The Moon without a telescope.
This section gives a brief discussion of the large moons in the solar system and the characteristics of the rings found around all of the jovian planets. The best known moon is the only other object that humans have explored directly---the Earth's moon. I will discuss the Moon first and then move to the large moons of the jovian planets. The two moons of Mars and most of the moons of the jovian planets are small, rocky objects about the size of a large city or smaller. Most of them are probably asteroids that wondered too closely to the planet and got trapped by the planet's gravity.
The Moon also has large, dark smooth areas covering about 17% of the Moon's surface that people originally thought were seas of liquid water so they are called mare (Latin for ``seas''---they are what make out the face on the Moon). Now it known that the mare are vast lava flows that spread out over many hundreds of square miles, covering up many craters that were originally there. The mare material is basaltic like the dark material on the Earth's ocean crust and that coming out many of our volcanoes (e.g., the Hawaiian islands). Mercury also has maria but they are lighter in color because of the different chemical composition and they do not stand out from its heavily cratered areas.
Liquid water cannot exist on the Moon because of the lack of an atmosphere---the Moon has only 1/81 the Earth's mass and about 1/6th the Earth's surface gravity. If there is any water to be found on the Moon, it will be in a frozen state in a place of constant shade such as deep craters near the poles. Recent missions have discovered some of those ice blocks near the poles. The ice blocks will be the source of water for any humans that decide to set up bases on the Moon.
Volcano craters are above the surrounding area on mountaintops while the craters from impacts are below the surrounding area with raised rims. The craters on all of the moons except Io, Mercury, and most of the ones on Mars are from impacts. The kinetic energy of the impacting meteorite or asteroid is converted into heat, sound, and mechanical energy---the projectile explodes on impact. The explosion is what carves out the crater so almost all craters are round (otherwise the great majority would be oblong in shape). See the "Not Round" page from the THEMIS site for what can make an impact crater not round (links will appear in a new window).
The rock on the surface of the planet or moon is bent backward, upward, and outward so the amount of material ejected is much larger than the projectile. Large craters will have a central peak formed by the rock beneath the impact point rebounding upward and they may also have terracing of the inner walls of the crater from the collapsing of the crater rim inward. The size of the craters having central peaks depends on the gravity of the planet or moon: on the Moon craters larger than about 60 kilometers in diameter have central peaks while the crater diameter on the Earth needs to be larger than just 1 to 3 kilometers.
The number of craters per unit area on a surface can be used to determine an approximate age for the planet or moon surface if there is no erosion. The longer the surface has been exposed to space, the more craters it will have. If you know how frequently craters of a given size are created on a planet or moon, you can just count up the number of craters per unit area. This assumes, of course, that the cratering rate has been fairly constant for the last few billion years. The heavy bombardment of about 3.8 billion years must be taken into account when using the crater age dating technique. For example, the highland regions on the Moon have ten times the number of craters as the maria, but radioactive dating (explained in the next chapter) shows that the highlands are approximately 500 million years older than the maria, not ten times older. Careful studies of how the craters overlap other craters and other features can be used to develop a history or sequence of the bombardment on the moons and planets.
Our knowledge about the Moon took a huge leap forward during the Apollo missions. One main science reason for going to the Moon was to return rock samples to find about their ages and composition. Using their knowledge of geology gained from the study of Earth rocks, scientists were able to put together a history for the Moon. The Apollo astronauts also left seismometers on the Moon to detect moonquakes that can be used to probe the interior using seismology.
The Moon's density is fairly uniform throughout and is only about 3.3 times the density of water. If it has an iron core, it is less than 800 kilometers in diameter. This is a sharp contrast from planets like Mercury and the Earth that have large iron-nickel cores and overall densities more than 5 times the density of water. The Moon's mantle is made of silicate materials, like the Earth's mantle, and makes up about 90% of the Moon's volume. The temperatures do increase closer to the center and may be high enough to partially liquify the material close to the center. Its lack of a liquid iron-nickel core and slow rotation is why the Moon has no magnetic field.
Lunar samples brought back by the Apollo astronauts show that compared to the Earth, the Moon is deficient in iron and nickel and volatiles (elements and compounds that turn into gas at relatively low temperatures) such as water and lead. The Moon is richer in elements and compounds that vaporize at very high temperatures. The Moon's material is like the Earth's mantle material but was heated to very high temperatures so that the volatiles escaped to space.
The giant impact theory proposes that a large Mars-sized object hit the Earth and blew mantle material outward which later recoalesced to form the Moon. The Earth had already differentiated by the time of the giant impact so its mantle was already iron-poor. The impact and exposure to space got rid of the volatiles in the ejecta mantle material. Such an impact was rare so is was not likely to have also occurred on the other terrestrial planets. The one ``drawback'' of the theory is that it has a lot of parameters (impactor size, speed, angle, composition, etc.) that can be tweaked to get the right result. A complex model can usually be adjusted to fit the data even if it is not the correct one (recall Ptolemy's numerous epicycles). But the giant impact theory is the only one proposed that can explain the compositional and structural characteristics of the Moon.
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last updated: February 26, 2008