The Solar System #2: Mercury
Weirdly, this is one of the planets we didn’t know an awful lot about for years. That’s because of a little problem called THE FLIPPING SUN!
“I WILL BURN ALL OF YOUR TELESCOPES AND RETINAS AND INTERFERE WITH YOUR SPECTRA” - The Sun. Probably.
So we had a lot of theories but no solid facts until 2011(!) when a probe flew by and took heaps of photos/readings. Most people thought it was a barren wasteland that had been baked by the sun for billions of years - it was similar to the moon. Most people, it turns out, were wrong.
We found out some INSANE things about it. For one thing (and this is my favourite I think) there’s ICE on Mercury. ICE. The thing about Mercury is because of its lack of significant tilt, the poles are pretty much in CONSTANT shadow and have been for a VERY long time. So Messenger (the probe) found evidence of water ice in these craters at the poles of Mercury. We think it’s leftover from comets crashing into the planet - it’s just never been hot enough to melt! Which is CRAZY when you think about how close it is to a flaming-hot ball of plasma.
Mercury was also seriously volcanic at some point in the distant past. Not in the same way as Mars or Earth with their big, mountainous volcanoes. No, Mercury had a whole bunch of volcanic basins that were filled with lava up until about 3.5 million years ago. As a result, Mercury’s surface is COVERED in Graphite - like the dark grey stuff in pencils. Hah!
Another weird thing about Mercury is that its core is REALLY big in comparison to the planet. To give you some perspective, we think Mercury’s core is more or less the same size as Earth’s. The most popular theory here is that Mercury was a MUCH bigger planet originally, that had most of its outer crust and inner layers stripped somehow. We’re not sure how though…
This theory’s actually backed up by looking at exoplanets. There are a bunch of stars we’ve found with planets orbiting REALLY fast and REALLY close - a lot of the exoplanets we know about orbit closer than Mercury does plus they’re about the size of Jupiter! No one knows why…
According to a New Scientist book, scientists have thrown a spanner into the works of that theory, finding volatile elements like potassium on the surface. If there’d been some sort of crazy bombardment then these elements would have been long gone.
Basically, space is annoying.
There’s another craft due to arrive in 2025 called “Bepi Columbo” (best name ever) so we might get some answers to questions Messenger raised.
Mercury also has an odd orbit around the sun. Instead of a circle (more or less circular like Earth anyway), Mercury has an ellipse. This actually made astronomers think there was something else pulling on it - there was another planet closer to the Sun! Despite looking for it for about 50 years in the 1800s, “Vulcan” was never found.
Image from the Smithsonian Magazine
The thing is, based on their knowledge of Newtonian physics, there was no other explanation for the weird orbit. I reckon it probably caused a lot of existential crises amongst astronomers… which I’m always a fan of. It wasn’t until Einstein came along with his theory of general relativity that they all looked at the weird orbit and went “OHHHHHHHHH, it’s being weird because of the SUN’s gravity.”
BUT the theory lives on. Although we’re not looking for a planet, some astrophysicists believe there are asteroids orbiting in between Mercury and the Sun. We’ve not found any evidence these “vulcans” exist yet but Star Trek fans hold out everlasting hope.
Tomorrow: Venus
Science Space is a not-for-profit organisation at the University of Wollongong. The work presented here was written by me as a part of a larger project spear-headed by my colleague Amanda Kruger.
