The Inner Planets
The first four planets out from the sun are the inner planets. They are the rocky planets, and only two of them have moons. The first out is tiny Mercury, followed by cloudy Venus. We are next, here on Earth, and we are the first planet out from the sun with a moon. The fourth rocky planet is Mars. Mars has two moons, but they sure aren't like our moon! Let's take a look at these four planets. Mercury is on this page and the other planets are on linked pages.
Mercury is the tiniest planet. It is less than half the size of Earth, with a diameter of only 3100 miles (Earth's diameter is 8000 miles). Mercury is so close to the sun that it is very hard to see or take any pictures of it because of the sun's glare. In the picture below, Mercury is the tiny black dot crossing the face of the sun. When a planet does this, it is called a transit. So this is a picture of the transit of Mercury.
How close is Mercury to the sun? 36 million miles. That sounds like a lot, but in space, that is a very short distance. The Earth is 93 million miles from the sun. We call that one astronomical unit, or 1 AU. So Mercury is only 0.39 AU from the sun.
Earth takes 365.25 days to go around the sun once. That is 1 Earth year. Mercury whips around the sun in just 88 of our days. So 88 days on Earth is one year on Mercury. Now the weird part. Every one of our days is 24 hours long, so we have 365 of them every year. Mercury spins once on its axis only once every 176 Earth days. So it's day is longer than its year. It's the only planet that does that. Mercury actually rotates 3 times on its axis for every 2 revolutions about the sun. This is called a 3:2 spin-orbit resonance. A very good animaltion of that can be seen here.
Most of the planets have a fairly circular orbit around the sun. But not Mercury. Its orbit looks like this:
This sort of an orbit is called elliptical. Mercury's orbit is highly elliptical. Because it is so close to the sun, its orbit swings a little bit each time it goes around. The way this is described scientifically is that its perihelion advances around the sun. "Perihelion" means the closest point to the sun. The farthest point from the sun for any orbit is called the "aphelion." So Mercury's aphelion also swings around, or advances, forming a sort of flower petal pattern through the years as the diagram on the right shows.
Here is an interesting fact. Mercury spins so slowly on its axis while it's whipping around the sun, that when it is close to the sun (which is when it is moving fastest), the sun appears to trace a slight loop in its sky. An excellent animation of what happens can be found here.
Every planet has an axis tilt. They look like this:
Mercury, as tiny as it is, is the planet that has the least axis tilt. It is almost completely "straight up and down."
The word "atmosphere" means the gases that float above the surface but are held onto the planet because of the planet's gravity. Little Mercury has very little gravity (about 1/3 of that of the earth). An object that weighs 100 pounds on Earth would only weigh 38 pounds on Mercury. It also has the least atmosphere of any of the planets. Its thin atmosphere is made up of 42% oxygen, 29% sodium, 22% hydrogen, and 6% helium. (This was measured spacecraft orbiting around Mercury) Atmosphere pressure on Mercury is 10-15 bars, or only 0.000000000000001 times the air pressure on earth at sea level.
Because of its thin atmosphere (it was boiled off by the heat of the sun), the sky would be black, not blue. In fact, if you hid in some shadows behind a pile of rocks, so that the sun’s rays were blocked out, the stars would be visible in the daytime sky.
Mercury has an enormous temperature range. That is because its rate of rotation is so slow that one half of it will face the sun for a very long time while the other have remains in the dark for a very long time. The sunny side can get up to over 660 degrees F. (remember water boils at 212, and 660 is enough to melt tin and lead). The dark side can go down to -274 degrees F. Mercury's thin atmosphere also allows it to heat and cool more completely and rapidly than a planet with a thicker atmosphere would do.
Mercury has a magnetic field. That means it has a magnetic north and a magnetic south pole. But this is not the same as the "top" of its axis. Its magnetic north is eleven degrees off its spin axis, so both the magnetic north and the magnetic south spin around the axis as well, as the planet rotates.
In the above illustration, the spin axis is shown by the narrow white line. Mercury's is almost straight up and down. But its magentic poles are eleven degrees off the spin axis, as shown by the thick blue line referred to as "beamed radiation" in this picture. (Actually, the thick blue line has nothing to do with beamed radiation, but the illustration is good otherwise.)
Mercury's density is 5.43 times that of water. This is because it has a huge nickel-iron core. Its core makes up 75% of the little planet's diameter. That means the core itself is about 40% of the planet! This is a bigger proportion of the planet than any other planet's core. Mercury also contains the highest percentage of iron of any of the other planets or moons in the solar system.
There are two parts to Mercury's core. The center is solid, and it is surrounded by a liquid iron outer core. The turning of the planet itself generates electric currents in the outer liquid core which, in turn, give rise to the magnetic field (Every electric current is circled by a magnetic field -- which is why it is called electromagnetism).
Mercury, like all the planets, is surrounded by its own ionosphere, or plasma sphere.
Unlike the other planets, Mercury's plasma sphere (or ionosphere) is very small in front. It is so close to the sun that it is literally battered constantly by the solar wind -- a stream of positive particles. However, like the other planets, the 'tail' stretches out for quite a long way. In fact, Venus can pass through the tail.
Mercury has the same number of craters per given area on its surface as our moon does. In short, it's loaded -- it is the most crater-pocked planet in the solar system. In some areas on Mercury there are plains between some of the craters. These are old, and are called inter-crater plains. There are other plains, called 'smooth plains,' which are more recent.
The Caloris Basin is the largest feature on Mercury. It is the site of a very large impact. It is about a thousand miles across.
On the exact opposite side of Mercury (the antipodal) side, we see the "weird terrain" or "broken terrain." There we find jumbled hills three to six miles across and about a mile high.
This weird terrain is the result of the force of the impact which formed the Caloris Basin. The shock traveled through the planet's interior and the result on the antipodal point was a massive series of Mercury-quakes which produced what you see in the photograph above.
A more complete explanation of cratering can be found in the section on Earth's Moon
Order of events on Mercury:
NOTE: The notation of "atomic years" is made because time measured by atomic processes is not the same as time measured by how many times we go around the sun. The difference is explained in Dating Methods.
Update April 8, 2015 -- Strange 'hollows' on Mercury revealed by NASA probe