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This means that the impactor can melt about 25 times its own mass (4.5 X 1014/1.7 X 1013 = 26).Although this is a rough calculation, it shows how effective accretion can be in heating up a growing body, and how it can help the body to attain a spherical shape and to internally differentiate into different compositional shells.

Though they know that the Earth is differentiated, scientists continue to argue about when it happened. Was the heat of accretion enough to melt or partially melt the planet, such that differentiation happened immediately upon accretion? Did the planet differentiate during the giant impact that formed the Moon (more on this in the Moon section)? Was differentiation a more gradual process, moved along by the giant Late Heavy Bombardment, a period of heavy meteorite impact that lasted until about 3.8 billion years ago? On the Moon there is almost irrefutable geochemical evidence for early, complete differentiation, but no equivalently strong evidence exists on Earth. Various researchers are attempting to answer when the Earth differentiated by examining the isotopic systems that can be found in Earth and planetary materials.

Based on its size, it is likely that the Earth heated from accretion sufficiently to melt at least the outer 600 miles (1,000 km) of the planet. If the planet was hot enough in general to allow iron and nickel to flow downward and form a core, the loss of potential energy (energy stored by being away from the center of gravity, and therefore able to be released by falling into the center of gravity) might have been enough to raise the temperature of the planet by 2,700 to 3,600°F (1,500 to 2,000°C). Heat of this magnitude may have melted the entire silicate portion of the Earth, turning the mantle into a massive magma ocean.These are encouraging numbers for early differentiation, but many scientists argue that such heat was unlikely, as these processes occurred over a long enough time that heat was lost by radiation to space and so the early Earth never experienced the peak temperatures at one time.

Short-lived isotopes are radioactive isotopes of certain elements that have short half-lives, short enough to measure events that happen in tens of millions of years (as opposed to billions of years, which can be

Elements and Isotopes A ll the materials in the solar system are made of atoms or of parts of atoms. A family of atoms that all have the same number of positively charged particles in their nuclei (the center of the atom) is called an element: Oxygen and iron are elements, as are aluminum, helium, carbon, silicon, platinum, gold, hydrogen, and well over 200 others. Every single atom of oxygen has eight positively charged particles, called protons, in its nucleus. The number of protons in an atom's nucleus is called its atomic number: All oxygen atoms have an atomic number of 8, and that is what makes them all oxygen atoms.

Naturally occurring nonradioactive oxygen, however, can have either eight, nine, or 10 uncharged particles, called neutrons, in its nucleus, as well. Different weights of the same element caused by addition of neutrons are called isotopes. The sum of the protons and neutrons in an atom's nucleus is called its mass number. Oxygen can have mass numbers of 16 (eight positively charged particles and eight uncharged particles), 17 (eight protons and nine neutrons), or 18 (eight protons and 10 neutrons). These isotopes are written as 16O, 17O, and 18O. The first, 16O, is by far the most common of the three isotopes of oxygen.

Atoms, regardless of their isotope, combine together to make molecules and compounds. For example, carbon (C) and hydrogen (H) molecules combine to make methane, a common gas constituent of the outer planets. Methane consists of one carbon atom and four hydrogen atoms and is shown symbolically as CH Whenever a subscript is placed by the symbol of an element, it indicates how many of those atoms go into the makeup of that molecule or compound.

Quantities of elements in the various planets and moons, and ratios of isotopes, are important ways to determine whether the planets and moons formed from the same material or different materials. Oxygen again is a good example. If quantities of each of the oxygen isotopes are measured in every rock on Earth and a graph is made of the ratios of 17O/16O versus 18O/16O, the points on the graph will form a line with a certain slope (the slope is 1/2, in fact). The fact that the data forms a line means that the material that formed the Earth was homogeneous; beyond rocks, the oxygen isotopes in every living thing and in the atmosphere also lie on this slope. The materials on the Moon also show this same slope. By measuring oxygen isotopes in many different kinds of solar system materials, it has now been shown that the slope of the plot 17O/16O versus 18O/16O is one-half for every object, but each object's line is offset from the others by some amount. Each solar system object lies along a different parallel line.

At first it was thought that the distribution of oxygen isotopes in the solar system was determined by their mass: The more massive isotopes stayed closer to the huge gravitational force of the Sun, and the lighter isotopes strayed farther out into the solar system. Studies of very primitive meteorites called chondrites, thought to be the most primitive, early material in the solar system, showed to the contrary that they have heterogeneous oxygen isotope ratios, and therefore oxygen isotopes were not evenly spread in the early solar system. Scientists then recognized that temperature also affects oxygen isotopic ratios: At different temperatures, different ratios of oxygen isotopes condense. As material in the early solar system cooled, it is thought that first aluminum oxide condensed, at a temperature of about 2,440°F (1,340°C), and then calcium-titanium oxide (CaTiO ), at a temperature of about 2,300°F (1,260°C), and then a calcium-aluminum-silicon-oxide (Ca2Al2SiO?), at a temperature of about 2,200°F (1,210°C), and so on through other compounds down to iron-nickel alloy at 1,800°F (990°C) and water, at -165°F (-110°C) (this low temperature for the condensation of water is caused by the very low pressure of space). Since oxygen isotopic ratios vary with temperature, each of these oxides would have a slightly different isotopic ratio, even if they came from the same place in the solar system.

The key process that determines the oxygen isotopes available at different points in the early solar system nebula seems to be that simple compounds created with l8O are relatively stable at high temperatures, while those made with the other two isotopes break down more easily and at lower temperatures. Some scientists therefore think that l7O and l8O were concentrated in the middle of the nebular cloud, and l6O was more common at the edge. Despite these details, though, the basic fact remains true: Each solar system body has its own slope on the graph of oxygen isotope ratios.

Most atoms are stable. A carbon-12 atom, for example, remains a carbon-12 atom forever, and an oxygen-16 atom remains an oxygen-16 atom forever, but certain atoms eventually disintegrate into a totally new atom. These atoms are said to be "unstable" or "radioactive." An unstable atom has excess internal energy, with the result that the nucleus can undergo a spontaneous change toward a more stable form. This is called "radioactive decay." Unstable isotopes (radioactive isotopes) are called "radioisotopes." Some elements, such as uranium, have no stable isotopes. The rate at which unstable elements decay is measured as a "half-life," the time it takes for half of the unstable

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Elements and Isotopes (continued) atoms to have decayed. After one half-life, half the unstable atoms remain; after two half-lives, one-quarter remain, and so forth. Half-lives vary from parts of a second to millions of years, depending on the atom being considered. Whenever an isotope decays, it gives off energy, which can heat and also damage the material around it. Decay of radioisotopes is a major source of the internal heat of the Earth today: The heat generated by accreting the Earth out of smaller bodies and the heat generated by the giant impactor that formed the Moon have long since conducted away into space.

measured by the uranium-lead decay series). An example of a shortlived radioactive isotope is samarium with mass number 146, meaning samarium with a total of 146 protons plus neutrons in its nucleus, denoted 146Sm. 146Sm decays into 142Nd (neodymium) with a half-life of 103 million years (for more on isotopes and half-lives, see the sidebar "Elements and Isotopes" on page 64). Half the 146Sm decays to 142Nd every 103 million years. If some differentiation event happened in the early solar system that could separate Sm from Nd before all the 146Sm decayed, then the material with excess 146Sm will end up with excess 142Nd, and the material without the 146Sm will end up with a deficit of 142Nd, compared to undifferentiated material. Neodymium isotopes, therefore, can tell us something about early solar system processes.

Using the Sm-Nd system, scientists Charles Harper and Stein Jacobsen at Harvard University found an excess of 142Nd in ancient crustal rocks that led them to conclude that there was a major terrestrial differentiation event, such as a magma ocean, 4.5 billion years ago, plus or minus 100 million years. This 142Nd anomaly has been corroborated by Maud Boyet, a scientist at the Carnegie Institute of Washington, and a team of French and Danish scientists, but they argue that the differentiation event had to have happened within just a few tens of millions of years of planetary accretion.This is a blinding-ly short amount of time, and consistent with a magma ocean that crystallized without very much of an insulating atmosphere to slow its heat loss. Boyet also concludes that the samarium-neodymium data requires that the mantle be differentiated from the rest of the planet completely by 100 million years after planetary accretion.

A second isotopic system, 182Hf-182W (hafnium decaying to tungsten), is useful for studying the time interval between planetary accretion and core formation because tungsten is carried into the core by the iron and nickel, while hafnium tends to stay in the silicate mantle. If core formation occurred after all the hafnium had decayed, then almost all the tungsten would have been carried into the core and little left in the mantle. If core formation occurred before all the 182Hf had decayed, then the mantle would contain more 182W in the long run, after the hafnium has decayed.There are tungsten excesses in the silicate mantle that indicate that the core formed within about 15 million years after planetary accretion, lending support to the magma ocean hypothesis by showing that all the heat of core formation would be available in a short period of time to heat the planet.

Another much-debated issue is the composition of the planetesi-mals that accreted to make the Earth. As discussed in the previous chapter, scientists have some idea of what the composition of the bulk Earth is, though many aspects are still being argued over. Using what is known about the composition of the bulk Earth, and comparing the Earth's composition to the compositions of meteorites, which are thought to represent early solar system material from the asteroid belt, some guesses can be made about the most likely material to have collected together to form the Earth. The meteorite closest to the Earth's bulk composition is a type called an enstatite chondrite. (For more on ways to compare different solar system materials, see the sidebar "Elements and Isotopes" on page 64.)

There are two main categories of theories for the formation of the atmosphere and oceans on Earth: The accretion hypothesis (that the atmosphere came from gases trapped in the original accreting plan-etesimals that made up the Earth); and the capture hypothesis (that the atmosphere came from early solar nebula, or from solar wind, or from comets that impacted the Earth after its formation). There are now better and better computer models for the formation of the planets, and they do indicate that within a few tens of millions of years after core formation the terrestrial planets should have been completely solid and should have formed an early crust. These formation models do indicate that there was so much heat during accretion and solidification that there is a good chance that most of the volatiles (gases and liquids) were lost from the early Earth into space. This weakens the accretion hypotheses for atmospheric formation.

At about 1 million years after its formation, the Sun probably went through a special stage of its evolution called the T-Tauri stage. During its T-Tauri stage a star's early contraction slows or ends, and a very strong outflow of charged particles is released. This outflow was thought to have been strong enough to sweep away all the gaseous atmospheres of the inner solar-system planets. Some scientists think that this is compelling evidence that the inner planets had to obtain their gaseous atmospheres by later additions, such as from comets, or from later outgassing of their interiors. Recent models and work on extinct isotopic systems indicate that the cores of the Earth, Mars, and the Moon all formed within about 15 million years of the beginning of the solar system, but even this early formation may have been long after the Sun's T-Tauri stage. Now, therefore, there is disagreement over whether the evolution of the Sun could have had a significant controlling influence over the formation or retention of atmospheres in the terrestrial planets.

Some researchers, such as Kevin Zahnle at the NASA Ames Research Center, think that the early Earth was accreted from both rocky planetesimals and icy cometary matter. In this model, huge quantities of water were added to the early Earth, so much that the vast amounts lost to space during the heat and atmospheric disruption of giant impacts, and the evaporation into space due to the large quantities of accretional heat still present in the early Earth still left plenty of water to form oceans. Even if the Earth was accreted entirely from ordinary chondrite meteoritic material, it would have started with about one-tenth of a percent of water by weight (0.1 percent, the amount naturally existing in ordinary chondrites), and this is still two to four times as much water as the Earth is thought to have today (though the amount of water in oceans and in ice on the surface of the Earth is well known, the amount of water existing in trace amounts in deep minerals in the Earth's interior is not known).

There was a period of heavy bombardment of inner solar system bodies by large bolides that lasted until about 3.8 billion years ago. The giant basins on the Moon, filled with dark basalt and so visible to the eye from Earth, are left over from that period, called the Late Heavy Bombardment. Some scientists believe that the Late Heavy

Bombardment was a specific period of very heavy impact activity that lasted from about 4.2 to 3.8 billion years ago, while others believe that it was just the tail end of a continuously decreasing rate of bombardment that began at the beginning of the solar system. In this continual bombardment model, the last giant impacts from 4.2 to 3.8 billion years ago simply erased the evidence of all the earlier bombardment.

If, alternatively, the Late Heavy Bombardment was a discrete event, then some reason for the sudden invasion of the inner solar system by giant bolides must be discovered.Were they bodies perturbed from the outer solar system by the giant planets there? If they came from the outer solar system, then more of the material was likely to be water-rich cometary material. If as much as 25 percent of the Late Heavy Bombardment was cometary material, it would have contributed enough water to the Earth to create its oceans. There is some evidence that there was liquid water on the Earth's surface earlier than 4.2 billion years ago. In the section titled "How Old Is the Earth's Surface?" (see page 71) the compositions of some very ancient mineral grains of a type called zircon are discussed.These zircons are 4.4 billion years old, and their oxygen isotopic compositions appear to indicate that liquid water was present at that time.

The theory of cometary accretion for ocean formation has another problem. Recent observations of the comets Halley, Hyakutake, and Hale-Bopp have revealed that their water is twice as enriched in deuterium than water on Earth (deuterium is water made with hydrogen that contains a neutron as well as a proton in its nucleus; this is also called heavy water, because the neutron added to each hydrogen in the water molecule increases its mass by about 10 percent).There is no explanation of why, if the Earth's oceans came from heavy cometary water, they have since preferentially lost the heavy water and arrived at their current low-deuterium composition. This indicates that the water on Earth probably came from sources other than comets.

Venus and Mars probably received water in the same way Earth did, since they are near neighbors in the solar system. On Venus, the water was lost because of the high input of solar energy: The heat of the Sun evaporated the water high into the atmosphere, where the heavy bombardment of ultraviolet radiation broke the water molecules into their constituent water and hydrogen atoms, which were then light enough to escape into space. In this way Venus, closer to the Sun, lost its water. Being far from the Sun is equally bad for oceans: If the planet is colder, water will freeze. Frozen water reflects more energy away from the planet and back into space, making the planet colder still. This may be part of the reason that, though Mars may once have had oceans, its water is now frozen into the soil.

The optimal distance from the Sun for a planet to maintain a liquid ocean has also changed over time. When the Sun first burst into nuclear life, it is thought to have been about 30 percent dimmer than it is today. This means that the critical range of radii for planetary orbits that would allow the retention of liquid water was closer to the Sun, and that this zone habitable for water-based life is moving away from the Sun as it continues to heat up and give off more radiation. If there were oceans on Mars today, it is more likely that the planet could retain them and avoid freezing. Similarly, in about a billion years, the Sun will have heated up to the point that the Earth will be too close to it to retain its oceans, and the atmosphere here on Earth will convert to something more like Venus, and will be much less hospitable to life.

On the early Earth, the atmosphere is thought to have been much richer in carbon dioxide and methane, both chemicals that help retain heat in the atmosphere and keep the surface of the planet warm (they are so-called greenhouse gases). As life developed on Earth and more and more organisms used photosynthesis, the oxygen content of the atmosphere was built up. Because of the oceans of water on Earth, excess carbon dioxide can be converted into carbonate minerals, such as calcite (CaCO ). Moderating the carbon dioxide content of the atmosphere helps keep the Earth's surface temperature within the range conducive to life.

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