How big is Mercury?
Mercury is about 2,900 miles across, bigger than our moon but smaller than Earth.

What is Mercury made of?
Mercury is a rocky, cratered planet. Pictures of it taken by the Mariner spacecraft show that it looks a lot like our moon. No spacecraft has visited the surface of Mercury, but we can guess that its rocks are similar to Earth's. There is no atmosphere.

How much does the temperature vary on Mercury?
The side of Mercury facing the sun is very hot, about 800° Fahrenheit! But the dark side gets very cold, about –280° Fahrenheit, with no sun to warm it and no atmosphere to hold in the heat.

Why does Mercury have such extreme temperature differences and Earth does not? Is it the atmosphere on Earth?
That is part of the answer. Earth's atmosphere and oceans help keep the its temperature moderate. Another BIG factor is that Mercury rotates very slowly. It rotates with respect to the sun only once every 59 earth days. So one side gets roasted for a long time by the sun, with nothing to protect it, before it gets any shade. Then on the night side, there is no sun to warm it for a long time and no blanket-like atmosphere to help.

What do you know so far about life on Mercury?
Mercury is scorched on one side by the hot sun for long periods of time, and is so hot that lead would melt. The other side is dark and extremely cold. There is no atmosphere. The surface looks similar to the moon — rocks and craters. I don't think life could exist on Mercury. I don't think we would even want to visit it!

Is Mercury hot or cold inside the planet?
This is a hard question. It isn't easy to measure the temperature of the inside of a planet far away in our solar system! We think that Mercury has a molten core, so that would mean that it is hot inside. The reason we think this is that the spacecraft Mariner 10 found that Mercury has a magnetic field, similar to Earth's but not as strong. We think that Earth's magnetic field is due to its molten core, so that means that Mercury's must also be due to a molten core.

Does Mercury have rings or any moons around it?
No, Mercury doesn't have either rings or moons. Neither does Venus! I think that this is because the planets are fairly close to the sun, and the sun's strong gravity would interfere with anything in orbit around those two planets. Good question! ALL the other planets have moons and some have rings!

When Mercury is directly in line with the sun and Earth, will it affect us on Earth?
Mercury is so small with respect to the sun that no one would notice Mercury's tiny shadow.

Do Mercury and Venus ever get between Earth and the sun? If so, what is it called?
Yes, this happens — it is called a "transit." Since the planets are so much smaller than the sun, it looks like a small black dot moving across the face of the bright sun. A transit of Mercury happens about 13 times in 100 years. Transits by Venus are more rare. There were two back in 1874 and 1882. The next ones you and I may see — they are coming up on June 8, 2004, and June 6, 2012.