AY 101 Lecture Notes - Lecture 17: Uranius, Radiometric Dating, Planetary System
The Age of Earth
The age of Earth (and by inference, the age of most other objects in the solar system) is
also not directly known. But related evidence can be studied, in this case, by the
technique of radioactive dating.Various elements (the parent element) are unstable
and decay to produce another (the daughter) element. The time in which one half of a ‐
parent sample decays into its daughter product is known as the half life‐ (t 1/2): it takes
4.5 billions years, for example, for one half of a sample of uranium 238 (the form of ‐ ‐
uranium with 238 nuclear particles) to become lead 206. Alternatively, uranius 235 ‐ ‐
decays much quicker, with one half of a sample becoming lead 207 in 710 million years.‐ ‐
After one half life, the parent/daughter ratio is‐ one half;‐after two half lives, the ratio is ‐
(1/2) 2 = 1/4, three half lives, (1/2)‐ 3 = 1/8, and so forth. Chemical analysis of a rock
sample thus yields the present abundance ratios and an age for the formation of the
rock. The oldest Earth rocks (which are rare due to the recycling of surface materials by
plate tectonics) have an age of 3.8 × 10 9 years, which is a lower limit to the age of the
planet and the solar system. A more correct estimate of the age of the solar system is
based on materials that have been unaltered since their original formation. Applying
radioactive dating to a specific class of meteorites believed to be unaltered since their
formation yields consistent dates for their origin of 4.6 ± 0.1 × 10‐ 9years. This solution is
adopted as the age of Earth and the solar system.
Origin of the Earth Moon System‐
The origin of the Earth Moon system is very much related to the origin of the solar ‐
system as a whole. The ancient lunar surface has preserved a record of events over the
last four billion years. Astronomers obtain relative crater ages from superimposition. For
example, younger craters are found on top of older craters. Ejecta rays from younger
craters also fall over older craters. Craters on lava flows (maria) similarly are younger
than the lava. The purpose of the Apollo lunar missions was to obtain rock samples
from different regions so that the relative age history of the lunar system could be
translated into one with absolute ages. The planet Mercury, which is also heavily
cratered with an apparently similar cratering history as the Moon, supplies additional
evidence to theorize the Moon's history and origin. This, and other evidence, points to a
process by which smaller objects ( planetesimals,or little planets) merged to form the
surviving planetary objects of today's solar system.
Earth and the Moon are so similar they can be thought of as forming a binary planetary
system. Study of their chemical makeup provides important information on how these
two objects became permanently associated with each other. The Moon is relatively
deficient in heavier elements (mean density 3.3 g/cm 3 compared to 5.5 g/cm 3 for
Earth). More specific chemical analysis of Moon rocks shows that the chemistry of the
two objects is otherwise very similar, but not identical. Traditionally, three theories
explain the association of the two objects. The theory of coeval formationargues that the
Moon and Earth coalesced together out of the same materials. The idea that their
chemistry is not identical poses a severe problem for this
theory. Fission theory suggests that a single, initially rapidly rotating object broke apart.
But this theory would require nearly identical chemical composition for the surviving
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