Our prospects of ever visiting Mercury are slim to none. It's a shame because the view would be unique. It is the closest planet to the Sun, and so our star would appear three times bigger than it does from Earth. Not that you would appreciate it if you saw it, because you'd be burned to a crisp.
It's scorching on the sunny side of Mercury, reaching 430 °C during the day. In stark contrast, in the dark craters at its poles night-time temperatures plunge to -180 °C. These shadowy spots look out into deep space and are locked in a permanent deep-freeze.
In cosmic terms, Mercury is one of our closest neighbours, but it is only in the last 5 years that we have found out very much about it. That's thanks to a NASA probe called Messenger, which spent 4 years studying the planet, finishing in May 2015.
Once thought of as a dull, burnt-out rock, it turns out Mercury is a distinctly odd planet, with a violent history. The latest findings go some way towards explaining why it is so peculiar. In the process, they could also help us understand why our own Earth is the way it is – and what to expect from planets elsewhere in the galaxy.
Mercury is almost impossible to study from Earth. Although it is visible to the naked eye, it is only visible close to sunrise and sunset, and doesn't rise far over the horizon. For most of human history, it has been nothing more than a moving dot.
How did it form, and how did it get a magnetic field?
That changed in the 1970s, when the Mariner 10 spacecraft visited the planet. It managed three flybys between 1974 and 1975, but because it had to protect itself from Mercury's intense heat, it only took photos of one side. As a result, we had only seen 45% of the planet's surface.
Mercury, it turned out, looked a lot like our own Moon. It is grey and barren. The surface is covered with craters, evidence of explosive collisions with asteroids and comets.
It has a very thin atmosphere, which explains the sharp temperature differences. The thin air cannot trap much of the Sun's heat, and cannot carry heat from the equator to the poles.
Mariner also found hints that Mercury had once been volcanically active. There was also evidence that it had a magnetic field.
But beyond that, the planet remained an enigma. How did it form, and how did it get a magnetic field – something most small planets don't have? What has happened to it over its 4.5-billion-year history?
Almost 20 years ago, NASA researchers began developing the Messenger probe, which would get much closer to Mercury and photograph the entire surface. Messenger finally entered Mercury's orbit in 2011.
Unable to maintain a stable orbit, it crashed into the planet
"We watched the first image come down, realising we were seeing a bit part of Mercury that had never been seen," says Sean Solomon, the mission's principal investigator and now director of the Lamont Doherty Earth Observatory at Columbia University in New York.
Messenger was only meant to last one year, but it ended up orbiting for four and sending home over 280,000 images. The mission finally ended in May 2015 when the spacecraft ran out of fuel. Unable to maintain a stable orbit while being buffeted by the Sun's powerful gravity, it crashed into the planet.
The most obvious revelations were about the surface of the planet. It is surely lifeless, but it is still host to some surprising things.
Despite its intense heat, some of the craters at Mercury's poles are filled with water ice. Messenger detected these mysterious dark patches in 2012. They are permanently shadowed from the Sun. The Moon may have something similar.
Comets could have dumped material on Mercury without actually hitting it
There's more. The water ice deposits at Mercury's poles are covered, in some places, by a mysterious dark material. It is slightly warmer than the ice, 20-30cm thick, and darker than anything else on Mercury.
It may be organic material. That doesn't mean something alive – Mercury is far too inhospitable for that – but simply chemicals containing carbon.
Many objects orbiting further from the Sun, such as comets and water-rich asteroids, contain organic material. Some of these may have crashed into Mercury, and left organic deposits behind.
"If the hypothesis is correct, this substance was delivered to Mercury by the impact of the same objects that brought water ice to Earth," says Solomon.
Comets could have dumped material on Mercury without actually hitting it. Other parts of the surface are much darker than our Moon, and a 2015 analysis proposes that passing comets gave off a sprinkling of carbon that has dusted the rocks.
Beyond this, Messenger's images of Mercury's surface rewrote our ideas about its history. Astronomers had thought that it had sat there, more or less inert, for billions of years, getting quietly baked. We now know its past was explosive.
"The early history of Mercury might have been very volcanic," says team member Paul Byrne of the Lunar and Planetary Institute in Houston, Texas.
We expected everything to be over almost four billion years ago
The evidence of past volcanic activity is abundant. There are lines scratched on Mercury's face. These are evidence that the surface has fractured. Messenger also found volcanic vents 25km (15.5m) long where hot lava once flowed, carving long ridges.
This volcanic activity went on until around one billion years ago – surprisingly recently. "We expected everything to be over almost four billion years ago," says Byrne.
There is even evidence of flood volcanism. Torrents of lava once poured onto Mercury's surface, and make up large swathes of its crust. The biggest such flood happened on its northern polar region, now known as the Northern Volcanic Plain. This great lava plain accounts for 6% of Mercury's surface.
These large flows buried what must have once been large craters, some of which are visible in the image above. Only traces of their rims remain, and these are called "ghost craters".
The strange thing is, most of Mercury's plains are about the same age, so they must have formed all at once.
Mercury has shrunk
"The last big phase of lava plain formation happened in one go," says Byrne. "This means there was an enormous amount of heat, an enormous amount of magma that came onto the surface."
Nowadays Mercury is not volcanically active. That's because the interior heat that powered the volcanic activity has cooled down.
As a result, Mercury has shrunk. Four billion years ago when the crust first solidified the planet was 7km bigger than it is today. Mariner 10's images had suggested this, but Messenger's show that it shrank much more than originally thought.
Despite cooling down, Mercury's interior still has enough energy to give it something hardly any other small planets have: a magnetic field.
Mercury is the only rocky planet, apart from Earth, to have one today – although its field is 100 times weaker than our own. There's evidence Mars once had one, but as it cooled down the field died.
We had been assuming that Mercury was a small version of Earth
Earth's is powered by the rotation of its core, a hot ball of liquid metal. Messenger verified that Mercury's field is generated the same way.
But there is something odd. Mercury's is not centred on the planet, but is offset by about 20% of its radius.
This had not been predicted by any previous models, says Solomon. "We had been assuming that Mercury was a small version of Earth."
Instead, Mercury turns out to be something quite different.
In some ways this finding simply highlights our ignorance about magnetic fields. We don't properly understand how planets generate them, and we don't know what Earth's magnetic field was like when the planet was young.
This ancient magnetic field was much stronger than the modern one
Today Earth's magnetic field is centred, and physicists had assumed that that had always been the case. But maybe it wasn't, says Solomon: maybe Earth once had a magnetic field that looked more like Mercury's.
In 2015 the Messenger team also discovered that Mercury's magnetic field is ancient, having been active almost four billion years ago. Catherine Johnson of the University of British Columbia in Vancouver and her colleagues examined magnetised rocks on the planet's surface, which record the history of the magnetic field.
This ancient magnetic field was much stronger than the modern one. It may have been as strong as Earth's field is today. Johnson says the mechanism generating it might have been subtly different in the planet's early days, allowing for a more powerful field.
Like our own Earth, Mercury is also about 4.5 billion years old. Yet the two planets are vastly different. Our other neighbours, Venus and Mars, tell other stories entirely.
Almost 2,000 "exoplanets" have been discovered
Astronomers aren't just trying to understand Mercury for its own sake, says Solomon. Doing so will help us understand planets in general.
If we want to figure out how the Earth formed, we need to understand all the processes that created the planets in our solar system. "In our own solar system alone," says Solomon, "common processes forming Earth-like planets have led to such different outcomes."
The same may well be true for planets outside our solar system. Almost 2,000 "exoplanets" have been discovered and estimates suggest there could be thousands more. If we can figure out the rules of planet formation, we can make informed guesses about what each exoplanet is like.
For instance, Earth's surface is divided into a mosaic of shifting plates, whereas Mercury is a one-plate planet.
Mercury could be the archetype for one-plate planets in other solar systems
That's because Earth has more internal energy than Mercury, powering the movement of the plates. As a result Earth still has volcanoes and earthquakes, and is altogether livelier.
If an exoplanet has enough energy to break the plates apart, it is more likely to look a bit like Earth. If it doesn't, it might resemble Mercury.
"Mercury could be the archetype for one-plate planets in other solar systems, so we can understand their geological history on the basis of what we see in Mercury today," says Byrne.
In other words, we are not just studying Mercury. We might actually be studying thousands of planets, most of which we will never see, but whose stories we can figure out by looking at Mercury.
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