For information on one of the planets please click on one of the links below

Mercury.

Mercury is the planet closest to the Sun. It's a small planet, only slightly larger than our Moon. Its "day" is nearly as long as its "year". In the daylight, the temperature is high enough to melt lead at its Equator. But the North Pole seems to be made of ice. Mercury has a surface similar to our moon with extensive cratering.

Mercury has always been hard to look at, because it is so close to the Sun. You can see Mercury for less than two hours before sunrise, or less than two hours after sunset. So the ancient astronomers actually believed that Mercury was two planets. Mercury was the planet seen in the early evenings, while Apollo was the planet seen in the early morning. The Romans called this planet after Mercury, the wing-footed messenger of the gods, because it moved so fast through the sky.

It takes Mercury about 88 Earth days to have a complete year. So there are three Mercury days every two Mercury years. The Mercury midday is very strange. If you were standing on the equator of Mercury you would see the Sun slow down as it moved towards high noon, then stop, then start up again in the opposite direction.

Only one spacecraft has ever visited Mercury. The Mariner 10 made three fly-pasts of Mercury in 1974/5. It took photographs of 45% of Mercury's surface. The main things the Mariner found on Mercury were craters and cliffs.

There are small craters less than 10 km across. They have bowl-shaped bottoms and they were caused by impacts with meteorites weighing from 10,000 to 100,000 tonnes. Craters between 10 and 20 km across have a flat bottom. Once the craters get bigger than 20 kilometres, they all have a flat bottom and a peak in the centre of the crater. Craters between 20 and 150 km across are caused by meteorites weighing between 1 and 100 billion tonnes. As the crater gets closer to 150 kilometres across, the central peak becomes taller and wider. When the craters are 150 to 200 km across, the peak gets so large that it turns into a central ring.

The Caloris Basin is 1,300 km across. It was caused by an enormous rock slamming into Mercury. The impact was so colossal that shock waves ran through the planet and focused to an area about 1,000 kilometres across, exactly on the opposite side of the planet, and that's how the huge Caloris Basin was made.