How can stones from the moon and Mars be found on the earth?
Just as the earth from time to time collides with asteroids, so do also the planets and moons in the solar system. In the asteroid belt between Mars and Jupiter, there are millions of stones of different sizes, and sometimes they come out of course and collide with each other, with planets or the earth.
When an asteroid that is some kilometres in size collides with the moon or Mars, it moves at a speed of perhaps 100,000 kilometres an hour, and causes an extremely intense and violent explosion. The asteroid and several hundred to several thousand cubic kilometres of rock vaporize, and huge amounts of stone are blown to a great height. Some of this will travel so far out that it will not fall again, but will disappear out into space. The release speed from the moon, for example, is 2.38 km per second. These stones may eventually fall down on other objects in space, for example Mars, the moon or the earth. On earth there have been found 65 stones from Mars and 40 from the moon, and it is likely that there may lie some types of rock from the earth on the moon. These meteorites are unique for scientists who in this way, for example, can investigate the surface of Mars without ever having been there, or estimate the age of rocks on other objects in space, thereby giving us further knowledge as to how our own solar system was formed and developed.
How can we know that the meteorites are from the moon or from Mars?
Analysis of chemical composition, isotopes, minerals and structure tell us where these meteorites come from. They greatly resemble earthly rock types such as basalt, but have a distinctive structure. Their appearance is quite different from that of chondrules, because they have been formed in a magma chamber where transformation and melting have formed a volcanic type of rock.
Volcanic activity can only take place on spatial objects of some size, and by means of ascertainment of age it has been proved that they are much younger than chondrites. As in stone on earth, the moon meteorites have a high proportion of the feldspar mineral plagioclase. Measurement of noble gasses in the meteorites also reveals where these come from. The Apollo program of NASA also had much material from the moon as the basis for research.
There is extensive research in this area, and much new knowledge has been gained in recent years. For example, in May 2008 an unmanned spaceship landed near the ice areas on the northern part of the planet Mars.
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