Mercury
Mercury is, surprisingly, one of the least well-studied objects in the solar system. It is so close to the Sun that when viewed from Earth it is never far from it. This means that it can only be viewed close to sunrise and sunset, and then low on the horizon. As a consequence, astronomers have to cope with a relatively bright sky, and also looking through a lot more air than looking straight up.
Even powerful telescopes like the Hubble, in orbit about the Earth and outside of the Earth's atmosphere, are not allowed to observe Mercury. This is because they have very delicate optics; pointing so close to the Sun is considered dangerous - a slight mishap and it could blind the telescope! But Mercury is an important body to understand - its place as the innermost planet in our solar system makes it an important "end member" of our family of planets. Understanding how Mercury formed and evolved is key to understanding all of the other innermost, rocky, "terrestrial" planets.
Despite its importance, Mercury has only been visited once for a brief fly-by. The Mariner 10 spacecraft encountered the planet in the mid-1970s and made 3 passes over the planet. Despite being relatively close, in astronomical terms, it is not distance, but energy that determines how easy it is to get to a planet. A spacecraft at the Earth's orbit must get rid of much of its orbital energy in order to "fall" towards the Sun. By the time it arrived at Mercury, Mariner 10 was travelling too fast and did not have enough fuel to enter orbit. Unfortunately, these 3 fly-bys took place over the same half of the planet, so we still only have pictures of half of the planet's surface!
The images that Mariner 10 returned show a grey, cratered body that looks much like the highland (lighter) regions on Earth's own Moon. Overall Mercury is somewhat darker than the Moon and lacks the marked contrast differences between the highland and Mare (seas) seen there. However, it also shows some unique features, such as long ridge features, and an area of fractured landscape known as the "Weird Terrain". We also know that Mercury has a different density from the other inner planets. Density tells us how much a given volume of material weighs, so to calculate it we need to know the mass and size. Mass can be calculated from the observed orbit and diameter from observations. From our knowledge of the Earth and other planets it seems most likely that this is because the central iron core of the planet is much larger, relative to Mercury as a whole, than at other planets - perhaps due to a catastrophic collision blasting off much of the outer crust.
Mercury also has a very odd orbit. Our own Moon is tidally locked so that one face always faces the Earth - through all of the phases of the Moon, we still see the same side. In the case of Mercury the situation is more complex: Mercury spins on its axis every 58.6 Earth days, and completes one orbit around the sun every 87.9 days. So Mercury has 3 complete days in exactly two years! This would make life rather interesting for any residents of Mercury - at some locations on the equator, they would see the Sun rise, change its mind and set again, before deciding that it was right the first time and carrying on with the day!
This peculiar "spin-orbit coupling" also has a big effect on the temperature of Mercury. You might expect that Mercury would be the hottest planet in the solar system, being so close to the Sun, but in fact that honour goes to Venus, who's runaway greenhouse keeps surface temperatures hot enough to ... However, Mercury does have the largest temperature variation - between 70K at night and 700K in the day!
The other big surprise from the Mariner 10 results was the existence of a planetary magnetic field. According to our best theories, magnetic field's like the Earth's are formed when a planet has a molten (and conductive) core. It was expected that Mercury, much like our Moon, should have cooled long ago and hence have no global field; the origin of Mercury's field is still unexplained and the answers will wait until we have another spacecraft (or two.) in orbit around the planet.
For many years Mercury was seen as a rather boring, small, ball of rock, albeit one about which we knew very little. But recent observations of the planet using powerful radar have shown some exciting results! The Arecibo radio telescope, famous both for its role in the film Contact and James Bond [which one?] directed a signal at Mercury and analysed the radio waves bounced back to Earth. Using this method they discovered reflections that looked like they came from ice! Further studies showed that the location of this ice agreed very well with the positions of craters observed by Mariner 10, all near either the north or the south poles. Much as the poles of the earth are very cold, the poles of Mercury receive little sunlight - and in fact the floor of such craters can be completely hidden from the Sun, meaning that they are always cold. cold enough for ice to survive for hundreds of thousands of years!
So Mercury is an important planet about which we know very little. However, this is soon to change! Two spacecraft are currently destined for Mercury - the NASA MESSENGER mission is already well underway, and the European BepiColombo pair of spacecraft will follow shortly. Watch this space for the answers to these questions, as well as many new ones!