31 May 2010

Solar System Tour: Mercury

Posted by Ryan Anderson

Mercury is the smallest planet in the solar system. It is 4,879 kilometers across. Compare that with our moon, which is 3456 km across, and you can see that Mercury is not very big. In fact, Jupiter’s moons Ganymede and Callisto and Saturn’s moon Titan are bigger! Even though those moons are bigger, Mercury weighs a lot more than they do because it is made of mostly metal and rock. In fact, Mercury has the most metals for its size of any planet in the solar system. Mercury is also the closest planet to the sun: it goes around the sun in 88 days. Mercury rotates on its axis (like a top) two times for every three times it goes around the sun. This means that days on Mercury are really weird! Since it rotates so slow and goes around the sun so fast, if you were standing on the surface of Mercury (I wouldn’t suggest it!) you would see the sun rise in the east for a while, then dip back down towards the horizon, then zip over to the west to set! The sun would be above the horizon for about 60 days. Check out this great simulation of a Day on Mercury to learn more.

Notice how the sun is slightly off-center because Mercury's orbit is more elliptical than most.

Mercury has an unusual orbit. It is more elliptical than most planets, which means it is more of an oval. For a long time, nobody could quite explain the way that Mercury orbited. Many astronomers thought there might be a smaller planet even closer to the sun that was tugging on Mercury and making its orbit disagree with their calculations. It turned out that they weren’t using the right physics! When Einstein came up with his theory of General Relativity, which was an improvement on Newton’s law of gravity, it gave exactly the right answers to explain Mercury’s orbit!

Mercury’s elliptical orbit means that its distance from the sun changes a lot. At the closest, it is 46 million kilometers from the sun. At the farthest point, it is 70 million km away. Even though the distance changes, no matter where it is in its orbit, Mercury is pretty darn close to the sun. On the daytime side, the temperature gets up to 400 degrees C (752 degrees Fahrenheit!). On the night side of the planet, the temperature falls down to -170 degrees C (-274 degrees F). That means that during the day on Mercury, a block of lead would melt into a puddle, and at night it is almost cold enough for oxygen to be a liquid (-182 Fahrenheit)!

Mercury is one of the rocky inner planets. It has a crust a lot like the earth, except mercury is not as geologically active. This means that the crust you’re seeing has not been “recycled” by erosion, volcanos, or plate tectonics. Below the crust is probably a mantle hot, almost-melted rocks. Because mercury is so heavy for its size, scientists think that it has a very large, metal core. By carefully studying how Mercury spins, scientists have figured out that Mercury’s core must be at least partially liquid.

Mercury is one of the most cratered objects in the solar system. The more craters something has, the older its surface is. (What does this tell you about the Earth’s surface?) You can also tell which craters are older than others. If there are craters on top of another crater, the one on the bottom is the oldest. Also, “fresh” craters have bright streaks coming out of them. These bright rays are debris that was blown into the sky when the crater was formed, and then fell back down onto the planet.

The largest crater on Mercury is called the “Caloris Basin”. This huge crater is 1300 kilometers (800 miles) across! It shows up clearly in this false-color view of Mercury:

Caloris means “heat” in latin. The basin got that name because it is near the place on Mercury that is pointed at the sun when the planet is closest to the sun. That means it gets really hot! The shock waves from the giant Caloris impact were so strong, that on the other side of the planet, the ground is all wrinkled and bunched up.

Until recently, there had only been one probe sent to Mercury, Mariner 10. It only got a chance to photograph half of mercury’s surface. For a long time, half of the planet was a complete mystery. But a probe called MESSENGER flew by Mercury a few times recently and took some beautiful pictures to fill in the missing information. It will go into orbit around Mercury in 2011, and we will finally be able to study mercury in detail!

A photograph from one of Messenger's flybys of Mercury.