun sneak through the planet鈥檚 magnetic field and hit the surface, sending tiny particles of dust and gas flying. 鈥淚t is continuously created and continuously fading away into space,鈥� says Mangano.
You read that right, Mercury is slowly shrinking. The process isn鈥檛 only powered by evaporation, either. As its molten core began to cool, the planet started to solidify, causing its surface to contract. Cliffs on the surface that can be up to about 1000 kilometres long and 3 kilometres high show that its diameter has shrunk by several kilometres since its crust formed, crumpling the landscape. As Mercury鈥檚 surface is all one shell rather than several tectonic plates, these cliffs probably couldn鈥檛 have formed any other way.
But despite millions of years of cooling, some of the core is still liquid. Part of the reason it has taken so long to freeze may be simply that the core is enormous. It takes up 85 per cent of Mercury鈥檚 radius 鈥� far more than any other planet we know of. Dig just a few hundred kilometres through the crust and you will hit molten iron-rich core. On Earth, you would have to dig almost 3000 kilometres to reach the outer edge of the core.
鈥淢ercury鈥檚 surface is continuously fading away into space鈥�
This thin veneer of rock over a huge core, combined with the unexpected volatiles, has led some researchers to conclude that Mercury may have formed further from the sun and then smashed into another, larger protoplanet as it migrated inward, stripping off its rocky outer layer. 鈥淚t could come in and hit the young Earth or the young Venus and lose its rock but not its volatiles in the process,鈥� says Rothery. Without this hit-and-run, Mercury might be a much bigger planet, and one more structurally similar to Earth.
Planetary scientists are still scratching their heads over the mysteries Messenger revealed. Which is where BepiColombo comes in. The hope is that it will answer some of those questions. BepiColombo consists of two orbiters that will separate when they reach Mercury and provide the most comprehensive global view we have ever had of the strange little world. It launched in 2018, but because of the convoluted path necessary to skim safely into orbit, it won鈥檛 arrive until the end of 2025.
Meanwhile, Venus has been unloved of late. 鈥淰enus is sort of the middle child,鈥� says Tracy Gregg of the University at Buffalo in New York, less loved than the favoured siblings of Mars and the gas giants. Of the 27 successful visits since 1962, only five occurred after 1990. That means our knowledge is even less up-to-date than it is for Mercury. It isn鈥檛 that Venus is particularly hard to get to 鈥� it is just hard to learn about once you get there.
鈥淭he Soviet landers lasted between an hour and an hour and a half on the surface before, essentially, they cooked,鈥� says Gregg. Surface temperatures are around 470掳C, with crushing pressure from the heavy atmosphere 90 times that at sea level on Earth.
Venus鈥檚 inhospitable conditions have very different origins to Mercury鈥檚. 鈥淯sually people think that Venus is so warm because it鈥檚 nearer to the sun than Earth, but it鈥檚 not true,鈥� says Pedro Machado at the Institute of Astrophysics and Space Sciences in Portugal. 鈥淚ts clouds reflect around 70 per cent of incoming radiation from the sun.鈥� Instead, Venus is hot because of a runaway greenhouse effect 鈥� its thick carbon dioxide atmosphere traps heat near the surface. That thick haze, which inexplicably rotates 60 times faster than the solid planet, means that the heat gets distributed all around the world instead of radiating away into the chill of space at night. Unlike Mercury, there is no cold side.
The atmosphere also hides the surface from orbiters, so we have very limited data on what lies below the Venusian clouds. 鈥淲e don鈥檛 know what Venus is made of,鈥� says Byrne. 鈥淲e don鈥檛 know if it鈥檚 still tectonically active, we don鈥檛 know what the cratering rate is, we don鈥檛 know why its atmosphere is moving so fast.鈥�
Venus is a place of mysteries. From radar data, we can see that it has what appear to be volcanic landforms: channels carved by lava, plains of volcanic rock and in excess of 1600 major volcanoes 鈥� more than anywhere else in the solar system, even though there is no evidence that they are active now. It is also home to the longest channel in the solar system, which once carried lava nearly 7000 kilometres. But nobody knows where the lava came from or where it went after creating the channel. 鈥淲e don鈥檛 see a big pile of lava at the end of these channels. We don鈥檛 see a volcanic mountain or a volcanic crater at the beginning of these channels,鈥� says Gregg. 鈥淭hese channels on Venus have no source, no sink, and yet there they are.鈥�
There are also strange bright areas of terrain called tesserae, which tend to be full of long ridges and troughs that form when the crust shifts due to tectonic activity. There have been hints from landers that these areas may be rich in silica, like continental crust on Earth. 鈥淚f we were to determine that the tesserae were continental crust, it would be a big finding, because it would mean there was much more complicated geology and chemistry than is happening now,鈥� says Byrne.
Crucially, this would also be one of many areas of similarity to Earth. 鈥淰enus and Earth should be twins,鈥� says Gregg. 鈥淭hey are made up of the same stuff, they are about the same distance from the sun, they are about the same size. And yet鈥︹��
If Venus and Earth formed close to where they are now, they should have about the same amount of water, whether they formed with it or got it from meteorites later on. There is no obvious reason for Venus to be so different, aside from maybe that a lack of plate tectonics made it difficult to sequester carbon dioxide in rocks. Early in its history, Venus may even have been pleasant for life, with surface water and a less dense atmosphere. Not now. 鈥淏eing on the surface is like diving in the ocean at a depth of around 1 kilometre, and it鈥檚 also like an incinerator,鈥� says Machado. 鈥淭here are better places to go for a holiday.鈥�
鈥淰enus and Earth should be twins. They are made up of the same stuff. And yet鈥︹��
Perhaps the implausibility of life on Venus and Mercury is why it has been hard to build interest in visiting them. 鈥淲e are probably not going to find life there,鈥� says Machado. 鈥淎nd we can鈥檛 go to the surface with an astronaut one day and put a flag in the soil.鈥�
Nevertheless, they may be important for us. 鈥淭he goal is not just to understand Mercury or Venus,鈥� says Byrne. 鈥淚t鈥檚 to understand our own world. It鈥檚 to understand other worlds around other stars.鈥� And to fathom those other worlds, it is important to figure out why the four rocky planets in our own solar system are so different from one another 鈥� and why only one seems to be right for life. 鈥淭o understand our own planet 鈥� and even exoplanets 鈥� we need to understand the three others as well,鈥� says Rothery. Yet for only one of them, Mars, do we even know what the surface is predominantly made of or where all the water went.
Comprehending the differences that seem so monumental here could help us parse the tiny dissimilarities we see between worlds circling other, distant stars. 鈥淎 seemingly Earth-like exoplanet could be a paradise planet like Earth or a hell planet like Venus,鈥� says Machado. To tell which is which, we are going to have to return to the two sizzling coals nearest the sun.
