Introduction
The moon was formed about 4.6 billion years ago. In the next 0.2 billion years, it was continual bombarded by huge chunks of materials from outer space. Much energy was released that the surface remained molten. Between 4.1 to 4.4 billion years ago, the amount of debris falling in has diminished, allowing the crust to solidify thereby producing the cratered highlands. However, below the surface, the moon was still molten. Between 3.9 to 4.1 billion years ago impacts of a few large meteoroids produced the huge basins with reduced thickness of crust. Subsequently, between 3.0 to 3.9 billion years ago, magma, which were produced by radioactive decay below the solid crust, flooded the basins thereby producing the maria. Since then, the moon has been relatively quiet with impacts of fewer and smaller meteoroids which not only produced the craters in the maria and more craters in the highland, but also eroded the surfaces (including the mountain ranges).Besides the meteorite impacts, erosion is caused by extreme temperature changes, impacts of cosmic rays, intense solar radiation and particles from the solar wind.
Features of the Moon
Our Moon has two types of prominent features visible to the naked eyes. These are:
the one and a half dozen or so large dark areas which resembles the seas occupying about 20% of the total surface area (maria), and
the light-colored regions resembles the continents occupying more than 70% of the surfaces (terra).
Maria
Instead of oceans or seas, the maria are actually flat dry lava plains.
They are relatively smooth and are punctured by few craters.
Most of these maria are confined to the northern part of the Earth-facing hemisphere: there are far fewer on the farside.
They can be regarded as basins filled with lava, so that in a sense they did resemble our seas at one time when the lava was molten.
These maria can be very large: the largest, Mare Imbrium, having a diameter of 1150 km.
These maria look dark because they consist of dark fine-grained rocks similar to terrestrial basalts. They are essentially igneous magnesium-iron silicates (Note: A basalt is a dark dense igneous rock of volcanic origin.).
Terra
The light-coloured areas are highlands which are elevated about three kilometers
(and not meters) above the maria (which, by contrast, are lowlands.)
They are made up of a light colored igneous rock with visible grains consisting of aluminium-calcium silicates.
The terra is heavily cratered and are older than those of the maria.
Craters
Craters
are found everywhere on the moon, ranging from microscopic size (10 microns
across)to more than 200 kilometers across. Most of them are found on the
highlands and far fewer on the maria. The largest crater on the Earth-facing
side is Clavius with a diameter of 240 km. It lies south of Tycho
which is a very impressive crater with a ray system that appears to stretch over
the whole Moon. Another prominent crater is Copernicus with a diameter of 93 km
and crater walls which are 3.7 km higher than the crater floor and a shark peak
at the center.
These craters were at one time thought to be extinct volcanoes.
The result is a roughly circular crater with a rim around the circular crater. Often there is a central peak inside the crater and a concentric ring of mountains outside. It is believed that the meteoroid vaporized material at the impact point and also produced a zone of melting around this point. Probably, a wavelike structure is created outside this zone which fractured bedrock and created the mountain ranges.
The energy released is colossal: for example, the meteoroid that produced the 90km diameter crater Copernicus released 1023 Joules (J) of energy. This is 1 million times more energetic than the volcanic eruption of Mt. St. Helena in May 1980 (with an energy yield of 33 megatons of TNT.) Note that the first atomic bomb released energy equivalent to 20 kilotons of TNT, where a kiloton of TNT is equivalent to 5 X1012J
The debris produced by the explosion can land on the surface producing more craters. They are responsible for the producing the rays which are bright streaks radiating from some craters such as Tycho.
Mountains
There are several mountain ranges, with most of them surrounding Mare Imbrium.
Rilles
These look like winding
valleys that resemble the canyons or dry river beds. These are probably produced
by flows of lava instead of water.
In the picture on the left, a volcanic rille is shown next to the mountains at the bottom right (the Apennine Mountains). This rille extends upwards and crosses into Maria Imbrium in which can be seen two prominent craters. The craters shown are named Autolycus and Aristillus (from centre to top).
Surface
The composition of the upper layer varies from place to place.
Lunar Eclipse
One of the most exciting events in the night sky is an eclipse of the moon. On the average about twice a year, when the moon is full, it sudden becomes darker and darker until it has almost lost all its light. It looks like it has been swallowed up. Slower, within an hour or two, the moon would reappear back into its full brilliance.
In the past, people thought that some supernatural events must be at work. Ancient astronomers of different civilizations were able to predict these events thereby gaining for themselves an air of ability, wisdom and power.A simple explanation is that a lunar eclipse occurs whenever the moon lies in the shadow of the Earth.
Should the moon wander into this shadow, the sunlight that would have fallen on it would be blocked by the Earth, thereby plunging the moon into darkness.
In the course of a lunar eclipse, more and more parts of the moon gets blocked away as it enters into the shadow. When the moon is completely in the shadow, the moon is completely blocked out. As the moon emerges from the shadow, it slowly regains its brightness and its full circular disk.