Oxygen Isotopes
Characterizing the relative amounts of different isotopes of an element can help identify where a sample came from.
Oxygen atoms have 8 protons and 8 electrons and exist as three main isotopes, which vary in the number of neutrons: 16O (8 neutrons), 17O (9 neutrons), and 18O (10 neutrons). The relative abundance of these stable isotopes can be expressed as ratios. 17O/16O is a measure of how much 17O there is compared to 16O (relative to a known standard and multiplied by 1000, because these are very small differences). Similarly, 18O/16O indicates how much 18O there is compared to 16O.
Natural processes separate the isotopes from each other on Earth, typically according to the mass of the isotopes. At different places on the planet, the amounts of the isotopes may differ, but their ratios will be the same. Therefore, their ratios fall along a line with a particular slope when plotted on a graph. The isotopic composition of nearly all rock samples from Earth fall along this line, which has a slope of 0.5.
Objects formed in different parts of the solar system contain different proportions of 16O, 17O, and 18O, so their rocks fall on different lines. This graph compares the oxygen isotope ratios for rock samples from the Moon, meteorites from Mars, and meteorites from asteroid Vesta, as compared to rock samples from Earth, represented by a line. Studies of other isotopes including silicon, chromium, titanium, and tungsten in Moon samples also show similarities with Earth samples.
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Take Notes
Based on the evidence, construct an argument on whether data about isotopic ratios support the giant impact hypothesis.
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