Origins of Moon Theories
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- Three theories on origin of the Moon:
- Earth and moon formed simultaneously, with moon orbiting around the earth.
- Early earth was spinning so fast that a chunk of it broke off from it and spun of into orbit and became the moon.
- Moon formed elsewhere in solar system and was later captured by earth’s gravitational field.
- The moon was formed 4. 5 billion years ago as a result of a collision between and a planet sized object in space. This the most accepted theory among scientists.
Development of Moon
- The debris from the impact of collision started to orbit around the earth and gravity of the debris started to pull each to its self. The debris then joined to form the moon.
- It is believed that this process took less than a year to happen.
- The moon then faced constant bombardment of meteoroids.
- This bombardment made the moon a huge magma ocean. Lighter materials floated to the top and heavier settled below it.
- As impacts of meteoroids reduced, the surfaces of the moon cooled and harden slowly.
- The new impacts then created craters or basins (very deep craters). Meteoroids that created basins caused lava flows on the moon. – covering the original surface
- It is believed that between 4 -3 billion years ago bombardment of meteoroids on moon reduced drastically.
- Magma (rich with iron) flowed on moon. As the magma cooled they created dark coloured plains and light coloured highlands.
- Slowly the moon interior became inactive (3 billion years ago).
- The moon turns once on its axis in the same time period in which it completes one orbit around Earth
- That’s why we only see one side of the moon – the front side
- The moon has three layers
- The core – 150km thick
- The mantle – 1400km thick
- And the crust – 65km thick
Information on The Moon
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- Moon turns on its axis in the same time period in which it completes its orbit around the earth – 27 days. This is why we always see only once side of the moon.
- Moon’s diameter – 3476 km (quarter of earth diameter.)
- Moon mass is 1/81 of the earth.
- · Moon density is 3.3g/cm3 < 5.5g/cm3 of earth.
- Density of earth and moon supports the collision theory that where by the moon would have formed from the less dense material from the outer layers of earth and the object that impacted it.
- They are the dark coloured areas on the moon.
- They are great basins and fractures on the moon by past giant impacts and lava flows. They are also level plains.
- Earlier telescope observers thought it was dark because it was filled with water.
- So they called it Maria(Latin for seas)
- Fist two Apollo missions brought back rocks from Maria. The rocks resembled basaltic rocks of Hawaii and contained mostly plagioclase feldspar and pyroxene. This rocks are called mare basalt.
- Some also contained Olivine and ilmenite.
- They range in age between 3.1- 3.8 billion years.
- Some areas of the moon have higher gravity, these were the areas over the more circular Maria.
- This gravity difference is caused due to different density of material under the Maria.
- The scientists believe this denser material is the same type of material that created mare basalts.
- These areas are called mascons (mass concentration)
- A special feature of Maria is Rille. Rilles are trench like valleys running though Maria bedrock. E.g. Hadley Rille on the floor of Mare Imbrium. Believed to be formed by tunnels of previous lava flows.
- They are light coloured rocks and reflect sunlight.
- There are mountains ranges and craters located in these highlands
- Some of them are Lunar Alps, Apennines.
- There usually 5 km high.
- Scientist believe this ranges were created by meteoroids impacts.
- The moon has no atmosphere to slow flying particles.
- Moon has a weak gravity pull.
- Rocks found in highlands are Breccias, coarsely crystalline igneous rocks.
- The composition of these rocks are similar to gabbro and anorthosite. Scientist thinks they make up the moon surface.
- Breccias are rocks made of angular fragments cemented together with fine material.
- In earth these rocks are created by volcanic eruptions.
- In moon it was mostly created by meteoroids impacts.
- The rocks of Lunar Highlands are usually 4.2 – 4.5 billion years old. This proves the hypothesis that lunar highlands are original lunar crust.
Lunar Craters and Rays
- Lunar craters are circular hollows on the moon’s surface created by meteoroids impacts.
- They create rugged cliffs.
- An example is Aitken Basin – 2100km.
- Rays radiates from craters. They are sometimes hundreds of kilometers long.
- They are shattered rocks and dust that were splashed out by meteoroid that created the crater.
- Scientist prefer to call it regolith which means loose rock materials.
- Grayish brown mixture that range in size between sand grains to fine dust.
- It has no water or organic materials.
- Formed from meteoroids impacts.
- The depth of soil is between 2 – 20 meters.
- It contains chips from different rocks and minerals.
- Regolith also contain glassy material which are droplets of molten rock of meteoroids that solidified too quickly. They sometimes form a glaze on other rocks.