Space exploration began in the 1960s and the early missions were sent out to the Moon, Venus and Mars.

Earth's Moon (Luna): the first Apoolo mission brought back 20kg of rock and soil. These rocks were much older then expected witht the oldest rocks dated as being 4400 Ma.

The now widely accepted theory of how the moon came into being is via the giant impact hypothesis whereby the very early Earth was struck by a body roughly the size of Mercury. The impact tore the Earth apart and ejected much of the lighter material into orbit around it, within just tens of thousands of years this material had accumulated into the Moon. This theory explains why the Moon is significantly less desnse than the Earth and while although it has a crust and mantle it has a very small core. The surface is made up of two different rock types:

the maria: dark areas of basalt lava flows which were generated by impacts of meteorites
the Lunar highlands: light coloured areas composed of plagioclase-rich rock called anorthosite

Mars
The first missions in the 1960s (Mariner 3-7) flew by Mars and they identified huge volcanoes on Mars including Olympus Mons, the largest volcanic structure in the solar system. Further missions such as Viking I and II gained higher resolution photographs but it was not until 2003 when the first spacecraft could land on the surface and move around to explore. These missions were highly successful and were only discontinued in 2011 when final contact was lost.

Venus
Venus is the planet most like the Earth. The two planets are similar in size, mass, composition, and distance from the Sun. Spacecraft have mapped Venus using radar and even landed so that the data on temperature and pressure are actual measurements. But Venus has no oceans and is covered by thick, rapidly spinning clouds that trap surface heat, creating a scorched greenhouse-like world with temperatures hot enough to melt lead. The clouds reflect sunlight making Venus the brightest planet in our sky.

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Undergraduate Geology - Part 1 (Earth Structure) Notiz am Exploring Space, erstellt von siobhan.quirk am 11/05/2013.