A fun fact about Mercury, the closest planet to the sun, is that you can only see it with the bare eye when the sun is rising or setting! Mercury is so close to the sun that it isn't visible at night (although the other planets are visible at night), so it can only be viewed when the sun is up.

Interestingly, even though Mercury is the closest planet to the sun, Venus is the hottest planet in the solar system. This is due to its proximity to the sun and its atmosphere, which is made up of dense gases that trap heat to the surface via the greenhouse effect. Venus has an average temperature of 864 degrees Fahrenheit! During the day, it gets to 872 F, which is hot enough to melt lead.

The four inner planets are thus named because they are closest to the sun. Unlike the outer planets which are gas giants, Mercury, Venus, Earth, and Mars all have rocky cores and solid surfaces. A lesser-known interesting fact about the planet Mercury is that scientists believe it may have formed far from the sun and somehow been pushed inward (i.e. by collision with another body) when the solar system was young. The reason for this theory is that recent data from the spacecraft, Messenger, show the existence of rather volatile elements such as sodium, sulfur, and potassium on Mercury's surface. If the planet had formed close to the sun, the sodium, sulfur, and potassium would not have been stable in their elemental forms. Instead, they would have formed various compounds, probably with oxygen, or been depleted through evaporation.

Volatile elements persisting on the surface of Mercury give scientists a clue that the planet's formation conditions were less like those of Venus, Earth, and Mars than they previously thought. (Source: chemical composition of Mercury ).

For me it's this: My college roommate worked on the first images of Mercury that were taken from space, in 1974. She loved finding the details of Mercury that had never been seen before, and she loved working on this exciting project in Pasadena CA. It's called 'image enhancement' - getting a clearer picture of a 'noisy' picture.

After imaging Venus, Mariner 10 used the planet’s gravitational pull to slingshot towards Mercury. The spacecraft entered Mercury’s orbit in March 1974, and took more than 2,800 pictures during three passes of the planet.

Earth is the most massive of the inner planets, but Venus has a denser atmosphere.