The Incredible Shrinking Planet

merc in sky

Right now, in the early evening, it is possible to see the closest planet to the Sun, Mercury. Assuming there are no clouds, in which case you probably don’t live very close to me! Mercury is not only the closest to the Sun, it is also the smallest planet but strangely enough, not the hottest. Let’s have a look at this little fellah, starting with some cold, (or should I say hot?) facts. And yes, that awful joke was intentional!

Mercury is, as I said, the closest planet to the Sun at a distance of 58,910,000 kilometres. It has a radius of only 2,440 kilometres compared to Earth’s  6,371, and its mass is only 0.055 that of Earth. Mercury has a day that lasts just over 58 Earth days and its year is only a little longer than its day at 88 Earth days. On Earth this would be like one day lasting from January to some time in the middle of September, so there’d be no complaining that there are never enough hours in the day to get things done! Now depending on where you are on Mercury, and the time of day, the temperature could be anything between 80K and 700K, (don’t forget that Kelvin starts at -273C so 700K is the same as 427C) so if you are planning on going there then you need to pack both Sun block and thick woolly underwear!. This is slightly cooler than Venus where the average temperature is 735K and never strays very far from that. The reason for this is that Venus has had a run-away greenhouse effect for a very long time so it’s not only about how much Sunshine you get, (actually the Earth should be quite a bit colder than it is, just under freezing actually, but we can thank a mild greenhouse effect for keeping us warm).

This is another connection between Mercury and the other planets, atmosphere. Mercury is too small to hold on to a real atmosphere, but it does have what is called an “exosphere”. This consists mostly of  hydrogen, helium, oxygen and other elements but is continuously being blasted into space by the intense solar wind which also helps replace that lost to space, ions being trapped by Mercury’s magnetosphere. The Aurora on Mercury must be an amazing sight, a real tourist attraction for future generations!

This also brings us to another odd thing about Mercury and which is why I called it “The Incredible Shrinking Planet” in the title. Mercury has, like all planets, a core. Mercury’s core is iron, just like Earth’s but unlike Earth where the core is just that, a small core in the middle of the planet, in the case of Mercury the core makes up 42% of the whole planet!


It is thought that Mercury’s core and mantle shrunk considerably as they cooled and this is backed up by the presence of long ridges in the crust, some of them hundreds of killometres long. Another theory postulates that Mercury was struck by  another planetoid very early on and that this impact stripped away the outer layers of the planet. In fact a new theory says that Mercury was just one of many planetoids very close to the Sun and that because they were so close to each other and orbiting the Sun at high speeds, collisions were frequent and violent enough to vaporise them, the debris being fine enough to be blown away by the solar wind. Whatever happened back then at the dawn of the Solar System, Mercury has somehow shrunk to something quite a bit smaller than it started out as.

Mercury has also helped us confirm one of Einstein’s theories. It was known from the work of Newton that planets orbit a star in an ellipse with the star at one of the foci of the ellipse and that other planets and the oblateness of the Sun, it’s slightly squashed shape due to its rotation, could cause the perihelion, a planets closest approach to the Sun, to move over time. This could all be calculated very accurately so Newton didn’t just get it right, he got it very right! Problem was, Mercury didn’t behave as it should, its closest approach to the Sun wasn’t where Newton said it should be. This had been known since 1859 and was considered to be a problem as it made a right old mess of the nice mechanical system the neat and orderly Victorians liked so much. Then along comes Einstein and explains why with his theory of General Relativity.

Einstein, I'm with stupid

So what does Mercury look like? Well it looks a lot like our Moon, with loads of impact craters where it has taken a beating over the millennia and some Mare or plains form ancient volcanism. The number of craters shows that Mercury has been geologically inactive for a very long time. The largest crater is called the Caloris Basin,is over 1500 kilometres across and the impact that created it was so massive that it  caused volcanic eruptions that left a ring 2 kilometres high around the crater. On the opposite side of the planet is a hilly area with the really cool name of the “Weird Terrain” and it is thought that this was caused by violent shock waves from the impact travelling around the planet and meeting on the other side.

230px-Mercury_weird_terrainThe Weird Terrain

It is only recently that we have seen such detail. Mercury is so small and appears so close to the Sun that it is difficult to see much of anything with a telescope, so it wasn’t until Mariner 10 in 1975 got a closer look that we saw Mercury close up. It wasn’t until Messenger arrived in 2008 that we could get some serious research done and we saw new details on the surface. The fact that we have not been there more is not just because other places are more interesting, it is also extremely difficult to get a probe into orbit around Mercury. Mercury is very close to the Sun so a probe experiences a powerful gravitational pull from the Sun and only a very tiny one from Mercury. This is made more complicated by Mercury’s rapid orbit compared to the outer planets. Mercury orbits the Sun at 48km/s compared to Earth’s 30 km/s so getting a probe close to Mercury at all is quite an achievement.

As a bit of a side track I will also tell you a couple of things that turned out not to be true. When Mariner 10 was approaching Mercury in 1974/75, it detected large amounts of Ultraviolet radiation near Mercury. This caused some researches to suggest that Mercury had a moon, but later on it was discovered that the UV light came from a star in the background so no, Mercury does not have a moon. The other thing is that for a while it was thought that Mercury was the second planet, that there was a planet, which was given the appropriate name of Vulcan, which would explain the deviance of Mercury’s orbit. It was of course our old mate Einstein and his Theory of General Relativity that eventually explained Mercury’s orbit and so there is no planet Vulcan, (sorry all you Trekkies out there!).

So there you have it. A small planet but a cool one, (sorry!!!), with lots to tell us about how it and the rest of our Solar System formed and developed.



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