composition of the material forming each planet. The relationship between density
and mineralogical and/or chemical composition is related to the way the atomic
structure of the matter is arranged (Broecker 1985).
Since the density of a planet depends on its mass, the ‘‘denser’’ or ‘‘heavier’’ a
planet is, the stronger its gravitational pull will be. Thus, since Venus and Earth are
larger than Mercury, gravity’s attraction on their surface will also be greater. The
pressure generated by a planet’s gravity will cause the atoms forming the planet’s
interior to contract in size. If a planet’s mass is increased, then its gravity will
increase and the contraction of the atoms forming the planet’s material will also
increase.
The information we have concerning the composition of the stars, planets, their
satellites (or moons) and comets has been obtained from astronomical observations
conducted from Earth and, more recently, from observations obtained by spacecraft launched from Earth to explore our galaxy. The astronomical observations on
the elements composing other celestial bodies are based on studies of light
refracted by a prism, in order to separate the light’s different colors into a spectrum. Each color of the prismatic light characterizes an element. Laboratory
experiments to define the various elements consist in calibrating the passage of
light from an electrical arc through a known gas mixture.
The most abundant elements in a star’s interior are hydrogen (H) and helium
(He), and the second most abundant elements are oxygen (O), carbon (C) and neon
(Ne) (Fig. 2.1). Volatiles in comets consist essentially of water and minor amounts
(about 1 %) of other constituents such as CO (carbon monoxide), CO 2 (carbon
dioxide), methanol, formaldehyde, ammonia and hydrogen sulfide (HS 2 ).
Compared to stars or comets, the most abundant elements forming the largest
planets are oxygen, magnesium, silicon and iron with nickel, sulfur, calcium,
aluminum, sodium, potassium, and titanium, in decreasing abundance. The
abundance of other elements drops rapidly, and becomes so insignificant that all
the other elements represent no more than one percent of a planet’s entire composition (White 2013). Due to these compositional variations, it is not surprising to
observe that when we are on Earth, we are walking around on a solid crust which is
mainly made up of elements such as O, Fe, Mg, Si and Al forming crystalline
compounds, which we call minerals and rocks.
Another direct source of knowledge about the composition of our planet comes
from the meteorites, which have fallen on Earth. These ‘‘space rocks’’ are
essentially made up of nickel, iron, silica, magnesium and oxygen, formed from
molten liquids which also contain small amounts of long-life radio-isotopes, which
can be used like a clock. By measuring the concentration of the radioactive elements and the time necessary for one radioactive isotope to decay and change into
another element, it is then possible to define the date of the meteorites’ formation.
It was determined that the meteorites from our galaxy were formed about 4.6
billion years ago (Myers and Crowley 2000). No rocks as old as this have as yet
been discovered on Earth, but analyzing a zircon compound in a metamorphic
formation in Canada, some scientists have determined an age of 4.1 billion years
(Myers and Crowley 2000). If, as according to astronomers, the age of the
The Birth of Planet Earth
25
and mineralogical and/or chemical composition is related to the way the atomic
structure of the matter is arranged (Broecker 1985).
Since the density of a planet depends on its mass, the ‘‘denser’’ or ‘‘heavier’’ a
planet is, the stronger its gravitational pull will be. Thus, since Venus and Earth are
larger than Mercury, gravity’s attraction on their surface will also be greater. The
pressure generated by a planet’s gravity will cause the atoms forming the planet’s
interior to contract in size. If a planet’s mass is increased, then its gravity will
increase and the contraction of the atoms forming the planet’s material will also
increase.
The information we have concerning the composition of the stars, planets, their
satellites (or moons) and comets has been obtained from astronomical observations
conducted from Earth and, more recently, from observations obtained by spacecraft launched from Earth to explore our galaxy. The astronomical observations on
the elements composing other celestial bodies are based on studies of light
refracted by a prism, in order to separate the light’s different colors into a spectrum. Each color of the prismatic light characterizes an element. Laboratory
experiments to define the various elements consist in calibrating the passage of
light from an electrical arc through a known gas mixture.
The most abundant elements in a star’s interior are hydrogen (H) and helium
(He), and the second most abundant elements are oxygen (O), carbon (C) and neon
(Ne) (Fig. 2.1). Volatiles in comets consist essentially of water and minor amounts
(about 1 %) of other constituents such as CO (carbon monoxide), CO 2 (carbon
dioxide), methanol, formaldehyde, ammonia and hydrogen sulfide (HS 2 ).
Compared to stars or comets, the most abundant elements forming the largest
planets are oxygen, magnesium, silicon and iron with nickel, sulfur, calcium,
aluminum, sodium, potassium, and titanium, in decreasing abundance. The
abundance of other elements drops rapidly, and becomes so insignificant that all
the other elements represent no more than one percent of a planet’s entire composition (White 2013). Due to these compositional variations, it is not surprising to
observe that when we are on Earth, we are walking around on a solid crust which is
mainly made up of elements such as O, Fe, Mg, Si and Al forming crystalline
compounds, which we call minerals and rocks.
Another direct source of knowledge about the composition of our planet comes
from the meteorites, which have fallen on Earth. These ‘‘space rocks’’ are
essentially made up of nickel, iron, silica, magnesium and oxygen, formed from
molten liquids which also contain small amounts of long-life radio-isotopes, which
can be used like a clock. By measuring the concentration of the radioactive elements and the time necessary for one radioactive isotope to decay and change into
another element, it is then possible to define the date of the meteorites’ formation.
It was determined that the meteorites from our galaxy were formed about 4.6
billion years ago (Myers and Crowley 2000). No rocks as old as this have as yet
been discovered on Earth, but analyzing a zircon compound in a metamorphic
formation in Canada, some scientists have determined an age of 4.1 billion years
(Myers and Crowley 2000). If, as according to astronomers, the age of the
The Birth of Planet Earth
25