Hubble’s Contributions
How did elements form in the first place? How did they get where we find them today? Perhaps the first real insight into these questions was made in 1929 by Edwin Hubble. Hubble determined the speed at which some galaxies (in the Virgo cluster) were moving away from Earth.
Hubble and others then noticed that the universe seems to be expanding; all matter is moving away from a central location. This led scientists to hypothesize that the expansion started at one place and time. Based on his observations, Hubble later estimated that one nearly instantaneous event created the universe about two billion years ago. Hubble’s calculations were not widely accepted, particularly because they conflicted with radioactive age dates, which suggested that the Earth was older than two billion years. In spite of the contradictions, Arno Penzias and Robert Wilson confirmed the basic validity of Hubble’s ideas in 1964, for which they subsequently won the Nobel Prize in Physics. Errors have since been found in some constants used in Hubble’s calculations. Current best estimates are that the universe originated in a fraction of a second, during the big bang, about 15 billion years ago (Figure 1). All of its mass and energy were created nearly instantaneously.
FIGURE 1 Time line for our universe.
In the Beginning
At its creation, the universe and all matter in it were at extremely high temperatures. As the universe expanded, it cooled to temperatures near a billion degrees and subatomic particles combined to form nuclei of hydrogen and helium, the lightest elements. This process was incredibly brief; most present-day chemists and cosmologists estimate it continued for only a half hour or so. No elements heavier than helium (Z = 2) formed. After another 750,000 years, scientists estimate that temperatures had cooled to 1,649 °C ( 3,000 °F ) and electrons began to attach themselves to nuclei. The cooler temperatures overall permitted some clumps of matter to begin to come together, even as the universe as a whole continued to expand. Stars, nebulae, and galaxies began to form, increasing their mass and gravitational attraction and the temperatures in their cores. Ultimately, temperatures became hot enough to sustain a “nuclear furnace” powered by hydrogen fusion. The nuclear reactions created heavier elements through a series of complicated reactions occurring in steps at different temperatures. Many stars acquired planets and other orbiting bodies. Today, the space between stars is filled with hydrogen and helium, dating from the original creation of the universe, and remnant heavier elements formed in the interior of stars that have exploded.
The Formation of Our Solar System
Our own solar system formed when a nebula condensed about six billion years ago. The process was very fast at first; hydrogen fusion in our Sun began during the first 100,000 years. The solar nebula contained primordial hydrogen (H) and helium (He) and, for the most part, these elements dominate our solar system today. As the nebula condensed, refractory (unreactive) elements remained in hotter regions. Pressure and temperature gradients led to differentiation of the solid/gas nebula elements that easily vaporized only remained in the cooler outer parts. Various clumps of matter condensed to form planetesimals (with radii ranging from a few meters to 1,000 km/620 mi.) with compositions varying predictably from the center of protosolar system to the outside (Figure 2).
FIGURE 2 Planets and planetesimals.
Minerals formed, with oxides and iron-nickel (Fe-Ni) alloy minerals collecting in the center of the protosolar system and magnesium-iron (Mg-Fe) silicates concentrating farther out. Water, methane, and other volatiles concentrated in the outermost sections. Today, remnant-heavy elements from previously existing stars are concentrated in the terrestrial planets (Mercury, Venus, Earth, and Mars) and parent bodies of meteorites (now asteroids). The Earth and other terrestrial planets seem to have condensed in stages. The core formed first from FeNi–rich planetesimals. More planetesimals, richer in silicon (Si), were added to the outside, thus giving the sharp compositional boundary between the core and mantle. Finally, planetesimals rich in volatile elements were added, ultimately leading to the Earth’s early atmosphere. It seems that, in its early history, the Earth was entirely molten, but it soon cooled and developed a crust.