Astronomy for Students
Lesson 3 -- The Solar System and Mercury
Our solar system is not just our sun and the planets. It also includes all the moons around the planets. It includes the asteroid belt between Mars and Jupiter and, way past Neptune, the small ‘dwarf planets’ and rocks that are part of the Kuiper Belt. Most of the meteorites come from the asteroid belt, and most of the comets come from the Kuiper Belt. Several times a year the earth passes through ‘debris fields’ left by comets in their orbits. This is when we see the ‘shooting stars.’ The night of August 12 will be a really good time to sleep out under the stars and watch them for as long as you can stay awake!
Going out from the Sun in order are Mercury, Venus, Earth, Mars, the Asteroid belt (with Ceres the largest member), Jupiter, Saturn, Uranus, Neptune and the Kuiper belt ( with Pluto as one of its main members).
This picture shows where the astroid belt is (this drawing does not show Mercury or Venus, both of which are between the earth and the sun):
As you can see, there is not just the main asteroid belt circling between Mars and Jupiter, but there are other groups of astroids, too, which orbit the sun in different ways. The groups that pass through the Earth's orbit are called "earth crossing asteroids" and they include the Apollo Group and the Armor Group shown above. In addition, Jupiter has captured some asteroids and the "Trojan asteroids" are in Jupiter's orbit with Jupiter.
Below is a diagram showing the Kuiper Belt (in brown) which is just outside Neptune's orbit.
The Kuiper Belt is so far out that if you were there the sun would only look like a bright star.
Here is a really excellent page to introduce you to the solar system. Make sure you click on each of the planets in turn. This link is good for all three levels. Introduction to our Solar System
The standard explanation for the asteroid belt and associated groups and for the Kuiper Belt objects is that they were bits and pieces left over from the formation of the solar system. The standard explanation is that the planets formed by something called accretion -- that bits of matter clung together and gradually attracted more and more matter until they were big enough to become planets and moons. This explanation, however, does not fit the evidence, which we will discuss later more thoroughly. The evidence actually indicates that there used to be a planet and its moon between Mars and Jupiter which exploded in several stages, giving us the rubble we call asteroids now. There is evidence that the material in the Kuiper Belt may be from the same sort of thing happening to one or more planets.
Before we go on to the first planets, here is something fun to look at. It is a comparison of size of the sun as seen from each of the planets:
Now for Mercury:
Because Mercury is so close to the sun, that is always how we will see it from earth. It will never be in a dark part of the sky, but always very near the sun either just before sunrise or just after sunset.
Mercury’s diameter is about 4875 km or3,050 miles. This is somewhat less than half the diameter of the earth or about one third larger than our Moon. It travels in an elliptical (oval shaped) orbit with an average distance from the Sun of 36 million miles. It orbits the Sun very quickly, once every 88 days. The axis is tilted at an angle of half a degree from the vertical; it is almost exactly upright. It rotates on its axis once every 59 days. But, because of this slow rotation and fast orbiting, strange things happen. The interval between one sunrise and the next on Mercury is 176 earth days. In other words Mercury orbits the Sun twice in the interval between successive sunrises seen from the planet. Imagine having a day that is twice as long as a year! Also, in that time between sunrises, Mercury has rotated on its axis 3 times! This is very different to what we are used to on Earth. Astronomers call this a “three to two spin orbit resonance” – its spins three times on its axis, while it goes round in its orbit twice.
Mercury also has a very elongated orbit. It is like it approaches the sun and then is flung away a bit, then approaches again. Because it is both so close to the Sun, and because its orbit is so elongated, the Sun’s gravitational pull causes Mercury’s orbit itself to swing around the Sun like this:
Mercury has a very thin atmosphere made up of small amounts of hydrogen, helium, oxygen and sodium.
Because Mercury is so small, its gravity is only about 1/3 of that on Earth. An object that weighs 100 pounds on Earth would only weigh 38 pounds on Mercury.
One of the first pictures of the planet Mercury along with an interesting article can be found here. Please note the bits about Mercury which puzzle scientists. We will be dealing with some of this later. One of the puzzles is why Mercury has such a large iron core compared to its size, and why it still has a magnetic field. Here is a picture of the comparitive core sizes of several of the planets:
From this diagram we see that the size of the nickel-iron cores of the planets becomes a smaller percentage of their total diameter as we go out from Mercury, past Venus to the Earth and Mars. The Moon is given for the purposes of comparison.
As shown above, Mercury's interior is dominated by a huge nickel-iron core. The interior is in 3 sections (1) a crust 100 to 300 km thick (about 180 miles). (2) A mantle 600 km thick (about 375 miles). (3) A core 1800 km radius (about 1100 miles).
The surface of Mercury has craters (some with rays coming from them like the one below -- the dark spot next to it may have been some kind of writing on the original picture) and smooth plains just like our Moon has.
The article connected with this picture tells what we have found after a year of circling Mercury -- and there are some surprises!
The major feature on Mercury is the giant Caloris Basin shown here (the large darkcircle in the middle):
This is clearly an impact crater. The Caloris basin is Mercury’s largest crater and measures 800 miles (1300 km) across.
On the opposite side of Mercury to the Caloris basin is the so-called “weird terrain” a chaotic jumble of hills and valleys, ridges and grooves. It formed as the shock pressure waves from the formation of the Caloris basin went right through the planet and focused on the opposite side. The “weird terrain” looks like this:
The fact that this is exactly opposite the Caloris Basin impact crater has an interesting explanation. The impact which caused the Caloris was transmitted as shock waves through the planet, displacing the crust on the opposite side. A very good explanation of this can be found in this article.
Order of events on Mercury:
NOTE 1 : These atomic dates come from Cosmic Ray Exposure (CRE) ages of meteorites and asteroids. This will be discussed in later studies about the two debris belts.
NOTE 2: The inter-crater plains are rough and have ridges and outlines of buried craters as the molten rock was poured out shortly after the craters were formed from the LHB. These plains underwent the disruption from the Caloris basin and subsequent events and so are rougher than the smooth plains which were formed later. These intercrater plains look like this:
The smooth plains resulted from the outpouring of magma associated with the Caloris basin event and covered over previously exposed material so they are smooth compared with the intercrater plains. Here is an area with the smooth plains:
Mercury has been full of surprises for scientists this past year, and because of the way they think the planets were formed, they are quite puzzled by what they have discovered. However, when we get to the formation of our solar system and the galaxies and stars, it will become apparent that when we let the data speak and tell us what happened, what MESSENGER has shown us should have been expected.