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Earth, and The Moon

The Earth is the third planet from the sun, and the largest of the terrestrial planets. The moon that accompanies it is so large (one fourth of earth's size) that the earth is sometimes considered to be a double-planet. Completeing an orbit in about 28 days, the moon shifts in phases, ranging from "new moon" to "full moon". The moon has helped mankind to strive for the stars, it has provided shelter against threats from space like meteors, asteroids and comets. Without it, life would be very different on Earth.
Even without a telescope or a pair of binoculars the moon displays two contrasting features: bright highlands and darker plains. Through a small pair of binoculars the eye can resolve many more craters on the lunar surface than are visible to the naked eye, especially at the edge of the lit area on the moon. Some notable ray craters are visible, like Copernicus and Tycho. Click here for a picture of ray craters.
It does not take many days before one realizes that the moon always shows the same face towards the earth. This, because it is tidally locked to the earth. The moon's orbit around the earth is exactly as long as one spin around its axis (a lunar day). The moon is gradually slowing down earth's day, which will become longer and longer as time passes.

On this page I will focus mainly on the moon.

Quick links

- Late Heavy Bombardment Illustration (P)
Creation of the moon - Space art 1 - Space art 2 - Space art 3
- Capture theory
- Coaccretion theory
- Fission theory
- Collision theory
Lunar exploration

The creation of the Earth-Moon system

The Earth and its companion were born approximately 4.6 billion years ago (the age is derived from radiometric age determinations of meteorites) from the solar nebula. There are many theories of how the Earth-Moon system was created.

The capture theory states that the moon was formed elsewhere in the solarsystem and was slowed down by debris close to the earth and then finally captured by Earth's gravity. This theory has some problems, for example that the only piece of evidence supported by the capture theory is the difference in composition between the earth and the moon, particularly the large difference in iron content. Another problem is that the encounter would have occurred at a relatively high velocity, making a capture impossible.
Thirdly, the samples of rocks brought back by the astronauts that were on the moon shows that the Earth and the moon have the same quantities of oxygen isotopes, suggesting a closer relationship. If the moon was captured, it would have had an extremely elliptic orbit, which it has not.

The co-accretion theory postulates that the moon was formed from an accretion disc of solid objects. The problem with this theory is that it cannot explain the angular momentum and the difference in chemical composition between the earth and the moon. If they would have formed at the same place they would have had a similar composition. It does not either explain why the Earth about 1.6 times denser than the moon. Also the moon orbits at a 5 09' inclination above the plane of earth's rotation. If it would have formed from the same disc, it would have orbited at the planar level (0 degrees above the plane).

The fission theory was announced by George H. Darwin (son of Charles Darwin). It states that the Earth spun and flattened so quickly that it ejected a large piece of material, which eventually became the moon. The Pacific Ocean basin is sometimes considered as the site from which the moon was ejected into space.
This theory explains the isotopic ratio and iron content similarities between Earth and Moon. However, the angular momentum today is too small to fit into this model.

The most popular theory is the collision theory, also know as the giant impact theory, which says that when the Earth had cooled down and got a solid crust a Mars-sized protoplanetary object hit it and once again melted the crust again. The matter from the crust that was thrown into space gathered together and gradually formed the moon. This would explain why the earth's crust and the moon has the same density and same materials. Another strength in this model is that it explains the lack of water and lightweight volatile compounds on the moon: The impact generated so much heat that the volatiles evaporated and escaped into space.
The object which is thought to have collided with the moon is believed to have originated close to the Earth, because of the low speed of the impact. Perhaps it came from one of Earth's five Lagrange points (points which hold some gravitationally stable orbits).
Giant impacts were not uncommon during the late stages of the terrestrial planets' formation. After this event, when the surface began to cool down, billions of years of bombardment by meteors, asteroids and comets followed, leaving craters as scars on the lunar surface. This bombardment is common to many objects in the solar system.

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Lunar Exploration

On the 20th of July, 1969 Neil Armstrong and his Apollo 11 crew landed on an area called "Sea of Tranquillity" on the moon. The team brought back samples of rocks to study. After this mission was over, six other Apollo missions headed for the moon. Apollo 13 was the only mission that failed to land (due to an explosion onboard) on the lunar surface. The last mission, Apollo 17, landed on the moon on December 11, 1972. After that no man has ever visited the moon.

There are many reasons to go back to the moon, one of them being that the moon is an important middle-step to other worlds, such as Mars. From the moon it would be cheaper to extract the fuel (oxygen and hydrogen from water) needed to go to Mars. The technology needed for the mission could also be tested more easily on the moon, which is 2-3 days away, rather than on Mars itself which is 6 months away.

The temperature on the moon varies from about -230 to 120 degrees Celsius, which would make water evaporate when it reaches above 100 degrees. For this reason water would be most easily found in permanently shadowed craters on the north/southpole.
In 1994, the spacecraft Clementine orbited the moon and mapped its surface. In one experiment, Clementine beamed radio signals into shadowed craters near the Moon's south pole. The reflections, received by antennas on Earth, seemed to come from icy material. Later, when astronomers tried to detect those signals again from the Arecibo radar in Puerto Rico, they couldnt find it. In 1998, NASA sent another spacecraft, Lunar Prospector, to search for the same signals again. Again the signal was found, which indicated the presence of hydrogen (hydrogen from water?). When it had ended its mission it plunged into a crater in order to make a cloud of water rise from the impact. But that cloud was never found, maybe there wasnt any water, or the water wasnt enough in quantity so that the astronomers could detect it.

There are other important reasons for us to go to the moon: Sooner or later, the atmosphere of the earth will get so polluted that it will be difficult to make studies of the night sky without worrying about light pollution. Astronomers have to look for remote and high grounds already today. Radioastronomers have to struggle with the media (suppliers of mobilephones, television, etc...) to get free frequency windows from which they can study the universe. That problem will be reduced when and if observatories are placed on the moon, but by choosing this tactics, new problems emerge that astronomers and space explorers will have to deal with.

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Previous: Venus.
Next: Mars.


Moons:"The moon".
Average distance from sun:149 597 890 km.
Equatorial Radius: 6 378.14 km.
Mass: 5.9737 x 10^24 kg.
Density: 5.515 g/cm^3.
Escape Velocity: 11.180 km/s.
Length of day: 23.934 hours.
Length of year: 365.24 Earth days.
Mean Orbit Velocity: 29.786 km/s.
Equatorial Inclination to Orbit: 23.45 degrees.
Surface Temperature: -88/58 (min/max) C.
Atmosphere: Nitrogen 77%, Oxygen 21%, other 2%.

Source: NASA.

Above: Earth as seen approximately 4.6 billion years ago just after the great collision, and the bodies had coalesced. During this phase, the Earth and the Moon were both bombarded very intensely by rocks, asteroids and comets.

This illustration is available upon request, as a Print (6000x4000 pixels, 300 dpi).

Above: Right after the collision that created the Earth, and the Moon, our neighbour in space was as close as 20 000 km, which resulted in massive tidal forces on the surface of the Earth. Volcanic activity was very common.

This illustration is available upon request, as a Print (3000x4000 pixels, 300 dpi).

Space art: Hadean Earth

Above: Earth as seen approximately four billion years ago when it was still at a primordial state. During this period heavy both the Earth and the moon were bombarded from debris.

This illustration is available upon request, as a Print (1600x1200 pixels, 300 dpi).

Space art: Hadean Moon

Above: The moon as seen approximately four billion years ago when it was still in a primordial state.

Space art: Hadean Moon

Space art: Hadean Moon

Above A hypothetic view of a Mars-sized object hitting the Earth, resulting in a collision that would form our Moon from the debris.
The image at the bottom is available in print size (5000 x 3000, 300 dpi) upon request!


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