It has such an unusual orbit and rotation period, the days and nights are, are very strange. A Mercurian year is just 88 earth days long, thanks to its quick sprint around the sun. But it rotates so slowly, a single day takes much longer.

 

The day on Mercury is more like half a year, eh, earth terms.

 

Although known to us since ancient times, for thousands of years we had little idea what the planet really looked like. Then in 1974, NASA's Mariner 10 sends back the first ever glimpses of its surface.

 

Pictures can be transmitted to tracking stations and on to the Jet Propulsion Laboratory in California.

 

Due to Mercury's slow\ rotation and its elliptic orbit around the sun, when it flew by three times, it saw the same half of the planet. So we've really only seen something like 45% of the planet at relatively low resolution.

 

 

Hidden in these fuzzy black and white postcards for over 30 years are clues that point to one of Mercury's biggest puzzles. It's been shrinking. Mercury doesn't have plate tectonics like the earth does. So we know that Mercury's crust is under a lot of compression. The only way it can really do that is if the planet shrank. And so you can think of it as the incredible, shrinking planet.

 

 

As Mariner 10 flies past, the mystery deepens. The spacecraft detects a vast iron core hidden inside